Thermochemistry from ion–molecule reactions of hydrated ions in the gas phase: a new variant of nanocalorimetry reveals product energy partitioning

Abstract

A new variant of nanocalorimetry is proposed for the thermochemical analysis of ion–molecule reactions of hydrated ions in the gas phase. The average number of water molecules evaporating during the reaction is extracted by quantitative modeling of the average number of water molecules in the reactant and product cluster distribution as a function of time, taking into account black-body radiation induced dissociation. The method is tested on reactions of (H₂O)_n⁻ with O₂ and CO₂, and the core exchange reaction of CO₂⁻(H₂O)_n with O₂ to yield O₂⁻(H₂O)_n and CO₂. Reproducible results are obtained for the number of water molecules evaporating. Nanocalorimetric analysis reveals a non-ergodic component of ΔE_ne = 59 ± 14 kJ mol⁻¹ in the core exchange reaction, most likely carried away by the neutral CO₂ product. Extrapolation to solution phase values suggests hydration enthalpies of ΔH_hyd = −375 ± 30 kJ mol⁻¹ for O₂⁻ and ΔH_hyd = −268 ± 27 kJ mol⁻¹ for CO₂⁻.