title: Probing the microsolvation of a quaternary ion complex: gas phase vibrational spectroscopy of (NaSO4(-))2(H2O)(n=0-6, 8).
abstract: We report infrared multiple photon dissociation spectra of cryogenically-cooled (NaSO4(-))2(H2O)n dianions (n = 0-6, 8) in the spectral range of the sulfate stretching and bending modes (580-1750 cm(-1)). Characteristic absorption bands and structural trends are identified based on a comparison to harmonic spectra of minimum-energy structures. The bare quarternary complex (NaSO4(-))2 exhibits a C2h structure containing two fourfold-coordinated sodium cations in-between the two chelating sulfate dianions. Its stepwise solvation is driven by an interplay of SO4(2-)-H2O and Na(+)-H2O interactions. The first water binds in a tridentate intersulfate-bridging fashion. The second and third water molecules bind to the sulfate groups at either end of the complex, which is followed by the onset of water hydrogen-bond network formation. In contrast to the binary ion pair, NaSO4(-), no clear evidence for the disruption of the quaternary ion complex upon microhydration is found up to n = 8, underlining its remarkable stability and suggesting that the formation of quaternary ion complexes plays a central role in the initial stages of prenucleation in aqueous Na2SO4 solutions.
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: Torsten
authors.initials: T
authors.lastname: Wende
authors.forename: Nadja
authors.initials: N
authors.lastname: Heine
authors.forename: Tara I
authors.initials: TI
authors.lastname: Yacovitch
authors.forename: Knut R
authors.initials: KR
authors.lastname: Asmis
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Neumark
authors.forename: Ling
authors.initials: L
authors.lastname: Jiang