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Isotopomer-selective spectra of a single intact H2O molecule in the Cs+(D2O)5H2O isotopologue: Going beyond pattern recognition to harvest the structural information encoded in vibrational spectra
Abstract
We report the vibrational signatures of a single H2O molecule occupying distinct sites of the hydration network in the Cs+(H2O)6 cluster. This is accomplished using isotopomer-selective IR-IR hole-burning on the Cs+(D2O)5(H2O) clusters formed by gas-phase exchange of a single, intact H2O molecule for D2O in the Cs+(D2O)6 ion. The OH stretching pattern of the Cs+(H2O)6 isotopologue is accurately recovered by superposition of the isotopomer spectra, thus establishing that the H2O incorporation is random and that the OH stretching manifold is largely due to contributions from decoupled water molecules. This behavior enables a powerful new way to extract structural information from vibrational spectra of size-selected clusters by explicitly identifying the local environments responsible for specific infrared features. The Cs+(H2O)6 structure was unambiguously assigned to the 4.1.1 isomer (a homodromic water tetramer with two additional flanking water molecules) from the fact that its computed IR spectrum matches the observed overall pattern and recovers the embedded correlations in the two OH stretching bands of the water molecule in the Cs+(D2O)5(H2O) isotopomers. The 4.1.1 isomer is the lowest in energy among other candidate networks at advanced (e.g., CCSD(T)) levels of theoretical treatment after corrections for (anharmonic) zero-point energy. With the structure in hand, we then explore the mechanical origin of the various band locations using a local electric field formalism. This approach promises to provide a transferrable scheme for the prediction of the OH stretching fundamentals displayed by water networks in close proximity to solute ions.
... At low temperature, the spectrum of the mass-selected ion packet consists of a heterogeneous combination of the spectra of individual isotopomers, each of which can be isolated using isotopomer-selective vibrational spectroscopic methods as discussed at length in previous studies (17)(18)(19)(20). Isotopomer selection is achieved by sequentially removing each isotopomer from the ion packet through infrared (IR) photodissociation at each of the five band positions with an initial IR laser with pulse width ∼8 ns (hereafter denoted the "bleach" laser). ...
... For reference, the evaporation time (1/ k evap ) is measured to be ∼17 ms at 200 K as described in detail in SI Appendix, Fig. S6. We note that the observed evolution of the spectra with temperature is consistent with nano-calorimetric measurements reported earlier (24) on H 3 O + ·(H 2 O) 20 . In that case, an inflection in the caloric curve near 135 K was interpreted to signal the cluster analog of a melting transition, which occurs below the onset of evaporation at around 150 K (24)(25)(26)(27)(28). ...
... In that case, an inflection in the caloric curve near 135 K was interpreted to signal the cluster analog of a melting transition, which occurs below the onset of evaporation at around 150 K (24)(25)(26)(27)(28). We note also that recent molecular dynamics simulations of H 3 O + ·(H 2 O) 20 by Korchagina et al. (29) indicate that the melting transition occurs over the 130 to 149 K range and that, although the PD structure dominates up to the melting region, cage structures containing four-membered rings begin to appear at 130 K. This is consistent with our temperature-dependent spectra, which display significant broadening of the bands as they are overcome by a diffuse background absorption between 120 and 150 K (Fig. 1 D and E). ...
Significance
Water’s vibrational spectrum is dynamic: The OH oscillator frequency changes spontaneously within a diffuse envelope on an ultrafast timescale. Here, we explore the mechanics that drive this “spectral diffusion” at the molecular level by following the time-dependent frequency of single OH oscillators, each embedded in a cage of 20 deuterated water molecules, as a function of temperature. These cages are isolated in the gas phase and incorporate a single, isotopically labeled OH group that can occupy many spectroscopically distinct sites. The rates of spontaneous change in the OH frequency reflect the pathways for migration of the isotopic label among these sites, which occurs on a remarkably long (approximately millisecond) timescale at the onset of large-amplitude motion near 100 K.
... Recently, we explored a QM description of electric fields and solvochromic frequency shifts of the OH vibrations in a low temperature Cs + (H 2 O) 6 cluster experimentally investigated by Johnson and coworkers. 56 As shown in Fig. 3, the projected electric fields at each water molecule's H sites are calculated as those arising from the QM charge densities of all the other atoms, Cs + (H 2 O) 5 , in the system excluding only those atoms (nuclei, electrons, and basis functions) of the H 2 O probe for which the field are being evaluated. Here it is important to note that in the QM case we are wanting to evaluate the ''effective'' fields when the total system is at a stationary point (i.e., a stable minimum energy configuration, as opposed to sampling from a thermal ensemble such as those investigated using molecular dynamics simulations 17,43,45,48 ). ...
... (Right) Comparison of the measured vibrational spectra (red) and that from the projected QM fields (black bars) and Lorentzian fits to QM fields (blue). See Wolke et al. 56 applying external fields to accurately determine the local field correction factor f. ...
The electric fields and potentials inside and at the interface of matter are relevant to many branches of physics, chemistry, and biology. Accurate quantification of these fields and/or potentials is essential to control and exploit chemical and physical transformations. Before we understand the response of matter to external fields, it is first important to understand the intrinsic interior and interfacial fields and potentials, both classically and quantum mechanically, as well as how they are probed experimentally. Here we compare and contrast, beginning with the hydrogen atom in vacuum and ending with concentrated aqueous NaCl electrolyte, both classical and quantum mechanical electric potentials and fields. We make contact with experimental vibrational Stark, electrochemical, X-ray, and electron spectroscopic probes of these potentials and fields, outline relevant conceptual difficulties, and underscore the advantage of electron holography as a basis to better understand electrostatics in matter.
... In this approach, a probe laser is tuned to a particular transition in the spectrum, and the population of ions responsible for that feature is monitored continuously while another, powerful pump laser is scanned through the entire spectrum upstream from the interaction with the probe laser. When the pump laser excites any transition shared by the species which is monitored by the probe frequency, the reduction in ion population yields a series of dips in the probe signal, and thus reveals all the transitions and line broadening (beyond the ~5 cm −1 bandwidth of the infrared lasers) associated with OH occupation in a particular site 38,39 . ...
The extremely broad infrared spectrum of water in the OH stretching region is a manifestation of how profoundly a water molecule is distorted when embedded in its extended hydrogen-bonding network. Many effects contribute to this breadth in solution at room temperature, which raises the question as to what the spectrum of a single OH oscillator would be in the absence of thermal fluctuations and coupling to nearby OH groups. We report the intrinsic spectral responses of isolated OH oscillators embedded in two cold (~20 K), hydrogen-bonded water cages adopted by the Cs+·(HDO)(D2O)19 and D3O+·(HDO)(D2O)19 clusters. Most OH oscillators yield single, isolated features that occur with linewidths that increase approximately linearly with their redshifts. Oscillators near 3,400 cm−1, however, occur with a second feature, which indicates that OH stretch excitation of these molecules drives low-frequency, phonon-type motions of the cage. The excited state lifetimes inferred from the broadening are considered in the context of fluctuations in the local electric fields that are available even at low temperature.
... Usually, these experiments monitor the dissociation of a weakly bound, non-interacting messenger (or "tag") upon irradiation to record a linear IR spectrum. [80][81][82][83] As has been shown very recently, similarly diagnostic ngerprints for glycans can also be obtained using these experimental setups 84 and even the development of userfriendly, commercial instruments is conceivable for the future. ...
Although there have been substantial improvements in glycan analysis over the past decade, the lack of both high-resolution and high-throughput methods hampers progress in glycomics. This perspective article highlights the current developments of liquid chromatography, mass spectrometry, ion-mobility spectrometry and cryogenic IR spectroscopy for glycan analysis and gives a critical insight to their individual strengths and limitations. Moreover, we discuss a novel concept in which ion mobility-mass spectrometry and cryogenic IR spectroscopy is combined in a single instrument such that datasets consisting of m/z, collision cross sections and IR fingerprints can be obtained. This multidimensional data will then be compared to a comprehensive reference library of intact glycans and their fragments to accurately identify unknown glycans on a high-throughput scale with minimal sample requirements. Due to the complementarity of the obtained information, this novel approach is highly diagnostic and also suitable for the identification of larger glycans; however, the workflow and instrumentation is straightforward enough to be implemented into a user-friendly setup.
