NONEQUILIBRIUM KINETICS OF BIMOLECULAR REACTIONS .6. TRANSIENT RATE COEFFICIENTS

The master equation is solved numerically for the time dependence of t he vibrational level populations of HCl (treated as a simple harmonic oscillator) during the bimolecular exchange reaction, Br + HCl --> HBr + Cl, taking into account the competition between reaction and vibrat ional equilibration subject to Landau-Teller T-V excitation. Strong de viations from Boltzmann distributions are found. A wide range of react ant concentrations, diluent concentrations and temperatures were explo red. It was found that no significant reaction occurs before the estab lishment of a steady vibrational population distribution, confirming t hat the rate coefficient for non-equilibrium bimolecular exchange reac tions can be determined from a simple analytical steady state treatmen t (J. Chem. Sec. Faraday Trans. 87, 229 (1991)). The rate of an isolat ed elementary bimolecular reaction, A + BC --> AB + C, under non-equil ibrium conditions can deviate seriously from the standard expression, k(eq)[A][BC], and is better given by the law d[BC]/dt = -k(eq)4[A] [BC ]/{1+1/(a+g[R]/[A])} where [R] is the concentration of the collisional equilibrator, R, and a and g are constants depending only on temperat ure. This generalized rate law describes not only the initial rate but also the rate all the way up to completion, in the absence of the rev erse reaction.