Accurate quartic force fields have been determined for the CCH- and NH2- molecular anions using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T). Very large one-particle basis sets have been used including diffuse functions and up through g-type functions. Correlation of the nitrogen and carbon core electrons has been included, as well as other small effects, such as the diagonal Born-Oppenheimer correction, and basis set extrapolation, and corrections for higher-order correlation effects and scalar relativistic effects. Fundamental vibrational frequencies have been computed using standard second-order perturbation theory as well as variational methods. Comparison with the available experimental data is presented and discussed. The implications of our research for the astronomical observation of molecular anions will be discussed.

Rich reactive resonances are found in a 3D quantum dynamics study of the N+N2 exchange reaction using our recently developed ab initio potential energy surface that has a shallow well between two transition states. An L2 analysis confirms that the quasibound states of ‘Lake Eyring’ are responsible for the reactive resonances with some quasibound states, mostly the bending motions, giving rise to strong reasonance peaks, whereas others contributing to the shoulders in the resonance structure. This is the first observation of reactive resonances from a ‘Lake Eyring’ feature in a potential energy surface.

The N+N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground A″4 state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N+N2(J=0). The J–K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasiclassical calculation using a London–Eyring–Polanyi–Sato PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate three-dimensional quantal dynamics study for the N+N2 reaction system and the ab initio PES reported here is the first such surface for N3.

The CCSD(T) method has been used to compute a highly accurate quartic force field and fundamental frequencies for all 16O and 18O isotopomers of the ozonide anion. The CCSD and CASPT2 methods have also been used to verify the reliability of the CCSD(T) fundamental frequencies. The computed fundamental frequencies are in agreement with gas-phase experiments, but disagree with matrix isolation experiments for the antisymmetric stretch, ν3. CASPT2 calculations show that the antisymmetric part of the O3- potential surface is sensitive to the external environment. It is concluded that the antisymmetric stretch exhibits a significant matrix shift in the matrix isolation experiments and that the matrix environment is not representative of the gas-phase environment for ozonide anion. It is hoped that the theoretical data provided here will aid in the interpretation of future high-resolution gas-phase experiments.

Some organic material in chondrites (primitive meteorites) exhibits a very low 14N/15N, suggesting that the compounds that carry this heavy nitrogen signature formed in the interstellar medium. Other organic components of the same chondrites show a more solar isotopic signature, suggesting they derive from an isotopically solar reservoir of nitrogen such as N2 or NH3 in the solar nebula. In this work, we model the chemistry of the shocks that have been hypothesized as the mechanism to melt chondrules. We find that such shocks (≈ 8 km/s) do not produce significant amounts of HCN and CN if all nitrogen is initially locked in N2 and all carbon is locked in CO. Only when NH3 or CH4 (or both) were present in the initial pre-shock nebula gas do CN and HCN form. We also find that C2H2 (acetylene) and C2H form in low abundances if the carbon is all locked in CO in the pre-shock gas. The presence of CH4 facilitates the formation of acetylene and related compounds. In the absence of CH4 or NH3, only negligible amounts of species containing CC or CN bonds form. Acetylene and cyanide-related compounds may be precursors to the organics that condensed into meteorites about 4.5 billion years ago. We find that CN bonds largely survive these shocks; thus, the very low interstellar 14N/15N signature can be preserved if the 15N is carried by CN-bearing interstellar compounds.

In their study of organic synthesis from impact shocks using the laser-induced-plasma (LIP) technique, McKay and Borucki(l) found that organic synthesis preferentially occurred in a reducing gas mixture rich in methane, and not in a mixture rich in carbon dioxide. This result means chemical models based on the thermodynamical equilibrium approach do not apply to shock chemistry. In this study, we employ the technique of reacting flow, i.e., chemical kinetics in a fluid flow, to simulate the chemistry occurring in LIP and in the wake region from comet or meteor impact. Three different air compositions have been used: (1) 1/3 CO2 and 2/3 H2, (2) pure CH4, and (3) 1/4 CH4, 1/4 CO2, and 1/2 H2O. The stoichiometric ratio of gas mixtures (1) and (3) are kept the same. For (1) we obtain equal mole fractions of CO and H2O as the major products and for (2) C2H2 is the major product. In both cases our results are in agreement with Ref. (1). For (3) we find an interesting case where the nature of chemicals produced to be critically dependent on the flowfield temperature. At the higher temperature part of the wake region, CO and H2O are the dominant products, whereas in the cooler region C2H2 is the dominant product. Further studies of these reactions, as well as for the gas mixture including N2, are being pursued.