... As such, we cannot with confidence establish the origin of a 3 /b 3 ; in addition to a smaller than expected IHB splitting, overtones of out-of-plane bends and combination bands between the IHB OH asym and soft modes all have precedents in strongly H-bonded anion hydrates arising from both mechanical and electrical anharmonicities, and should be considered possible alternative candidates for weaker bands in this energy region. [41][42][43][44] A useful approach to resolve some of this uncertainty would be to experimentally establish the coupling between the OH oscillators using site-specific isotopic substitution, 45,46 and such studies are currently underway. ...
We report the isotope-dependent vibrational predissociation spectra of the H2-tagged OH⁻ ⋅ (H2O)n=2,3 clusters, from which we determine the strongly coordination-dependent energies of the fundamentals due to the OH groups bound to the ion and the intramolecular bending modes of the water molecules. The HOH bending fundamental is completely missing in the delocalized OH⁻ ⋅ (H2O) binary complex but is recovered upon adding the second water molecule, thereby establishing that the dihydrate behaves as a hydroxide ion solvated by two essentially intact water molecules. The energies of the observed OH stretches are in good agreement with the values predicted by Takahashi and co-workers [Phys. Chem. Chem. Phys. 17, 25505 (2015); 15, 114 (2013)] with a theoretical model that treats the strong anharmonicities at play in this system with explicit coupling between the bound OH groups and the O–O stretching modes on an extended potential energy surface. We highlight a surprising similarity between the spectral signatures of OH⁻ ⋅ (H2O)3 and the excess proton analogue, H3O⁺ ⋅ (H2O)3, both of which correspond to completed hydration shells around the proton defect. We discuss the origin of the extreme solvatochromicity displayed by both OH⁻ and H⁺ in the context of the anomalously large “proton polarizabilities” of the H5O2 ⁺ and H3O2 ⁻ binary complexes.
Decoding the structural information contained in the interfacial vibrational spectrum of water requires understanding how the spectral signatures of individual water molecules respond to their local hydrogen bonding environments. In this study, we isolated the contributions for the five classes of sites that differ according to the number of donor (D) and acceptor (A) hydrogen bonds that characterize each site. These patterns were measured by exploiting the unique properties of the water cluster cage structures formed in the gas phase upon hydration of a series of cations M+·(H2O)
n
(M = Li, Na, Cs, NH4, CH3NH3, H3O, and n = 5, 20-22). This selection of ions was chosen to systematically express the A, AD, AAD, ADD, and AADD hydrogen bonding motifs. The spectral signatures of each site were measured using two-color, IR-IR isotopomer-selective photofragmentation vibrational spectroscopy of the cryogenically cooled, mass selected cluster ions in which a single intact H2O is introduced without isotopic scrambling, an important advantage afforded by the cluster regime. The resulting patterns provide an unprecedented picture of the intrinsic line shapes and spectral complexities associated with excitation of the individual OH groups, as well as the correlation between the frequencies of the two OH groups on the same water molecule, as a function of network site. The properties of the surrounding water network that govern this frequency map are evaluated by dissecting electronic structure calculations that explore how changes in the nearby network structures, both within and beyond the first hydration shell, affect the local frequency of an OH oscillator. The qualitative trends are recovered with a simple model that correlates the OH frequency with the network-modulated local electron density in the center of the OH bond.
The 1,3-diaza-2,4-diborobutane (NBNB) molecule serves as the smallest model complex of an intramolecular “dihydrogen bond,” which involves a nominally hydrogen-bonding interaction between amine and borane hydrogen atoms. In the present study, the role of this dihydrogen bond in influencing the inherent molecular dynamics of NBNB is investigated computationally with ab initio molecular dynamics and path integral molecular dynamics techniques, as well as vibrational spectra analysis and static quantum chemistry computations. These simulations indicate that the dihydrogen-bonding interaction impacts both the high- and low-frequency motions of the molecule, with the dominant motions involving low-frequency backbone isomerization and terminal amine rotation. Geometric isotope effects were found to be modest. The analysis also addresses the paradoxical fostering of amine rotation via a relatively strong dihydrogen-bond interaction. Electrostatic and geometric factors most directly explain this effect, and although some orbital evidence was found for a small covalent component of this interaction, the dynamics and electronic structure suggest that electrostatic contributions are the controlling factors for molecular motion in NBNB.
Wet surface sightings in clusters
In principle, the surface structure of water (H 2 O) should be discernable from the O–H vibrations. In practice, however, so many configurations rapidly interconvert that the bands are bewilderingly broad. Yang et al. studied a cluster of 20 H 2 O surrounding a cesium ion, using isotopomers that vary in the position of one H 2 O amid 19 heavy water (D 2 O) molecules. Precisely assigned spectral features from contributing configurations mapped well onto a bulk surface spectrum.
Science , this issue p. 275
The IR predissociation spectra of microsolvated glycine and L-alanine, GlyH+(H2O)n and AlaH+(H2O)n, n=1-6, are presented. The assignments of the solvation structures are aided by H2O/D2O substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the interactions between water and amino acid as well as water and water molecules, and the subtle effects of the methyl side-chain in L-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1-2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen-bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in L-alanine. For n = 4-6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominate isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretch on the amino acid, as well as the first observation of H2O-H2O interactions in the spectra of n = 5. Finally, we note that calculations at the DFT level show excellent agreement with the experiment for the smaller clusters. But when water-water interaction competes with water-amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared with MP2 results.
The structure of hydrogen bonded networks is intimately intertwined with their dynamics. Despite the incredibly wide range of hydrogen bond strengths encountered in water clusters, ion-water clusters and liquid water, we demonstrate that the previously reported correlation between the change in the equilibrium bond length of the hydrogen bonded OH covalent bond and the corresponding shift in its harmonic frequency in water clusters is much more broadly applicable. Surprisingly, this correlation describes the ratios for both the equilibrium OH bond length/harmonic frequency, and the vibrationally averaged bond length/anharmonic frequency in water, hydronium water and halide water clusters. Consideration of harmonic and anaharmonic data leads to a correlation of -19 ± 1 cm⁻¹/0.001 Å. The fundamental nature of this correlation is further confirmed through the analysis of ab initio Molecular Dynamics (AIMD) trajectories for liquid water. We demonstrate that this simple correlation for both harmonic and anharmonic systems can be modeled by the response of an OH bond to an external field. Treating the OH bond as a Morse oscillator, we develop analytic expressions, which relate the ratio of the shift in the vibrational frequency of the hydrogen-bonded OH bond to the shift in OH bond length, to parameters in the Morse potential and the ratio of the first and second derivatives of the field-dependent projection of the dipole moment of water onto the hydrogen-bonded OH bond. Based on our analysis, we develop a protocol for reconstructing the AIMD spectra of liquid water from the sampled distribution of the OH bond lengths. Our findings elucidate the origins of the relationship between the molecular structure of the fleeting hydrogen-bonded network and the ensuing dynamics, which can be probed by vibrational spectroscopy.
Although several mechanisms concerning the biological function of lithium salt, a drug having tranquilizing abilities, have been proposed so far, the key mechanism for its selectivity and subsequent interaction with neurotransmitters has not been established yet. We report ultraviolet (UV) and infrared (IR) spectra under ultra-cold conditions of Li+ and Na+ complexes of noradrenaline (NAd, norepinephrine), a neurotransmitter responsible for the body’s response to stress or danger. A detailed analysis of the IR spectra, aided by quantum chemical calculations, reveals that the Li+-noradrenaline (NAd-Li+) complex forms only the extended structure, while the NAd-Na+ and protonated (NAd-H+) complexes form both the folded and extended structures. This conformational preference of the NAd-Li+ complex is further explained from considering specific conformational distributions in solution. Our results clearly discern the unique structural motifs that Li+ adopts when interacting with NAd compared with other abundant cations in the human body (Na+) and can form the basis towards a molecular level understanding of the selectivity of Li+ in biological systems.
