Unraveling the Mechanisms of Carboxyl Ester Bond Hydrolysis Catalyzed by a Vanadate Anion

Abstract

The mechanism of p-nitrophenyl acetate (pNPA) hydrolysis promoted by vanadate ions was investigated utilizing both density functional theory and ab initio methods. In accordance with experiments, suggesting pure hydrolytic ester bond cleavage involving a nucleophilic addition in the rate-limiting transition state, four possible B(AC)2 (acyl-oxygen bond cleavage) mode reaction pathways were modeled. Moreover, two alternative reaction modes were also considered. Geometry optimizations were carried out using B3LYP, BP86, and MPWB1K functionals, conjugated with a 6-31++G(d,p) basis set and a Stuttgart effective core potential (ECP) for the vanadium atom. Single-point calculations were performed utilizing M06, B3LYP-D, and BP86-D functionals as well as B2PLYP-D and MP2 methods with a 6-311++G(2d,2p) basis set (with and without ECP). To address bulk solvation effects, the universal solvation model (SMD) and the conductor-like polarizable continuum model were applied, using the parameters of water. All levels of theory predict the same reaction mechanism, B(AC)2-1, as the lowest-energy pathway on the potential energy surface for pNPA hydrolysis catalyzed by the H(2)VO(4)(-) ion in aqueous media. The B(AC)2-1 pathway passes through two transition states, the first associated with the nucleophilic addition of H(2)VO(4)(-) and the second with the release of p-nitrophenoxide ion (pNP(-)), linked with a tetrahedral intermediate state. The intermediate structure is stabilized via protonation of the acyl oxygen atom by the vanadate and formation of an intramolecular hydrogen bond. The first and second barrier heights are 24.9 and 1.3 kcal/mol respectively, as calculated with the SMD-M06 approach. The theoretically predicted B(AC)2-1 mechanism is in good agreement with the experiment.