Inelastic scattering calculations in polyatomic systems using an ab initio intermolecular potential energy surface: The CO₂^∗(0,0,1,0)+H₂(D₂)→CO₂(0,0,0,0)+H₂(D₂) systems

Abstract

An ab initio computation of the energy transfer dynamics in the (CO₂,H₂) and (CO₂,D₂) systems has been carried out. The intermolecular potential energy hypersurface has been obtained from the results of ab initio SCF computations using extended Gaussian basis sets. The potential energy has been computed for 1053 different geometries. Previously formulated cubic spline fitting procedures are employed to effect surface interpolation and to extract surface gradients. At small CO₂-H₂ center-of-mass separations, the potential energy is repulsive and nearly exponential in form. At larger separations, small attractive wells are found. At a given separation, the most stable conformation is a planar, parallel structure of C_2v symmetry. Vibrational deexcitation probabilities, energy transfer mechanisms, and isotope ratios for the relaxation of the first excited state of the doubly degenerate bending mode of CO₂ have been computed as a function of temperature by quasiclassical trajectories. In the temperature range below 600 K, the computed deexcitation probabilities are in fair to good agreement with the experimental shock tube data. At higher temperatures up to 1500 K, the computed results are too large by a factor of 3 or less. This error is interpreted to result at least partly from the assumption of classical motion. Computed isotope ratios are in fair accord with experiment over a 1000 K temperature range. The major relaxation mechanism is found to be V→R energy transfer for T?700 K and V→T transfer for 700 K<T?1500 K.