We report vibrational spectra of the cryogenically cooled H9O4+ cation along with those of the D2-tagged HD8O4+ isotopomers using two variations on a two-color, IR-IR double resonance photoexcitation scheme. The spectrum of the isolated H9O4+ ion consists of two sharp features in the OH stretching region that indicate exclusive formation of the "Eigen" cation, the H3O+·(H2O)3 isomer that corresponds to the filled hydration shell around the hydronium ion. Consistent with this structural assignment, the spectrum of the HD8O4+ isotopologue is resolved into contributions from two isotopomers: one with the single OH group on one of the three solvent water molecules and another in which it resides on the hydronium core ion. The latter spectrum is dominated by a broad feature assigned to the isolated hydronium OH stretching fundamental with an envelope that is similar to that displayed by the H3O+·(H2O)3 isotopologue. The feature appears with a diffuse band ~380 cm-1 above it, which is assigned to a combination band involving the frustrated translation mode of the HD2O+ core and one of the solvating water molecules. These trends are analyzed with anharmonic calculations involving four mode coupling on a realistic potential surface, and interpreted in the context of vibrationally adiabatic potentials.
We describe a two-color, isotopomer-selective IR-IR population labeling method that can monitor very slow spectral diffusion of OH oscillators in H-bonded networks and apply it to the I⁻·(HDO)·(D2O) and I⁻·(H2O)·(D2O) systems, cryogenically cooled and D2-tagged at an ion trap temperature of 15 K. These measurements reveal very large (>400 cm⁻¹), spontaneous spectral shifts despite the fact that the predissociation spectra in the OH stretching region of both isotopologues are sharp and readily assigned to four fundamentals of largely decoupled OH oscillators held in a cyclic H-bonded network. This spectral diffusion is not observed in the untagged isotopologues of the dihydrate clusters that are generated under the same source conditions, but does become apparent at about 75 K. These results are discussed in the context of the large amplitude “jump” mechanism for H-bond relaxation dynamics advanced by Laage and Hynes in an experimental scenario where rare events can be captured by following the migration of OH groups among the four available positions in the quasi-rigid equilibrium structure.
Processes of N2O5 in water media are of great importance in atmospheric chemistry, and have been the topic of extensive research for over two decades. Nevertheless, many physical and chemical properties of N2O5 at the surface or in bulk water are unknown, or not microscopically understood. This paper presents extensive new results on physical properties of N2O5 in or at the surface of water, with a focus on their microscopic basis. The main results are obtained using ab initio molecular dynamics and calculations of a potential of mean force. These include: (1) Collisions of N2O5 with water at 300 K lead to trapping at the surface, for at least 25 ps with 95% probability. (2) During that time, there is no hydrolysis, evaporation, or entry into the bulk of N2O5. (3) There is a charge separation between the NO2 and NO3 groups of N2O5, which fluctuates significantly with time. (4) Energy accommodation of the colliding N2O5 at the surface takes place within picoseconds. (5) The binding energy of N2O5 to a nanosize amorphous ice particle at 0 K is on the order of 15 kcal mol-1, for the main surface site. The binding is due to one weak hydrogen bond, and to interactions between partial charges on the N2O5 and on water. (6) The free-energy profile was calculated for transporting N2O5 from the gas phase through the interface and into bulk water. The corresponding concentration profile exhibited a propensity for N2O5 at the aqueous surface. The free energy barrier for entry from the surface into the bulk was determined to be 1.8 kcal mol^-1. The above findings are used for the interpretation of recent experiments. These are discussed with focus on implications to atmospheric chemistry.
We present the infrared predissociation (IRPD) study of microsolvated GlyH⁺(H2O)n and GlyH⁺(D2O)n clusters, formed inside a cryogenic ion trap via condensation of H2O or D2O onto the protonated glycine ions. The resulting IRPD spectra, showing characteristic O-H and O-D stretches, indicate that H/D exchange reactions are quenched when the ion trap is held at 80K, minimizing the presence of isotopomers. Comparisons of GlyH⁺(H2O)n and GlyH⁺(D2O)n spectra clearly highlight and distinguish the vibrational signatures of the water solvent molecules from those of the core GlyH+ ion, allowing for quick assessment of solvation structures. Without the aid of calculations, we can already infer solvation motifs and presence of multiple conformations. The use of a cryogenic ion trap to cluster solvent molecules around ions of interest and control H/D exchange reactions is broadly applicable and should be extendable to studies of more complex peptidic ions in large solvated clusters.
Macroion-droplet interactions play a critical role in many settings such as ionization techniques of samples in mass spectrometry analysis and atmospheric aerosols. The droplets under investigation are composed of a polar solvent, primarily water, a charged macroion and, possibly, buffer ions. We present highlights of our research on the relation between the charge state of a macroion and the droplet morphologies. We have determined that depending on the charge on the macroion and certain macroscopic properties of the solvent, such as its dielectric constant and surface tension, a droplet may obtain striking conformations such as droplets with extruded tails, ``pearl-necklace'' conformations and multi-point ``star'' shapes. The shapes of the droplet containing the macroion influence the charging mechanism of the macroion in a reciprocal manner. The understanding of the macroion-droplet interactions play a central role in explaining the origin and the magnitude of the charge in spectra obtained in electrospray ionization mass spectrometry experiments and in controlling the stability of complexes of nucleic acids and proteins in droplets.
We reveal the microscopic mechanics of iodide ion microhydration by recording the isotopomer-selective vibrational spectra of the I⁻·(H2O)·(D2O), I⁻·(HOD)·(D2O), and I⁻·(DOH)·(H2O) isotopologues using a new class of ion spectrometer that is optimized to carry out two-color, IR-IR photodissociation in a variety of pump-probe schemes. Using one of these, we record the linear absorption spectra of a cryogenically cooled cluster without the use of a messenger “tag”. In another protocol, we reveal the spectra of individual H2O and D2O molecules embedded in each of the two possible binding sites in the iodide dihydrate, as well as the bands due to individual OH and OD groups in each of the four local binding environments. Finally, we demonstrate how temperature dependent isotopic scrambling among the spectral features can be used to monitor the onset of large amplitude motion, heretofore inferred from changes in the envelope of the OH stretching vibrational manifold.
Die Vielfalt der in Glykanen und ihren Konjugaten vorhandenen stereochemischen Isomere stellt eine Herausforderung für ihre Strukturanalyse dar. Üblicherweise wird Massenspektrometrie (MS) in Kombination mit Flüssigchromatographie oder Ionenmobilitätstrennung verwendet. Allerdings machen strukturell ähnliche Isomere, die häufig koexistieren, eine eindeutige Identifizierung unmöglich. Andere Methoden wie Gasphasen-Infrarot-Spektroskopie waren bisweilen auf kleinere Glykane beschränkt, da die räumliche Flexibilität und die thermische Aktivierung während der Messung eine mangelnde spektrale Auflösung verursachen. Dieses Defizit kann durch Kaltionen-Spektroskopie überwunden werden. Die spektralen Fingerabdrücke kalter Oligosaccharid-Ionen weisen eine beträchtliche Anzahl hochaufgelöster Absorptionsbanden auf, die selbst minimale Strukturvariationen anzeigen. Kaltionen-Spektroskopie könnte sich in Verbindung mit sequenzieller MS als Schlüsseltechnologie zur Strukturbestimmung komplexer Glykome erweisen.
The diversity of stereochemical isomers present in glycans and glycoconjugates poses a formidable challenge for comprehensive structural analysis. Typically, sophisticated mass spectrometry (MS)-based techniques are used in combination with chromatography or ion mobility separation. However, coexisting structurally similar isomers often render an unambiguous identification impossible. Other powerful techniques such as gas-phase infrared (IR) spectroscopy have been limited to smaller glycans, as conformational flexibility and thermal activation during the measurement result in poor spectral resolution. Here, we show that this limitation can be overcome using cold-ion spectroscopy. The vibrational fingerprints of cold oligosaccharide ions exhibit a wealth of well-resolved absorption features that are diagnostic for minute structural variations. The unprecedented resolution of cold-ion spectroscopy coupled with tandem MS may render this the key technology to unravel complex glycomes.
