Solvent effects on methyl transfer reactions. 1. The Menshutkin Reaction

A full quantum mechanical description of the Menshutkin Reaction has been obtained for gas phase and solution by using density functional theory (DFT) and the self-consistent isodensity polarizable continuum model (SCI-PCM). Ammonia and pyridine are compared as nucleophiles, and methyl chloride and bromide are used as methyl transfer reagents. In the gas phase, all of the reactions proceed via an initial dipole complex, followed by a transition state leading to an ion pair. Methyl bromide shifts the position of the transition state to an earlier position than that found with methyl chloride. In the reaction with methyl chloride, replacing ammonia with pyridine stabilizes the transition state by 3 kcal/mol and stabilizes the ion pair by 17 kcal/mol. In the SCIPCM solvent effect calculations, the dipole complex disappears in both cyclohexane and DMSO. The transition state is shifted to an earlier stage of the reaction and is stabilized with respect to the gas phase. The ion pair product is strongly stabilized, and in DMSO it is calculated to dissociate into free ions. The reactions also were studied using Monte Carlo free energy perturbation. The results were in good agreement with the reaction field calculations. The rates of reaction between pyridine and methyl bromide were determined at 25 degrees C in cyclohexane, di-n-butyl ether, and acetonitrile and compared with the computational results. Activation free energies calculated using the SCRF-SCIPCM model agree remarkably well with the experimental values.