We investigate the perchlorate anion, ClO4−, microhydrated with up to eight water molecules using infrared photodissociation (IRPD) spectroscopy aided by density functional theory computations. The features observed in the IRPD spectra of D2-tagged ClO4−(H2O)n with n = 0−8 in the O–H stretching (2800–3800 cm−1) and fingerprint (800–1800 cm−1) spectral ranges are assigned to vibrational modes of the solvent, i.e., free and hydrogen-bonded O–H stretching and H2O bending modes, and of the solute, i.e., perchlorate stretching modes. The splitting of the triply degenerate antisymmetric stretching mode in the bare ClO4− (1102 cm−1) upon microhydration directly reports on the symmetry of the local solvation network, while the red-shift of the hydrogen-bonded O–H stretching bands in comparison to those of free O–H oscillators (>3700 cm−1) reflects the strength of the anion-water (3550–3650 cm−1) and water-water (3240–3540 cm−1) hydrogen bonds. The stronger water-water interaction leads to an early onset of water-water hydrogen bond formation (n = 2) and eventually asymmetric solvation of ClO4−, evidenced by the population of the second and third hydration shells at n = 5 and n = 8, respectively, before the first hydration shell is completed. The present study provides a bottom-up view of the initial stages of hydrogen-bond network formation around a high surface propensity anion, and these insights are discussed in the context of recent results obtained for the water-vapor interface of perchlorate solutions.
Probing the propensity of perchlorate anions for surface solvation by infrared photodissociation spectroscopy
John T. Kelly, Martin Mayer, Adam C. Kennedy, Christian Schemel, Knut R. Asmis; Probing the propensity of perchlorate anions for surface solvation by infrared photodissociation spectroscopy. J. Chem. Phys. 14 June 2018; 148 (22): 222840. https://doi.org/10.1063/1.5028416
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