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Absorption Spectra of Ethynyl, Ethenyl, and Phenyl Peroxyl Radicals
Abstract
Both the ethenyl and phenyl peroxyl radical absorb in the visible region in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were initially surprising. Ab initio determination of the electronic structure of the ground and excited states of the ethynyl, ethenyl, and phenyl peroxyl radicals provides a fundamental understanding of these electronic transitions. The electronic excited states of these radicals are low in energy because the pi-type open-shell orbital localized on the oxygen atoms in the ground state couples to a relatively low-energy pi or conjugated orbital system in the excited state. In the case of both the ethynyl and phenyl peroxyl radicals, the excitation of the C-C bond pi orbital is relatively high in energy, and the in vacuo prediction for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because all the radicals are polar in the ground state, and the dipole moment, in the relevant excited state is substantially larger. In addition, for the ethenyl peroxyl radical there are two isomeric forms whose ground states are close in energy, and the observed spectrum can be assigned to the convoluted spectra of both isomers.
... 2,12,13 Some of the earliest spectroscopic data for phenylperoxy were obtained in the solution phase, using pulsed radiolysis to generate the radical. 15,16 A broad absorption band, with a maximum near 470 nm, was reported. Yu and Lin 13 then observed an absorption feature in a gas phase experiment that was attributed to phenylperoxy. ...
... There have been theoretical studies of the three lowest energy electronic transitions of phenylperoxy. 14,16,17 Electronic structure calculations predict that the molecule is planar in the ground and the lower energy excited states, with C s symmetry. The lowest energy transition, Ã2A′−X̃2A″ is attributed to promotion of an electron between orbitals that are mostly localized on the peroxy moiety 14,17 (the singly occupied molecular orbital (SOMO) of the ground state receives the excited electron). ...
The visible absorption bands of the phenyl peroxy radical in the gas phase have been investigated using cavity ring-down spectroscopy. Jet-cooling was used to reduce the spectral congestion. Structured spectra spanning the range from 17500 - 19000 cm(-1) are reported for the first time. Analyses of these data have been guided by the results from time-dependent density functional calculations. The observed spectrum was found to be dominated by the bands of the transition. An analysis of the rotational contour for the origin band yielded a homogeneous linewidth of 2.2 cm(-1), corresponding to a decay rate of 4.1x10(11) s(-1). The results provide a rationale for the lack of structure in room temperature spectra that have been previously attributed to phenyl peroxy. They also indicate that the lower energy region of the spectrum may show resolvable structure at room temperature. If so, this would provide a more definitive signature for monitoring phenyl peroxy in kinetic measurements.
Photoelectron spectra of cryogenically cooled X∼1A′ tert-butyl peroxide anions are obtained using slow electron velocity-map imaging. The spectra show highly structured bands corresponding to detachment to the X∼2A″ and A∼2A′ electronic states of the neutral radical and represent a notable improvement in resolution over previous photoelectron spectra. We report an electron affinity of 1.1962(20) eV and a term energy T0(A∼2A′) of 0.9602(24) eV for the tert-butyl peroxy radical. New vibrational structure is resolved, providing several frequencies for both neutral states. Additionally, the threshold behavior of the photodetachment cross section is investigated within the context of Dyson orbital calculations.
Peroxy radicals (RO2) are intermediates in fuel combustion, where they engage in efficiency-limiting autoignition reactions. They also participate in atmospheric chemistry leading to the formation of unwanted tropospheric ozone. Advances in spectroscopic techniques have allowed for the possibility of employing the lowest () electronic transition of RO2 as a tool to selectively monitor these species, enabling accurate kinetic values to be obtained. Herein, high-level ab initio methods are employed to systematically refine spectroscopic predictions for the methyl peroxy radical (CH3O2), one of the most abundant peroxy radicals in the atmosphere. In particular, vibrationally corrected geometries and anharmonic vibrational frequencies for both the ground () and first excited () state are predicted using coupled-cluster theory with up to perturbative triples [CCSD(T)] and large atomic natural orbital basis sets. Equation-of-motion coupled-cluster theory is utilised to compute vertical transition properties; a radiative lifetime of 4.7 ms is suggested for the excited state. Finally, we predict the adiabatic excitation energy (T0) via systematic extrapolation to the complete basis limit of coupled-cluster with up to full quadruples (CCSDTQ). After accounting for several approximations, and including an anharmonic zero-point vibrational energy correction, we match experiment for this transition to within 9 cm−1.
