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Four reactions of potential importance in the catalytic recombination of H + OH in the presence of phosphorus compounds have been studied by ab initio quantum chemical and RRKM methods. These are the reactions HOPO + OH → (HO)2PO → H2O + PO2 (3), HOPO + H → P(OH)2 → H2O + PO (4), (HO)2PO + H → P(OH)3 → H2O + HOPO (5), and HOPO2 + H → (HO)2PO → H2O + PO2 (6). Each of these reactions takes place by recombination with small (<5 kcal mol^-1) or no barrier to form an adduct. The subsequent decomposition of the adducts occurs by the elimination of H2O through four-centered transition states. The thermochemistry of these reactions including the heats of formation of each of the adducts and the appropriate transition states was computed by the Gaussian G3X and G3X2 methods. Overall rate coefficients for each of these reactions at temperatures from 1000 to 2000 K and at pressures between 1 and 10 000 Torr were computed by the MultiWell code developed by Barker (Barker, J. R. Int. J. Chem. Kinet. 2001, 33, 232). For each reaction, there is very little stabilization of the adduct; hence the reactions are essentially pressure-independent. For reaction 6, its overall rate coefficient for the addition/decomposition mechanism greatly exceeds that previously derived for an abstraction transition state (Haworth, N. L.; Bacskay, G. B.; Mackie, J. C. J. Phys. Chem. A 2002, 106, 1533−1541).

A theoretical investigation of the effects of PO2 on the H + OH radical recombination reaction is reported. The focus of the study is the computation of rate coefficients by ab initio quantum chemical and RRKM methods for the H + PO2 and OH + PO2 recombination reactions and for the H + HOPO → H2 + PO2, OH + H2 → H + H2O, and H + HOPO2 → H2O + PO2 abstraction reactions, which constitute a catalytic pathway for the H + OH recombination reaction (Twarowski, A. Combust. Flame 1993, 94, 91). These are a subset of Twarowski's reaction model (Twarowski, A. Combust. Flame 1995, 102, 41) that consists of 175 individual reactions and includes 17 phosphorus-containing molecules. The thermochemistry of this complete reaction model was computed using the Gaussian methods:  G2, G3, and G3X. While G3X was found to be superior to G2 and G3 for the prediction of heats of formation of phosphorus-containing molecules, it underestimates the heat of formation of HOPO2 by at least 3.6 kcal mol^-1, when compared with the extensive coupled cluster computations of Bauschlicher (Bauschlicher, C. W., Jr. J. Phys. Chem. 1999, 103, 11126). Consequently, the rate coefficients reported in this work are based on Bauschlicher's thermochemical data for PO2, HOPO, and HOPO2. The computed rate coefficients are consistent with the available experimental data and the results of modeling studies by Twarowski (Twarowski, A. Combust. Flame 1995, 102, 41) and Korobeinichev et al. (Korobeinichev, O. P.; Ilyin, S. B.; Bolshova, T. A.; Shvartsberg, V. M.; Chernov, A. A. Combust. Flame 2001, 125, 744). The recombination reactions are found to be substantially into the fall-off region at near atmospheric pressures.

The heats of formation of a range of chlorofluoro-, bromofluoro-, and iodofluoromethanes, methyls, and carbenes were computed using the Gaussian-3 (G3) and Gaussian-2 (G2) methods. From a number of computed isodesmic reaction enthalpies, the heats of formation (at 298 K) of the key methyl species in the decomposition reactions of difluoromethanes, namely CF2Cl, CF2Br, and CF2I, were determined as −64.7 ± 2.0, −52.3 ± 2.0, and −39.3 ± 2.0 kcal mol^-1, respectively, while our recommended heat of formation for CI4 is 77.0 ± 3.0 kcal mol^-1. Using the available thermochemistry the energetics of the dissociation reactions of CF2X2 (X = Cl, Br, I) corresponding to the product channels CF2X + X and CF2 + X2 (as well as CF2 + 2X) were thus computed and compared. The energetics favor the molecular channel but only by ~10−12 kcal mol^-1 in the case of CF2Cl2 and CF2Br2. However, no transition states were found in these systems for the molecular elimination of Cl2 and Br2. By contrast, H2 elimination in CF2H2 was found to be favored by ~ 40 kcal mol^-1 (at the G2 level) with the corresponding transition state predicted to be 17.3 kcal mol^-1 below the radical products CHF2 + H. The vertical excitation energies of CF2Cl2 and CF2Br2 (to the lowest singlet and triplet B1, A2, B2, and A1 excited states) were computed using a range of methods including CASPT2 and EOM-CCSD in conjunction with the cc-pVTZ basis set. The results obtained for the lowest energy, viz., Ã(¹B1) ← &Xtilde;(¹A1), transitions are consistent with the available experimental data. The excited state potential energy surfaces, as calculated at the CASPT2 level of theory, are found to be repulsive with respect to a single C−Cl or C−Br bond stretch, but have shallow local minima in C2v symmetry, i.e., when both C−Cl or C−Br bonds are stretched to ~2.3 Å, that suggest the existence of weakly bound metastable states.

