Evidence for water rings in the hexahydrated sulfate dianion from IR spectroscopy.

Multiply charged anions, such as sulfate, play important roles in interfacial and condensed-phase chemistry. Sulfates act as nuclei for cloud formation,1 and the presence of water on the Martian surface has been inferred from sulfate geochemistry.2 Despite the stability of sulfate in many condensed-phase environments, calculations indicate that the isolated dianion is unstable, with an electron tunneling lifetime of 1.6 × 10-10 s.3 This may explain why the isolated dianion has not been observed experimentally.4-6 Sulfate dianions solvated by three or more water molecules can be formed by electrospray ionization (ESI)4-7 via evaporation from more extensively hydrated droplets.8 The structures and reactivities of these hydrated dianions have been extensively studied.4-7,9-12 Activation of [SO4(H2O)n] results in fragmentation either by the loss of a water molecule or by the loss of a solvated hydroxide ion.4,6 The latter is the only pathway observed during blackbody infrared radiative dissociation (BIRD) at room temperature for n e 4, whereas water loss is the only pathway observed for n > 6.6 Water loss is entropically favored,6 and the formation of [SO4(H2O)2] has been reported from more energetic activation methods.4 This ion has been inferred to be electronically unstable based on photoelectron spectra of slightly larger clusters.5 BIRD experiments indicated that [SO4(H2O)6] and [SO4(H2O)12] are more stable than neighboring hydrates,6 although equilibrium measurements indicated no special stability for the latter ion at elevated internal energies.9 The stability of these ions indicates that they may have complete solvent shells or especially stable networks of hydrogen bonds.6 Multiple structures of [SO4(H2O)6] have been proposed to account for its special stability. In one of these, all of the water molecules donate two hydrogen bonds to the sulfate core (Td symmetry).6 In the remaining structures, water molecules interact with each other in addition to solvating the ion. [SO4(H2O)6] can adopt structures containing one (C3 symmetry)5 or two (C2 symmetry)10 trimeric water rings, in which each water molecule donates one hydrogen bond to an oxygen atom of the sulfate core and one hydrogen bond to a neighboring water molecule. Additional structures have been proposed that have larger water rings and more inter-water hydrogen bonding (e.g., structure C1). The structures of hydrated sulfate dianions were recently investigated by Zhou et al. using elegant infrared (IR) action spectroscopy experiments in the 540-1850 cm-1 region.7 For [SO4(H2O)6], the calculated spectra for the Td and C3 symmetry structures in this region are similar, but the photodissociation spectrum was assigned to the Td symmetry structure based on the absence of a band near 865 cm-1 calculated for the C3 symmetry structure. Similarly, the spectrum for [SO4(H2O)7] was assigned to a structure in which the seventh water molecule binds to the Td symmetry core found for [SO4(H2O)6], although the presence of an alternative structure that has a water ring could not be ruled out.7 The spectrum of [SO4(H2O)12] was assigned to a T symmetry structure with a water trimer on each of the four faces of the sulfate core, like that found computationally by molecular mechanics6 and ab initio10,12 calculations. Here, an IR action spectrum of [SO4(H2O)6] in the hydrogen stretch region and complementary calculations provide new insights into the structure of this ion (Figure 1). Experiments were performed on a Fourier-transform ion cyclotron resonance instrument,13-15 and general experimental methods are described elsewhere.14 [SO4(H2O)n] ions formed by ESI are trapped in a cylindrical ion cell, which is cooled to 130 K with a regulated flow of liquid nitrogen.15 [SO4(H2O)6] is isolated and subsequently exposed to 70-1200 pulses of IR radiation (8-21 mJ per ∼7 ns pulse) from a tunable 10 Hz optical parametric oscillator/amplifier (LaserVision, Bellevue, WA). The IR action spectrum includes all product channels16 and is obtained by plotting the powerand timecorrected photodissociation intensity as a function of laser frequency.14 All calculations were performed using Gaussian 03,17 and initial structures were based on previously proposed structures.6,10,11 Comparisons between the photodissociation spectrum and spectra calculated for candidate structures provide insights into the structure of this ion (Figure 1, relative energies in Table 1). All structures are energetically competitive (within 8 kJ/mol with zero-point energy corrections). The structures with water rings are slightly lower in energy than the Td symmetry structure, but this result Figure 1. Photodissociation spectrum of [SO4(H2O)6] and the spectra for five low-energy conformers from B3LYP/AUG-cc-pVDZ calculations. Calculations use the harmonic oscillator approximation, and vibrational frequencies are scaled by 0.962, a factor used previously to compare results from similar calculations with photodissociation spectra of [H(H2O)n] in this frequency region.18 Individual oscillators are broadened using 20 cm-1 fwhm Lorentzian distributions. Published on Web 02/01/2007