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

Supercollisions are collisions which transfer large quantities of ener gy in a single gas-phase collision. Single events which transfer more than 30 kcal/mol were, in the past, observed experimentally and obtain ed in trajectory calculations. The present work reports on the contrib ution of a small fraction of supercollisions to the overall rate coeff icient of gas-phase chemical reactions. Model calculations of the isom erization of cyclobutene to butadiene and the fission of cyclobutane t o ethylene in a bath of a weak collider with average energy transferre d per one down collision of 300 cm-1 are performed when small fraction s (5 X 10(-3) and 10(-3)) of supercollisions are taking place. It is f ound that small quantities of supercollisions cause large changes in t he value of the rate coefficient for reaction. For example, it is foun d that the rate coefficient for cyclobutene isomerization varies, in t he 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 quant ities and in the collisional efficiency. The latter can change by as m uch as a factor of 1 0 at high temperatures. The fact that, in spite o f their small probability of occurrence, supercollisions play a major role in chemical reactions forces a reevaluation of our understanding of how chemical reactions occur.