Incorporation of the unnatural D-proline ((D)P) stereoisomer into a polypeptide sequence is a typical strategy to encourage formation of ?-hairpin loops because natural sequences are often unstructured in solution. Using conformation-specific IR and UV spectroscopy of cold (? 10 K) gas-phase ions, we probe the inherent conformational preferences of the (D)P and (L)P diastereomers in the protonated peptide [YAPAA+H](+), where only intramolecular interactions are possible. Consistent with the solution phase studies, one of the conformers of [YA(D)PAA+H]+ is folded into a charge-stabilized ?-hairpin turn. However, a second predominant conformer family containing two sequential ?-turns is also identified, with similar energetic stability. A single conformational isomer of the LP diastereomer, [YA(L)PAA+H]+, is found and assigned to a structure that is not the anticipated "mirror image" ?-turn. Instead, the (L)P stereo center promotes a cis alanine-proline amide bond. The assigned structures contain clues that the preference of the (D)P diastereomer to support a trans-amide bond and the proclivity of (L)P for a cis-amide bond is sterically driven and can be reversed by substituting glycine for alanine in position 2, forming [YG(L)PAA+H](+). These results provide a basis for understanding the residue-specific and stereo-specific alterations in the potential energy surface that underlie these changing preferences, providing insights to the origin of ?-hairpin formation.
The structure and properties of the proton in water are of fundamental importance in many areas of chemistry and biology. The high mobility of the proton in an aqueous solution is understood in terms of its “hopping” between neighboring water molecules, as suggested by the two-century-old Grotthuss mechanism. The barrier for this process intimately depends on the proton's surrounding environment, which is manifested by the connectivity of the immediate hydrogen-bonding network as well as its dynamics caused by thermal fluctuations. On page 1131 of this issue, Wolke et al. ( 1 ) shed new light on the role that the proton's water neighbors play toward facilitating positive charge translocation within a hydrogen-bonded network in a cold water cluster.
Gas phase reactions of O2•‐(H2O)m and CO2•−(H2O)m with HCl as well as HNO3 are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. HCl and HNO3 are efficiently taken up by the clusters. In the reactions of O2•‐(H2O)m, a characteristic mass change is observed which is consistent with the evaporation of a hydroperoxyl radical HO2, most likely formed via proton transfer to O2•−. Using CO2•−(H2O)m as the ionic reactant, products indicating the formation of HOCO are hardly observed in the HCl reaction. With HNO3, however, evidence is found for elimination of HOCO, resulting from proton transfer to CO2•‐, when the clusters have lost almost all water molecules. Thermochemical cycles using literature thermochemistry show that the reactions are exothermic.
We describe an ultrabroadband IR-visible sum-frequency (SF) setup that allows simultaneous acquisition of the entire vibrational spectrum of water molecules at mineral surfaces in the OH stretching region without ever tuning the IR laser pulses. Our newly developed 800-nm pumped noncollinear optical parametric amplifier (NOPA) generates broadband mid-IR pulses (~1800-3500 nm, or ~2900 – 6000 cm⁻¹) with bandwidths >600 cm⁻¹ at half-maximum at near 3500 cm⁻¹. Using the ultra-broadband IR NOPA, we constructed a sum-frequency vibrational spectrometer that allowed the acquisition of spectra of the OH stretches of water at hydrophilic and hydrophobic silica surfaces, over the frequency range ~2900 – 3800 cm⁻¹, within 60 s, much shorter than with scanning SFG spectrometers. The ultra-broadband SFG spectrometer reported here can be potentially applied to time-resolved measurements of kinetics at interfaces.
In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
Pyridine containing water clusters, H(+)(pyridine)(m)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. In the experiments, H(+)(pyridine)(m)(H(2)O)(n) with m = 1-4 and n = 0-80 are observed. For the cluster distributions observed, there are no magic numbers, neither in the abundance spectra, nor in the evaporation spectra from size selected clusters. Experiments with size-selected clusters H(+)(pyridine)(m)(H(2)O)(n), with m = 0-3, reacting with D(2)O at a center-of-mass energy of 0.1 eV were also performed. The cross-sections for H/D isotope exchange depend mainly on the number of water molecules in the cluster and not on the number of pyridine molecules. Clusters having only one pyridine molecule undergo D(2)O/H(2)O ligand exchange, while H(+)(pyridine)(m)(H(2)O)(n), with m = 2, 3, exhibit significant H/D scrambling. These results are rationalized by quantum chemical calculations (B3LYP and MP2) for H(+)(pyridine)(1)(H(2)O)(n) and H(+)(pyridine)(2)(H(2)O)(n), with n = 1-6. In clusters containing one pyridine, the water molecules form an interconnected network of hydrogen bonds associated with the pyridinium ion via a single hydrogen bond. For clusters containing two pyridines, the two pyridine molecules are completely separated by the water molecules, with each pyridine being positioned diametrically opposite within the cluster. In agreement with experimental observations, these calculations suggest a "see-saw mechanism" for pendular proton transfer between the two pyridines in H(+)(pyridine)(2)(H(2)O)(n) clusters.
Many chemical reactions in atmospheric aerosols and bulk aqueous environments are influenced by the surrounding solvation
shell, but the precise molecular interactions underlying such effects have rarely been elucidated. We exploited recent advances
in isomer-specific cluster vibrational spectroscopy to explore the fundamental relation between the hydrogen (H)–bonding arrangement
of a set of ion-solvating water molecules and the chemical activity of this ensemble. We find that the extent to which the
nitrosonium ion (NO+)and water form nitrous acid (HONO) and a hydrated proton cluster in the critical trihydrate depends sensitively on the geometrical
arrangement of the water molecules in the network. Theoretical analysis of these data details the role of the water network
in promoting charge delocalization.
It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D(2)O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H(2)O indicates that this fast feature is due to an overtone transition of Zundel-like H(3)O(2)(-) states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2-3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.
Electric field fluctuations play a major role in dissociation reactions in
liquid water and determine its vibrational spectroscopic response. Here, we
study the statistics of electric fields in liquid water using molecular
dynamics computer simulations with a particular focus on the strong but rare
fields that drive dissociation. Our simulations indicate that the important
contributions to the electric field acting on OH bonds stem from water
molecules less than 7 {\AA} away. Long-ranged contributions play a minor role.
We review the role that gas-phase, size-selected protonated water clusters, H(+)(H2O)n, have played in unraveling the microscopic mechanics responsible for the spectroscopic behavior of the excess proton in bulk water. Because the larger (n≥10) assemblies are formed with three-dimensional cage morphologies that more closely mimic the bulk environment, we report the spectra of cryogenically cooled (10 K) clusters over the size range 2<n<28, over which the structures evolve from two-dimensional arrangements to cages at around n=10. The clusters that feature a complete, second solvation shell around a surface-embedded hydronium ion yield similar spectral signatures of the proton defect to those observed in dilute acids. The origins of the large observed shifts in the proton vibrational signature upon cluster growth were explored with two types of theoretical analyses. First, we calculate the cubic and semi-diagonal quartic force constants and use these in vibrational perturbation theory calculations to establish the couplings responsible for the large anharmonic red shifts. We then explore how the extended electronic wavefunctions that are responsible for the shapes of the potential surfaces depend on the nature of the H-bonded networks surrounding the charge defect. These considerations indicate that, in addition to the sizeable anharmonic couplings, the position of the OH stretch most associated with the excess proton can be traced to large increases in the electric fields exerted on the embedded hydronium ion upon formation of the first and second solvation shells. The correlation between the underlying local structure and the observed spectral features is quantified using a model based on Badger's rule as well as via the examination of the electric fields obtained from electronic structure calculations.
We studied the Stark effect on the hydroxyl stretching vibration of water molecules in ice under the influence of an external electric field. Electric fields with strengths in the range of 6.4 × 10**7 - 2.3 × 10**8 V·m−1 were applied to an ice sample using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the field-induced spectral changes of vibrationally decoupled O-H and O-D bands of dilute HOD in D2O and H2O-ice, respectively. The spectral changes of the hydroxyl bands under applied field were analyzed using a model that can simulate the absorption of a collection of Stark-shifted oscillators. The analysis shows that the Stark tuning rate of ν(O-D) is 6.4 - 10 cm−1/(MV·cm−1) at a field strength of 1.8 × 10**8 - 6.4 × 10**7 V·m−1, and the Stark tuning rate of ν(O-H) is 10 - 16 cm−1/(MV·cm−1) at a field strength of 2.3 × 10**8 - 9.2 × 10**7 V·m−1. These values are uniquely large compared to the Stark tuning rates of carbonyl or nitrile vibrations in other frozen molecular solids. Quantum mechanical calculations for the vibrations of isolated water and water clusters show that the vibrational Stark effect increases with the formation of intermolecular hydrogen bonds. This suggests that that the large vibrational Stark effect in ice is due to its hydrogen-bonding network, which increases anharmonicity of the potential curve along the O-H bond and the ability to shift the electron density under applied electric field.
