Classical trajectory studies of the reaction F + I2

Abstract

The reaction F + I₂→ IF + I has been studied using classical trajectory methods. A procedure is presented for selecting an appropriate extended L.E.P.S. surface for the reaction, in which the well depth is fixed at the value corresponding to the known stability of I₂F. When this is done, the Sato parameters are uniquely related and a systematic variation of one of them in a series of trajectory calculations allows a surface to be selected by matching the calculated reaction properties with the corresponding experimental values. It is found that, for this exothermic reaction with a well located in the exit valley of the surface, as the minimum of the well is moved into the exit valley, the vibrational excitation of the products is increased. An extended trajectory analysis was performed to calculate the form of the product recoil angular and energy scattering distributions, the IF vibrational state distribution, and the values of the reaction cross-section and thermal rate coefficient. Good agreement is found between the calculated and experimental molecular-beam angular and translational energy distributions. At a collisional energy of 8.4 kJ mol^–1, the I₂F intermediate is apparently not sufficiently long-lived to give statistical energy partitioning. The IF vibrational distribution is strongly inverted and yields a better linear surprisal plot when F_V² is used as a variable rather than F_V. The calculations do not reproduce the experimentally observed second peak in the IF vibrational distribution at v= 0. This may arise from a second reaction pathway on another surface, possibly involving the production of I^*(²P_1/2), which has a cross-section which may be up to twice as large as that for the pathway populating the high vibrational states of IF.