O–H anharmonic vibrational motions in Cl−⋯(CH3OH)1−2 ionic clusters. Combined IRPD experiments and AIMD simulations

Abstract

The structures of Cl−–(Methanol)1,2 clusters have been unraveled combining Infrared Predissociation (IR-PD) experiments and DFT-based molecular dynamics simulations (DFT-MD) at 100K. The dynamical IR spectra extracted from DFT-MD provide the initial 600cm−1 large anharmonic red-shift of the O–H stretch from uncomplexed methanol (3682cm−1) to Cl−–(Methanol)1 complex (3085cm−1) as observed in the IR-PD experiment, as well as the subtle supplementary blue- and red-shifts of the O–H stretch in Cl−–(Methanol)2 depending on the structure. The anharmonic vibrational calculations remarkably provide the 100cm−1 O–H blue-shift when the two methanol molecules are simultaneously organized in the anion first hydration shell (conformer 2A), while they provide the 240cm−1 O–H red-shift when the second methanol is in the second hydration shell of Cl− (conformer 2B). RRKM calculations have also shown that 2A/2B conformers interconvert on a nanosecond time-scale at the estimated 100K temperature of the clusters formed by evaporative cooling of argon prior to the IR-PD process.