Delocalized electrons in infrared intensities

•How the IR intensities of strongly IR-active modes are generated is described.•Coupled nuclear and electronic motions are shown schematically and theoretically.•Intermolecular charge flux induces IR intensity enhancement upon H and X bonding.

In the sense that the main features of infrared (IR) spectra are governed by strongly IR active modes, one needs to know how those strong IR intensities are generated, in order to fully extract the information on the properties of molecular systems from their vibrational spectra. In fact, for many of those modes, the IR intensities arise mainly from the charge flux term, meaning that delocalized electronic motions are strongly coupled. The present article shows how we can understand those electronic motions qualitatively by simple schemes, and quantitatively on the basis of well founded theories, especially by analyzing the electron density derivatives. The cases of the peptide group, hydrogen-bonded water, and an aromatic hydrocarbon are shown as typical examples.