Analytical expressions for the sCIS (scaled configuration interaction singles) and UCIS (unrestricted CIS) energy gradients are presented for the semiempirical method MSINDO. The theoretical background of the derivation of the analytical gradients is presented, and the implementation into the MSINDO program package is described. The computational efficiency of the underlying Z-vector method is greatly enhanced by making use of the transpose-free quasiminimal residual (TFQMR) algorithm. Benchmark timing tests are compared to the widely used TD-B3LYP approach. For a statistical evaluation of the accuracy of MSINDO-sCIS, geometry optimizations are performed for a small set of organic molecules in selected excited states. The obtained results are compared to CASPT2 and TD-B3LYP/TZVP. In order to demonstrate the applicability of the present approach to periodic systems within the cyclic cluster model, we present first calculations of the excited state structure of ethyne adsorbed on the NaCl (100) surface.
The reaction of the ethynyl radical with molecular oxygen has been examined using density functional theory. Two major reaction routes are open to the chemically activated HCCOO adduct : dissociation to HCCO+O and formation of the thermodynamically most stable products HCO + CO : HCCOO → dioxirenyl → oxyrenyloxy → oxo-ketene → HCO + CO → H + 2CO . The CCSD(T)/6-311++G(d,p)// B3LYP/6-311++G(d,p) energies of the respective rate controlling transition states, and , indicate that the route leading to H + CO + CO should dominate. Several other [C2,H,O2] isomers and other, minor pathways have also been characterised. The present study reveals this reaction to be a capture-limited association-elimination reaction with a high and pressure-independent rate coefficient.
This chapter describes the chemistry of reactive radical intermediates in combustion and the atmosphere. Combustion processes convert chemical energy into heat and work, playing several important roles in society. Oxidation processes provide power to beneficiaries ranging from automobiles to electrical generators and atmospheric oxidation reactions impact a wide range of environmental phenomena. At high temperatures, methane oxidation is initiated through hydrogen atom abstraction by hydroxyl radical, oxygen atom, or hydrogen atom. Peroxy radical chemistry plays a substantial role in low-temperature combustion as opposed to the alkoxy radical chemistry of high-temperature combustion.The peroxy radicals constitute an important class of reactive intermediates with significant implications for low temperature combustion and atmospheric reactions. Phenylperoxyradical, originally assumed to be a factor in low-temperature combustion only, has actually been shown to play a substantial role in dictating the overall combustion trends of benzene. The chapter suggests that both hydrogen atom and alkyl group loss can contribute to the combustion of coal in initiation reactions.
