Multidimensional OH local mode calculations for OH−(H2O)3—Importance of intermode anharmonicity

We present theoretical calculations on the vibrational spectra between the energy range of 2000–4000 cm−1 for 4 isoenergetic conformers of gas phase OH−(H2O)3 and OH−(H2O)3·Ar clusters. The peak positions and associated intensities of the OH stretching vibrations were calculated using an extended local mode model on multi-dimensional ab initio potential energy surfaces and dipole moment functions obtained by MP2/6-311++G(3df,3pd). Furthermore, to simulate the experimental spectra directly, both the homogeneous and inhomogeneous line widths were determined theoretically. For the ionic hydrogen bonded OHs, which directly bind to the OH−, anharmonic coupling between the OHs on different water units is crucial for the reproduction of experimentally observed features. On the other hand, the coupling between free OH stretching vibrations is so small that the usual 1-dimensional local mode model provides a good and economical way to obtain the spectra. By comparing the theoretical spectra for 4 isoenergetic conformers, we found that the ionic hydrogen bonded OH stretching peaks can act as a descriptor for the subtle conformational differences in the first solvation shell. Furthermore, we showed that the coupling between the ionic hydrogen bonded OH and low frequency O⋯O stretching vibrations can cause fairly strong combination bands, and that the weakly bound argon, which was used as a messenger in the experiment by Robertson et al. [Science; 2003, 299, 1367], causes shifts on the peak positions for the ionic hydrogen bonded OHs. In addition, we quantified the effect of counterpoise correction on the simulated spectra for the ionic hydrogen bonded OHs.