Calculations have been carried out for the reaction of SiCl2 and SiHCl with H and Cl atoms. In each case, the stationary point geometries and harmonic frequencies were characterized using CASSCF/derivative methods and the cc-pVDZ basis set. Accurate energetics were obtained by combining the CCSD(T) results using the aug-cc-pVTZ basis set with an extrapolation to the basis set limit using the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets at the MP2 level. The geometries, energetics, and harmonic frequencies were used to obtain rate constants using conventional transition state theory or a Gorin-like model. In each case, we find direct abstraction pathways compete with an addition elimination pathway. In the case of SiClH + H, the two direct pathways are H abstraction which is barrierless and Cl abstraction with a barrier of 13.5 kcal/mol, whereas the addition elimination process has a barrrier of 26.9 kcal/mol. In the case of SiCl2 + H, the direct pathway is Cl abstraction with a barrier of 16.4 kcal/mol, whereas the addition elimination pathway has a barrier of 29.6 kcal/mol. In the case of SiClH + Cl, the direct pathway is H abstraction which is barrierless and the addition elimination pathway has a barrier of 2.0 kcal/mol.

Calculations have been carried out for the thermal decomposition of silane, chlorosilane; dichlorosilane, and trichlorosilane. In each case, the stationary point geometries and harmonic frequencies were characterizing using CASSCF/derivative methods and the cc-pVDZ basis set. Accurate energetics were obtained by combining the CCSD(T) results using the a-cc-pVTZ basis set with an extrapolation to the basis set limit using the a-cc-pVDZ, a-cc-pVTZ, and a-cc-pVQZ basis sets at the MP2 level. The geometries, energetics, and harmonic frequencies were used to obtain rate constants using conventional transition state theory. The barrier heights obtained in the present work (kcal/mol) are the following: SiH4 --> SiH2 + H-2 (61.9); SiClH3 --> SiClH + H-2 (66.7); SiClH3 --> SiH2 + HCl (76.9); SiCl2H2 --> SiCl2 + H-2 (77.2); SiCl2H2 --> SiClH + HCl (74.8); SiCl3H --> SiCl2 + HCl (72.7). The computed barrier heights are believed to be accurate to within 1 kcal/mol. The rate coefficients obtained in the present work are in fair accord with most of the experimental results.

Vertical electronic excitation energies for singlet states have been computed for the high energy density material TdN4 in order to assess synthetic routes that originate from excited states of N2 molecules. Based on linear response coupled-cluster calculations, the lowest six excited states are 9.35(1 1T1), 10.01(1 1T2), 10.04(1 1A2), 10.07(1 1E), 10.12(2 1T1), and 10.42(2 1T2) eV above the ground state. Comparison with the energies of excited states of N2+N2 fragments, leads us to propose that the most likely synthetic route for TdN4 involving this mechanism arises from combination of two bound quintet states of N2.

Trends in chlorine/oxygen and bromine/oxygen single and double bonds are examined for several molecules of interest in stratospheric halogen chemistry. Specifically, the relationships between bond distance and quadratic force constant, and bond distance and ionic bonding character are examined, together with bond energies. Similar to a previous study of FO bonding, it is found that the relationship between bond distance and force constant for Cl—O and Br—O single bonds is unusual and distinctly nonlinear. This is attributed to the through space interaction of halogen lone-pair electrons with the remainder of the molecule. Supporting evidence for this assertion is given by the fact that for X=O (X is Cl, Br) double bonds, where there are fewer halogen lone-pair electrons due to hypervalent bonding, this relationship is approximately linear. A detailed explanation for chlorine and bromine hypervalent bonding is presented which is consistent with all available data and with the trends studied here. In this model, chlorine or bromine hypervalent bonding is a result of the p → d promotion of one or two lone-pair halogen electrons followed by the formation of two or four additional pd hybrid halogen bonds. This model explains why -XO2 species are particularly stable while the -XO and -XO3 species are not. Finally, the implications of this new understanding of chlorine and bromine chemistry for stratospheric reservoir species are discussed.

