Computed High-Temperature Rate Constants for Hydrogen-Atom Transfers Involving Light Atoms

Abstract

The procedure of Johnston and Parr for computing the potential energy and structure of a triatomic linear activated complex containing hydrogen as its central atom has been combined with the Eyring—Polanyi rate-constant expression to calculate rate constants at 1000° to 4000°K of gas-phase hydrogen-atom transfers for every possible reaction between the ground states of the atoms H, Li, Be, B, C, N, O, F, Na, Cl, Br, I, and any of the diatomic hydrogen compounds of these elements. The relation between activated complex theory and collision theory for reactions of this sort is examined in detail. The usual form of activated complex theory breaks down for reactions of low activation energy and high temperature, and the conditions for this failure in terms of bending vibrational amplitude are given. For these cases rate constants are evaluated by hard-sphere collision theory. Although calculated activation energy and other parameters differ from one reaction to another, these differences become unimportant at very high temperature. Almost all calculated rate constants have a value within a factor of 4 of the single value 2×1013 cc/mole·sec at 2500°K.