HYDROGEN DEUTERIUM-EXCHANGE REACTIONS OF METHOXIDE METHANOL CLUSTERS

The tandem flowing afterglow-selected ion flow tube has been employed to measure rate coefficients for a series of 12 bimolecular isotope-exchange reactions of methoxide-methanol clusters. From these kinetic measurements the dissociation energies of several hydrogen-bonded dimers were determined: CH3O-·DOCH3 (28.3 ± 0.1 kcal/mol), CH3O-·HOCD3 (28.5 ± 0.1 kcal/mol), CH3O-·DOCD3 (28.1 ± 0.1 kcal/mol) CD3O-·HOCD3 (28.9  0.1 kcal/mol), and CD3O- ·DOCD3 (28.5 ± 0.1 kcal/mol). Differences in the bond strengths of these cluster ions can be understood in terms of the acidity of the neutral molecule and the basicity of the anion involved in the cluster; a given ion makes a stronger bond with a stronger acid while a given neutral molecule makes a stronger bond with a stronger base. By use of literature values of the electron affinity of the methoxy radicals along with the known RO-H and RO-D bond strengths, the gas-phase acidities (.delta.H°acid) of CD3OD (383.9 ± 0.7 kcal/mol), CH3OD (383.5 ± 0.7 kcal/mol), and CD3OH (382.0 ± 0.7 kcal/mol) were calculated. A simple model is employed to predict reaction efficiencies and branching ratios for these isotope-exchange reactions. © 1990, American Chemical Society. All rights reserved.