To explore the extent of molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of the bare NH4(+)(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4(+)(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, the rotational fine structure on the NH and OH asymmetric stretching vibrations, which arise from free internal rotation of the -OH2 and -NH3 groups, also remains intact in the adducts. To establish the location and quantum mechanical delocalization of the He atoms, we carried out Diffusion Monte Carlo (DMC) calculations of the vibrational zero point wavefunction, which indicate that the barriers between the three equivalent minima for He attachment are so small that the He atom wavefunction is delocalized over the entire -NH3 rotor, effectively restoring C3 symmetry for the embedded -NH3 group.
Over the past decade, we have developed a spectroscopic approach to measure electric fields inside matter with high spatial (<1 Å) and field (<1 MV/cm) resolution. The approach hinges on exploiting a physical phenomenon known as the vibrational Stark effect (VSE), which ultimately provides a direct mapping between observed vibrational frequencies and electric fields. Therefore, the frequency of a vibrational probe encodes information about the local electric field in the vicinity around the probe. The VSE method has enabled us to understand in great detail the underlying physical nature of several important biomolecular phenomena, such as drug–receptor selectivity in tyrosine kinases, catalysis by the enzyme ketosteroid isomerase, and unidirectional electron transfer in the photosynthetic reaction center. Beyond these specific examples, the VSE has provided a conceptual foundation for how to model intermolecular (noncovalent) interactions in a quantitative, consistent, and general manner.
The strong temperature dependence of the I⁻∙(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process are determined using van't Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I⁻∙(H2O)2 system sample the extended potential energy surface with increasing temperature.
The results of experiments with water clusters of the type M+(H2O)n (M = Li, Na, K, Rb and Cs, n = 1–30) are presented. On the basis of analysis of two complementary data sets, namely the abundance spectra and the metastable ion dissociation spectra, both obtained using a quadrupole time-of-flight mass spectrometer, we report consistent rates for water evaporation. The results show enhanced kinetic stability for n = 20 and 27 for all alkali metals except sodium, and for n = 25 for Li, K and Rb. The structural relationships among alkali-metal ion centred water-clusters and protonated water cluster in terms of these so-called magic numbers are discussed.Reactions of size-selected M+(H2O)n (n = 1–30) with D2O were also investigated. It was observed that the rates of protium/deuterium exchange for water clusters containing alkali metal ions, when properly corrected for contamination of the D2O, are consistently extremely low, even for the largest clusters. These results are valuable in providing solid support for the fact that alkali-metal–water clusters are essentially inert in H/D exchange reactions with water, making them ideal for benchmarking in such studies.Quantum chemical calculations performed for A(H2O)n (A = H2O, Li+, Na+, n = 3–8) revealed significant energy barriers towards proton transfer, in good agreement with the experimental observations. Interestingly, it appears that proton mobility in a water cluster containing an alkali metal ion is lower than in pure water clusters of the same size. Furthermore, the lithium ion behaves like the other alkali metals, showing no similarity with hydrated H+ or D+.
Significance
Understanding the mechanics underlying the diffuse OH stretching spectrum of water is a grand challenge for contemporary physical chemistry. Water clusters play an increasingly important role in this endeavor, as they allow one to freeze and isolate the spectral behavior of relatively large assemblies with well-defined network morphologies. We exploit recently developed, hybrid instruments that integrate laser spectroscopy with cryogenic ion trap mass spectrometry to capture the H 3 O ⁺ and Cs ⁺ ions in cage structures formed by 20 water molecules. Their infrared spectra reveal a pattern of distinct transitions that is unprecedented for water networks in this size range. Theoretical analysis of these patterns then reveals the intramolecular distortions associated with water molecules at various sites in the 3D cages.
Voltages inside matter are relevant to crystallization, materials science, biology, catalysis, and aqueous chemistry. The variation of voltages in matter can be measured by experiment, however, modern supercomputers allow the calculation of accurate quantum voltages with spatial resolutions of bulk systems well beyond what can currently be measured provided a sufficient level of theory is employed. Of particular interest is the Mean Inner Potential (Vo) - the spatial average of these quantum voltages referenced to the vacuum. Here we establish a protocol to reliably evaluate Vo from quantum calculations. Voltages are very sensitive to the distribution of electrons and provide metrics to understand interactions in condensed phases. In the present study, we find excellent agreement with measurements of Vo for vitrified water and salt crystals and demonstrate the impact of covalent and ionic bonding as well as intermolecular/atomic interactions. Certain aspects in this regard are highlighted making use of simple model systems/approximations. Furthermore, we predict Vo as well as the fluctuations of these voltages in aqueous NaCl electrolytes and characterize the changes in their behavior as the resolution increases below the size of atoms.
We rely on a hierarchical approach to identify the low-lying isomers and corresponding global minima of the pentagonal dodecahedron (H2O)20 and the H3O+(H2O)20 nanoclusters. Initial screening of the isomers is performed using classical interaction potentials, namely the Transferable Interaction 4-site Potential (TIP4P), the Thole-Type Flexible Model, versions 2.0 (TTM2-F) and 2.1 (TTM2.1-F) for (H2O)20 and the Anisotropic Site Potential (ASP) for H3O+(H2O)20. The nano-networks obtained with those potentials were subsequently refined at the density functional theory (DFT) with the Becke-3-parameter Lee–Yang–Parr (B3LYP) functional and at the second order Møller–Plesset perturbation (MP2) levels of theory. For the pentagonal dodecahedron (H2O)20 it was found that DFT (B3LYP) and MP2 produced the same global minimum. However, this was not the case for the H3O+(H2O)20 cluster, for which MP2 produced a different network for the global minimum when compared to DFT (B3LYP). The low-lying networks of H3O+(H2O)20 correspond to structures having 9 ‘free’ OH bonds and the hydronium ion on the surface of the nanocluster. The IR spectra of the various networks are further analysed in the OH stretching (‘fingerprint’) region and the various bands are assigned to structural arrangements of the underlying hydrogen bonding network. © 2012 Canadian Society for Chemical Engineering
The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (∼1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.
Vibrational predissociation spectra of D2 "tagged" Mg(2+)OH(-)(H2O)n=1-6 and Ca(2+)OH(-)(H2O)n=1-5 clusters are reported to explore how the M(2+)OH(-) contact ion pairs respond to stepwise formation of the first hydration shell. In both cases, the hydroxide stretching frequency is found to red-shift strongly starting with addition of the third water molecule, quickly becoming indistinguishable from nonbonded OH groups associated with solvent water molecules by n=5. A remarkably broad feature centered around 3200 cm(-1) and spanning up to ~1000 cm(-1) appears for the n≥4 clusters that we assign to a single-donor ionic hydrogen bond between a proximal first solvent shell water molecule and the embedded hydroxide ion. The extreme broadening is rationalized with a theoretical model that evaluates the range of local OH stretching frequencies predicted for the heavy particle configurations available in the zero-point vibrational wavefunction describing the low frequency modes. The implication of this treatment is that extreme broadening in the vibrational spectrum need not arise from thermal fluctuations in the ion ensemble, but can rather reflect combination bands based on the OH stretching fundamental that involve many quanta of low frequency modes whose displacements strongly modulate the OH stretching frequency.
Blackjack water cluster detected
Spectroscopy of protonated water clusters has played a pivotal role in elucidating the molecular arrangement of acid solutions. Whereas bulk liquids manifest broad spectral features, the cluster bands tend to be sharper. The 21-membered water cluster has for decades inspired particular interest on account of its stability and its place in the transition from two-dimensional to three-dimensional hydrogen-bonding network motifs, but the spectral signature of its bound proton has proved elusive. Fournier et al. have now detected this long-sought vibrational feature by applying an innovative ion cooling technique.