The absorption spectrum and cross sections of vinylperoxy (C2H3O2, ethenylperoxy) radical have been determined, for the first time in the gas phase, in the spectral range 220−550 nm. The spectrum exhibits a relatively broad and intense absorption centered at about 232 nm, followed by at least two identifiable and weaker absorptions with maxima at about 340 and 420 nm. The absorption tail persists at longer wavelengths into the visible region. To discern competition between the stabilization of the vinylperoxy isomers and reaction, the effect of pressure on the absorption has been examined. Vinylperoxy radicals in these experiments were produced through photochemical production of vinyl radicals followed by reaction of vinyl radicals with molecular oxygen. Vinyl bromide (C2H3Br) photolyzed at 193 nm was used as the precursor of vinyl radicals, and methyl vinyl ketone (CH3COC2H3) photolyzed at 193 nm was used as a precursor of both methyl and vinyl radicals. In the latter system identical concentrations of methyl and vinyl peroxy radicals were produced, and by employing comparative methods and using the literature values for methylperoxy absorption cross sections, the absolute absorption cross sections for vinylperoxy were determined. Ab initio molecular orbital calculations of CH3O2, C2H3O2, C2H3O, and HCO have been employed to characterize the observed spectrum and to identify the species and assign transitions contributing to the spectrum. These calculations suggest that the observed spectrum can primarily be assigned to two stable isomers (conformations) of the vinylperoxy radical with the O−O bond in a cis or trans position relative to the C−C bond, with a minor contribution to the absorption spectrum from the vinoxy and formyl radicals. For unsaturated radicals and the weak bonds involved here, accurate geometries are difficult to calculate, but the geometry obtained by gradient optimization using the multiconfiguration self-consistent-field method yields excitation energies that most closely agree with experiment. The relative theoretical oscillator strengths of all relevant vinylperoxy and vinoxy transitions have been evaluated and assist the analysis of the pressure dependence of the absorption spectrum.
Removal of the nitrogen atom from aromatic nitrogen-containing molecules takes place via hydrogenation of the aromatic rings, CN bond cleavage of the resulting saturated amines by NH3 elimination, and alkene (de)hydrogenation. The separate reaction steps cannot be combined into one kinetic equation, because every reaction takes place on a different catalytic site which differs in its ability to bind reactants, intermediates and products. A multi-site Langmuir-Hinshelwood rate model has to be used in the kinetic modeling. Depending on the type of molecule, aniline or pyridine-like, the rate determining step is phenyl hydrogenation or ring opening elimination, respectively. The HDN behaviour of polycyclic aromatic N-containing molecules is based on a combination of the HDN reactions of aniline and pyridine-like molecules.The catalyst components influence the rate as well as the adsorption constants. Ni has a positive effect on all reaction steps for all reactant molecules, while the effect of phosphorus depends on the type of rate determining reaction, and thus on the reactant. Hydrogenation reactions are promoted by P (aniline and alkene), and elimination reactions are negatively influenced (piperidine and decahydroquinoline). The reverse is true for H2S, suggesting that phosphorus increases the number of sulfur vacancies which are considered to be the catalytic sites for (de)hydrotreating reactions.
The ultraviolet absorption spectrum and cross sections of propargyl (C3H3) radicals have been, for the first time, determined in the gas phase in the spectral range of 230–300 nm. The propargyl radicals were produced from 193 nm excimer laser photolysis of C3H3Cl, 248 nm photolysis of C3H3Br and from reaction of Cl-atoms with propargyl chloride. The spectra obtained from the three systems were nearly identical. The ultraviolet spectrum for the propargyl radicals exhibits a relatively broad absorption with a maximum cross section of 1.2 × 10−17 cm2 molecule−1 at 242 nm. The electronic transitions in propargyl radical have also been calculated by ab initio quantum chemical methods at the CASSCF, CASPT2, CIS, and EOM_CCSD levels of theory and assign the new observed band to the in plane π∗(b2)←π(b2) allowed transition. In addition, we report here the absorption spectra and cross sections for propargyl chloride and propargyl bromide, precursors of propargyl radicals, in the spectral range of 160–230 nm.
The room-temperature gas-phase ultraviolet absorption spectrum and cross-sections of vinyl (C2H3) radicals have been determined in the spectral range 225–238 nm, employing cavity ring-down absorption spectroscopy. Vinyl radicals in these experiments were produced from the 193 nm excimer laser photolysis of methyl vinyl ketone (CH3COC2H3) and vinyl bromide (C2H3Br). The spectra obtained from the two systems were nearly identical. The observed spectrum exhibits a relatively broad and featureless absorption with a cross-section of 5.3×10−18 cm2 molecule−1 at 230 nm. A combined uncertainty of ∼25% for cross-section values has been assessed. The electronic transitions in the vinyl radical have been calculated by ab initio quantum chemical methods at the CIS, EOM-CCSD, CASSCF and CASPT2 levels of theories which assign the new observed band to the highly allowed and `in plane' π*(2a″) ← π(1a″) transition.
