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Spin--orbit ab initio study of alkyl halide dissociation via electronic curve crossing
Abstract
An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-sigma* states are studied including spin-orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH3I and CF3I are in agreement with the experimental observation. The curve crossing region is around 2.371 Angstrom for CH3I and CF3I. The potential curves of the repulsive excited states have larger slope for CF3I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH3Br and CH3Cl. (C) 2004 American Institute of Physics.
... Furthermore, judging from the appearance of the corresponding absorption spectra (Keller-Rudek et al. 2013) the excited state energies lower as the number of halogen atoms increases. The corresponding potential energy curves as a function of the carbon-halogen (C-X) bond energy for the methyl-monohalides are found to be repulsive, which forms the basis for a halogen atom formation by photodissociation (CH 3 Cl (Granucci et al. 2010); CH 3 Cl and CH 3 I (Ajitha et al. 2004;Eden et al. 2007); CH 3 Br (Van Veen et al. 1985;Blanchet et al. 2009;Escure et al. 2009); CH 3 Br (Van Veen et al. 1985;Hafliðason et al. 2018Hafliðason et al. , 2019; CH 3 I (Matthíasson et al. 2020)). ...
... Absorption spectra (Causley and Russell 1975;Eden et al. 2007;Keller-Rudek et al. 2013), photofragmentation studies (Van Veen et al. 1985;Blanchet et al. 2009;Hafliðason et al. 2018Hafliðason et al. , 2019 and/or potential energy calculations (Ajitha et al. 2004;Escure et al. 2009;Granucci et al. 2010) for chlorine-and bromine-containing halomethane molecules reveal the nature of excited electronic states in the energy range within the photoexcitation used here (290 nm) and lower limits of the S 1 and T 1 states of melatonin as detected here (see Fig. 4a, b). These excited states might serve as the donors in the proposed electron energy transfer of concern. ...
Studies of melatonin photoreactivity in water solutions: An effect of an external heavy atom I ⁻ on UV/Vis absorption, fluorescence and phosphorescence spectra is explored. The data allowed determination of relevant energetics for the system.The heavy atom effect (HAE) of I ⁻ on melatonin is clearly found to induce an intersystem crossing from the lowest energy singlet state to the lowest energy triplet state ( T 1 ) by a state mixing. Lifetime for the first excited triplet states of melatonin in association with I ⁻ and quenching rates for halomethanes (CH 2 X 2 , CHX 3 , CY 4 , X = Cl, Br, Y = Cl) are determined from Time-Correlated Single-Photon Counting decay curves for the phosphorescence. Dramatic alterations in quenching rate constants with quenchers as CH 2 X 2 < CHX 3 < CX 4 and Cl < Br are attributed to energy transfer from an I −… M e* ( T 1 ) complex to low-lying electronic states of the halomethanes followed by dissociation to form R and X fragments. Relevance of the melatonin photoreactivity to photosensitizer properties in organic media is discussed.
Graphical abstract
... 40 The obtained vertical excitation energies for X → 1 Q 1 , 3 Q 0 and 3 Q 1 (photon absorption peaks for the three electronic states) are 4.97, 4.70, and 4.25 eV, respectively, in good agreement with Gedanken's experimental results. 30 In our calculation, the potential curves 3 Q 0 and 1 Q 1 cross at E c = 31385 cm −1 at the R C−I = 2.358 Å. Ajitha et al. 41 calculated the potential energy at the curve crossing is 28785 cm −1 at R C−I = 2.372 Å. While Yu 42 calculated E c = 30012 cm −1 at R C−I = 2.401 Å, and Alekseyev et al. 43 calculated E c near 27600 cm −1 at R C−I = 2.497 Å. ...
... The E c will change a lot, while the C−I bond length changes a little at the crossing region. 44 Ajitha et al. 41 also suggested that many factors, (such as the zero-point corrections for spectator modes, the umbrella vibrational mode excitation), altogether make a difference of 3000 cm −1 between calculated potential curves and the experimentally ones are quite reasonable. 3. 6. ...
The photodissociation of CF3I→CF3(v1,v2)+I*/I has been investigated at 248, 266, and 277 nm with our high resolution mini-TOF photofragment translational spectrometer. Based on the theoretical calculations of Clary and of Bowman et al., now in this manuscript, we assign 701 cm⁻¹ to the CF symmetric stretch (breathing) v1 mode, and 1086 cm⁻¹ to the umbrella v2 mode of the CF3 fragment. In the obtained TOF spectra of I⁺ from the I* channel, situated in the 701 cm⁻¹ gaps between the original series of (v1, 0) vibrational peaks, a new series of weaker (v1, 1) vibrational peaks are partially resolved. These observed new peaks with 1086 cm⁻¹v2 mode excitation have never been reported in previous literature. In the TOF spectra of I⁺ from the I channel, the new series of (v1, 1) peaks are also partially resolved. But these spectra of I channel are less satisfactory, because for higher Eavl and higher ET, the higher resolution of PTS is required.The potential energy at the curve crossing point, and the excitation of CF3(v1, 2) and (v1, 3) vibrational states have been also analyzed.
... Halomethanes have served as a long-standing prototypical molecular system for experimental and theoretical chemists, since they are highly related to environmental issues [1,2] and useful for the understanding of photochemical reaction mechanism. Accordingly, numerous theoretical [3][4][5][6][7][8][9][10][11] and experimental [12][13][14][15][16][17] studies have been undertaken to elucidate their electronic structure and photodynamical characters. In addition, when a halomethane contains heavy halogen atom such as Br and I, the spin-orbit coupling (SOC) may play a key role in the interpretation of experimental results. ...
... In contrast, the SO ab initio methods based on a multiconfigurational approach [24,25] reasonably work in calculations of the excited states as well as the ground states, but additional work is required for geometry optimization and vibrational frequency calculation due to the lack of the analytical gradient. Thus, the latter method has been mainly used for simple diatomic molecules to calculate the potential energy surfaces (PESs) [26,27] and for halomethanes to calculate PES along the dissociation coordinate [4,5,9,10]. Both approaches provide reasonable results for the calculations of SO states. ...
Spin–orbit coupling plays a crucial role in the determination of molecular structure and calculation of vibrational frequencies
of CH2ClI+. We performed the geometry optimizations and vibrational frequency calculations of both the lower and upper spin–orbit (SO)
states using an ab initio SO method based on multiconfigurational wave function. The multistate complete active space perturbation
theory second-order SO (MS-CASPT2-SO) method reasonably describes the structures of the lower SO state, yielding the C–I distance
and the I–C–Cl angle close to the experimental values. The geometrical parameters of the upper SO state is quite similar to
that of the lower SO state, whereas structures of two states differ substantially in calculations prior to the introduction
of SO coupling. The MS-CASPT2-SO method reproduces the difference between the lower and upper SO states for the I–C–Cl bending
frequency. The vibrational frequencies calculated by MS-CASPT2-SO generally overestimate in comparison with the experiments.
The energy gap between the two SO states calculated by MS-CASPT2-SO is reasonably close to the experimental value. To the
best of our knowledge, this is the first attempt to calculate vibrational frequencies of two SO states of CH2ClI+, and the first time to apply MS-CASPT2-SO method to the geometry optimization and vibrational frequency calculation of polyatomic
molecule.
KeywordsDihalomethane–Cation–Spin–orbit coupling–Ab initio
... Iodotrifluoromethane (CF 3 I) belongs to the C 3v point group and consists of three fluorine atoms and one iodine atom, all of which are bonded to a central carbon atom (see figure 1. 3). The presence of the large iodine atom forces the molecule into a trigonal pyramidal geometry, with an optimised umbrella angle of 108.55°, as determined by Ajitha et al. [92] using the complete active space self consistent field (CASSCF) theory. At equilibrium, each C-F bond is 1.344Å [18] long while the C-I bond measures 2.101Å [18]. ...
... It has been experimentally found, using photofragment imaging 2,4-6 and time-of-flight techniques, 7 that these nonadiabatic transitions play an important role in methyl bromide photodissociation through the à band. The 1 Q 1 and 3 Q 0 curve crossing has been estimated to be at a C-Br internuclear distance of around 2.445 Å. 10 The most recent and comprehensive theoretical study on CH 3 Br photodissociation in the à band was done by Escure et al. 11 They report that the spin-orbit coupling is strong enough to induce nonadiabatic transitions between 3 Q 1 and 1 Q 1 states during the dissociation. They also state that nonadiabatic transitions at the conical intersection between 1 Q 1 and 3 Q 0 states play a minor role in this system. ...
