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

Abstract

Infrared photodissociation (IRPD) spectra of SO42−(H2O)n, n=9–13, recorded in the cooled cell of a Fourier-transform ion cyclotron resonance mass spectrometer, between 2900 and 3800cm−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 SO42−(H2O)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 SO42−(H2O)11 and SO42−(H2O)13 deriving from that of SO42−(H2O)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 3350–3650cm−1 range, plus a small band near 3100cm−1. The 3350–3650cm−1 massif, which includes most bands arising from second sphere molecules, acquires larger intensity relative to that at 3150–3350cm−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.