Double proton transfer in formic, thioformic and dithioformic acid dimers: An energetic, mechanistic and kinetic investigation

A quantum chemical investigation of energetic, mechanistic, and kinetic aspects of the double proton transfer reactions in homogenous and mixed dimers of formic acids and its mono and dithio derivatives have been studied employing CCSD(T)-F12/cc-pVTZ-F12//ωB97X-D/aug-cc-pV(D + d)Z level of theory. The findings agree closely with existing results for formic acid dimer. A total of ten dimers, 4 homogeneous and 6 mixed, have been studied and 7 transition states have been identified. The bimolecular proton transfer barriers vary linearly with the binding energies of the dimers. Reaction force and Wiberg bond index analyses show that proton transfers in homogeneous dimers are synchronous, whereas the same in mixed dimers are asynchronous; The latter type of proton transfer is always initiated by dissociation of SH bond. The bimolecular rate constant of proton transfer exhibits a systematic dependence on the barrier height, supporting the observed correlation between the barrier height and binding energy.