The photodissociation of methyl bromide at 193 nm is studied using slice imaging. From the measured photofragment translational energy and angular distributions we were able to extract methyl-state-specific dissociation channel yields and crossing probabilities between (3)Q0 and (1)Q1 surfaces. The angular distributions for the Bromine photofragments show a strong dependence on the total translational energy release. Nonadiabatic transition probabilities from the (3)Q0 to (1)Q1 surface dominate the dynamics in this excitation energy region and for most of the methyl vibrational states probed.
Dans cette thèse nous étudions théoriquement l'interaction de la lumière avec la matière en phase diluée, et plus particulièrement les dynamiques nucléaires et électroniques suivant l'absorption d'un ou plusieurs photons par une molécule, ces dernières se déroulant à l'échelle de la femtoseconde (10^-15 s). Mes travaux ont consisté aux développement de modèles numériques complets permettant de reproduire au mieux les propriétés physico-chimiques des espèces considérées, et à résoudre exactement l'équation de Schrödinger dépendante ou indépendante du temps. Dans une première partie, nous nous intéressons à la description du continuum d'ionisation moléculaire grâce à des méthodes permettant de calculer les fonctions d'onde sélectionnées par l'absorption d'un photon. Nous appliquons cette approche stationnaire à de petites molécules modèles à deux dimensions, nous permettant de jouer sur l'anisotropie avec une grande liberté. Les retards d'ionisation sont calculés par cette approche, ceux-ci nous permettent de faire le liens avec l'approche dépendante du temps et de valider cette description. Dans une deuxième partie, plus appliquée, nous cherchons à interpréter les résultats d'une expérience de spectroscopie pompe-sonde où la dynamique de dissociation de la molécule d'iodométhane (CH3I) est sondée grâce au processus d'ionisation multiphotonique. Nous construisons un modèle théorique comprenant deux degrés de liberté : électronique et nucléaire, et montrons qu'il permet de comprendre une grande partie des observations expérimentales.
Ab initio molecular dynamics approach has been extended to multi‐state dynamics on the basis of the spin–orbit coupled electronic states that are obtained through diagonalization of the spin–orbit coupling matrix with the multi‐state second‐order multireference perturbation theory energies in diagonal elements and the spin–orbit coupling terms at the state‐averaged complete active space self‐consistent field level in off‐diagonal elements. Nonadiabatic transitions over the spin–orbit coupled states were taken into account explicitly by a surface hopping scheme with utilizing the nonadiabatic coupling terms calculated by numerical differentiation of the spin–orbit coupled wavefunctions and analytical nonadiabatic coupling terms. The present method was applied to the A‐band photodissociation of methyl iodide, CH3I + hv → CH3 + I (²P3/2)/I* (²P1/2), for which a pioneering theoretical work was reported by Amatatsu, Yabushita, and Morokuma. The present results reproduced well the experimental branching ratio and energy distributions in the dissociative products. © 2018 Wiley Periodicals, Inc.
The Effective Relativistic Coupling by Asymptotic Representation (ERCAR) approach is a new method developed by us over the past few years that allows for the accurate diabatic representation of a molecular Coulomb and spin-orbit Hamiltonian and yields an analytic potential energy surface (PES) model for use in quantum dynamics simulations. So far, we focused on the single one dissociation coordinate defining the asymptote for diabatic representation and corresponding to removing a single, strongly relativistic atom from the remaining fragment. In the present study, we extend this approach to multiple dimensions for the first time. To this end, a 3D PES model is developed for the methyl iodide (CH3I) system accounting for all totally symmetric coordinates (C–I stretch, CH3 umbrella, and CH3 breathing modes). The model parameters are fitted with respect to high-level ab initio reference data for the spin space (“spin-free”) states which are reproduced with very good accuracy. The ERCAR method also yields the fine structure states and energies which are not computed ab initio. This is particularly important for the ¹Q1 and ³Q0 fine structure states of CH3I which form an intersection that is considered key for the photodissociation dynamics of the system. Our new model shows that this intersection is considerably curved in the 2D subspace of the C–I stretch and CH3 umbrella coordinate. This will certainly affect the complicated nonadiabatic photodissociation dynamics of CH3I. The construction of a full 9D diabatic PES model is currently in progress.
As one of the simplest alkyl halides, methyl iodide is extensively investigated in the research fields of the photodissociation and photoionization. In the experimental investigations of ionization and dissociation, many molecular fragments, such as Iq+(q≤3), CHn⁺(n≤3), H⁺, etc., are observed in the mass spectrum of CH3I. While the mechanisms for dissociation and ionization are not completely understood. As the doubly-ionized product, CH3I²⁺ exhibits different isomer structures and isomerization reactions. The dissociation channels of different isomers in combination with the corresponding transition states of CH3I²⁺ are helpful for better understanding the dissociation and ionization dynamics of CH3I in an intense laser field. In our present work, the dissociation channels of CH3I²⁺ are investigated by the density functional and couple cluster theory. The geometries and energies corresponding to the local isomers and the transition states of CH3I, CH3I⁺ and CH3I²⁺ are computed. The first and second ionization energies we measured are in good agreement with experimental values. Our computational results show that the ground state of the CH3I²⁺ is a triplet one with ³A2 symmetry. Totally 11 two-body and 15 three-body dissociation channels of the CH3I²⁺ on both the lowest singlet and the lowest triplet potential energy surfaces are computed and analyzed in detail. Our computations indicate that seven two-body dissociations channels, i.e., six singlet and one triplet ones, are exergonic, in which CH3I²⁺(¹A')→CH2⁺+HI⁺(⁴A1) is the easiest process to achieve; four exergonic three-body dissociation channels with three on singlet potential energy surface and one on triplet potential energy surface are found. The possible mechanisms for producing the dissociative ionized fragments observed in experiments, CH3⁺, H⁺, and I⁺, are presented; furthermore, the dissociation channels generating other ions not observed in experiments, such as H3⁺ et al, are also given for further experimental study. The detailed information about dissociation channels and fragments is summarized for further experimental comparisons. In the computations, we find that the density functional theory and CCSD(T) methods give different energy orders for a few dissociation potential energy surfaces; and in this work, the analysis and discussion are performed based on the CCSD(T) results. Our computations indicate that the dissociation channels on singlet and triplet potential energy surface exhibit different behaviors.
The effective potential energy curves correlated to 2I3/20 and 2I1/20 * atom limits are calculated using second order spin-obit multiconfigurational quasidegenerate perturbation theory (SO-MCQDPT). The absorption spectra of CH3I molecule are calculated, photodissociation processes of CH3I molecule are analyzed and the quantum yields of 2I1/20 * atom are estimated. The present calculations can be used to interpret the experimental results.
Employing the multiconfiguration self-consistent field method and the multiconfiguration quasi-degenerate perturbation method, the adiabatic potential energy curves and vertical excitation energies of alkyl iodides CF3I and C2H2F3I are calculated for the low-lying states, respectively. It is found that the low-lying excited states of the two molecules are repulsive, and the calculated dissociation energies for their ground states are 2.473 eV and 2.835 eV, respectively, in which the former one agrees well with the experimental results.
Although for carbon and hydrogen the basis set of atomic natural orbitals (ANO-S) has been used in the all calculations, the basis set of ANO and effective core potential (ECP) have been used for the bromine, respectively. Using complete active space self-consistent field (CASSCF) method with these two species correlation consistent basis set, the ground state and low-lying excited states of ethyl bromide molecule and cation were studied. Meanwhile, taking into account the dynamic correlation effects, the second-order perturbation (CASPT2) method was used to obtain more reliable energies. For the ethyl bromide molecule, the optimized geometry of the ground state with ANO basis set agrees better with the experimental values than those with ECP basis set. The adiabatic ionization energies were obtained from the corresponding molecule and cation states optimized, the first and second ionization energies and the energetic gap of the first two electronic states were of somewhat difference with the experimental values at the CASSCF level. The results were improved with the CASPT2 method, in which the first two ionization energies differ from the experimental data by just 0.3593 and 0.4496 eV using the ANO basis set. Furthermore, the harmonic frequencies of the tide molecule were calculated at the CASSCF/ANO level, the frequencies agree well with the experimental values.
