Hydration of the sulfate dianion in size-selected water clusters: From SO42−(H2O)9 to SO42−(H2O)13

International audienceIn celebration of Jose Riveros' many essential contributions to gas phase ion chemistry. Keywords: Hydrated sulfate IRPD spectroscopy AMOEBA polarizable force field Second hydration shell Mixed quantum classical modelling O H stretching frequency a b s t r a c t Infrared photodissociation (IRPD) spectra of SO 4 2− (H 2 O) n , n = 9-13, recorded in the cooled cell of a Fourier-transform ion cyclotron resonance mass spectrometer, between 2900 and 3800 cm −1 , are reported. The structures, energetics and infrared spectra of n = 9 and 11-13 were investigated by a combination of classical polarizable molecular dynamics and static quantum chemical calculations. Low-energy structures are mainly determined by the strong structuring effect of the sulfate ion, however, the highest cohesion is achieved when strong water-water interactions are present as well. As a result, the sulfate ion in the most stable structures for n = 9, 11 and 12 is on the surface of the water cluster. While SO 4 2− (H 2 O) 9 involves a mixture of isomers, the other sizes are found to be described by a single structural family, with the most stable structures of SO 4 2− (H 2 O) 11 and SO 4 2− (H 2 O) 13 deriving from that of SO 4 2− (H 2 O) 12 by removal and addition of a water molecule, respectively, without substantial reorganization. An important feature of these structures is that the number of water molecules in the second solvation sphere increases with cluster size, up to 3 for n = 12 and 4 for n = 13. This is directly reflected in the IRPD spectra. All spectra display two main features in the 3150-3350 and 3350-3650 cm −1 range, plus a small band near 3100 cm −1. The 3350-3650 cm −1 massif, which includes most bands arising from second sphere molecules, acquires larger intensity relative to that at 3150-3350 cm −1 which is mainly composed of stretches in first sphere molecules. Whereas most water molecules have ADD coordination (where A stands for acceptor and D stands for donor of a hydrogen bond), special cases, including DD and AADD account for bands at the red and blue ends of the spectra. Computed IR spectra are able to account for most experimental features, especially when anharmonicities are taken into account for the largest red shifts. Finally, the higher abundance of n = 12 relative to other sizes is related to a lower water evaporation rate constant, in good agreement with the water binding energy which is computed to be larger for n = 12 than for 13