Conformational stability and vibrational assignment of propanal

Abstract

Variable temperature (−52 to −100°C) studies of the infrared spectra (3500–400 cm −1) of propanal, CH 3CH 2CHO, dissolved in liquid xenon have been recorded and bands due to both the cis and gauche conformers have been identified. From these data, the enthalpy difference has been determined to be 370±21 cm −1 (4.43±0.25 kJ mol −1) with the cis conformer (methyl group eclipsing the oxygen atom) the more stable form. Utilizing the new infrared data from the xenon solution, along with some additional Raman data, and ab initio predictions from MP2/6-31G* calculations, several reassignments of the fundamentals have been made for the cis conformer. Additionally, several bands which have been identified as arising from the gauche conformer have also been assigned. Infrared (3500–80 cm −1) and Raman (3500–30 cm −1) spectra of gaseous, liquid and/or solid CD 3CD 2CHO are also reported and the fundamentals assigned for the cis conformer. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.