Science , this issue p. 1009
Infrared photodissociation (IRPD) spectra of M(+)(H2O)nAr (M= Rb, Cs; n = 3-5) with simultaneous monitoring of [Ar] and [Ar+H2O] fragmentation channels are reported. The comparison between the spectral features in the two channels and corresponding energy analysis provide spectral assignments of the stable structural conformers and insight into the competition between ion---water electrostatic and water---water hydrogen bonding interactions. Results show that as the level of hydration increases, the water---water interaction exhibits the tendency to dominate over the ion---water interaction. Cyclic water tetramer and water pentamer sub-structures appear in Cs(+)(H2O)4Ar and Cs(+)(H2O)5Ar systems, respectively. However, cyclic water tetramer and pentamer structures were not observed for Rb(+)(H2O)4Ar and Rb(+)(H2O)5Ar systems, respectively due to the stronger influence of the rubidium ion---water electrostatic interaction. The energy analysis, including the available internal energy and the IR photon energy, helped provide an experimental estimate of water binding energies.
We report the first optimum geometries and harmonic vibrational frequencies for the ring pentamer and several water hexamer (prism, cage, cyclic and two book) at the coupled-cluster including single, double, and full perturbative triple excitations (CCSD(T))∕aug-cc-pVDZ level of theory. All five examined hexamer isomer minima previously reported by Mo̸ller-Plesset perturbation theory (MP2) are also minima on the CCSD(T) potential energy surface (PES). In addition, all CCSD(T) minimum energy structures for the n = 2-6 cluster isomers are quite close to the ones previously obtained by MP2 on the respective PESs, as confirmed by a modified Procrustes analysis that quantifies the difference between any two cluster geometries. The CCSD(T) results confirm the cooperative effect of the homodromic ring networks (systematic contraction of the nearest-neighbor (nn) intermolecular separations with cluster size) previously reported by MP2, albeit with O-O distances shorter by ∼0.02 Å, indicating that MP2 overcorrects this effect. The harmonic frequencies at the minimum geometries were obtained by the double differentiation of the CCSD(T) energy using an efficient scheme based on internal coordinates that reduces the number of required single point energy evaluations by ∼15% when compared to the corresponding double differentiation using Cartesian coordinates. Negligible differences between MP2 and CCSD(T) frequencies are found for the librational modes, while uniform increases of ∼15 and ∼25 cm(-1) are observed for the bending and "free" OH harmonic frequencies. The largest differences between CCSD(T) and MP2 are observed for the harmonic hydrogen bonded frequencies, for which the former produces larger absolute values than the latter. Their CCSD(T) redshifts from the monomer values (Δω) are smaller than the MP2 ones, due to the fact that CCSD(T) produces shorter elongations (ΔR) of the respective hydrogen bonded OH lengths from the monomer value with respect to MP2. Both the MP2 and CCSD(T) results for the hydrogen bonded frequencies were found to closely follow the relation -Δω = s [middle dot] ΔR, with a rate of s = 20.2 cm(-1)∕0.001 Å for hydrogen bonded frequencies with IR intensities >400 km∕mol. The CCSD(T) harmonic frequencies, when corrected using the MP2 anharmonicities obtained from second order vibrational perturbation theory, produce anharmonic CCSD(T) estimates that are within <60 cm(-1) from the measured infrared (IR) active bands of the n = 2-6 clusters. Furthermore, the CCSD(T) harmonic redshifts (with respect to the monomer) trace the measured ones quite accurately. The energetic order between the various hexamer isomers on the PES (prism has the lowest energy) previously reported at MP2 was found to be preserved at the CCSD(T) level, whereas the inclusion of anharmonic corrections further stabilizes the cage among the hexamer isomers.
Crystalloluminescence, the long-lived emission of visible light during the crystallization of certain salts, was first observed over 200 years ago, however, the origin of this luminescence is still not well understood. The observations suggest that the process of crystallization may not be purely classical but also involves an essential electronic structure component. Strong electric field fluctuations may play an important role in this process by providing the necessary driving force for the observed electronic structure changes. The main objective of this work is to provide basic understanding of the fluctuations in charge, electric potentials, and electric fields for concentrated aqueous NaCl electrolytes. Our charge analysis reveals that the water molecules in the 1st solvation shell of the ions serve as a sink for electron density originating on Cl-. We find that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our results show that the potential and field distributions are largely independent of concentration. We also find that the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged non-zero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence.
The potential energy surface for the ground electronic state of the HO2 system has been characterized using extended basis sets with a recently introduced density functional incorporating gradient corrections and some Hartree–Fock exchange. All the structural, thermodynamic and spectroscopic properties of the hydroperoxide radical and of its molecular fragments (OH, O2) are in close agreement with experiment. The saddle points for HO2 isomerization and OO–H dissociation, together with the hydrogen bonded OH–O structure, have been fully characterized. Refined post Hartree–Fock computations have been performed to further validate density functional results. The two series of quantum mechanical computations are in good agreement and suggest some refinement of the most recent semiempirical surfaces developed for dynamical studies. This task can be made easier by the force fields of all the stationary points computed in the present work. These findings together with the very favorable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of reactions involving large, chemically significant species.
The calculation of accurate electron affinities (EAs) of atomic or molecular species is one of the most challenging tasks in quantum chemistry. We describe a reliable procedure for calculating the electron affinity of an atom and present results for hydrogen, boron, carbon, oxygen, and fluorine (hydrogen is included for completeness). This procedure involves the use of the recently proposed correlation-consistent basis sets augmented with functions to describe the more diffuse character of the atomic anion coupled with a straightforward, uniform expansion of the reference space for multireference singles and doubles configuration-interaction (MRSD-CI) calculations. Comparison with previous results and with corresponding full CI calculations are given. The most accurate EAs obtained from the MRSD-CI calculations are (with experimental values in parentheses) hydrogen 0.740 eV (0.754), boron 0.258 (0.277), carbon 1.245 (1.263), oxygen 1.384 (1.461), and fluorine 3.337 (3.401). The EAs obtained from the MR-SDCI calculations differ by less than 0.03 eV from those predicted by the full CI calculations.
A refined version of the ''shape consistent'' effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin--orbit operators for the elements Rb through Xe. Particular attention was given to the partitioning of the core and valence space and, where appropriate, more than one set of potentials is provided. These are tabulated in analytic form. Gaussian basis sets with contraction coefficients for the lowest energy state of each atom are given. The reliability of the transition metal AREPs was examined by comparing computed atomic excitation energies with accurate all-electron relativistic values. The spin--orbit operators were tested in calculations on selected atoms.
ital initio averaged relativistic effective core potentials (AREP) and spin--orbit (SO) operators are reported for the elements Cs through Rn. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definitions thereby permitting the treatment of core--valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets for the lowest energy state of each atom are tabulated. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic excitation energies and SO splitting energies with all-electron relativistic values. Calculated atomic excitation energies are found to agree to 0.12 eV and SO energies to 3.4%.
The position and the intensity of electronic bands are influenced by an electric field. Pronounced changes in the position of absorption bands are mainly due to the dipole moment of the molecule in the ground state and the change in the dipole moment during the excitation process, and pronounced changes in intensity are due to the field dependence of the transition moment, which can be described by the transition polarizability. The effect of an external electric field on the optical absorption (electrochromism) of suitable molecules can be used to determine the dipole moment in the ground state, the change in dipole moment during the excitation process, the direction of the transition moment of the electronic band, and certain components of the transition polarizability tensor. These data largely determine the strong solvatochromism (solvent-dependence of the position and intensity of electronic bands), which is observed in particular with molecules having large dipole moments. Smaller contributions to solvatochromism result from dispersion interactions, which predominate in the case of nonpolar molecules. The models developed have been experimentally checked and verified by a combination of electro-optical absorption measurements (influence of an external electric field on absorption) and investigation of the solvent-dependence of the electronic bands.
The modulation of proton transfer reactions by environmental effects has been investigated for the case of keto -enol isomerization of formamide. Formation of long-lived adducts with a single water molecule or of formamide dimers enhances the reaction rate and shifts the equilibrium towards the lactam form. The effect of bulk solvent on these adducts is small, whereas it further stabilizes the keto form of free formamide. Zero point and entropy effects on the thermo-dynamics and kinetics of the reaction are generally negligible and do not modify the above general trends. Significant reaction rates are obtained well under the energy barrier due to tunneling. They are, however, smeared out when going from microcanonical to canonical ensembles.