The 304 nm photodissociation of the C-H symmetric stretch excited CH3I [v
1 = 1, v
2 = 0] (v
1 denotes the C-H symmetric stretch mode, and v
2 denotes the umbrella mode) is studied with our simple photofragment translational spectrometer. An IR laser is used to excite the ground state CH3I [0, 0] to the C-H symmetric stretch excited CH3I [1, 0]. With IR laser OFF and ON, the fractions of photofragments CH3 (ν
1, ν
2) from the 304 nm photodissociation of CH3I [1, 0] have been determined through the photofragment translational spectra (PTS) from measuring I and I* and also through the PTS from measuring CH3 (0, 0) (1, 0) (0, 1) and (1, 1). The experimental results show that the C-H symmetric stretch vibration (v
1 = 1) in parent molecules is about 66% retained in the photofragments in the I channel, but only 24% in the I* channel. The populations of photofragments CH3 (0, 2) and (0, 3) are higher than CH3 (0, 0) and (0, 1), showing strong inverted population both in I and I* channels.
In this contribution the application of ultrafast laser pulses to reveal the dynamics of chemical reactions on metal oxide surfaces will be discussed. A combination of an optical pump-probe configuration with time-of-flight mass spectrometry is employed to monitor the mass and the relative velocity of intermediates and products of a photoinduced surface reaction in real time. Starting from a well defined reactant adsorption geometry representing the initial "collision complex" of the reactive encounter, this approach enables the observation of the coherent nuclear motion through the transition state to the emerging reaction products and thus provides insight into the elementary steps of complex surface chemical reaction mechanisms. Results will be presented for the application of this technique to the photodissociation dynamics of methyl iodide and methyl bromide adsorbed on magnesia ultrathin films on a Mo(100) single crystal surface.
We investigate the potential energy surfaces and activation energies for
reactions between methyl halide molecules CH$_{3}X$ ($X$ = F, Cl, Br, I) and
alkali-metal atoms $A$ ($A$ = Li, Na, K, Rb) using high-level {\it ab initio}
calculations. We examine the anisotropy of each intermolecular potential energy
surface (PES) and the mechanism and energetics of the only available exothermic
reaction pathway, ${\rm CH}_{3}X+A\rightarrow{\rm CH}_{3}+AX$. The region of
the transition state is explored using two-dimensional PES cuts and estimates
of the activation energies are inferred. Nearly all combinations of methyl
halide and alkali-metal atom have positive barrier heights, indicating that
reactions at low temperatures will be slow.
Photolysis of CH3I is studied in the range of excitation wavelengths λ = 266−333 nm using a three-dimensional wave packet method. The aim
is to try to clarify some discrepancies arisen in different experiments for the vibrational distributions of the CH3 (ν
2) fragment (being ν
2 the umbrella bend mode or the symmetric deformation mode of the CH3 group) produced through the channel CH3 (ν
2) + I(2P3/2). The simulations predict non-statistical vibrational distributions peaking at ν
2 = 1 in the range λ = 266−315 nm, and a change to statistical behavior with a maximum at ν
2 = 0 for longer wavelengths. The calculations agree qualitatively with all the available experiments for λ = 266−280 nm. For λ ≥ 298 nm theory is consistent with one of the experiments and disagrees with the other two.
Experimental investigation has been carried out for
dissociation/ionisation of methyl bromide using time of flight mass
spectrometer, then, the mass signals were assigned to H+,
CHm+ (m= 0-3), iBr+ (i = 79, 81), and
the main processes of multi-photon ionization and dissociation of CH3Br
were given.
Experimental investigation have been carried out for dissociation/ionisation of methyl bromide using time of flight mass spectrometer, and the mass signals m/e = 12, 13, 14, 15, 79, 81, 129 and 131 were assigned to H+, C+, CH+, CH2+, CH3+, Br-i(+) (i = 79, 81), (CH)(2)Br-i(+) (i=79, 81). Next, two resonant peaks of Br-79(+)* / Br-81(+)* were observed at 280.41m and 281.74nm due to spin-forbidden 5p(4)P(3/2)<--4P(2)P(1/2) and 5p(4)P(5/2)<--4P(2)P(1/2) which have not been reported before. At last, the C REMPI spectra found near 280.34nm, 286.94nm and 284.21m were assigned, too, which compared with the predicted wavelength values.
We performed a theoretical study of the photodissociation dynamics of CH3Br in the A∼ band using a wavepacket propagation technique on coupled ab initio potential energy surfaces corresponding to the 3Q1 and 1Q1 states correlated at large distance with the Br ground spin–orbit state, as well as the 3Q0 and 4E states correlated to the excited one. The methyl umbrella vibrational product state distributions are found in very good agreement with experimental results. They are hotter for Br than for Br∗, and this is related to the shapes of the 3Q0 and 1Q1 potentials.
The C–Br photo-fragmentation of bromo-3,5-difluorobenzene (Br-3,5-diFBz) has been investigated using ab initio methods. A reaction coordinate combining a carbon–bromine bond stretch and a bromine out-of-plane bending on the S1 surface has been found with an activation energy of 2.96kcal/mol, compatible with the observed picosecond time scale.
The photodissociation of CF3I has been studied near 304nm. The partially resolved vibrational peaks in the I* and I channels reveal that the ν2 umbrella mode of CF3 is preferentially excited. Besides, in the I* channel, the extra vibrational peaks assigned to the umbrella mode of CF3 from the hot band ν3′=1,2 states of CF3I are also partially resolved. The Eint/Eavl shows a gradual increase as the state of CF3I changes from ν3′=0 to ν3′=2. The anisotropy parameter is also obtained and discussed with the interrelation of the 3Q1, 3Q0, and 1Q1 potential energy surfaces.
The photoinduced homolytic cleavage of the Sn-I bond in iodotrimethylstannane (CH3)(3)SnI, observed after UV irradiation, is investigated by means of spin-orbit ab initio calculations based on CASSCF (complete active space self-consistent field) and MS-CASPT2 (multi-state complete active space 2(nd) order perturbation theory) methods. The absorption electronic spectrum is characterized by ten low-lying (1,3)A ' and (1,3)A '' spin eigenstates corresponding to p(y)(I),p(x)(I)-> sigma*(SnI);-> sigma(SnI)-> sigma *(SnI) and p(y)(Sn), p(x)(Sn) -> sigma*(SnI), where sigma(SnI) and sigma*(SnI) are the bonding and anti-bonding orbitals of the Sn-I bond along the p(z) axis. From the (1)A ' electronic ground state and these ten spin eigenstates, 21 spin-orbit states are generated leading to various deactivation channels of (CH3)(3)SnI, corresponding to the formation of radicals (CH3)(3)Sn center dot and center dot I and to the ionic species (CH3)(3)Sn+ and I-. Irradiation into the upper band at 175 nm should lead to the heterolytic cleavage of the Sn-I bond to form the ionic primary products exclusively, whereas absorption into the shoulder at 250 nm induces the homolytic breaking with formation of the radical products.
From the photofragment translational spectra of C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] photodissociatioin at 277.5 nm, the vibrational distribution of photofragments CH3 (v1 = 0, v2 = 0), (0, 1), (1, 0), (1, 1) in the I* channel are measured to be 0.02, 0.02, 0.47, 0.25, and of CH3 (1, 0), (1, 1) in the I channel are 0.04, 0.05, respectively. It shows that most of the dissociated CH3I [1, 0] retain the C-H symmetric stretch vibration v1 = 1 in the photofragments CH3 and the vibrational distribution in umbrella bending mode is not seriously affected by the original C-H symmetric stretch excitation. The photodissociation of CH3I [1, 0] mainly follows the vibrationally adiabatic process. The original vibrational excitation [v1 = 1] of CH3I is quite like a spectator, and the internal vibrational redistribution (IVR) does not play obvious part during photodissociation.
The theoretical treatment of state-state interactions and the development of coupled multi-dimensional potential energy surfaces (PESs) is of fundamental importance for the theoretical investigation of nonadiabatic processes. Usually, only derivative or vibronic coupling is considered but the presence of heavy atoms in a system can render spin-orbit (SO) coupling important as well. In the present study, we apply a new method recently developed by us [J. Chem. Phys. {\bf 136}, 034103 (2012) and J. Chem. Phys. {\bf 137}, 064101 (2012)] to generate SO coupled diabatic PESs along the C--I dissociation coordinate for methyl iodide (CH$_3$I). This is the first and mandatory step towards the development of fully coupled full-dimensional PESs to describe the multi-state photodynamics of this benchmark system. The method we use here is based on the diabatic asymptotic representation of the molecular fine structure states and an effective relativistic coupling operator. It therefore is called Effective Relativistic Coupling by Asymptotic Representation (ERCAR). This approach allows the efficient and accurate generation of fully coupled PESs including derivative and SO coupling based on high-level {\em ab initio} calculations. In this study we develop a specific ERCAR model for CH$_3$I that so far accounts only for the C--I bond cleavage. Details of the diabatization and the accuracy of the results are investigated in comparison to reference {\em ab initio} calculations and experiments. The energies of the adiabatic fine structure states are reproduced in excellent agreement with {\em ab initio} SO-CI data. The model is also compared to available literature data and its performance is evaluated critically. This shows that the new method is very promising for the construction of fully coupled full-dimensional PESs for CH$_3$I to be used in future quantum dynamics studies.