The singles and doubles coupled-cluster method that includes a perturbative correction for connected triple excitations, denoted CCSD(T), is used in conjunction with an spdf quality one-particle basis set to determine an accurate quartic force field for cyclopropenylidene. A second-order perturbation theory treatment of vibrational anharmonicities, together with proper treatment of Fermi resonances, is used to predict fundamental vibrational frequencies of cyclopropenylidene and its 13C and deuterium isotopomers. Agreement between theory and the available experimental data is excellent. It is demonstrated that four vibrational bands assigned to cyclopropenylidene in 1984 matrix isolation experiments are correct, contrary to a recent suggestion. The anharmonic progression in the C-H stretches is examined and found to be similar for both the symmetric and antisymmetric C-H stretches, contrary to the findings from another recent study.

The spectroscopic properties of the HNO→HON transition structure have been studied using density functional theory (DFT) and a coupled-cluster method [CCSD(T)]. The barrier height relative to the HNO minimum has been found to be 77.8 and 73.4 kcal mol−1 at the DFT and CCSD(T) levels of theory, respectively. Accordingly, the isomerization to HON should not occur to any appreciable degree even in rather severe combustion environments.

The singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), together with a triple zeta double polarized (TZ2P) one-particle basis set is used to determine the geometries, harmonic frequencies, infrared intensities, and dipole moments of HOF, F2O, HOOF, FOOF, ClOOF. Agreement with experiment is very good, with the exception that the currently accepted experimental assignment of the symmetric and antisymmetric O-F stretches in FOOF is shown to be reversed (and to be consistent with an earlier experimental study). Very accurate heats of formation of HOOF, FOOF and ClOOF are also computed using the CCSD(T) method in conjunction with large atomic natural orbital basis sets. The F-O bond distances, quadratic force constants, bond energies, and fluorine and oxygen atomic charges from the above five molecules and six previously studied molecules (FONO2, trans-FONO, cis-FOND, FOCI, FOBr and FON) are compared and used to deduce a simple model of F-O bonding. The unusual relationship between the F-O bond distance and quadratic force constant shows that PO bonding is a function of at least three effects, which are degree of covalent character, degree of ionic character, and extent of lone electron-pair repulsions. All of the data are qualitatively consistent with this simple model. The bonding in cis-FONO is even more complicated, involving also dispersion interactions between fluorine and the terminal oxygen. It is suggested that the general importance of lone pair repulsions in F-O bonding and the additional importance of intra-molecular dispersion interactions explains why many density functionals have difficulty in describing the geometry of cis-FONO.

The XCN and XNC (X = F, Cl) isomers have been investigated using the CCSD(T) method in conjunction with correlation consistent basis sets. Equilibrium geometries, harmonic frequencies, anharmonic constants, fundamental frequencies, and heats of formation have been evaluated. Agreement with experiment for the fundamental frequencies is very good, even for nu(2) for ClCN, which is subject to a strong Fermi resonance with 2 nu(3). It is also shown that a second-order perturbation theory approach to solving the nuclear Schrodinger equation gives results in excellent agreement with essentially exact variational calculations. This is true even for nu(2) Of ClCN, provided that near-singular terms are eliminated from the perturbation theory formulas and the appropriate Fermi interaction energy matrix is then diagonalized. A band at 615 cm(-1), tentatively assigned as the Cl-N stretch in ClNC in matrix isolation experiments, is shown not to be due to ClNC. Accurate atomization energies are determined and are used to evaluate accurate heats of formation (3.1 +/- 1.5, 33.2 +/- 1.5, 72.6 +/- 1.5, and 75.9 +/- 1.5 kcal/mol for FCN, ClCN, FNC, and ClNC, respectively). It is expected that the theoretical heats of formation for FCN, FNC, and ClNC are the most accurate available.

We show that the geometric phase arising from a conical intersection of the lowest potential energy surfaces of HO2 causes its bending vibrational wave functions to be double-valued, which enables them to be locally symmetric on one side of the intersection and locally antisymmetric on the other.

The heat of formation of HNO is determined from ab initio calculations to very high accuracy. Two independent approaches have been used to verify the reliability. The singles and doubles coupled-cluster approach that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), has been used in conjunction with very large one-particle basis sets that include up to g functions. Correlation of the core electrons has also been investigated. The two approaches agree to within 0.4 kcal/mol, and our best estimate for ΔHf,0o (ΔHf,298o) is 26.7±0.8 kcal/mol (26.0±0.8 kcal/mol). This value is 2.2 kcal/mol larger than the currently accepted experimental value, indicating an error in the experimental determination.