Calculation of OH-stretching band intensities of the water dimer and trimer

We have calculated fundamental and overtone OH-stretching vibrational band intensities of the water dimer and trimer. The intensities were determined with a simple harmonically coupled anharmonic oscillator (HCAO) local mode model and ab initio dipole moment functions. The dipole moment functions we re calculated at the self-consistent-field Hartree-Fock and quadratic confi guration interaction including single and double excitations levels of theo ry with the 6-31G(d), 6-311+G(d, p), and 6-311++G(2d, 2p) basis sets. The o vertone spectra of the dimer and trimer have not been observed and a method of obtaining local mode parameters from scaled ab initio calculations has been suggested. We find that the falloff in absolute intensity with increas ing overtone of the dimer and trimer is similar to the falloff observed for the monomer. Our calculations show that the total overtone intensity of th e dimer and trimer, although distributed differently, is close to two and t hree times the total intensity of the monomer for a given region. One signi ficant difference between the monomer and the dimer and trimer is the appea rance of the red shifted hydrogen bonded OH-stretching band in the dimer an d trimer spectra. We suggest that these red shifted bands are ideal for att empts to observe the water dimer in the atmosphere. The method presented ca n provide an accurate estimate of the OH-stretching intensities for molecul es for which vibrational spectra have not been observed. Such calculations are of importance in atmospheric solar energy absorption models. (C) 1999 A merican Institute of Physics. [S0021-9606(99)00317-7].