Vibrational and Conformational Studies of Cyclopropanecarboxaldehyde

The infrared (3500 to 40 cm-l) and Raman (3200 to 20 cm") spectra have been recorded for gaseous and solid cyclopropanecarboxaldehyde, c-C3H5CHO. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. It has been determined that the conformer with the oxygen atom cis to the three-membered ring is the more stable form at ambient temperature in the gaseous phase but in the Iiquid phase the trans is more stable and is the only conformer present in the spectra of the solid. From a temperature study of the Raman spectrum of the liquid an energy difference of 405 ± 20 cm? (1.16 kcal/mol) was obtained whereas the energy difference for the gaseous state was found to be approximately 60 cm". The observed changes in !'lH in going from the gas to the liquid state indicate strong intermolecular association in the condensed phases which can account for the trans being more stable in the liquid and the stable conformer in the solid state. The asymmetric torsion of the trans conformer was observed in the far infrared spectrum as a type B band centered at 126 cm? and the corresponding torsion for the cis conformer was observed at 113 cm? as a type C band with six accompanying hot bands falling at lower frequencies. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1 = 61 ± 6, V2 = 1494 ± ± 15, V3= -7 ± 6, and V4= -39 ± 6 cm'. The calculated trans to cis and cis to trans barriers were determined to be equivalent with a value of 1521 crrr '. A complete vibrational assignment is proposed for both conformers based on in fra red band contours, depolarization values, and group frequencies. These results are compared to similar quantities in several related molecules.