Anharmonicity and hydrogen bonding II - A near infrared study of water trapped in nitrogen matrix

The infrared spectra of H-2 O-16 and (H2O)-O-18 trapped in solid nitrogen were measured in the range 8000-80 cm(-1). Concentration and annealing effects enable monomer, dimer and polymer bands to be distinguished. Those due to the monomer, easily assigned on the basis of O-16/O-18 isotopic substitution, are subdivided in three classes: intramolecular fundamentals and multiquanta transitions, librations and combinations with librations, combinations with the matrix vibron. For the dimer the assignment of the bands of the proton acceptor (PA) molecule follows that of the monomer while the assignment of the proton donor (PD) bands has to take into account the decoupling between the two oscillators, one being hydrogen bonded (OHb), the other quasi-free (OHf). The same decoupling occurs for polymeric species (H2O)(n), n greater than or equal to 3. The most remarkable result is the non-observation of the first overtone of the OHb oscillators while the corresponding fundamental transitions give rise to strong absorptions. The data analysis is focused on three points: determination of the vibrational spectral term after correction from resonances, determination of the dipole moment function (first and second derivatives of the dipole moment), and simultaneous transitions. For the monomer and PA the data are enough numerous for a comprehensive analysis. For PD the discussion is limited to the upsilon (1)/2 omicron (2), upsilon (1) + upsilon (2)/3 upsilon (2) Fermi resonances and to the evolution of the dipole moment function accounting for the weakness of the 2 upsilon OHb signal. For larger aggregates the same analysis as that carried out for PD can be developed, leading to comparable conclusions about the effect of hydrogen bonding on the dipole moment function. (C) 2001 Elsevier Science B.V. Ah rights reserved.