We establish the argon solvent size dependence of the Cl−·H2O·Arn predissociation spectra, and discuss the discrepancies between previously reported predissociation spectra of the Cl−·H2O and Cl−·H2O·Ar3 complexes [Choi et al., J. Phys. Chem. A 102 (1998) 503; Ayotte et al., J. Am. Chem. Soc. 120 (1998) 12361]. The argon-induced shift in the ∼3130 cm−1 ionic H-bonded OH stretching band, calculated to be large (>30 cm−1/Ar red-shift) by Satoh and Iwata [Chem. Phys. Lett. 312 (1999) 522], is found to be quite small (IHB band center=3128±3 cm−1 for 1⩽n⩽5). We compare this result with similar behavior displayed by the bare versus argon-solvated bromide monohydrate.
We propose a general procedure for the numerical calculation of the harmonic vibrational frequencies that is based on internal coordinates and Wilson's GF methodology via double differentiation of the energy. The internal coordinates are defined as the geometrical parameters of a Z-matrix structure, thus avoiding issues related to their redundancy. Linear arrangements of atoms are described using a dummy atom of infinite mass. The procedure has been automated in FORTRAN90 and its main advantage lies in the nontrivial reduction of the number of single-point energy calculations needed for the construction of the Hessian matrix when compared to the corresponding number using double differentiation in Cartesian coordinates. For molecules of C1 symmetry the computational savings in the energy calculations amount to 36N - 30, where N is the number of atoms, with additional savings when symmetry is present. Typical applications for small and medium size molecules in their minimum and transition state geometries as well as hydrogen bonded clusters (water dimer and trimer) are presented. In all cases the frequencies based on internal coordinates differ on average by <1 cm-1 from those obtained from Cartesian coordinates.
Evaporative cooling, which is the usual mode of formation for many ion–molecule complexes, typically results in high internal energies. This in turn leads to a broadening of vibrational or vibronic spectra of these species. By incorporating argon into the nascent ion cluster, it is possible to significantly reduce the internal energy and thus simplify the spectra. This approach has been applied to the Cs+(H2O) cluster ion. The binding of argon lowers the internal energy to an effective temperature of 125 K. Rotational structure in the asymmetric stretch can be analyzed to conclude that the structure of Cs+(H2O)Ar is quasi-linear with the heavy atoms in an Ar–Cs+–O configuration and the two hydrogen atoms symmetrically displaced off–axis, pointing away from the ion. © 2002 American Institute of Physics.
Theoretical considerations suggest that the absorption and emission spectra of certain dyes may be shifted by hundreds of angstroms upon application of a strong electric field. The effect could be called ``electrochromism,'' in analogy to ``thermochromism'' and ``photochromism.'' The theory of the effect is outlined and is discussed in terms of compounds which might be expected to show it most strongly.
Infrared spectra of Cs+(H2O)1−5 were obtained from vibrational predissociation of mass‐selected cluster ions in a triple quadrupole mass spectrometer using a pulsed‐tunable infrared laser in the 2.6–3.0 μm region. By comparison to size‐selective infrared spectra of neutral water clusters, the structure of hydrogen‐bonded water clusters complexed to the Cs+ can be observed for cluster ions with three or more water molecules. The onset of hydrogen bonding is also marked by the presence of structural isomers. There is also evidence for an unusual change in the vibrational transition moments for the symmetric and asymmetric O–H stretch, for isolated (non‐hydrogen‐bonded) water molecules, where the symmetric stretch is substantially enhanced. © 1996 American Institute of Physics.
Reactions of M(+)(H(2)O)(n), n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D(2)O are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Isotopically highly enriched metals are used as applicable. Isotopic scrambling with formation of HDO is not observed for M = Cr, Fe, Co, Ni, Cu, and Zn, which indicates that these hydrated metal ions consist of a singly charged metal center and a hydration shell of intact, inactivated water molecules. In the vanadium case, HDO formation is observed in the size region where also hydroxide formation with evolution of molecular hydrogen occurs. For manganese, HDO formation occurs in the size regime n ≈ 8-20. Additional experiments show that, in this size regime, Mn(+)(H(2)O)(n) is slowly converted into HMnOH(+)(H(2)O)(n-1) under the influence of room temperature blackbody radiation. The reaction is mildly exothermic; ΔH ≈ -21 ± 10 kJ mol(-1).
We report the spectral signatures of water molecules occupying individual sites in an extended H-bonding network using mass-selective, double-resonance vibrational spectroscopy of isotopomers. The scheme is demonstrated on the water heptamer anion, (H2O)7¯, where we first randomly incorporate a single, intact D2O molecule to create an ensemble of isotopomers. The correlation between the two OD stretching frequencies and that of the intramolecular DOD bending transition is then revealed by photochemical modulation of the isotopomer population responsible for particular features in the vibrational spectrum. The observed patterns confirm the assignment of the dominant doublet, appearing most red-shifted from the free OD stretch, to a single water molecule attached to the network in a double H-bond acceptor (AA) arrangement. The data also reveal the unanticipated role of accidentally overlapping transitions, where the highest-energy OD stretch, for example, occurs with its companion OD stretch obscured by the much stronger AA feature.Keywords (keywords): hydrated electron; hole burning; dip-infrared spectroscopy; gas phase; supersonic
By isolating I-·Wn clusters in the gas phase, we eliminate the fluctuations and heterogeneity present in solution, allowing us to follow how water molecules (W) sequentially lock into position and form supramolecular complexes at the early stages of hydration. We report vibrational predissociation spectra of the I-·Wn, 1 ≤ n ≤ 3, complexes quenched close to their minimum energy configurations by complexation with argon. Structures are extracted from these spectra by comparison with the band patterns expected for the various geometries.
Molecular structure and bonding interactions at the vapor−water interface of varying H2O/HOD/D2O composition has been calculated using molecular dynamics simulations. From these simulations a surface vibrational sum frequency (VSF) spectrum of the OH stretch region has been generated and compared with experimental VSF results of similar isotopic mixtures. The peak frequency of the uncoupled, solvated OH stretch mode determined from the computational spectrum of the vapor−HOD interface shows excellent agreement with these experimental results. With the addition of H2O, the calculations performed in this work provide information as to how various OH stretch modes at the vapor−water interface are impacted by the coupling effects that are induced by hydrogen bonding to adjacent OH oscillators. The results of these calculations demonstrate the frequency shifting, spectral broadening, and changes in transition strength exhibited by the OH stretch modes of interfacial water species that occur with increased intermolecular and intramolecular coupling, providing an improved understanding of the different types of water species present at the vapor−water interface that have been difficult to assign in previous VSF experimental studies.
The effects of uniform electric fields on equilibrium interatomic separations, vibrational energy levels, and infrared transition moments are summarized for diatomic molecules in a matrix in which random orientation is maintained in both ground and excited states. The dependence of each property on field strength F can be expressed as a function of a linear and a quadratic term in the field strength. For vibrational energy levels, these terms are conventionally but imprecisely described as the vibrational dipole moment change and the vibrational polarizability change, respectively, accompanying excitation to a given vibrational excited state; for the dipole moment gradient, which determines the intensity, the analogous terms correspond to the transition polarizability and transition hyperpolarizability, respectively. Parameters describing the electric field response can be defined in terms of the quadratic and cubic force constants, and the first, second, and third gradients of the dipole moment, polarizability, and first hyperpolarizability of the molecule considered. This develops the earlier work of Hush and Williams (1974), Gready, Bacskay and Hush (1977-8), Lambert (1983-91), and the more recent work of Bishop et al. (1993-). The electric field response can, in principle, be measured in a manner analogous to that of electronic transitions in electroabsorption spectroscopy. The general form of this, based on the analysis of Liptay for the Stark effect on electronic transitions, is outlined for the corresponding vibrational case, and computational strategies are discussed. Stark vibrational spectral parameters for CO calculated at different levels of quantum calculation and by different strategies for analysis of the electric field perturbation data are presented.
The infrared anharmonic spectra for the H+(H2O)3, H+(H2O)4, and H+(H2O)21 water clusters have been reported using vibrational second-order perturbation theory at the B3LYP level with 6-31+G(d) and 6-311++G(3df,3pd) basis sets. The anharmonicity results crucial for the evaluation of the protonated water clusters and the anharmonic corrections can be larger than 500 cm−1, resulting in a shift of the H3O+ asymmetric stretchings near the region of 2000 cm−1.
