Electronic structure effects on the double proton transfer reactions: a case study for substituted formic acid dimer

A comprehensive theoretical study to understand the changes in the electronic structures and their effects on promoter modes for double proton transfer reaction in bare and substituted formic acid dimer (FAD) is presented. In FAD, the dimer-stretch acts as a promoter mode which modulates the effective barrier to proton transfer reactions. Motivated by the fact that the modifications in the electron densities of the system significantly affect the proton transfer process and reaction barrier, the topology of the reactions is investigated by perturbing the electronic environment by substituting the terminal hydrogens attached to the carbon atoms of FAD with electron-donating (ed-) and withdrawing (ew-) groups. The variations in the electronic structure along the intrinsic reaction coordinate (IRC) are examined and compared for three systems: FAD, ed-FAD, and ew-FAD. Quantum mechanical calculations using the B3LYP and MP2 methods were performed to investigate structural features, charge distribution, and natural bond orbital (NBO) analysis, to comprehend the essential electronic distributional changes throughout the reaction. Additionally, reaction force analysis was conducted to gain insights into the electronic activities occurring along the reaction pathway. Among the three systems studied, ed-FAD exhibits the lowest activation energy for the double proton transfer reaction, followed by FAD and ew-FAD, accompanied by several distinct characteristic changes in their electronic and vibrational structures. Substitutions modulated the extent of electron delocalization from the acceptor lone pair to the antibonding orbital of the covalent bond between donor and hydrogen atoms. Reaction force analysis also revealed that reaction work (W) associated with the activation energy is much lower for ed-FAD compared to FAD and ew-FAD.