A quantum chemical investigation of energetic, geometric, mechanistic, and kinetic aspects of multiple proton transfer in cyclic H2O, H2S, and H2O-H2S clusters

Multiple proton transfers in cyclic homogenous and mixed H2O-H2S clusters from trimers to pentamers have been investigated at an affordable and accurate level of theory. Reaction force and Wiberg bond index analyses showed the process to be nearly synchronous in all the homogenous clusters, while asynchronous in the mixed clusters; always initiated by movement of the S-H protons to the H-bonded O atoms. The barriers for proton transfers exhibited a complex pattern, decreasing initially from the homogeneous H2O cluster upon replacement of one H2O by an H2S moiety and subsequently rising monotonically with each subsequent replacement(s) to becoming highest in pure H2S clusters. Finally, the impact of the energetic and mechanistic modulations on the proton transfer kinetics has been studied.