Use of polarizability and chemical hardness to locate the transition state and the potential energy curve for double proton transfer reaction: A DFT based study

Density functional theory (DFT) based calculations on a series of double and single proton-transfer reactions e.g. formamide (FA), acetamide (AA) and trifluoro acetamide (TFA) dimmers are performed to understand the potential energy surfaces during proton transfer processes. Apart from using the N–H distances as proton transfer co-ordinate we have computed the variations in polarizations and chemical hardnesses of the species involved to locate the transition state structures during the double proton transfer reactions. The average polarizability (αav) and the chemical hardness (η) show their optimum value at the same N–H distance and it corresponds to the transition state for all the three titled complexes. The maximum polarizability and minimum chemical hardness at the transition state (TS) are due to maximal charge separation at TS. We observe that computation of maximum polarizability and minimum chemical hardness along the reaction co-ordinate are the easiest way to locate the transition state during the proton transfer processes.