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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 C_2h structure containing two fourfold-coordinated sodium cations in-between the two chelating sulfate dianions. Its stepwise solvation is driven by an interplay of SO₄²⁻–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.