Tag Archives: Ljupco Pejov

Different structures give similar vibrational spectra: The case of OH− in aqueous solution

Authors: Pavlin D. Mitev, Philippe A. Bopp, Jasmina Petreska, Kaline Coutinho, Hans Ågren, Ljupco Pejov, and Kersti Hermansson

We have calculated the anp5aharmonic OH(aq) vibrational spectrum in aqueous solution with a “classical Monte Carlo simulation + QM/MM + vibrational” sequential approach. A new interaction model was used in the Monte Carlo simulations: a modified version of the charged-ring hydroxide-water model from the literature. This spectrum is compared with experiment and with a spectrum based on CPMD-generated structures, and the hydration structures and H-bonding for the two models are compared. We find that: (i) the solvent-induced frequency shift as well as the absolute OH frequency are in good agreement with experiment using the two models; (ii) the Raman and IR bands are very similar, in agreement with experiment; (iii) the hydration structure and H-bonding around the ion are very different with the two ion-water interaction models (charged-ring and CPMD); (iv) a cancellation effect between different regions of the hydration shell makes the total spectra similar for the two interaction models, although their hydration structures are different; (v) the net OH frequency shift is a blueshift of about +80 cm−1 with respect to frequency of the gas-phase ion.

J. Chem. Phys. 138, 064503 (2013);
http://dx.doi.org/10.1063/1.4775589

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