Modeling of molecular charge distribution on the basis of experimental infrared intensities and first-principles calculations: the case of CH bonds.

DFT calculations are used to predict CH stretching infrared (IR) intensities for about 50 small molecules. B3LYP and PBE1PBE functionals and different basis sets are tested to obtain the best agreement with the experimental absolute IR intensities available. PBE1PBE functional in particular predicts average CH stretching intensities very close to the experimental ones. On the basis of a simple analytical model, it is shown how it is possible to extract atomic charges directly from computed atomic polar tensors (APTs): these IR charges are very close in value to the experimentally derived ones and faithfully reproduce peculiar molecular phenomena. DFT-derived IR charges are also compared with the charges obtained by population schemes such as Mulliken population analysis, natural population analysis (NPA), and with charges obtained by fitting the electrostatic potential according to the Merz-Kollman (MK), CHELP, and CHELPG models. IR charges are found to be similar especially to the charges obtained by these last methods: therefore they can be used as an alternative scheme for the determination of the molecular charge distribution while being strictly connected to experimentally measurable properties. Moreover, the analytical model is further developed to obtain a method for the calculation of the charge fluxes, which take place along the chemical bonds during molecular vibrations.