Rotational and Vibrational Energy Effects on Ion−Molecule Reactivity As Studied by the VT-SIFDT Technique

Abstract

A method is described for measuring internal energy dependences of gas phase ion−molecule reactions in a variable temperature-selected ion flow drift tube (VT-SIFDT) instrument. Numerous studies have been conducted to examine the effects of both rotational and vibrational excitation on rate constants and branching ratios. Rotational and translational energy are found to be equally efficient at driving endothermic reactions. For exothermic reactions, large rotational effects are found only when one or both of the reagents have a large rotational constant. This indicates that changing from a low to moderate J value can affect reactivity but that changing from moderate to high J has little influence on reactivity. Vibrational effects are more varied. In some reactions, vibrational excitation in the anticipated reaction coordinate strongly affects reactivity, while in other cases it does not. Vibrational effects are often observed in charge transfer reactions, presumably due to energy resonances and Franck−Condon arguments. For the SN2 reactions studied to date, vibrations play no role in governing reactivity for halide ions reacting with methyl halides, while vibrations are equivalent to other types of energy in influencing the reactivity of non-methyl halide systems.