A theoretical determination of the methyl and aldehydic torsion far-infrared spectrum of propanal-d0 with the vibrational zero point correction

Abstract

The methyl and aldehydic torsions far-infrared (FIR) spectrum of standard propanal is determined theoretically taking into account the vibrational zero point energy (ZPE) of the remaining small amplitude vibration modes in an anharmonic approach. For this purpose, the potential energy function obtained in a previous calculation at the 6-311(3df,p) restricted Hartree–Fock/second-order Møller–Plesset level [J. Chem. Phys. 109, 2279 (1998)] is modified for the anharmonic ZPE corrections. With this corrected potential for the ZPE, the kinetic parameters, as well as the electric dipole moment variations, the FIR frequencies, and intensities for the methyl and aldehyde torsions of propanal were determined theoretically using a two-dimensional approach. The calculated torsional band structures of propanal determined without and with the ZPE corrections were compared between themselves and with the available experimental data. It is found that the theoretical calculations permit one to reproduce and assign 24 of the 25 observed bands. In the same way, the introduction of the ZPE corrections is seen to yield more satisfactory frequency values for the aldehyde torsion, double quanta, and methyl–aldehyde combination bands.