Low frequency vibrational spectra, methyl torsional potential functions, and internal rotational potential of propanal

Abstract

The low frequency Raman and infrared spectra of gaseous propanal have been recorded in the region 300–50 cm−1. A complex far infrared spectrum was observed, and a substantial number of bands are assigned to the asymmetric and methyl torsional modes of both the s-cis and gauche conformers. Analysis of these bands allowed calculation of the torsional potential functions present in this molecule. The results indicated that the potential functions calculated previously for this molecule are inadequate, and that an unusual interaction exists in the s-cis conformer between an in-plane bending mode and the asymmetric torsion. The potential function coefficients for the asymmetric torsion of propanal were calculated (in cm−1) to be: V2=559±7, V3=651±8, V4=−105±4, and V6=−46±6. This potential function has an enthalpy difference between the more stable s-cis and the higher energy gauche conformers of 241±10 cm−1, with the gauche dihedral angles being about ±135 °.