A density functional theory study of double proton transfer reactions in formamide, formamide-formic acid and formic acid dimers

We have performed a theoretical study on the double proton transfer (DPT) reactions for the formamide-formamide, formamide-formic acid and formic acid-formic acid dimeric complexes by using the ωB97XD/aug-cc-pVDZ level of density functional theory (DFT). The DFT calculated reaction energy barriers are calibrated by the high-level MP2 and CCSD(T) methods at the complete basis set limit. The reaction energy profiles along the intrinsic reaction coordinates have been analyzed using the symmetry adapted perturbation theory. The reaction mechanism is elucidated by examining the reaction force, the chemical potential and the reaction electronic flux. Our results show that the DPT reactions are essentially synchronous which can form an alternative reaction pathway to monomer tautomerization. The reactions proceed with significant intermolecular bond shortening and charge accumulation, yielding large increment in the exchange energy toward the formation of the reaction energy barrier. Once the transition state is reached, the energetic constraint is released by the attractive energies.