Gas phase peroxyl radicals are central to our chemical understanding of combustion and atmospheric processes and are typically characterized by strong absorption in the UV (λmax ~ 240 nm). The analogous maximum absorption feature for arylperoxyl radicals is predicted to shift to the visible but has not previously been characterized nor have any photoproducts arising from this transition been identified. Here we describe the controlled synthesis and isolation in vacuo of an array of charge-substituted phenylperoxyl radicals at room temperature, including the 4-(N,N,N-trimethylammonium)methyl phenylperoxyl radical cation (4-Me3N([+])CH2-C6H4OO(•)), using linear ion-trap mass spectrometry. Photodissociation mass spectra obtained at wavelengths ranging from 310-500 nm reveal two major photoproduct channels corresponding to homolysis of aryl-OO and arylO-O bonds resulting in loss of O2 and O, respectively. Combining the photodissociation yields across this spectral window produces a broad (FWHM ~ 60 nm) but clearly resolved feature centered at λmax = 403 nm (3.08 eV). The influence of the charge-tag identity and its proximity to the radical site are investigated and demonstrate no effect on the identity of the two dominant photoproduct channels. Electronic structure calculations have located the vertical B←X transition of these substituted phenylperoxyl radicals within the experimental uncertainty and further predict the analogous transition for unsubstituted phenylperoxyl radical (C6H5OO(•)) to be 457 nm (2.71 eV): nearly 45 nm shorter than previous estimates and in good agreement with recent computational values.
The electronic structure and spin density distribution of peroxyl radicals are investigated by density functional theory (DFT) at the B3LYP level. Results found for superoxide anion and tert-butyl peroxyl radicals at a variety of basis sets suggest that 6-31G is the most appropriate basis set for calculation of hyperfine coupling constants (hfcc's) of carbon-based peroxyl radicals. Calculation of parallel 17O hfcc's [A(17O)] for a series of substituted methyl peroxyl radicals with the 6-31G basis set yielded calculated values with a maximum deviation of 2.2% from experiment. Spin density distributions estimated from experiment A(17O) are compared to theoretical estimates from Mulliken orbital population analysis. Electronegative substitution at the carbon alpha to the peroxyl group results in an increase of terminal oxygen hyperfine coupling and spin density, shortening of C−O, and lengthening of O−O. In cases involving significant steric hindrance, however, C−O bond shortening was prevented. A(17O) values for the terminal peroxyl oxygen atom correlate well with Taft σ* substitutent parameters for the R group in the peroxyl radicals (ROO•). Thiyl peroxyl radicals are reinvestigated using B3LYP for comparison to previous theoretical work at UHF level. This resulted in confirmation that the effect of the addition of an electron pair donor (hydroxide ion) to CH3SOO• is to alter the spin density distribution in the peroxyl group. Structural models of lipid peroxyl radicals show that vinyl peroxyl radicals may be distinguished from saturated, allylic, and ester-based peroxyl radicals on the basis of hyperfine coupling constants.
