EFFECT OF SUPERCOLLISIONS ON CHEMICAL-REACTIONS IN THE GAS-PHASE

Supercollisions are collisions which transfer large quantities of energy in a single gas-phase collision. Single events which transfer more than 30 kcal/mol were, in the past, observed experimentally and obtained in trajectory calculations. The present work reports on the contribution of a small fraction of supercollisions to the overall rate coefficient of gas-phase chemical reactions. Model calculations of the isomerization of cyclobutene to butadiene and the fission of cyclobutane to ethylene in a bath of a weak collider with average energy transferred per one down collision of 300 cm-1 are performed when small fractions (5 X 10(-3) and 10(-3)) of supercollisions are taking place. It is found that small quantities of supercollisions cause large changes in the value of the rate coefficient for reaction. For example, it is found that the rate coefficient for cyclobutene isomerization varies, in the low-pressure regime, by a factor of 4 at 1000 K and by a factor of 11 at 1500 K for 0.5% of 10000-cm-1 supercollisions. Large changes are found, as well, in the values of the average energy transferred quantities and in the collisional efficiency. The latter can change by as much as a factor of 1 0 at high temperatures. The fact that, in spite of their small probability of occurrence, supercollisions play a major role in chemical reactions forces a reevaluation of our understanding of how chemical reactions occur.