A classical prediction on the existence of adiabatic barriers even where no saddle point exists on the potential energy surface is verified using a purely quantal calculation. The adiabatic surfaces are then used for a vibrationally adiabatic transition state theory computation of reaction probabilities. Comparison with exact quantal results shows that the barriers suffice for explaining the sohyphen;called rsquo;rsquo;dynamic barriersrsquo;rsquo; to reaction. Since the barriers are in a region where the adiabatic assumption is valid, the adiabatic transition state theory provides an approximate upper bound to the exact reaction probabilities. Finally, it is shown that adiabatic transition state theory coupled with a purely classical transmission factor suffices for explaining most of the oscillatory nature of the exact quantal probability.
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