An analytical expression for temperature-dependent infrared intensities in fully coupled anharmonic molecular systems

Abstract

We present the analytical expression for determining the temperature evolution of absolute integrated infrared (IR) intensities of fundamental bands in fully coupled anharmonic molecular systems. Only harmonic frequencies and integrated IR intensities (within the double harmonic approximation) are needed to predict the absolute temperature-dependent IR intensities of the fundamental bands. These quantities can be readily obtained from ab initio electronic structure calculations for relatively large molecules. In order to illustrate the accuracy of the expression we compare the results from this formula using harmonic frequencies and intensities from a tight-binding potential energy surface with the results from semiclassically prepared molecular dynamics (SCP-MD) simulations on the same potential energy surface for the naphthalene molecule. For a favorable comparison with experimental data we use the same analytical expression with the more accurate B3LYP/cc-pVDZ harmonic frequencies and intensities. We conclude that the IR intensity changes that our proposed formula predicts in the fundamental vibrational bands are mainly due to intramolecular vibrational energy redistribution (IVR).