Role of Hartree-Fock Exchange in Spontaneous Proton Transfer Reactions

The electric field experienced by a protic group depends primarily on the solvent configuration and those configurations in which the electric field along the protic group exceeds a critical value results in spontaneous proton transfer. Electronic structure calculations using DFT result in artifacts in estimating the critical electric fields for the spontaneous proton transfer process, often leading to bistable behaviour, in contrast to MP2 level calculations. The discrepancies in assessing the critical electric fields using the DFT method can be attributed to the under-representation of Hartree-Fock exchange in many commonly used functionals such as B3LYP and M06-2X, whose effect is precipitative in the proton transferred structures. Using the (critical) electric field values obtained by the MP2 level of theory as a benchmark, it is shown that the B3LYP functional altered to include 40% Hartree-Fock exchange omits the bistable behaviour and calculates reasonably accurate critical electric fields.