A Fourier Transform Ion Cyclotron Resonance Study of theTemperature and Isotope Effects on the Kinetics of Low-Pressure Association Reactions of Protonated Dimethyl Ether with Dimethyl Ether

Abstract

The temperature dependence of the low-pressure association reaction of dimethyl ether with protonated dimethyl ether has been investigated using Fourier transform ion cyclotron resonance mass spectrometry. The unimolecular dissociation of nascent proton-bound dimers is complicated by two factors: (1) the presence of another unimolecular dissociation route producing trimethyloxonium cation and methanol through a highenergy isomer of the proton-bound dimer and (2) the presence of at least two high-energy isomers of the proton-bound dimer en route to dissociation of the nascent proton-bound dimer. RRKM modeling of the experimental temperature dependence of the unimolecular dissociation of nascent proton-bound dimers strongly suggests that dissociation of the nascent proton-bound dimer proceeds through a high-energy isomer. The possible existence of such species is also shown by ab initio calculations. The original mechanism for the ion/molecule reaction and analysis of radiative association kinetics used in the past was found to be too simple for accurate modeling of the reaction between protonated dimethyl ether and dimethyl ether. A slightly more complicated mechanism is proposed which more accurately accounts for the temperature dependence of the unimolecular dissociation to re-form reactants. As well, three isotopomeric variants of the protonated dimethyl ether/dimethyl ether reaction were examined experimentally and theoretically.