Theoretical assignments of the spectra of the H and OH adduct radicals of cytosine are obtained with ab initio multiconfiguration self-consistent-field calculations. The C6OH isomer is substantially more stable than the C5OH. The C6H isomer is slightly more stable than C5H, but both are much less stable than the N3H isomer. The C5 and C6 adducts for both H and OH are found to support transitions in entirely different spectral regions. Visible absorption transitions are found for the C6 adducts in which the open shell orbital is delocalized by conjugating to a neighboring double bond or system of bonds. In C5 adducts, the open-shell orbital remains localized giving rise to absorption transitions in the UV. The calculations show that the very broad absorption observed at 440 nm is the convolution of two transitions for the C6OH isomer and that the observed peak at 340 nm has contributions from both the C5OH and C6OH isomers but is dominated by the absorption of C5OH. The similarity of the absorption pattern of the cytosine and uracil adducts suggest a general behavior in the spectroscopy of pyrimidine adducts. Base radical calculations were reported at the 8th International Congress of Quantum Chemistry, Prague, June 20, 1994.
We present theoretical confirmation of an intramolecular charge-transfer (CT) state in peridinin in agreement with experimental results of Frank and co-workers (J. Phys. Chem. B 1999, 103, 8751 and J. Phys. Chem. B 2000, 104, 4569). Quantum chemical calculations using time-dependent density functional theory under the Tamm−Dancoff approximation were made on two structures: fully optimized peridinin and a molecule from the crystal structure of peridinin−chlorophyll−protein. The CT state appears as the third and second excited singlet state, respectively, for the two structures. A dipole-in-a-sphere model is used to estimate the solvation stabilization energies of each state in a variety of solvents. The energy of the CT state is shown to decrease dramatically in solvents of increasing polarity while the energy of the dark S1 (Ag--like) state remains comparatively constant.
New catalysts for deep hydrodesulfurization (HDS) of diesel oil were prepared by loading the active metal components on the TiO2−SiO2 composite supports, which were synthesized by a sol−gel and supercritical fluid drying method. The HDS performance of the prepared catalysts for RFCC (resid fluid catalytic cracking) diesel oils was evaluated in a fixed-bed reactor under different conditions. The major sulfur compounds in the RFCC diesel oils are alkyl substituent benzothiophenes (BTs), dibenzothiophene (DBT), and alkyl substituent dibenzothiophenes (DBTs), in which the alkyl DBTs with alkyl groups at the 4 and/or 6 positions, such as 4,6-DMDBT and 4-MDBT, are the refractory compounds. The effects of the catalyst properties on the HDS performance for different sulfur compounds, especially the refractory compounds, in the diesel oils were studied. It was found that the catalysts supported on TiO2−SiO2 composites with higher ratios of Lewis acid sites and with Ni−W active components presented better HDS performance for the alkyl DBTs. The HDS performance of the catalysts for removing of alkyl DBTs can be further improved through increasing the concentration and intensity of Lewis acid by adding F and P into the catalysts.
To promote aromatic hydrogenation of distillates, a new composite support containing presynthesized crystalline AlPO4 structure was developed. This composite support causes easier reducibility of metal oxides and impedes nickel aluminate formation effectively compared with pure alumina counterparts. Moreover, the composite support exhibits higher surface area, larger pore volume, and better surface characteristic than the phosphoric acid-modified alumina to improve metal−support interactions and facilitate the polymeric tungsten species formation. The evaluation results show that the composite supported catalyst exhibits the highest conversion of tetralin, in comparison to the corresponding catalysts with pure alumina or phosphoric acid-modified alumina as supports.