The heats of formation of ~120 C1 and C2 hydrofluorocarbon and oxidized hydrofluorocarbon molecules, as well as of hexafluoropropene and the hexafluoropropyl radical, were computed using the Gaussian-3 (G3) method, along with two approximations to G3, denoted G3[MP2(full)] and G3(MP4SDQ), and the G2(MP2) method. The performance of G3 is clearly superior to that of the other methods when the heats of formation are computed via atomization energies, and in general, the G3 results agree with the available good-quality experimental and theoretical data to within 2 kcal mol^-1. The use of isodesmic reaction schemes improves the overall accuracy of the computed heats of formation and results in a consistent set of predictions that are largely method-independent. Although, for the majority of molecules, the G3 data agree well with the earlier theoretical predictions of Zachariah et al. (J. Phys. Chem., 1996, 100, 8737), who used the bond-additivity-corrected MP4 (BAC-MP4) method, there are significant discrepancies as well. The heats of formation of a group of small molecules consisting of the fluoroacetylenes (HCCF and C2F2, as well as C2H2) and the C2H, C2F, and formyloxyl (HCOO) radicals were also computed using the coupled-cluster method with basis sets ranging from cc-pVDZ to aug-cc-pVQZ and cc-pCVQZ, followed by extrapolation to the CBS limit and corrections for core-valence correlation and scalar relativistic effects. The predicted CBS heats of formation (in kcal mol^-1) are Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%298%@be@%0%@sxx@%%@mx@% (HCCF) = 24.6 ± 1.0, Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%298%@be@%0%@sxx@%%@mx@% (C2F2) = 0.5 ± 1.0, Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%298%@be@%0%@sxx@%%@mx@% (C2H) = 135.9 ± 1.0, Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%298%@be@%0%@sxx@%%@mx@% (C2F) = 109.1 ± 1.0, and Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%298%@be@%0%@sxx@%%@mx@% (HCOO) = −30.1 ± 1.0, in good agreement with the G3 results. The current work on formyloxyl provides strong support for the experimental value of Δf%@mt;sys@%%@ital@%H%@rsf@%%@sx@%0%@be@%0%@sxx@%%@mx@% = 28.6 ± 0.7 kcal mol^-1 obtained by Langford et al. (J. Chem. Soc., Faraday Trans. 1997, 93, 3757).

The molecular vibrations of 1,4,5,8-tetraazanaphthalene (1458-TAN), 2,3,6,7-tetraazanaphthalene (2367-TAN) and pteridine are analyzed using a combination of infrared and Raman spectroscopy, ab initio calculations at the HF/6-31G* and MP2/6-31G* levels of theory, and B3LYP/4-31G and B3LYP/6-31G* density-functional calculations. New spectra are reported, and almost all of the ground-state normal modes have been identified for 1458-TAN and pteridine; the analysis for 2367-TAN is less complete. Calculated B3LYP/6-31G* vibrational frequencies provide the best correlation between theory and experiment for these compounds, but corresponding calculations using the smaller 4-31G basis are also satisfactory. The spectra of 1458-TAN and pteridine spectra exhibit a substantial number of Fermi resonances which complicate but do not prevent a detailed analysis.

The kinetics of pyrolysis of furan have been investigated theoretically by ab initio quantum chemical techniques and by detailed chemical kinetic modeling of previously reported experimental results. [Organ, P. P.; Mackie, J. C. J. Chem. Soc., Faraday Trans. 1991, 87, 815.] The kinetic model, containing rate constants derived from the ab initio calculations, can satisfactorily model the species profiles that had been obtained in shock tube experiments at three initial concentrations of furan. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio calculations carried out at CASSCF, CASPT2, and G2(MP2) levels of theory. The calculations suggest that two parallel processes, initiated by 1,2-H transfers that result in the formation of cyclic carbene intermediates and lead to the decomposition products CO + propyne and C2H2 + ketene (as major and minor channels, respectively), are the dominant pathways and enable the quantitative modeling of the kinetics of furan disappearance and the formation of the major products. Direct ring scission in furan, either on a singlet or triplet surface, is found to be much too energetic to contribute to any appreciable degree. No evidence was found for significant participation of a third channel producing HCO + C3H3. H atoms and C3H3 radicals arise essentially by CH fission of propyne. Hydrogen abstraction from furan by methyl radicals is, however, significant and represents the principal source of methane in the products.