Vibrational predissociation spectroscopy of the HOOC(CH2)10COO− and −OOC(CH2)10COO− anions is carried out by predissociation of weakly bound H2 molecules. The HOOC(CH2)10COO− (H2)2 and −OOC(CH2)10COO− (H2)10 cluster ions are formed by H2 attachment to the electrospray-generated bare ions in an ion trap cooled to below 20 K using a closed cycle helium cryostat. The photofragmentation behavior indicates that the H2 binding energy is about 600 cm−1, which is similar in strength to that found in Ar-tagged ions. The spectra indicate that the monoanion adopts a cyclic structure through the formation of an asymmetrical, internal anionic H-bond.Graphical abstract.View high quality image (122K)Research highlights▶ H2 tagging of electrosprayed ions in cryogenically cooled RF ion trap. ▶ Singly and doubly charged ions analyzed by vibrational predissociation spectroscopy. ▶ Characterization of intramolecular hydrogen bonding in cyclic monoanion.
The latest release of NWChem delivers an open-source computational chemistry package with extensive capabilities for large scale simulations of chemical and biological systems. Utilizing a common computational framework, diverse theoretical descriptions can be used to provide the best solution for a given scientific problem. Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures. This paper provides an overview of NWChem focusing primarily on the core theoretical modules provided by the code and their parallel performance.
Vibrational spectroscopy can provide important information about structure and dynamics in liquids. In the case of liquid water, this is particularly true for isotopically dilute HOD/D2O and HOD/H2O systems. Infrared and Raman line shapes for these systems were measured some time ago. Very recently, ultrafast three-pulse vibrational echo experiments have been performed on these systems, which provide new, exciting, and important dynamical benchmarks for liquid water. There has been tremendous theoretical effort expended on the development of classical simulation models for liquid water. These models have been parameterized from experimental structural and thermodynamic measurements. The goal of this paper is to determine if representative simulation models are consistent with steady-state, and especially with these new ultrafast, experiments. Such a comparison provides information about the accuracy of the dynamics of these simulation models. We perform this comparison using theoretical methods developed in previous papers, and calculate the experimental observables directly, without making the Condon and cumulant approximations, and taking into account molecular rotation, vibrational relaxation, and finite excitation pulses. On the whole, the simulation models do remarkably well; perhaps the best overall agreement with experiment comes from the SPC/E model.
The harmonic approximation provides a powerful approach for interpreting vibrational spectra. In this treatment, the energy and intensity of the 3N- 6 normal modes are calculated using a quadratic expansion of the potential energy and a linear expansion of the dipole moment surfaces, respectively. In reality, transitions are often observed that are not accounted for by this approach (e.g. combination bands, overtones, etc.), and these transitions arise from inherent anharmonicities present in the system. One interesting example occurs in the vibrational spectrum of H(2)O((l)), where a band is observed near 2000 cm(-1) that is commonly referred to as the "association band". This band lies far from the expected bend and stretching modes of the water molecule, and is not recovered at the harmonic level. In a recent study, we identified a band in this spectral region in gas-phase clusters involving atomic and molecular adducts to the H(3)O(+) ion. In the current study we probe the origins of this band through a systematic analysis of the argon-predissociation spectra of H(3)O(+)·X(3) where X = Ar, CH(4), N(2) or H(2)O, with particular attention to the contributions from the non-linearities in the dipole surfaces, often referred to as non-Condon effects. The spectra of the H(3)O(+) clusters all display strong transitions between 1900-2100 cm(-1), and theoretical modeling indicates that they can be assigned to a combination band involving the HOH bend and frustrated rotation of H(3)O(+) in the solvent cage. This transition derives its oscillator strength entirely from strong non-Condon effects, and we discuss its possible relationship to the association band in the spectrum of liquid water.
IR probes have been extensively used to monitor local electrostatic and solvation dynamics. Particularly, their vibrational frequencies are highly sensitive to local solvent electric field around an IR probe. Here, we show that the experimentally measured vibrational frequency shifts can be inversely used to determine local electric potential distribution and solute-solvent electrostatic interaction energy. In addition, the upper limits of their fluctuation amplitudes are estimated by using the vibrational bandwidths. Applying this method to fully deuterated N-methylacetamide (NMA) in D(2)O and examining the solvatochromic effects on the amide I' and II' mode frequencies, we found that the solvent electric potential difference between O(═C) and D(-N) atoms of the peptide bond is about 5.4 V, and thus, the approximate solvent electric field produced by surrounding water molecules on the NMA is 172 MV/cm on average if the molecular geometry is taken into account. The solute-solvent electrostatic interaction energy is estimated to be -137 kJ/mol, by considering electric dipole-electric field interaction. Furthermore, their root-mean-square fluctuation amplitudes are as large as 1.6 V, 52 MV/cm, and 41 kJ/mol, respectively. We found that the water electric potential on a peptide bond is spatially nonhomogeneous and that the fluctuation in the electrostatic peptide-water interaction energy is about 10 times larger than the thermal energy at room temperature. This indicates that the peptide-solvent interactions are indeed important for the activation of chemical reactions in aqueous solution.
We present infrared photodissociation spectra of two protonated peptides that are cooled in a ~10 K quadrupole ion trap and "tagged" with weakly bound H(2) molecules. Spectra are recorded over the range of 600-4300 cm(-1) using a table-top laser source, and are shown to result from one-photon absorption events. This arrangement is demonstrated to recover sharp (Δν ~6 cm(-1)) transitions throughout the fingerprint region, despite the very high density of vibrational states in this energy range. The fundamentals associated with all of the signature N-H and C=O stretching bands are completely resolved. To address the site-specificity of the C=O stretches near 1800 cm(-1), we incorporated one (13)C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each C=O oscillator contributes a single distinct band, effectively "reporting" its local chemical environment. For both peptides, analysis of the resulting band patterns indicates that only one isomeric form is generated upon cooling the ions initially at room temperature into the H(2) tagging regime.
Theoretical studies predict that [Al·nH(2)O](+) clusters are present as hydride-hydroxide species HAlOH(+)(H(2)O)(n-1) in gas-phase experiments, energetically favoured by 200 kJ mol(-1) over Al(+)(H(2)O)(n). After collisions with D(2)O, however, no H/D scrambling occurs between H(2)O and D(2)O in clusters with n > 38, indicating that large clusters are present as the higher-energy isomers Al(+)(H(2)O)(n).
Infrared laser action spectroscopy is used to characterize divalent Mg, Ca, and Ba ions solvated by discrete numbers of water molecules in the gas phase. The spectra of the hexahydrated ions are very similar and indicate that all six water molecules directly solvate the metal ion. The spectra of the heptahydrated ions indicate the presence of populations of structures that have a water molecule in the outer shell for all ions and an average coordination number (CN) for Ba that is higher than that for Ca or Mg. Comparisons between CN values obtained from M06 density functional and local MP2 theory indicate that the B3LYP density functional favors smaller CN values. The spectra of clusters containing at least 12 water molecules indicate that the relative abundance of single-acceptor water molecules for a given cluster size decreases with increasing metal ion size, indicating that tighter water binding to smaller metal ions disrupts the hydrogen bond network and results in fewer net hydrogen bonds. The spectra of the largest clusters (n = 32) are very similar, suggesting that intrinsic water properties are more important than ion-water interactions by that size, but subtle effects of Mg on surface water molecules are observed even for such large cluster sizes.
Copper-water ion-molecule complexes with attached argon atoms, Cu(+)(H(2)O)Ar(2), are produced in a supersonic molecular beam by pulsed laser vaporization. These systems are mass-selected in a reflectron time-of-flight spectrometer and studied with infrared photodissociation spectroscopy. The vibrational spectra for these complexes are characteristic of many cation-water systems, exhibiting symmetric and asymmetric O-H stretch fundamentals, and weaker features at higher frequency that have been tentatively assigned to combination bands. Using isotopically substituted spectra and model potential calculations, we are able to assign the combination bands to a water torsional vibration (frustrated rotation) in combination with the asymmetric stretch fundamental. This combination band assignment is likely to apply to IR spectra of many cation-water complexes.