Correlation between the Rate Order and the Number of Molecules in the Reaction of Trimethyl Phosphite with Water in Acetonitrile Solvent

Density functional theory calculations of the title reaction, P(OCH3)(3) (H2O)(n) in CH3CN, were conducted, where n is the number of water molecules. Two routes, the routes suggested by (A) Aksnes and (B) Arbuzov, were traced with various a values. Both routes consist of two transition states (TSs) and one intermediate. Route B was found to be more likely than route A. In the former, the activation free energy (Delta G(double dagger)) of n = 3 is slightly smaller than that of n = 2. The n = 3 TS geometry is composed of a nucleophile H2O, a proton donor H2O, and an auxiliary one. Indeed, the geometry appears to be plausible for ready proton relays along hydrogen bonds, but it is inconsistent with the observed third-order rate constant. Catalytic water molecules were added to the n = 2 and 3 bond-interchange circuits. Then route B with n = 2 + 2 was found to be best. By n = 2 + 10 and n = 3 + 12 models, the n = 2 based route B was confirmed to be likely.