Ab initio and Density Functional Theory (DFT) calculations were performed to determine the equilibrium geometries, charge distributions, spin density distributions, dipole moments, electron affinities (EAs), and C–O bond dissociation energies (BDEs) of CH2ClO2• CHCl2O2•, CCl3O2•, CF2ClO2•, CFCl2O2•, and CHFClO2• peroxyl radicals. The C–H BDEs of the parent methanes were calculated using the same levels of theories. Both MP2(full) and B3LYP methods, using the 6-31G(d,p) basis set, were found to be capable of accurately predicting the geometries of peroxyl radicals. The B3LYP/6-31G(d,p) method was found to be comparable to high ab initio levels in predicting C–O BDEs of studied peroxyl radicals and C–H BDEs of the parent alkanes. The progressive chlorine substitution of hydrogen atoms in methyl peroxyl radicals results in an increase (decrease) of the spin density on the terminal (inner) oxygen, a decrease in dipole moments, and an increase in electron affinities. The substitution of fluorine by chlorine in the series CF3O2• – CCl3O2• was found to lengthen (destabilize) the C–O bonds. Both C–O BDEs and EAs of peroxyl radicals (RO2•) were found to correlate well with Taft σ* substituent constants for the R groups. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005
The equilibrium geometries, harmonic vibrational frequencies, charge distributions, spin density distributions, dipole moments, electron affinities (EAs), and CO bond dissociation energies (BDEs) of HO, CH3O, CH2FO, CHF2O, and CF3O peroxyl radicals have been calculated using ab initio molecular orbital theory and density functional theory (DFT) at the B3LYP level. The CH bond dissociation energies of the parent fluoromethanes have been calculated using the same levels of theory. Both the MP2(full) and B3LYP methods, using the 6-31G(d,p) basis set, are found to be capable of accurately predicting the geometries of peroxyl radicals. Electron correlation accounts for ∼25% of the CH BDE of fluoromethanes and for ∼50% of the CO BDE of the corresponding peroxyl radicals. The B3LYP/6-31G(d,p) method is found to be comparable to high ab initio levels in predicting CO BDEs of studied peroxyl radicals and CH BDEs of the parent alkanes. The progressive fluorine substitution of hydrogen atoms in methyl peroxyl radicals results in shortening of the CO bond, lengthening of the OO bond, an increase (decrease) of the spin density on the terminal (inner) oxygen, a decrease in the dipole moments, and an increase in electron affinities. Both CO BDEs and EAs of peroxyl radicals (RO) correlate well with Taft σ* substituent constants for the R group in peroxyl radicals. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
Vibrationally excited CO (v), HCO (ν1, ν2), HNC (ν3) and HCN (ν3) were observed as products of the reaction of C2H with NO by using time-resolved FTIR emission spectroscopy. Three exothermic reaction channels leading to HCN+CO, HNC+CO and CN+HCO are identified, verifying an association-elimination reaction mechanism. The nascent products of CO (v⩽10) and CO2 (ν3, v⩽3) were observed for the reaction of C2H with O2. A yield ratio of CO/CO2 is estimated as 9. The experimental observations suggest that reaction is rapid and forms CO and HCO.
\maketitle Peroxy radicals are key intermediates in bothcombustion and atmospheric chemistry. We have previously studied small alkyl peroxy radicals (methyl, ethyl peroxy} \underline{\textbf{112}}, 10695, (2000).}, 1- and 2-propyl peroxy} accepted}) via their near IR electronic transition using cavity ringdown spectroscopy (CRDS). The reaction of phenyl radical (C$_6$H$_5$) with oxygen is postulated to play a particulary important role in the soot formation process inherent in hydrocarbon combustion. In this talk, we report CRDS of the near IR $\widetilde{A}^{2}$A$^{\prime}$-$\widetilde{X}^{2}$A$^{\prime\prime}$ electronic transition of phenyl peroxy radical. \textit{Ab initio} calculations have been carried out to help assign T$_{00}$ band origin, as well as other vibrational bands.
