Effect of curvature of the reaction path on dynamic effects in endothermic chemical reactions and product energies in exothermic reactions

Abstract

Collinear quasiclassical trajectories are examined for two realistic potential energy surfaces for atom−diatomic molecule reactions for two reaction attributes: (1) vibrational energy of the products of a thermal−energy exothermic reaction; (2) threshold energy for endothermic reaction of ground−state reagents. Eight different mass combinations are studied. The potential energy surfaces differ primarily in the amount of potential energy released in an exothermic reaction before and in the region of large curvature of the minimum−energy path and in the curvature of the repulsive potential energy contours when all three atoms are close. For attribute (1), we find the results are qualitatively correlated by the theory of Hofacker and Levine although, contrary to previous work, one potential energy surface shows high mixed energy release (in the language of Polanyi and co−workers) but low excitation to product vibration for five different mass combinations. For reaction attribute (2), we find one surface has a high translational threshold (or no reaction at any energy) for six mass combinations, while the other surface shows this behavior in only three cases. Thus, this type of surface provides an exception to previous generalizations that extra vibrational energy is required for very endothermic reactions with late barriers. This demonstrates the importance of the location of the curvature of the reaction channel for such reaction attributes. Very accurate determinations of potential energy surfaces will be required to make reliable predictions of reaction attributes such as (1) and (2) for real systems. Analysis of the details of the trajectories shows that the high threshold can generally be attributed to reflection before the saddle point of the surface rather than to recrossing the saddle point region. The vibrational excitation of reagents in nonreactive collisions is also strongly effected by curvature of the minimum−energy path.