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

Abstract

We have calculated the anharmonic 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.

Why does the IR spectrum of hydroxide stretching vibration weaken with increase in hydration?

“…As can be seen from the comparison of the OH À (H 2 O) n versus the bare OH À (also see Table S1, ESI †), all methods show blue shifting when OH À is hydrated and this has been examined in detail by Hermansson and coworkers. 30,31,54,55 However a more drastic change is seen in the large decrease in the intensity with increasing n for OH À (H 2 O) n . In particular, at n = 3 the intensity becomes two orders smaller than the bare OH À .…”

“…However recent studies by Hermansson and coworkers showed that such a water molecule exists mainly due to the water network and not due to the direct interaction with hydroxide. 30,31 Thereby, we believe that the gas phase model given in Fig. 1 is a good zeroth order picture to provide a qualitative understanding of the electronic and vibrational states of hydroxide in the aqueous phase.…”

“…24 Numerous theoretical studies have been performed on these small hydrated hydroxide clusters, however there are no simple explanations concerning this interesting feature of the IR spectra. [25][26][27][28][29] Recently, Hermansson and coworkers 30,31 have reported that the blue shift in hydroxide upon hydration is due to the electric field formed by the solvating water molecules, but there is no previous analysis of the IR absorption intensity. Joseph and Jemmis have discussed that in some cases of so-called improper hydrogen bonding systems, fundamental XH (X = C, N, and so on) stretching vibration shows blue shift with decreased intensities upon hydrogen bonding.…”

Isotropic Magnetic Shielding of Al(OH) − 4 in Aqueous Solution: A Hybrid Monte Carlo - Quantum Mechanical Computational Model

“…For Al 3+ cation, the potential parameters from [10] were used, while the hydroxide ions were modeled by the so-called "simple-charge" potential parameters described in details in [18].…”

Local and Collective Reaction Coordinates in the Transport of the Aqueous Hydroxide Ion

“…This investigation is also motivated by an interest in establishing connections between proton transfer dynamics in aqueous hydroxide and recent two-dimensional infrared spectroscopy (2DIR) of the participating O–H stretching vibrations. , Considerable evidence indicates that the solvent electric field exerted along an O–H bond in neat water determines the O–H stretch vibrational frequency, − and thereby the shape of the proton potential. It is therefore likely that the same collective variables that influence the proton transfer dynamics also describe the IR spectroscopy.…”