Alkylperoxyl and alkoxyl radicals absorb only in the UV region, whereas vinylperoxyl, phenylperoxyl, alkylthiylperoxyl and benzyloxyl radicals have strong absorptions in the visible region. Using the UTD/B3LYP/6–31G+(d,p) method, we calculated the long-wavelength absorption maxima of these and related radicals in vacuo and in aqueous solution. The latter were accounted for with a continuum model (SCRF=PCM). The vinylperoxyl radical long-wavelength absorption band is due to a −2β[π] → 0β[n(O)z] transition. The n(O)y orbital lies above the π orbital, but since it is orthogonal to the n(O)z orbital, the oscillator strength of the −1β[n(O)y]→0β[n(O)z] transition is close to zero. Upon chlorine substitution, the first absorption band is red shifted. The π orbital is now raised above the n(O)y orbital, and the transition is denoted −1β[π]→0β[n(O)z]. The absorption of the phenylperoxyl radical in the visible region is accounted for by two nearby transitions of the same type, i.e. −2β[π] → 0β[n(O)z] and −1β[π] → 0β[n(O)z]. Owing to the charge-transfer character of these transitions, there is a marked red shift on going from the gas phase to aqueous solutions. The benzyloxyl and phenoxylmethyl radicals are related to the phenylperoxyl radicals in so far as one of the peroxyl oxygens is replaced by a methylene group. The benzyloxyl radical also absorbs in the visible region and the transition is of the −2β[π] → 0β[n(O)z] plus −1β[π] → 0β[n(O)z] type, i.e. it is closely related to that of the phenylperoxyl radical. The phenoxymethyl radical only absorbs in the UV region (λmax = 320 nm; spectrum obtained by pulse radiolysis) and this absorption band is due to 0α[π] → +2α[π*] plus −1β[π] → 0β[π] transitions. The absorption in the visible region of the alkylthiylperoxyl radical is a −1β[n(S)z]→0β[n(O)z] transition. Alkylthiyl and some other sulfur- and carbon-centered radicals react reversibly with O2. The energetics of these reactions were addressed by DFT quantum-chemical calculations. Copyright © 2005 John Wiley & Sons, Ltd.
We investigate the assignment of electronic transitions in alkyl peroxy radicals. Past experimental work has shown that the phenyl peroxy radical exhibits a transition in the visible region; however, previous high level calculations have not reproduced this observed absorption. We use time dependent density functional theory (TDDFT) to characterize the electronic excitations of the phenyl peroxy radical as well as other hydrocarbon substituted peroxy radicals. TDDFT calculations of the phenyl peroxy radical support an excitation in the visible spectrum. Further, we investigate the nature of this visible absorption using electron attachment/detachment density diagrams of the peroxy radicals and present a qualitative picture of the origin of the visible absorption based on molecular orbital perturbations. The peroxy radical substituent is also compared against isoelectronic radical groups. The visible absorption is determined to be dependent on mixing of the alkyl and radical substituent orbitals.
A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.
The absorption and emission solvatochromism of five indole derivatives was studied by referring the shifts to the corresponding vapor-phase wave numbers. The emission results clearly demonstrate, in three cases, the change in the nature of the emitting state when the solvent polarity is increased. In absorption, a large red shift in the 1La band is observed in all polar solvents, and this cannot be explained by classical solvent–solute interaction forces. The temperature dependence of the emission shift in a viscous solvent (glycerol) confirms that most of the emission shift is due to solvent relaxation. The 1La–1Lb level inversion model applied to ground-state polar solvent–solute complex, can account for most of the solvatochromism features observed.
As part of a study of the mechanism of the 1,3-migration in allylperoxyl radicals, the equilibrium geometries, dipole moments, charge distributions, spin density distributions, and C-O bond dissociation energies of a variety of peroxyl radicals have been calculated with extended basis sets. It is shown that the effects of electron correlation on the structures of the peroxyl radicals are smaller than in the corresponding peroxides and that electron correlation accounts for about one-half of the C-O bond dissociation energy. The peroxyl radicals are shown to be Ï-radicals with large dipole moments in the 2.3-2.6 D range. The majority of the negative charge resides on the inner oxygen, while the spin density is higher on the terminal oxygen. The C-O bond dissociation energy decreases with the degree of saturation of the carbon adjacent to the peroxyl group and also when the hydrocarbon radical product is resonance stabilized.