Photodissociation dynamics of methyl iodide (CHI) adsorbed on both amorphous solid water (ASW) and porous amorphous solid water (PASW) has been investigated. The ejected ground-state I(P) and excited-state I(P) photofragments produced by 260- and 290-nm photons were detected using laser resonance-enhanced multiphoton ionization. In contrast to gas-phase photodissociation, (i) the I(P) photofragment is favored compared to I(P) at both wavelengths, (ii) I(P) and I(P) have velocity distributions that depend upon ice morphology, and (iii) I is produced on ASW. The total iodine [I(P)+I(P)+I] yield varies with substrate morphology, with greater yield from ASW than PASW using both 260- and 290-nm photons. Temperature-programmed desorption studies demonstrate that ice porosity enhances the trapping of adsorbed CHI, while pore-free ice likely allows monomer adsorption and the formation of two-dimensional CHI clusters. Reactions or collisions involving these clusters, I atomic fragments, or I-containing molecular fragments at the vacuum-surface interface can result in I formation.
The multireference spin-orbit (SO) configuration interaction (CI) method in its Λ-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CF(3)I relevant to its photodissociation in the lowest absorption band (A band). The computed equilibrium geometry for the X[combining tilde]A(1) ground state and vibrational frequency ν(3) for the C-I stretch mode agree well with available experimental data. The (3)Q(0(+)) state dissociating to the excited I((2)P(1/2)) limit is found to have a minimum of 1570 cm(-1) significantly shifted to larger internuclear distances (R(C-I) = 5.3 a(0)) relative to the ground state. Similar to the CH(3)I case, this makes a single-exponent approximation commonly employed for analysis of the CF(3)I recoil dynamics unsuitable. The 4E((3)A(1)) state possessing an allowed transition from the ground state and converging to the same atomic limit as (3)Q(0(+)) is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the (3)Q(1), (3)Q(0(+)), and (1)Q states indicate that the A-band spectrum must lie approximately between 31 300 and 45 200 cm(-1), i.e., between 220 and 320 nm. This result is in very good agreement with the measured absorption spectrum.
The real time photodissociation dynamics of CH3I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH3 fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH3(ν = 0) and I(2P3/2) and I*(2P1/2) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I* channels when the CH3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.
Methyl iodide and methyl bromide molecules were adsorbed at submonolayer coverages on an ultrathin MgO(100) film on Mo(100) and photoexcited by 266 nm femtosecond-laser irradiation. The subsequent photodissociation and desorption dynamics were probed by time delayed multi photon ionization mass spectrometric detection of the emerging reaction products. The pronounced difference in the appearance times of the methyl radical fragments from methyl iodide and bromide is discussed on the basis of the different molecular adsorption geometries on magnesia. The surface adsorption structure also defines the alignment of the encounter complex for the observed bimolecular formation of the halogen molecules I2 and Br2 within about 1 and 2 ps, respectively. Finally, photoexcitation of co-adsorption layers of CH3I and CH3Br resulted in a heteronuclear bi-molecular reaction yielding IBr molecules.
Scanning Tunnelling Microscopy (STM) is opening up a new field of reaction dynamics, followed one-molecule-at-a-time, only recently applied to reaction at a metal surface. Here we combine experiment with theory in studying the motions involved in the successive breaking by electron-induced reaction of the two carbon-halogen bonds, C-Cl or C-I, in physisorbed p-dihalobenzene, to form chemisorbed halogen-atoms and organic residue on Cu(110) at 4.6 K. We characterize the geometry of the physisorbed initial state, p-dichlorobenzene (pDCB) and p-diiodobenzene (pDIB), at the copper surface, as well as the successive final states of both chemisorbed reaction products: electron #1 giving rise to the first halogen-atom and a chemisorbed halophenyl and electron #2 giving a second halogen-atom and a chemisorbed phenylene. The major findings reported are (a) the distance and angular distributions of the chemisorbed reaction products relative to the physisorbed reagent molecule, (b) an approximate ab initio calculation, coupled with classical molecular dynamics (MD), of the repulsion between the products on the excited potential-energy surfaces, pes*, following excitation by electrons #1 or #2, and subsequently MD on the ground-state pes with inclusion of inelastic surface-interaction as a means to understanding the above, (c) observation of the changing dynamics with the chemistry of the halogen-atom, and (d) characterization of the effects of secondary encounters among the reaction products in the constrained space of the more highly localized reaction of pDIB. Item (d) shows clear evidence of high reactivity in surface-aligned collisions with restricted impact parameter, termed Surface Aligned Reaction, SAR, characterized by STM.
An ab initio investigation of electronic curve crossing in a methyl iodide molecule is carried out using Spin–Orbit multiconfigurational quasidegenerate perturbation theory. The one-dimensional rigid potential curves and optimized effective curves of low-lying states, including Spin–Orbit coupling and relativistic effects, are calculated. The Spin–Orbit electronic curve crossing between 3Q0+and 1Q1, and the shadow minimum in potential energy curve of 3Q0+ at large internuclear distance are found in both sets of the curves according to the present calculations. The crossing position is in the range of RC–I = 0.2370 ± 00001 nm. Comparisons with other reports are presented.
The ionic bond in molecules containing an electropositive moiety and an electronegative moiety originates from a coupling between the ionic and the covalent contributions. Some representative cases in the example of LiF, LiCl, NaF, NaCl, KF, KCl, LiCH3, CH3F, LiCN, NaCN and KCN are calculated by ab initio and density functional methods. The resulting bond energy can be improved a posteriori by a recently proposed first-order method using the dipole moment or the effective charge. For the ab initio calculations, this method brings about a systematic improvement of the bond energy with respect to the experimental value. While the density functional method gives qualitatively mixed results, application of this first-order method generally improves the bond energy.
In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266 nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266 nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed.
The photodissociation of CH3Cl is an important source of atmospheric chlorine atoms. To more fully understand this reaction, potential energy surfaces of the ground state X(1)A' and the first two excited singlets (2(1)A' and 1(1)A '', corresponding to the degenerate E-1 at the ground state C-3 nu equilibrium geometry) of CH3Cl have been constructed through ab initio Complete Active Space SCF calculations. AX(1)A'/2(1)A' conical intersection was located, and the importance of this feature in the photolysis of the C-H bond after photoexcitation at 193 nm is discussed.
This paper shows the results of combined experimental and theoretical work that have unravelled the mechanism of ultrafast ejection of a methyl group from a cluster, the methyl iodide dimer (CH(3)I)(2). Ab initio calculations have produced optimized geometries for the dimer and energy values and oscillator strengths for the excited states of the A band of (CH(3)I)(2). These calculations have allowed us to describe the blue shift that had been observed in the past in this band. This blue shift has been experimentally determined with higher precision than in all previously reported experiments, since it has been measured through its effect upon the kinetic energy release of the fragments using femtosecond velocity map imaging. Observations of the reaction branching ratio and of the angular nature of the fragment distribution indicate that two main changes occur in A-band absorption in the dimer with respect to the monomer: a substantial change in the relative absorption to different states of the band, and, more importantly, a more efficient non-adiabatic crossing between two of those states. Additionally, time resolved experiments have been performed on the system, obtaining snapshots of the dissociation process. The apparent retardation of more than 100 fs in the dissociation process of the dimer relative to the monomer has been assigned to a delay in the opening of the optical detection window associated with the resonant multiphoton ionization detection of the methyl fragment.
The photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) and 2-(bromomethyl) hexafluoro-2-propanol (BMHFP) have been studied at 234 nm, and the C-Br bond dissociation investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS). Br formation is a primary process and occurs on a repulsive surface involving the C-Br bond of BTFP and BMHFP. Polarization dependent time-of-flight profiles were measured, and the translational energy distributions and recoil anisotropy parameters extracted using forward convolution fits. A strong polarization dependence of time-of-flight profiles suggest anisotropic distributions of the Br((2)P(3/2)) and Br((2)P(1/2)) fragments with anisotropy parameter, β, of respectively 0.5 ± 0.2 and 1.2 ± 0.2 for BTFP, and 0.4 ± 0.1 and 1.0 ± 0.3 for BMHFP. The measured velocity distributions consist of a single velocity component. The average translational energies for the Br((2)P(3/2)) and Br((2)P(1/2)) channels are 9.2 ± 1.0 and 7.4 ± 0.9 kcal/mol for BTFP, and 15.4 ± 1.8 and 15.1 ± 2.0 kcal/mol for BMHFP. The relative quantum yields of Br((2)P(3/2)) and Br((2)P(1/2)), which are 0.70 ± 0.14 and 0.30 ± 0.06 in BTFP and 0.81 ± 0.16 and 0.19 ± 0.04 in BMHFP, indicate that the yield of the former is predominant. The measured anisotropy parameters for the Br((2)P(3/2)) and Br((2)P(1/2)) channels suggest that the former channel has almost equal contributions from both the parallel and the perpendicular transitions, whereas the latter channel has a significant contribution from a parallel transition. Non-adiabatic curve crossing plays an important role in the C-Br bond dissociation of both BTFP and BMHFP. The estimated curve crossing probabilities suggest a greater value in BTFP, which explains a greater observed value of the relative quantum yield of Br((2)P(1/2)) in this case.
The photodissociation of methyl iodide in the A band is studied by full-dimensional (9D) wave packet dynamics calculations using the multiconfigurational time-dependent Hartree approach. The potential energy surfaces employed are based on the diabatic potentials of Xie et al. [J. Phys. Chem. A 2000, 104, 1009] and the vertical excitation energy is taken from recent ab initio calculations [Alekseyev et al. J. Chem. Phys.2007, 126, 234102]. The absorption spectrum calculated for exclusively parallel excitation agrees well with the experimental spectrum of the A band. The electronic population dynamics is found to be strongly dependent on the motion in the torsional coordinate related to the H(3)-C-I bend, which presumably is an artifact of the diabatic model employed. The calculated fully product state-selected partial spectra can be interpreted based on the reflection principle and suggests strong coupling between the C-I stretching and the H(3)-C-I bending motions during the dissociation process. The computed rotational and vibrational product distributions typically reproduce the trends seen in the experiment. In agreement with experiment, a small but significant excitation of the total symmetric stretching and the asymmetric bending modes of the methyl fragment can be seen. In contrast, the umbrella mode of the methyl is found to be too highly excited in the calculated distributions.
The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).
Quantum chemical calculations of CF(3)Br and the CF(3) radical are performed using density functional theory (DFT) and time-dependent DFT (TDDFT). Molecular structures, vibrational frequencies, dipole moment, bond dissociation energy, and vertical excitation energies of CF(3)Br are calculated and compared with available experimental results. The performance of six hybrid and five hybrid meta functionals in DFT and TDDFT calculations are evaluated. The ωB97X, B3PW91, and M05-2X functionals give very good results for molecular structures, vibrational frequencies, and vertical excitation energies, respectively. The ωB97X functional calculates well the dipole moment of CF(3)Br. B3LYP, one of the most widely used functionals, does not perform well for calculations of the C-Br bond length, bond dissociation energy, and vertical excitation energies. Potential energy curves of the low-lying excited states of CF(3)Br are obtained using the multiconfigurational spin-orbit ab initio method. The crossing point between 2A(1) and 3E states is located near the C-Br bond length of 2.45 Å. Comparison with CH(3)Br shows that fluorination does not alter the location of the crossing point. The relation between the calculated potential energy curves and recent experimental result is briefly discussed.
An approach has been developed to study steric effects in ion molecule reactions. The molecular orientation has been controlled by laser alignment. A first scattering experiment of ions with laser aligned molecules has been performed. In der Arbeit wurde ein experimenteller Ansatz entwickelt, sterische Effekte in Ion-Molekül Reaktionen zu untersuchen. Die Moleküle wurden dazu mit Hilfe starker Laserfelder ausgerichtet. An den ausgerichteten Molekülen wurde ein erstes Streuexperiment durchgeführt.
Excitation of the A-band low-lying electronic states in the methyl halides, CH(3)I, CH(3)Br, CH(3)Cl, and CH(3)F, has been investigated for the (n-->sigma*) transitions, using electron energy loss spectroscopy (EELS) in the range of 3.5-7.5 eV. For the methyl halides, CH(3)I, CH(3)Br, and CH(3)Cl, three components of the Q complex ((3)Q(1), (3)Q(0), and (1)Q(1)) were directly observed, with the exception of methyl fluoride, in the optically forbidden EELS experimental conditions of this investigation. The effect of electronic-state curve crossing emerged in the transition probabilities for the (3)Q(0) and (1)Q(1) states, with spin-orbit splitting observed and quantified against results from recent ab initio studies.
Quantum chemical calculations of CF(2)ICF(2)I and (*)CF(2)CF(2)I, model systems in reaction dynamics, in the gas phase and methanol solvent are performed using the density functional theory (DFT) and multiconfigurational ab initio methods. Molecular geometries, vibrational frequencies, and vertical excitation energies (T(v)) are computed and compared with available experimental results. We also evaluate the performance of four hybrid and one hybrid meta DFT functionals. The T(v) values calculated using time-dependent DFT vary depending on the exchange-correlation functionals, with the degree of variation approaching approximately 0.7 eV. The M05-2X functional well predicts molecular geometries and T(v) values, while it overestimates the vibrational frequencies. The T(v) values calculated using the M05-2X are similar to those calculated by the CASPT2. All low-lying excited states in CF(2)ICF(2)I are characterized by the excitation from the nonbonding to antibonding orbital of C-I. The excited states of (*)CF(2)CF(2)I are different in their character from those of CF(2)ICF(2)I and have considerable double excitation characters. The spin-orbit coupling of (*)CF(2)CF(2)I is larger than that of CF(2)ICF(2)I.
We performed configuration interaction ab initio calculations on the valence and 5s, 5p(a(1)), and 5p(e) Rydberg bands of the CH(3)Br molecule as a function of the methyl-bromide distance for frozen C(3v) geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the CH(3)Br(+) ion with a minimum at short distance. One state emerging from the 5p(e) band has valence and ion-pair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ion-pair production experiments. In each band, there is a single state, which has a non-negligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the 5p(e) band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methyl-bromide distance decreases due to a mixing between valence and 5s Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the (1)Q(1) state is dominant. In each Rydberg band, however, spin-orbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state ((1)Q(0)) with significant parallel transition moment induced by spin-orbit interaction with the ground state at large distance.
We performed a theoretical study of the photodissociation dynamics of CH(3)Br in the A band using a wave packet propagation technique on coupled ab initio potential energy curves. The present model involves the (3)Q(1) and (1)Q(1) excited states which can be populated from the ground state by a perpendicular transition and which are correlated at large methyl-bromide distance to the ground bromide spin-orbit state, as well as the (3)Q(0) and 4E states which can be excited by a parallel and perpendicular transition (respectively) and both correlate to excited Br(*) spin-orbit state. The model provides absorption cross sections and branching ratios in excellent agreement with experimental results. Due to weak spin-orbit interaction, the (1)Q(1) state is the dominant contributor to the absorption cross section, except for the red wing of the band where (3)Q(0) and (3)Q(1) states have significant absorption. However, spin-orbit coupling is strong enough to induce nonadiabatic transitions between the (3)Q(1) and (1)Q(1) states during the dissociation process which should be experimentally detectable in the alignment properties of the fragments. Nonadiabatic transitions at the conical intersection between (3)Q(0) and (1)Q(1) are shown to play a minor role in this system.
We employ the velocity map imaging technique to measure kinetic energy and angular distributions of state selected CH(3) (v(2)=0,1,2,3) and Br ((2)P(3/2), (2)P(1/2)) photofragments produced by methyl bromide photolysis at 215.9 nm. These results show unambiguously that the Br and Br( *) forming channels result in different vibrational excitations of the umbrella mode of the methyl fragment. Low energy structured features appear on the images, which arise from CH(3)Br(+) photodissociation near 330 nm. The excess energy of the probe laser photon is channeled into CH(3) (+) vibrational excitation, most probably in the nu(4) degenerate bend.