The results of an ab initio quantum chemical study, carried out at the Gaussian-2 (G2) level of theory, of the potential energy surface governing the isomerization and dissociation reactions of the CHNO isomers (to CH + NO, NH + CO, and OH + CN) are reported. In general, several pathways were found that interconnect each open-chain or cyclic isomer with the others. In particular, two bifurcation points were located; the first connects HCNO to both HNCO and cyclic N(H)CO, the second connects HOCN to HNCO and cyclic N(H)CO. The lowest energy pathways for the isomerization of the open-chain species are two-step reactions that proceed via cyclic intermediates. Comparison of the G2 energies with those obtained in density functional (DFT) and QCISD(T)/cc-pVTZ calculations show a reasonable level of consistency, although differences up to ~9 kcal/mol were found between G2 and DFT.

Following the theoretical work on the isomerization reactions of the formaldiminoxy (CH2NO) radical, this paper reports the results of the ab initio quantum chemical study of the decomposition reactions of the various isomers, using the Gaussian-2 (G2), CASSCF, CASPT2, and QCISD(T) methods and basis sets that range from cc-pVDZ and cc-pVTZ to the 6-311+G(3df,2p) of G2. A total of 24 dissociation reactions of 11 isomers and their rotamers have been investigated. The lowest energy reactions occur for molecules with a NCO-chain backbone, which yield HCN, HNC, OH, NH2, CO, HNCO, and H as primary products. The reaction that could compete with the above is the CH bond fission in CH2NO, giving H + HCNO as products.

A detailed ab initio quantum chemical investigation of the potential energy surface associated with the isomerization reactions of the formaldiminoxy (CH2NO) radical is reported. CH2NO(²A‘) is the direct product of the reaction between triplet methylene and “prompt” nitric oxide. The quantum chemical calculations were performed at the Gaussian-2 (G2), CASSCF, CASPT2, and QCISD(T) levels of theory using basis sets that range from cc-pVDZ and cc-pVTZ up to the 6-311+G(3df,2p) of G2. A total of 11 isomers (plus 9 rotamers) and 28 transition states linking them are identified and characterized on the potential energy surfaces. The isomers and their rearrangement reactions are conveniently divided into two classes:  those with CNO- and NCO-chain backbones. The latter class of molecules are generally more stable, with the NH2CO isomer being the most stable with a predicted heat of formation of −3.6 ± 1 kcal/mol. Interconversion between the two groups occurs via a cyclic isomer of CH2NO. The energetics of the various isomerization pathways are expected to influence the subsequent dissociation reactions of the formaldiminoxy isomers.

The kinetics of pyrolysis of pyrrole have been investigated theoretically by ab initio quantum chemical techniques and by detailed chemical kinetic modeling of previously reported experimental results. [Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, M. Int. J. Chem. Kinet. 1991, 23, 733.] The overall kinetics can be successfully modeled by a 117 step kinetic model that gives good agreement with temperature profiles of major products and also provides an acceptable fit for minor products. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio calculations carried out at CASSCF, CASPT2, and G2(MP2) levels of theory. Several reaction pathways were investigated. The major product, HCN, arises principally from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of this carbene leads to an allenic imine precursor of HCN and propyne. This is the decomposition pathway of lowest energy. Pyrolysis is preceded by the facile tautomerization of pyrrole to 2H-pyrrolenine. The latter can undergo CN fission to form an open chain biradical species, which is the precursor of the butenenitrile isomeric products, cis- and trans-crotononitrile and allyl cyanide. The biradical can also undergo facile H-fission to form cyanoallyl radical, which is an important precursor of acetylene, acetonitrile, and acrylonitrile. H2 also arises principally from H-fission of the biradical.

The pyrolysis kinetics of acetonitrile dilute in argon has been studied in the temperature range 1400−2100 K at an average pressure of 12 atm in single-pulse shock tube experiments. The principal products are HCN, C2H2, CH4, and H2, while the minor products include HCCCN, H2CCHCN, C2H4, and C4H2. The overall kinetics is successfully simulated by an 87 step kinetic model that accurately accounts for the temperature profiles of the major products and also provides an acceptable fit for the minor products. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio quantum chemical calculations carried out at CASSCF, CASPT2, and Gaussian-2 levels of theory. Several distinct reaction pathways were studied, whereby the geometries, vibrational frequencies, and energies of approximately 70 molecular species representing reactants, products, intermediates, and transition states were computed. The pyrolysis of acetonitrile is initiated by CH bond fission, forming a cyanomethyl radical. This reaction is the most sensitive one in the kinetic model. On the basis of sensitivity analyses of the model as well as ab initio calculations, the heat of formation of cyanomethyl has been revised as %@mt;sys@%Δ%@sb@%f%@sbx@%%@mh;1.5q@%%@ital@%H%@rsf@%%@mh;1.5q@%%@sx@%298%@be@%0%@sxx@%%@mx@% (CH2CN) = 263 ± 9 kJ mol^-1. The limiting high-pressure value of the corresponding rate constant, as obtained by ab initio variational transition state calculations, is k1∞ = 1.2 × 10¹⁶ exp(−413 kJ mol^-1/RT) s^-1, which is in good agreement with our extrapolated experimental measurement. A number of the observed products, including HCCCN and H2CCHCN, largely arise from the decomposition of succinonitrile, a key intermediate, that forms by the recombination of two cyanomethyl radicals.

