Reinvestigation of the photodissociation kinetics of m-iodotoluene molecular ion

Abstract

Photodissociation kinetics of the m-iodotoluene molecular ion has been investigated on a nanosecond time scale by mass-analyzed ion kinetic energy spectrometry. Photodissociation rate constants have been determined at molecular ion internal energies of 3.7–4.3 eV. The influence of the collisional relaxation of the molecular ion occurring in the ion source on the rate constant has been corrected, which was not considered in the previous photodissociation experiment. The kinetic energy release distributions (KERDs) determined from the ion kinetic energy profiles are composite due to the participation of two competing reaction channels, one leading to the formation of m-tolyl ion and the other to tropylium ion. By separating these bimodal KERDs, the branching ratios for the two channels have been obtained. From the overall rate constants and the branching ratios, rate constants for each channel have been determined on a nanosecond time scale. It has been found that the rate-energy data for each channel can be explained adequately by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The production of m-tolyl ion proceeds via a loose transition state while that of tropylium ion proceeds via a tight transition state. Refined experimental data together with the microsecond data reported by Lin and Dunbar have been found to be essential to arrive at the correct mechanistic pictures for these reactions