In the 254 nm photolysis of air-saturated aqueous solutions of tetrachloroethene, the major products (quantum yields in parentheses) are as follows: chloride ion (2.05), carbon dioxide (0.62), trichloroacetic acid (0.41), dichloroacetic acid (0.08) and hypochlorite (0.08). Trichloroacetic acid is not formed in the absence of O2; it is suppressed by the addition of hydrogen donors, such as tert-butanol and also by carbonate/bicarbonate ions. The quantum yield of dichloroacetic acid remains unaffected by O2 and hydrogen donors.It is concluded that there are two (or three) major primary processes CCl2=CCl2+hv→CCl2=CCl+Cl (1) CCl=CCl2+hv+H2O→CCl2(OH)CCl2H (2) CCl=CCl2+hv→CCL++Cl− (3) From scavenging experiments with alcohols, with and without O2, φ(1) = 0.23 ± 0.03 has been determined. Reactions (2) and (3) will finally both yield dichloroacetic acid and no distinction can be made between the two routes: φ(2+3)=0.08.In the absence of scavengers, the Cl atoms add to tetrachloroethene yielding the pentachloroethyl radical which, in the presence of O2, is converted to the corresponding peroxyl radical, the precursor of trichloroacetic acid. In the presence of O2, a short chain reaction sets in. The chain carrier is the Cl atom which is liberated in the bimolecular termination reactions of the various peroxyl radicals formed in the present system. The rate constants k(Cl.+C2Cl4)=2.8 × 108 dm3 mol−1 s−1, (Cl.CO2−3) = 5.0 × 108 dm3 mol−1 s−1 and k(Cl.+HCO−3)=2.2 × 108 dm3 mol−1 s−1 have been arrived at in this study. Thus bicarbonate prevents the formation of trichloroacetic acid from potentially present tetrachloroethene in the UV disinfection of drinking water.
The spectrum of a substance in a solution is changed compared to its spectrum in the vapor state. Solvent-shift effect is caused by a weak interaction between the solute and surrounding solvent molecules. The change in energy of the system because of this interaction will differ depending on if the solute molecule is in its ground or excited state and consequently there will be the change in excitation energy observed experimentally. The solvent–solute interaction, being weak, is best treated using quantum–mechanical perturbation theory that allows it to be divided into electrostatic type interactions and dispersion interactions. The electrostatic type interactions relate the solvent shift to the change in dipole moment and polarizability of the solute molecule on excitation from its ground state to its excited state and thus, can give experimental values-albeit once removed-of excited-state dipole moments and polarizabilities. The order-of-magnitude values for excited-state dipole moments can be obtained from solvent shift data. All solvent-shift theories are based on the assumption that solvent and solute molecules are sufficiently well separated that overlap of electronic distribution can be neglected. The chapter proves with the help of calculations that Abe's theory and the reaction-field method differ by very little in the final analysis. The solvent-shift data can also be used to obtain the order of magnitude estimates of excited-state dipole moments.
The planar cis conformer is calculated as lowest in energy but the trans is sufficiently close that both species may be present at room temperature. The difference in the peak absorption energy for the strong 1 1A′ → 2 1A′ transition is calculated to be sufficiently large between the conformers to distinguish them or determine their composition. The transition is dissociative in the gas phase but in the solution photo-isomerization is possible to the much more stable nitrate anion. Thermal conversion in the gas phase from cis to trans is calculated to require 65kJ by rotation about the NO bond.
Compact effective potentials, which replace the atomic core electrons in molecular calculations, are presented for atoms in the first and second rows of the periodic table. The angular‐dependent components of these potentials are represented by compact one‐ and two‐term Gaussian expansions obtained directly from the appropriate eigenvalue equation. Energy‐optimized Gaussian basis set expansions of the atomic pseudo‐orbitals, which have a common set of exponents (shared exponents) for the s and p orbitals, are also presented. The potentials and basis sets have been used to calculate the equilibrium structures and spectroscopic properties of several molecules. The results compare extremely favorably with corresponding all‐electron calculations.