The electronic energies of the lowest 3 A′ and 3 A′′ states of the O( 3 P) + H 2 system were calculated for 951 geometries using MOLPRO. The calculations were fitted by a rotating Morse spline method and independently by a generalized London-Eyring-Polanyi-Sato (LEPS) double-polynomial method. A higher accuracy calculation for 112 of these geometries was also performed for both 3 A′ and 3 A′′ to obtain correction potential energy surfaces (PESs) used to raise the accuracy of the original surfaces to about 0.3 kcal/mol. The resulting fitted PESs are presented and compared to each other and to a previous empirical LEPS surface.

We present ab initio calculations of geometries, energies, and normal mode frequencies for complexes and saddle points along the minimum energy reaction path for the reaction C + HCCH → C3H2 → C3H + H. We also present ab initio calculations along the minimum energy reaction path in the entrance channel for the reactions C + HCCH, CH + HCCH, and 1CH2 + HCCH. These results and those presented earlier by Walch are used to calculate rate constants for the reactions of C, CH, and 1CH2 with acetylene, using variational RRKM theory. The rate constants obtained agree well with experimental results for all three reactions. Unimolecular lifetimes for intermediate complexes associated with each reaction path are also presented. Among the more stable C3 isomers are propargyl and propargylene, which have lifetimes of 25 and 1900 ps, respectively, under thermal reaction conditions.

Complete active space SCF/contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N+O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A′ surface and 18.0 kcal/mol for the 4A′ surface. The O+N2 reaction is endoergic by 75 kcal/mol. The 3A″ surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A′ surface is calculated to have a late barrier of 14.4 kcal/mol relative to NO+N.

The quenching of electronically excited OH A 2Σ+ radicals has been investigated in complexes of OH with molecular hydrogen, deuterium, and nitrogen and through complementary theoretical calculations. Many of the intermolecular vibrational levels supported by the OH A 2Σ+ (v‘ = 0, 1) + H2, D2, and N2 potentials have been characterized by laser-induced fluorescence and fluorescence depletion measurements of the complexes in the OH A 2Σ+−X 2Π 1−0 and 0−0 spectral regions. Homogeneous line broadening of the spectral features yields picosecond lifetimes for complexes prepared in levels derived from OH A 2Σ+ (v‘ = 0) as a result of electronic quenching and/or chemical reaction. More extensive line broadening is observed for complexes excited to levels correlating with OH A 2Σ+ (v‘ = 1). The corresponding decay rates are 10−75 times faster than obtained for v‘ = 0 due to the opening of the vibrational predissociation channel and/or enhancement of the quenching/reaction processes upon OH vibrational excitation. Ab initio calculations of the OH (A 2Σ+, X 2Π) + H2 and N2 potential energy surfaces reveal the minimum energy configurations, T-shaped O−H--H2 and linear O−H--N⋮N, and the large increases in interaction energy upon electronic excitation of OH. The theoretical calculations also identify specific orientations, T-shaped H−O--H2 and linear H−O--N⋮N, that lead to conical intersections between the ground- and excited-state surfaces and give rise to quenching of OH A 2Σ+ by hydrogen and nitrogen.

Quasiclassical trajectory studies of N+OH, O+NH, and H+NO collisions using global ab initio potential energy surfaces