Acid–base equilibrium dynamics in methanol and dimethyl sulfoxide probed by two-dimensional infrared spectroscopy

Abstract

Two-dimensional infrared (2DIR) spectroscopy, which has been proven to be an excellent experimental method for studying thermally-driven chemical processes, was successfully used to investigate the acid dissociation equilibrium of HN₃ in methanol (CH₃OH) and dimethyl sulfoxide (DMSO) for the first time. Our 2DIR experimental results indicate that the acid–base equilibrium occurs on picosecond timescales in CH₃OH but that it occurs on much longer timescales in DMSO. Our results imply that the different timescales of the acid–base equilibrium originate from different proton transfer mechanisms between the acidic (HN₃) and basic (N₃⁻) species in CH₃OH and DMSO. In CH₃OH, the acid–base equilibrium is assisted by the surrounding CH₃OH molecules which can directly donate H⁺ to N₃⁻ and accept H⁺ from HN₃ and the proton migrates through the hydrogen-bonded chain of CH₃OH. On the other hand, the acid–base equilibrium in DMSO occurs through the mutual diffusion of HN₃ and N₃⁻ or direct proton transfer. Our 2DIR experimental results corroborate different proton transfer mechanisms in the acid–base equilibrium in protic (CH₃OH) and aprotic (DMSO) solvents.