Conformational barriers to internal rotation and vibrational assignment of cyclopropylcarbonyl fluoride

Abstract

The infrared (3500 to 50 cm−1) and Raman (3200 to 10 cm−1) spectra have been recorded for the gaseous and solid states of cyclopropylcarbonyl fluoride, c-C3H5CFO. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. The asymmetric torsion for the conformer which has the carbonyl group cis to the cyclopropane ring has been observed at 88.5 cm−1 in the far infrared spectrum of the gas with five accompanying hot bands, and the corresponding torsion of the trans conformer was observed at 91.0 cm−1 with two additional hot bands occurring at 90.6 and 90.2 cm−1. 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=−618±6, V2=1970±27, V3=399±6, and V4=−48±10 cm−1. It has been determined that the cis conformer is the predominant form at ambient temperature in the gas phase and the enthalpy difference between the cis and the trans conformers is 222 cm−1 (629 cal/mol) for the vapor. The calculated trans-cis barrier is 1948 cm−1 (5.57 kcal/mol) and the cis-trans barrier is 2168 cm−1 (6.20 kcal/mol). From a temperature study of the Raman spectrum of the liquid an energy difference of 92±20 cm−1 (263 cal/mol) has been determined with the trans conformer being more stable in the liquid phase due to intermolecular interactions. Additionally, an investigation of the vibrational spectrum of the solid state indicates that both conformers persist even to 30 K but the trans conformer is more stable. The number of lattice modes observed in the Raman spectrum of the solid indicates at least two molecules per primitive cell. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. These results are compared to similar quantities in some related molecules.