Vibrational spectroscopy of the hexahydrated sulfate dianion revisited: role of isomers and anharmonicities

Abstract

We report on the gas phase vibrational spectroscopy of the hexahydrated sulfate dianion, SO₄²⁻(H₂O)₆, and its fully deuterated isotopologue, SO₄²⁻(D₂O)₆, using infrared photodissociation (IRPD) spectroscopy of the D₂-tagged dianions in combination with density-functional-theory calculations on minimum-energy structures as well as finite temperature ab initio molecular dynamics (AIMD) simulations. The IRPD spectra were recorded at an ion trap temperature of 12 K and in the spectral range from 650 to 3800 cm⁻¹, covering the intramolecular modes of the solvent (OH/OD stretches and H₂O/D₂O bends) at higher energies, those of the solute (sulfate stretches) at intermediate energies and the intermolecular solute librational modes at the lowest energies. Isomer-specific double resonance in combination with messenger-tag dependent IRPD spectra show that only a single isomer is contributing significantly and that this isomer is not the highly symmetric T_d but rather the lower symmetry C₃ isomer. Temperature-dependent IR multiple photon dissociation spectra of bare SO₄²⁻(H₂O)₆ suggest that the C₃ isomer remains the most stable one up to 200 K. The AIMD simulations reveal that the IRPD spectra can only be fully understood when anharmonic effects as well as entropy-driven hydrogen bond network fluctuations are considered.