A Fourier transform ion cyclotron resonance study of the temperature and isotope effects on the kinetics of low-pressure association reactions of protonated dimethyl ether with dimethyl ether

The temperature dependence of the low-pressure association reaction of dime thyl ether with protonated dimethyl ether has been investigated using Fouri er transform ion cyclotron resonance mass spectrometry. The unimolecular di ssociation of nascent proton-bound dimers is complicated by two factors: (1 ) the presence of another unimolecular dissociation route producing trimeth yloxonium cation and methanol through a high-energy isomer of the proton-bo und dimer and (2) the presence of at least two high-energy isomers of the p roton-bound dimer en route to dissociation of the nascent proton-bound dime r. RRKM modeling of the experimental temperature dependence of the unimolec ular dissociation of nascent proton-bound dimers strongly suggests that dis sociation of the nascent proton-bound dimer proceeds through a high-energy isomer. The possible existence of such species is also shown by ab initio c alculations. The original mechanism for the ion/molecule reaction and analy sis of radiative association kinetics used in the past was found to be too simple for accurate modeling of the reaction between protonated dimethyl et her and dimethyl ether. A slightly more complicated mechanism is proposed w hich more accurately accounts for the temperature dependence of the unimole cular dissociation to re-form reactants. As well, three isotopomeric varian ts of the protonated dimethyl ether/dimethyl ether reaction were examined e xperimentally and theoretically.