Photodissociation dynamics of ethyl iodide in the A-band is investigated at wavelengths between 294 and 308 nm using resonance-enhanced multiphoton ionization technique combined with velocity imaging detection. The I/I* branching, translational energy disposals, anisotropy parameters, and curve crossing probabilities of the dissociation channels are determined. The I(5p(2)P(3/2))-product channel is found to have hotter internal states of C(2)H(5), and the I*(5p(2)P(1/2)) channel is accompanied by colder C(2)H(5). Anisotropy parameters (beta) for the I* channel remain at 2.0, indicating that the I* production should originate from the (3)Q(0) state. The beta values are from 1.6 to 1.9 in the I-product channel, which comprises two components of direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probability rises rapidly around the conical intersection but remains almost constant after passing through the curve crossing. The (1)Q(1) and (3)Q(0) states in the exit region are thus expected to cross almost parallel along the dissociation coordinate. As compared to the case of CH(3)I, the nonadiabatic transition probabilities are slightly enhanced by an ethyl-substituted group.
The multireference spin-orbit (SO) configuration interaction (CI) method in its Lambda-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CH3I relevant to the photodissociation process in its A absorption band. The computed equilibrium geometry for the X A1 ground state, as well as vibrational frequencies for the nu2 umbrella and nu3 symmetric stretch modes, are found to be in good agreement with available experimental data. The 3Q0+ state converging to the excited I(2P1/2o) limit is found to possess a shallow minimum of 850 cm(-1) strongly shifted to larger internuclear distances (RC-I approximately 6.5a0) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the CH3I fragmentation dynamics unsuitable. The 4E(3A1) state dissociating to the same atomic limit is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the 3Q1, 3Q0+, and 1Q states indicate that the A-band spectrum must lie approximately between 33,000 and 44,300 cm(-1), i.e., between 225 and 300 nm. This result is in very good agreement with the experimental findings. The lowest Rydberg states are computed to lie at >or=49,000 cm(-1) and correspond to the ...a(1)2n3a1(6sI) leading configuration. They are responsible for the vacuum ultraviolet absorption lines found experimentally beyond the A-band spectrum at 201.1 nm (49,722 cm(-1)) and higher.
A diabatic representation is convenient in the study of electronically nonadiabatic chemical reactions because the diabatic energies and couplings are smooth functions of the nuclear coordinates and the couplings are scalar quantities. A method called the fourfold way was devised in our group to generate diabatic representations for spin-free electronic states. One drawback of diabatic states computed from the spin-free Hamiltonian, called a valence diabatic representation, for systems in which spin-orbit coupling cannot be ignored is that the couplings between the states are not zero in asymptotic regions, leading to difficulties in the calculation of reaction probabilities and other properties by semiclassical dynamics methods. Here we report an extension of the fourfold way to construct diabatic representations suitable for spincoupled systems. In this article we formulate the method for the case of even-electron systems that yield pairs of fragments with doublet spin multiplicity. For this type of system, we introduce the further simplification of calculating the triplet diabatic energies in terms of the singlet diabatic energies via Slater’s rules and assuming constant ratios of Coulomb to exchange integrals. Furthermore, the valence diabatic couplings in the triplet manifold are taken equal to the singlet ones. An important feature of the method is the introduction of
The photodissociation of rotationally state-selected methyl bromide is studied in the wavelength region between 213 and 235 nm using slice imaging. A hexapole state selector is used to focus a single (JK=11) rotational quantum state of the parent molecule, and a high speed slice imaging detector measures directly the three-dimensional recoil distribution of the methyl fragment. Experiments were performed on both normal (CH(3)Br) and deuterated (CD(3)Br) parent molecules. The velocity distribution of the methyl fragment shows a rich structure, especially for the CD(3) photofragment, assigned to the formation of vibrationally excited methyl fragments in the nu(1) and nu(4) vibrational modes. The CH(3) fragment formed with ground state Br((2)P(3/2)) is observed to be rotationally more excited, by some 230-340 cm(-1), compared to the methyl fragment formed with spin-orbit excited Br((2)P(1/2)). Branching ratios and angular distributions are obtained for various methyl product states and they are observed to vary with photodissociation energy. The nonadiabatic transition probability for the (3)Q(0+)-->(1)Q(1) transition is calculated from the images and differences between the isotopes are observed. Comparison with previous non-state-selected experiments indicates an enhanced nonadiabatic transition probability for state-selected K=1 methyl bromide parent molecules. From the state-to-state photodissociation experiments the dissociationenergy for both isotopes was determined, D(0)(CH(3)Br)=23 400+/-133 cm(-1) and D(0)(CD(3)Br)=23 827+/-94 cm(-1).
The UV photodissociation of bromo-3-fluorobenzene under collisionless conditions has been studied as a function of the excitation wavelength between 255 and 265 nm. The experiments were performed using ultrafast pump-probe laser spectroscopy. To aid in the interpretation of the results, it was necessary to extend the theoretical framework substantially compared to previous studies, to also include quantum dynamical simulations employing a two-dimensional nuclear Hamiltonian. The nonadiabatic potential energy surfaces (PES) were parameterized against high-level MS-CASTP2 quantum chemical calculations, using both the C-Br distance and the out-of-plane bending of the bromine as nuclear parameters. We show that the wavelength dependence of the photodissociation via the S0-->1pipi*-->1pisigma* channel, accessible with a approximately 260 nm pulse, is captured in this model. We thereby present the first correlation between experiments and theory within the quantitative regime.
The recently implemented second-order perturbation theory based on a complete active space self-consistent field reference function has been extended by allowing the Fock-type one-electron operator, which defines the zeroth-order Hamiltonian to have nonzero elements also in nondiagonal matrix blocks. The computer implementation is now less straightforward and more computer time will be needed in obtaining the second-order energy. The method is illustrated in a series of calculations on N2, NO, O2, CH3, CH2, and F−.
Molecular beams of CH3I and CD3I are photodissociated by 248 nm light. Dissociation takes place into CH3 (CD3) and II = I(2P)] or I*[= I(2P)]. The quantum yields for I* formation are respectively 0.71 and 0.81. The CH3 and CD3 fragments are found to be vibrationally excited in the umbrella mode. Both distributions for CH3 peak at υ = 2. The distribution in the I* channel equals that at 266 nm, found by other authors, in sharp contrast with earlier theoretical predictions by Shapiro and Bersohn. The distribution for CH3+I is broader and contains vibrations at least up to υ = 7. In the case of CD3I the vibrational distribution of the CD3 fragments peaks at υ = 3 for the I* channel and around υ = 5 in the I channel. Simple models are developed which give a fair description of the excitation of the umbrella mode of the CH3 and the CD3 radicals in both the I and I* channels. The angular distributions of the CH3 and CD3 fragments exhibit a predominantly parallel character in both channels, the I signal being slightly less anisotropic than the I* distribution. The anisotropy parameters for the CH3 fragments are β(I) = 0.72±0.02 and β(I*) = 0.76±0.02. For the CD3 fragments β(I) = 0.77±0.07 and β(I*) = 0.83±0.04. The main absorption is due to the 3Q0 ← 1A1 transition. The I signal is almost exclusively due to curve crossing between the 3Q0 and the 1Q states. Additionally, accurate values for the CI bond strength in CH3I and CD3I are obtained. These values are respectively 2.30±0.01 and 2.33±0.01 eV.
The spin-orbit coupling of molecular states of CH3I is introduced through an effective Hamiltonian and with pseudopotentials (CIPSO algorithm). Results are presented for the first states of CH3I which are correlated to the CH3(2A″2) + I(2P or 2P) dissociation limits. The dissociation energy for CH3I molecules is in good agreement with observation.
The fragment angular distribution is calculated for photodissociation by a beam of plane-polarized radiation of an ensemble of oriented symmetric-top molecules selected in a specific |JKM > state. For prompt dissociation the resulting expression, proportional to (σ/4π) PJKM (cos θ) [1 + βP2 (cos θ)], where PJKM (cos θ) is the probability of finding the symmetric-top precursor molecule with its figure axis in a given orientation, agrees with that given previously by Choi and Bernstein (J. Chem. Phys. 85 (1986) 150).
The MCD (magnetic circular dichroism) of CF3I, C2H5I and (t-BuI has been studied in the A band region (n→σ* continuum). A complex MCD signal was observed for all these molecules. The contribution of the excited electronic states to the A band is resolved and the results are compared with data obtained using other experimental techniques.