Using quantum chemical methods, the thermodynamic stabilities of benzoic, o-, m-, and p-hydroxybenzoic, and acetic acids and their conjugate bases were studied in both gas phase and aqueous solution, enabling the computation of the pKa's of the above acids. The electronic structure calculations were carried out at the Hartree−Fock SCF and MP2 levels of theory, using 6-31G(d,p) and 6-311+G(2d,p) basis sets. Solvation energies were calculated using two dielectric continuum methods:  SCIPCM and PCM. o-Hydroxybenzoic acid and its anion were found to possess intramolecular hydrogen bonds, which in the case of the anion are so strong as to result in a “bridged” structure in the gas phase with the proton effectively midway between the oxygens of the carboxylic and hydroxide groups. As a consequence of this strong hydrogen bond, the ortho anion appears to have the lowest solvation energy, lower (in magnitude) by ~8−10 kcal/mol than that of benzoate itself, despite the presence of a potentially hydrophilic hydroxy group. The trends in the calculated solvation energies of the anions provide an explanation for the unusual binding strength of o-hydroxybenzoate to surfactant films. The computed pKa's of the acids were analyzed in terms of the contributing energetics, and while these calculations have not yielded quantitatively accurate predictions of the pKa's, they do exhibit the correct qualitative trend among the acids studied.

The polarizability and its first and second derivatives have been calculated for methane at the following levels of approximation: ab initio SCF, singles and doubles CI and averaged coupled pair functional method; correlation and basis set effects have been investigated. The calculated polarizability surface predicts to experimental accuracy several observables related to the refractive index. The correlated first derivatives provide improved predictions of Raman scattering cross sections for the fundamentals. First and second derivatives are used to calculate scattering cross sections for selected overtone and combination bands including all totally symmetric bands in the pentad region. A comparison is made between cross sections based on perturbation expressions and on variational vibrational state functions. Deficiencies in the vibrational potential derived from rovibrational spectra are pointed out.

Ab initio self-consistent field calculations of the electric field gradients (efg) at the nuclei in hydrogen bonded pentamers of H2O are reported, and the shifts relative to the isolated molecule discussed with reference to the importance of the electrostatic, polarization, exchange and charge transfer mechanisms. The results of electrostatic model calculations on ice structures in which a single H2O molecule as well as a hydrogen bonded pentamer (treated quantum mechanically) are surrounded by a point charge representation of the remaining molecules in the crystal lattice are also presented. The calculated efg shifts are compared with the observed solid state shifts in the nuclear quadrupole coupling constants for normal ice (Ih) and the high pressure polymorph ice VIII, assuming that the 17O and 2H nuclear quadrupole moments are -2·63 fm2 and 0·286 fm2 respectively. A new estimate of the 17O nuclear quadrupole coupling constant in liquid water is also proposed; this is χ(17O) = 8·5 ± 0·5 MHz.

Decomposition of the benzyl radical at a range of temperatures, ~1450−1650 K, has been investigated using ab initio quantum chemical and experimental (shock tube) techniques. Four possible decomposition mechanisms are considered:  (a) via a norbornadienyl intermediate, (b) via a cycloheptatrienyl intermediate, (c) via direct ring opening, and (d) via a 6-methylenebicyclo[3.1.0]hex-3-en-2-yl (MBH) intermediate. On the basis of the quantum chemical calculations, mechanisms c and d are found to be the dominant reaction channels. A theoretically derived rate constant for the overall disappearance of benzyl is k = 10^16.6±0.3 exp(−97 ± 3 kcal mol^-1/RT) s^-1, in reasonable agreement with that obtained in previous studies. The experiments were carried out by shock heating benzyl bromide to temperatures between 1050 and 1650 K, followed by analysis of the spectral components of benzyl bromide, benzyl, and benzyl “fragments”. The rate constants derived from these experiments by using a simple two-step kinetic model are in good agreement with the theoretical values.