Anharmonic vibrational frequencies of proton transfer coordinates in the clusters of aromatic molecules with water

The vibrational frequencies of OH and NH groups which form hydrogen bonds t o a water molecule are calculated using the complete active space self-cons istent field (CASSCF) method and the second order perturbation theory based on the CASSCF reference (CASPT2). As model systems indole(H2O), phenol(H2O ), and their corresponding cations are chosen. Furthermore, different isome rs of the 3-aminophenol(H2O)(+) and 4-aminophenol(H2O)(+) clusters are cons idered. For all these clusters IR spectra are known, but the IR spectra of the ionic clusters in the region of the hydrogen-bonded OH and NH groups ha ve not yet been well interpreted. Due to the strong anharmonicity of the OH and NH vibrations it is not possible to predict their frequencies by a sim ple scaling of harmonic values. By using different active spaces including both pi -orbitals as well as sigma -orbitals it is shown that the vibration al frequencies can be derived from potentials obtained from CASSCF single-p oint calculations along the OH and NH stretching modes. These vibrations ca n be regarded as proton-transfer coordinates. The calculated vibrational fr equencies are in excellent agreement with the experimental values obtained for the investigated clusters. Furthermore, a strong shift of the OH stretc hing frequency is predicted by going from phenol(H2O)(+) to aminophenol(H2O )(+) clusters, explaining the completely different IR spectra of the invest igated species. (C) 2001 American Institute of Physics.