A molecular beam photodissociation study has been made of a number of simple aryl iodides and bromides. The angular distribution of photofragments is characterized by an anisotropy parameter β which was measured for each molecule. The values of β depend on the lifetime of the excited state compared to the rotational correlation times of the molecule as well as on the orientation of the transition dipole with respect to the C☒X bond. Knowing these orientations we can extract excited state liftimes as follows: methyl iodide, 0.07 picoseconds (psec); iodobenzene, 0.5 psec; α‐iodonaphthalene, 0.9 psec; and 4‐iodobiphenyl, 0.6 psec. It is concluded that methyl iodide directly dissociates but that the aryl compounds predissociate. The analogous aryl bromides have small anisotropy parameters and it is estimated that they live in the excited state perhaps two orders of magnitude longer than the aryl iodides. The data suggest a mechanism for predissociation in which S2 undergoes intersystem crossing to a triplet state localized on the C☒I bond either directly or via S1.
The n→σ∗ transition in CH3I at 257 nm produces mainly excited I(2P1/2) atoms and the available energy is mainly released as translation. A recent experiment by Y. Lee and co‐workers measured the distribution of the methyl radicals over the bending umbrella vibration, the only mode excited. This dissociation process is attractively simple because it can be expressed in terms of two linear coordinates rC–I, the distance between the carbon and iodine nuclei and rC–H3, the distance between the carbon nucleus and the plane of the three protons. The potential surface chosen for the excited surface dissociating to I(2P1/2) atoms was (atomic units) An essentially exact close coupling calculation using this potential energy together with an appropriate ground state potential fitted the absorption curve very well and the vibrational distribution approximately. Both the experimental and theoretical distributions for λ=266 nm peak at v=2. Calculations for other wavelengths show that the umbrella mode distribution will peak at higher (lower) v when a more (less) energetic photon is absorbed.
The dipole strengths for certain perpendicular‐type transitions N→Q in the mixed halogens, the hydrogen and monovalent metal halides, and the alkyl halides, are calculated theoretically by the LCAO and by the AO approximations. The scanty experimental absorption coefficient data on these halides (particularly the hydrogen and alkyl bromides and iodides) are critically examined, and acceptable experimental dipole strengths are obtained for the bromides. These show, very gratifyingly, the same kind of agreement with the calculated values as was found in IX of this series for the N→Q transitions in F2, Cl2, and Br2. The iodides, however, just like I2, show anomalously low strengths for the true N→Q part of the intensity (N→3II1 and N→1II), together with high strength for N→3II0+. These anomalies are ascribed here, as in I2, to partial case c coupling (partial preservation of atomic J's). The comparison between theory and experiment confirms the interpretation of the ultraviolet continua of the hydrogen and alkyl halides as N→Q transitions. Dipole strength calculations for the well‐known 2Σ→2II transition in OH have also been made. The agreement with Oldenberg and Rieke's experimental values for OH is of much the same kind as for the other molecules. A general equation for the N→Q dipole strength in AX or OH is obtained covering any degree of ionicity and of polarity. This includes the AO and LCAO approximations, with any degree of polarity, as special cases. It is found that the intensity is on the whole not especially sensitive to polarity. The magnitudes of overlapping and dipole strength integrals in relation to molecular stability and to principal quantum number of the valence orbital are discussed. The integrals are notably larger for molecules containing the heavier halogens (3p, 4p, 5p orbitals) than for those containing fluorine (2p orbital). They are also larger for hydrogen halides than for corresponding halogens.
We present a study of the dissociation of CH3I on coupled repulsive electronic potential energy surfaces by the technique of polarized emission spectroscopy. We excite CH3I at 266 nm and disperse the photons emitted from the dissociating molecule by both frequency and angular distribution with respect to the polarization direction of the excitation laser. We thus measure the polarization of the first 12 C–I stretching emission features, corresponding to the spectral region between 266 and 317 nm. We also obtain the rotational envelope of selected emission features in higher resolution scans and model the lineshapes with parameters derived from the polarization results. The polarization measurements show the emission into the first few low‐lying C–I stretching vibrational levels is via a transition moment parallel to the absorbing one, consistent with excitation to and emission from the 3Q0(2A1) repulsive surface. Emission to higher C–I stretching overtones shows an increasing contribution from emission via a transition moment perpendicular to the absorbing one, consistent with emission from a repulsive surface of E symmetry following excitation to the 3Q0(2A1) state.
The tert‐butyl iodide molecule is readily focused with the electrostatic hexapole, via its first‐order Stark effect as a pseudo‐symmetric top. The pulsed, seeded supersonic focused beam, characterized by 〈cos θ〉=Vth/ V0 (where θ is the angle between the molecular dipole axis μ and the electric field E; ±V0 the hexapole ‘‘rod voltage,’’ and Vth the so‐called threshold voltage), passes into a small homogeneous electric field in which it is oriented. The degree of laboratory orientation achieved is measured using the method of linearly polarized laser‐induced photofragmentation [S. R. Gandhi, T. J. Curtiss, and R. B. Bernstein, Phys. Rev. Lett. 59, 2951 (1987)], operating (at three laser wavelengths) on the I(2P3/2) and I(2P1/2) as well as the t‐C4H9 radical photofragments. The results show that the oriented beam molecules of t‐butyl iodide (at a rotational temperature near 15 K) have a higher degree of orientation than the prototype CH3I molecules (JKM state‐selected and focused similarly), explainable by the greater importance of the so‐called hyperfine disorientation effect for the prolate symmetric top (CH3I) than for the t‐C4H9I. For the latter, orientations with photofragment up–down asymmetry ratios as large as a factor of 10 can be achieved, suggesting that t‐C4H9I is an excellent candidate reagent for reactive asymmetry studies.
The full nine‐dimensional potential energy surfaces (PESs) of the 3Q0 and 1Q1 states of CH3I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociation dynamics of both CH3I and CD3I in the A‐continuum. The results are in general good agreement with the recent experimental findings. The reasonable I∗/(I∗+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the 1Q1 state is considered. The rotational distribution of the CH3 and CD3 fragments and its I∗/(I∗+I)‐channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection region. The vibrational distribution of umbrella mode is closely related to the shape of PESs for the umbrella angle; the sudden switch of reaction coordinate from 3Q0 to 1Q1 at the conical intersection is the origin of vibrational excitation in the I∗ channel. The larger umbrella excitation of the CD3 fragment in both I and I∗ channels, in comparison with the CH3 fragment, is related to the larger separation of the reaction coordinate from the Franck–Condon geometry. The symmetric stretching energy increases during the dissociation, which is related to the shape of PESs with respect to this coordinate, and the excitation of symmetric stretching mode seems to be possible. © 1996 American Institute of Physics.
The photodissociation of CF3I cooled in a supersonic molecular beam has been investigated at 277 nm by state‐selective photofragment imaging. Fragmented iodine atoms of two spin–orbit states are state‐selectively ionized and projected onto a two‐dimensional position‐sensitive detector, to obtain their speed and angular distribution. The anisotropy parameter for an excited iodine atom I∗(2P1/2), β(I∗), is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150–350 fs from rotational correlation function. Contrary to earlier reports, the parallel‐like distribution for the ground state iodine atom I(2P3/2) at this wavelength, shows a more favorable curve‐crossing dissociation path (68%) from 3Q0 to 1Q1 and a less favorable direct dissociation path (32%) from 3Q1. The recoil energy distribution of I is found to be broader than that of I∗ and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previous works, and the strong photon energy dependence of the energy partitioning in CF3+I∗ channel and curve crossing are interpreted in terms of the final state interaction and curve crossing probability, respectively. © 1996 American Institute of Physics.
The photodissociation of CH3I at 266 nm is investigated by means of high resolution photofragment spectroscopy. The resolution is sufficient to determine the vibrational population of the CH3 umbrella motion for the I∗(2P1/2)+CH3 product channel. An approximate vibrational distribution for the I(2P3/2)+CH3 product channel is also determined. The rotational energy distribution for the CH3+I∗(2P1/2) channel is estimated to be on the order of or less than 600 cm−1 wide for each of the CH3 vibrational states. A refined value for the C–I bond dissociation energy of 53.3±0.7 kcal/mole is determined from the energy threshold for the I∗+CH3 channel. The vibrational energy distribution for the I∗+CH3 channel is discussed in relation to a recent model calculation by Shapiro and Bersohn and possible explanations for the discrepency between the calculated and the measured distributions are considered.
New basis sets of the atomic natural orbital (ANO) type have been developed for the main group and rare gas atoms. The ANO's have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the dimer at its equilibrium geometry. Scalar relativistic effects are included through the use of a Douglas−Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and the ground-state potentials for the dimers. These calculations include spin−orbit coupling using the RASSCF State Interaction (RASSI-SO) method. The spin−orbit splitting for the lowest atomic term is reproduced with an accuracy of better than 0.05 eV, except for row 5, where it is 0.15 eV. Ionization energies and electron affinities have an accuracy better than 0.2 eV, and atomic polarizabilities for the spherical atoms are computed with errors smaller than 2.5%. Computed bond energies for the dimers are accurate to better than 0.15 eV in most cases (the dimers for row 5 excluded).
