CALCULATION AND INTERPRETATION OF THE VIBRATIONAL SPECTRA OF INTERACTING MOLECULES

A system of programs has been developed to predict and interpret the infrared spectra of molecules and different chemically interacting species of interest. The frequency parameters (or force constants) and the intensity parameters (or Atomic Polar Tensors (APT) are calculated using the Gaussian 76 program for ab initio quantum mechanical calculations with a 4-31G basis set. The resultant frequencies and intensities have been used to predict the fundamental infrared spectrum of water molecules interacting with different molecular and ionic species. The corresponding frequency shifts and intensity changes from the spectrum of the non-interacting water molecule are dependent upon both the geometrical orientation and the electron-donating properties of the interacting species. The intensity parameters can be further analyzed by using the charge, charge-flux, overlap (CCFO) interpretation of the APT. This interpretation is helpful to pin-point the exact source of the perturbation that produces the intensity change. The significance will be discussed of each of these CCFO terms in predicting infrared intensities of molecules undergoing intermolecular interaction.