The vibrational spectrum of (H2O)2:: comparison between anharmonic ab initio calculations and neon matrix infrared data between 9000 and 90 cm-1

From the spectrum of water ((H2O)-O-16 and (H2O)-O-18) trapped in neon matrix recorded between 50 and 9000 cm(-1) 29 vibrational transitions from the ground state have been identified for the water dimer. Twenty measured in the mid- and near infrared have been assigned to one-, two- and three-quanta transitions of the intramolecular modes, five in the far infrared to intermolecular modes and four in the mid infrared to binary intra + inter combinations. These assignments are based on O-16/O-18 isotopic shifts and on the comparison with the spectrum of the acetonitrile:water one to one complex in which the vibrational properties of the water subunit are very close to that of the proton donor molecule in (H2O)(2). The comparison of the results in the mid- and far infrared with those obtained in the gas phase shows that the Ne matrix induced perturbations are very small for the intramolecular vibrations and do not exceed 20% for the intermolecular ones. Accordingly this set of data can be used to test the ability for a new version of the Gaussian program to account for the anharmonicity of vibrations and its evolution upon hydrogen-bonding. The results show that the anharmonicity coefficients are generally well reproduced for the intramolecular modes and that the highly anharmonic low frequency intermolecular modes are calculated less than 20% higher than observed in the gas phase. (C) 2004 Elsevier B.V. All rights reserved.