The OH- ion in water is studied using a CPMD/BLYP + QM electronic + 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry