Non-conservation of activation energy barriers in the same chemical process: a cooperative (effect) proton transfer on (HF)n molecular aggregates

Abstract

The formation of (HF)n aggregates with n = 2, 3, 4, 5 and 6 and concerted proton transfer processes in these aggregates were systematically analyzed. It was verified that, by a cooperative effect, the barrier associated with the proton transfer process decreases for aggregates with a larger number of molecules, indicating that the activation energy for proton transfer depends on the molecularity of the process. Natural bond orbital and quantum theory of atoms in molecules were used to characterize the strength of the hydrogen bonds established in the aggregates, which verified a general increase in the delocalization energy as a function of increasing aggregate size. A deformed Eyring (d-Eyring) equation was used to calculate the proton transfer rate constants, where the d-Eyring equation adequately described the proton transfer kinetics. Analysis of the rate constants showed that proton transfer became faster as the cluster size increased. Arrhenius and d-Arrhenius plots showed a decrease in the dependence of the rate constants on temperature, particularly for the tetramer, pentamer, and hexamer. The d-Arrhenius plots, for which the d parameter was included in the Eyring equation, suggest non-Arrhenius behavior for proton transfer in the HF aggregates at low temperatures.