Location of energy barriers. VII. Sudden and gradual late-energy-barriers

Abstract

In the earlier papers of this series it was noted that barriers of "type II"("Late barriers", associated with substantially endothermic reaction) exhibited a large cross section if a given reagent energy was present predominantly as vibration in the bond under attack. It was found, however, that it was advantageous to retain in the reagents an amount of translation sufficient to surmount that part of the endothermic energy barrier which lay in the coordinate of approach; this could be identified with the repulsive part of the energy-release in the reverse, exothermic, reaction. In the present 3D classical trajectory study we compare S_r(V')_T'+V', for two endothermic surfaces with almost identical barrier heights and late barrier-crest locations, but differing fractions of the barrier in the coordinate of approach. For the surface IIS with a "sudden" rise to the barrier crest (implying substantial attractive energy-release in the reverse direction), S_r(V'_T'+V' increased continually with V', despite very low T'. By contrast surface IIG with "gradual" rise to the barrier crest (highly repulsive in the reverse direction), exhibited an S_r(V'_T'+V' that passed through a maximum. Two further surfaces were investigated; surface IIHS resembling IIS but with a substantially higher barrier, and surface I, IIG with an intermediate barrier-location and hence a large fraction of the barrier along the coordinate of approach. The dynamics exhibited S-type (sudden) behaviour on IIHS, and G-type (gradual) behaviour on I, IIG. The presence of a significant fraction of the barrier on G-type surfaces along the endothermic coordinate of approach correlated with a more gradual curvature of the repulsive wall. This is likely to contribute to the greater availability of translation for barrier-crossing on G-type surfaces. Increased reagent rotation, R', increased the reactive cross-section for endothermic reaction; the indications are that the mechanism by which rotation is effective involves vibration-rotation interaction.