Transverse optical to longitudinal optical splitting and dipole moment derivatives from infrared spectra of thin films of molecular solids

For a number of molecular solids as thin films, longitudinal optical (LO) and transverse optical (TO) frequencies have been observed for molecular vibrations absorbing in the mid-infrared spectral region. From the LO-TO splitting, dipole moment derivatives have been calculated from equations previously derived for molecules on sites of tetrahedral or greater symmetry. For spherical top molecules these calculated derivatives are compared with values calculated from intensities reported in the literature. The agreement is remarkably good considering that most of the intensity data was determined for the gas phase and most of the solids have the molecules on sites of symmetry lower than tetrahedral. This offers a useful technique for determining dipole moment derivatives and infrared absorption intensities for gas-phase species as well as molecular and ionic solids.