Properties of local vibrational modes: the infrared intensity

For the local (adiabatic) vibrational modes of Konkoli and Cremer (Int J Quantum Chem 67:29–40, 1998), infrared intensities are derived by setting up the appropriate adiabatic conditions. It is shown that the local mode intensities are independent of the coordinates used to describe a molecule and correctly reflect the molecular symmetry and isotope composition. Normal mode intensities are related to local mode intensities via an adiabatic connection scheme (ACS). The ACS reveals intensity changes due to local mode mixing and avoided crossings, which are easily identified and quantified. The infrared intensities of simple molecules such as H2O, CH4, O3, HOOH, CH3OH, and the water dimer are discussed, and the influence of isotopes is quantified.