The vibrating hydroxide ion in water

Kersti Hermansson, Philippe A. Bopp, Daniel Spångberg, Ljupco Pejov, Imre Bakó, Pavlin D. Mitev

scienceThe OH ion in water is studied using a CPMD/BLYP + QMelectronic + QMvibrational approach. The ion resides in a cage of water molecules, which are H-bonded among each other, and pinned by H-bonding to the ion’s O atom. The water network keeps the ‘on-top’ water in place, despite the fact that this particular ion-water pair interaction is non-binding. The calculated OH vibrational peak maximum is at ∼3645 cm−1 (experiment ∼3625 cm−1) and the shift with respect to the gas-phase is ∼ +90 cm−1 (experiment +70 cm−1). The waters molecules on each side of the ion (O and H) induce a substantial OH vibrational blueshift, but the net effect is much smaller than the sum. A parabolic ‘frequency-field’ relation qualitatively explains this non-additivity. The calculated ‘in-liquid’ ν(OH) anharmonicity is 85 cm−1.

Chemical Physics Letters, Vol. 514, 2011, Pages 1–15
http://dx.doi.org/10.1016/j.cplett.2011.07.042

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