Multidimensional local mode calculations for the vibrational spectra of OH(-)(H2O)2 and OH(-)(H2O)2·Ar.

Multidimensional local mode calculations are performed for OH stretching vibrations of the gas phase OH(-)(H2O)2 and OH(-)(H2O)2·Ar clusters in the 1000-4000 cm(-1) energy range. The potential energies and the associated dipole moment values are calculated with MP2/6-311++G(3df,3pd). To fully take into account the anharmonic effects for the stretching vibrations of the ionic hydrogen bonded OHs (IHB OHs), those donating H to the O atom in OH(-), the vibrational Hamiltonian represented by the discrete variable representation (DVR) technique is diagonalized without using any truncation/contraction scheme for the basis. The necessary potential energies and dipole moment values at the DVR grid points are supplied by the polynomial inter- and extrapolations based on the values calculated at fine spatial grid points. We found that the peaks at 2700 cm(-1) should be assigned to the first overtone (ν: 0 → 2) of the IHB OH stretching vibrations rather than the previous assignment of the fundamental of the IHB OH based on harmonic frequencies. The relevant fundamental peaks should be observed around 1600-2000 cm(-1) where no experimental observation has been performed. This prediction of the fundamental peak positions leads to a simple correlation between the magnitude of the red-shift of the IHB OH stretching vibrational peak position and the cluster size of OH(-)(H2O)n for n = 1-3. Furthermore, to determine important contributions toward the assignment of the experimental spectrum, detailed analyses are performed from the following 3 viewpoints: (1) mode coupling between the inter water IHB OH stretching vibrations, (2) coupling between the IHB OH and the low-frequency OO stretching vibrations and (3) argon attachment to OH(-)(H2O)2. We found that the overall shape of the vibrational spectrum can be essentially described by considering only factor (1). However, fairly large peak shifts are caused by factors (2) and (3).