Modelization of vibrational spectra beyond the harmonic approximation from an iterative variation–perturbation scheme: the four conformers of the glycolaldehyde

Abstract

This paper presents the computed anharmonic frequencies and IR intensities in the mid-infrared region for the four conformers of glycolaldehyde (Cis cis, Trans trans, Trans gauche and Cis trans forms). The fundamental transitions and their connected overtones and combination bands through strong anharmonic couplings (Fermi resonances) are provided. The results are stemmed from an iterative variational–perturbational resolution of the vibrational problem implemented in the VCI-P code. The four potential electronic surfaces are built as a Taylor series truncated to the fourth order around each minimum geometry. The second derivatives with respect to the normal coordinates were computed at the CCSD(T)/cc-pVTZ level, while the third and fourth derivatives were estimated with the B3LYP/6-31 + G(d,p) model chemistry. For the most stable Cc form, an average deviation of about 10 cm⁻¹ is obtained with respect to the unambiguous experimental values. Furthermore, some of the transitions observed in the CH stretchings region were reassigned. The theoretical values calculated for the Tt and Tg forms are compared to the experimental data obtained from the irradiation of the Cc conformer isolated in Ar matrix with an IR source.