Finite-temperature IR spectroscopy of polyatomic molecules: a theoretical assessment of scaling factors

With a recently developed simulation method (Basire, M.; et al. J. Phys. Chem. A 2009, 113, 6947), the infrared vibrational spectra of several polyatomic molecules are calculated over a broad range of temperature, taking into account quantum, anharmonic, and couplings effects. Anharmonic force fields, generated from static first-principle calculations, are sampled in the microcanonical ensemble to provide energy-resolved absorption intensities and their finite temperature analogues after Laplace transformation. Effective anharmonic frequencies are characterized as a continuous function of temperature for vinyl fluoride, the N-acetyl-Phe-NH(2) peptide, and protonated naphthalene. These frequencies generally deviate increasingly from the harmonic value with increasing temperature, although the overestimation due to the harmonic approximation is particularly salient for high-frequency modes. Anharmonicities may also be sufficient to alter structural assignment of experimental spectra with respect to empirically scaled harmonic bands. These results emphasize some possible limitations and inaccuracies inherent to using such static scaling factors for correcting harmonic IR spectra.