Interpretation of infrared intensity changes on molecular complex formation. I. Water dimer

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Abstract
The infrared intensities for the three fundamental vibrations of the H2O monomer have been analyzed to obtain experimental values for the atomic polar tensors (APTs) for the atoms in the monomer. These APTs have been compared with values calculated for the monomer by an ab initio quantum mechanical treatment using IBMOL‐5 with a 4‐31G basis set. The agreement is satisfactory enough to extend the ab initio calculation to the atoms in the linear water dimer. It is argued that the differences in the APTs from monomer to dimer (and, hence, the intensities for the intramolecular vibrations of the water dimer in terms of the corresponding intensities for the monomer) can be predicted with reasonable certainty by this treatment. The spectrum predicted for the water dimer compares very favorably with the experimental spectrum for the dimer in a N2 matrix, reported by Tursi and Nixon. The calculated APTs for the atoms in the monomer and in the dimer are subjected to a ‘‘charge–charge flux–overlap’’ (CCFO) analysis in order to see just what is responsible for the observed changes in intensity. The only significant change occurs in Pyy(CF), the charge flux contribution to the change in the dipole moment along the O–H–O hydrogen bond for motion of the hydrogen‐bonded H atom along that bond. This result is analyzed in terms of contributions from the change in charge transfer that ocurs when the O–H bond stretches and contributions from the change in polarization of the EA and ED molecules. This analysis shows that the contribution from changing intermolecular charge transfer in the water dimer accounts for approximately one‐half of the APT increase that gives rise to the characteristic infrared intensification observed for vibrations (particularly the O–H stretch of the hydrogen‐bonded H atom) on hydrogen bond formation.

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