Conformational diversity in deprotonated water clusters and anharmonic infrared spectra

Abstract

ABSTRACTInfrared spectroscopy has become a powerful tool for structural assignment of ionic compounds in the gas phase. Using the reactive OSS2 potential, the stable conformers of deprotonated water clusters (HO)OH− have been explored for n = 6–9 and n = 20 using different computational approaches, namely conventional molecular dynamics, replica-exchange molecular dynamics (REMD), and the harmonic superposition approximation (HSA). The stable structures generally exhibit a rather open character, the missing proton giving rise to either a well-localised hydroxide ion or, occasionally, to a bihydroxide anion with one proton shared between two oxygens. At finite temperature, the predictions of REMD and HSA agree reasonably well with one another. The anharmonic infrared absorption spectra obtained from the equilibrium samples of conformers were also determined from the three methods. At 100 K, the spectra resulting from conventional MD sampling are found to be possibly unreliable owing to isomerisation over times scales that exceed fluxional motion which we evaluate to be on the order of a few tens of picoseconds.