Characteristic Substituent Effect Model for the Infrared Intensities of the XCY (X = H, F, Cl, Br; Y = O, S) Molecules.

Many years ago, the gas-phase infrared fundamental intensities of ClCS were determined within experimental error from the experimental intensities and frequencies of FCO, ClCO, and FCS. An additive characteristic substituent shift relationship between atomic polar tensors of these molecules formed the basis for these calculations. Here, QCISD/cc-pVTZ-level Quantum Theory of Atoms In Molecules (QTAIM) individual charge, charge transfer, and polarization contributions to these atomic polar tensor elements are shown to obey the same basic relationship for the extended XCY (Y = O, S; X = H, F, Cl, Br) family of molecules. QTAIM charge and polarization contributions, as well as the total equilibrium dipole moments of the XCY molecules, also follow this characteristic substituent shift model. The root-mean-sqaure error for the 231 estimates of these parameters is 0.14 or only about 1% of the total 10 range of the Atomic Polar Tensor (APT) contributions determined from the wave functions. The substituent effect APT contribution estimates were used to calculate the infrared intensities of the XCY molecules. Although one serious discrepancy was observed for one of the CH stretching vibrations of HCS, accurate values were within 45 km·mol or about 7% of the 656 km·mol intensity range predicted by the QCISD/cc-pVTZ wave functions. Hirshfeld charge, charge transfer, and polarization contributions are also found to follow this model, although their charge parameters do not follow electronegativity expectations.