Second-order perturbation theory based on a CASSCF reference state is derived and implemented. The first-order wave function includes the full space of interacting states. Expressions for the contributions to the second-order energy are obtained in terms of up to four-particle density matrices for the CASSCF reference state. The zeroth-order Hamiltonian reduces to the Møller-Plesset Hamiltonian for a closed-shell reference state. The limit of the implementation is given by the number of active orbitals, which determines the size of the density matrices. It is presently around 13 orbitals. The method is illustrated in a series of calculations on H 2, H 2O, CH 2, and F -, and the results are compared with corresponding full CI results.
A method to compute spin–orbit coupling between electronic states is presented. An effective one-electron spin–orbit Hamiltonian is used, based on atomic mean field integrals. The basic electronic states are obtained using the restricted active space (RAS) SCF method. The Hamiltonian matrix is obtained by an extension of the restricted active space state interaction (RASSI) method. Several hundred states can be included. Tests for atoms and molecules from the entire periodic system show accurate results. Computed spin–orbit effects on relative energies are normally accurate within a few percent. The method has been included in the MOLCAS-5.0 quantum chemistry software.
Generally contracted basis sets for second row atoms have been constructed using the Atomic Natural Orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over several atomic states, positive and negative ions, and atoms in an external electric field. The contracted basis sets give virtually identical results as the corresponding uncontracted sets for the atomic properties, which they have been designed to reproduce. The design objective has been to describe the ionization potential, the electron affinity, and the polarizability as accurately as possible. The result is a set of well balanced basis sets for molecular calculations. The starting primitive sets are 17s12p5d4f for the second row atoms Na-Ar. Corresponding ANO basis sets for first row atoms have recently been published.
A new one-particle zeroth-order Hamiltonian is proposed for perturbation theory with a complete active space self-consistent field (CASSCF) reference function. With the new partitioning of the Hamiltonian, reference functions dominated by a closed-shell configuration, on one hand, and an open-shell configuration, on the other hand, are treated in similar and balanced ways. This leads to a better description of excitation energies and dissociation energies. The new zeroth-order Hamiltonian has been tested on CH2, SiH2, NH2, CH3, N2, NO, and O2, for which full configuration interaction (FCI) results are available. Further, excitation energies and dissociation energies for the N2 molecule have been compared to corresponding multireference (MR) CI results. Finally, the dissociation energies for a large number of benchmark molecules containing first-row atoms (the G1 test) have been compared to experimental data. The computed excitation energies compare very well with the corresponding FCI and MRCI values. In most cases the errors are well below 1 kcal/mol. The dissociation energies, on the other hand, are in general improved in the new treatment but have a tendency to be overestimated when compared to other more accurate methods.
Generally contracted basis sets for first row atoms have been constructed using the Atomic Natural Orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over several atomic states, positive and negative ions, and atoms in an external electric field. The contracted basis sets give virtually identical results as the corresponding uncontracted sets for the atomic properties, which they have been designed to reproduce. The design objective has been to describe the ionization potential, the electron affinity, and the polarizability as accurately as possible. The result is a set of well-balanced basis sets for molecular calculations. The starting primitive sets are 8s4p3d for hydrogen, 9s4p3d for helium, and 14s9p4d3f for the heavier first row atoms.
The technique of linearly polarized laser-induced photofragmentation for the measurement of the degree of orientation of rotationally state-selected symmetric top molecules [Phys. Rev. Lett.59, 2951 (1987)] has been used to study the retention of molecular orientation in optical frequency AC and homogeneous DC electric fields. For CH3I beams, state-selected by the electrostatic hexapole focuser in several specific low-J parent states, recoupling of the iodine nuclear spin with the molecular rotational angular momentum occurs rapidly in weak fields, leading to some loss of orientation, but the resulting degree of orientation (i.e., theM
F
distribution) is retained in both DC and optical frequency electric fields. The direction of the orientation of the molecular axis can be inverted with 100% efficiency by changing the sign of the DC orienting field. The up/down asymmetry of the photofragment angular distribution can be observed with either parallel (vertical) or perpendicular (horizontal) laser polarization.
An ab initio study of the vertical electronic excitations in CX3I, C6X5H, and C6X5I (X=H and F) is presented. All-electron basis sets are used and the relativistic effects are accounted for with the relativistic elimination of small components scheme. The structures are optimized with the complete active space self-consistent field approach and the excitation energies are computed with the spin-orbit multiconfiguration quasidegenerate perturbation theory. The n-sigma(*) transitions of CX3I, low-lying pi-pi(*) transitions of C6X5H, and low-lying n-sigma(*), pi-pi(*), and pi-sigma(*) transitions of C6X5I are elucidated. For CH3I, energy values of parallel and perpendicular transitions differ from experimental values by 455 and 1156 cm(-1), respectively. Effects of fluorination are emphasized, it is found that fluorination increases the gap between (3)Q(0) and (1)Q(1) transitions and increase is substantially more in aryl iodides than in alkyl iodides. Electronic factors influencing increased I-* quantum yield in the photodissociation on fluorination of alkyl iodides is attributed to increased gap between (3)Q(0) and (1)Q(1) transitions reducing curve crossing probability and for aryl iodides there is additional role by phenyl transitions. A correlation diagram illustrating transitions of aryl iodides is presented. (C) 2002 American Institute of Physics.
A density matrix formulation of the super-CI MCSCF method is presented. The MC expansion is assumed to be complete in an active subset of the orbital space, and the corresponding CI secular problem is solved by a direct scheme using the unitary group approach. With a density matrix formulation the orbital optimization step becomes independent of the size of the CI expansion. It is possible to formulate the super-CI in terms of density matrices defined only in the small active subspace; the doubly occupied orbitals (the inactive subspace) do not enter. Further, in the unitary group formalism it is straightforward and simple to obtain the necessary density matrices from the symbolic formula list. It then becomes possible to treat very long MC expansions, the largest so far comprising 726 configurations. The method is demonstrated in a calculation of the potential curves for the three lowest states (1Σ+g, 3Σ+u and 3Πg) of the N2 molecule, using a medium-sized gaussian basis set. Seven active orbitals were used yielding the following results: De: 8.76 (9.90), 2.43 (3.68) and 3.39 (4.90) eV; re: 1.108 (1.098), 1.309 (1.287) and 1.230 (1.213) Å; ωe: 2333 (2359), 1385 (1461) and 1680 (1733) cm−1, for the three states (experimental values within parentheses). The results of these calculations indicate that it is important to consider not only the dissociation limit but also the united atom limit in partitioning the occupied orbital space into an active and an inactive part.
An extension of the multiconfigurational second-order perturbation approach CASPT2 is suggested, where several electronic states are coupled at second order via an effective-Hamiltonian approach. The method has been implemented into the MOLCAS-4 program system, where it will replace the single-state CASPT2 program. The accuracy of the method is illustrated through calculations of the ionic-neutral avoided crossing in the potential curves for LiF and of the valence-Rydberg mixing in the V-state of the ethylene molecule.
A method is described to calculate the matrix elements of one- and two-electron operators for CASSCF wavefunctions employing individually optimized orbitals. The computation procedure is very efficient, and matrix elements between CAS wavefunctions with up to a few hundred thousand configurations are easily evaluated. An implementation of this method is presented, which sets up and solves the Hamiltonian secular problem in a basis of independently optimized CASSCF wavefunctions. In the program, the only restriction imposed is that the AO basis set and the number of active orbitals is the same for all states. The method is illustrated by calculations on π-excited states of some aromatic molecules.
All-electron spin-orbit mean-field integrals have been modified to be used with effective core potential valence wave-functions. Two different effective core potentials, one of the conventional Huzinaga type and the other an ab initio model potential, have been employed in our investigation. In all cases the full nodal structure of the valence orbitals has been kept. The applicability of the present approach has been tested for the 3D and 1D (d9s1) and the 3F (d8s2) electronic states of atomic platinum, the 2D d9 state of Pt+, and the low-lying states of platinum hydride. Spin-orbit matrix elements evaluated for ab initio model potential wavefunctions are found to agree with all-electron results to within better than 3%, and the corresponding agreement for the spectroscopic constants of PtH is excellent.
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