QUANTAL REACTIVE SCATTERING

The difficulties and complexities of the ab initio calculation of reaction rates are well known. Accurate 3-D quantal calculations of only the simplest atom-diatom system, H + H(,2), have been reported to date. For larger systems accurate and efficient approximate quantal theories of reactive scattering are needed. An approximate quantal theory of reactive scattering based on centrifugal and energy sudden treatments of the molecular rotational degrees of freedom but a close coupling treatment of the vibrational degree of freedom is presented. Integral and differential cross sections are calculated for the 3-D H + H(,2) reaction for H(,2) in the vibrational states v = 0,1 on the Porter-Karplus potential energy surface. Qualitative agreement with exact quantal results was achieved. The limited success of this method arises from a matching condition assumption and from neglect of the triatomic's bending motion.; Collinear exact quantum rate constants and transmission coefficients are presented for the 0(('3)P) + H(,2)((nu)), (nu) = 0,1 reaction on five different potential energy surfaces. A vibrationally adiabatic analysis is found to be useful in interpreting the reaction threshold energies as well as the oscillations in the reaction probabilities. The collinear exact quantum transmission coefficients were used to correct conventional 3-D transition state theory(TST). The corrected TST rate constants and the conventional TST rate constants calculated for the various surfaces are compared with experiment. Only the DIM(diatomics-in-molecules) surface leads to rate constants which do not show good agreement with experiment. This result is explained as arising from the great difference in saddle point location between the DIM surface and the other surfaces.; A statistical-dynamical theory of reactive molecular collision processes is developed. The theory utilizes either statistical or dynamical treatments of selected motions as appropriate. Two versions are presented which differ only in their treatment of bending motions---one version treats these motions dynamically, while the other treats these motions statistically. Application is made to the H + H(,2) reaction and results are compared with exact quantal calculations. The dynamical bending version is superior at low energies, but both agree well with exact results at high energies. Although the theory does not provide results that can be directly compared with experiment, it can provide estimates of degeneracy averaged integral cross sections, which can be useful as a guide for experimentalists.