Conformational studies of cyclopropylcarbonyl chloride from temperature-dependent FT-IR spectra of xenon solutions

Abstract

Variable temperature (−60 to −100°C) studies of the infrared spectra (3500 to 400 cm −1) of cyclopropylcarbonyl chloride, c-C 3H 5CClO, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the cis and trans conformers, the enthalpy difference has been determined to be 102±19 cm −1 (292±54 cal mol −1), with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From this Δ H value, along with assigned torsional transitions of 87.4 and 54.7 cm −1 for the trans and cis conformers, respectively, and with excited state transitions for torsional modes for both conformers, the potential function governing the conformational interchange has been calculated. The cis-to-trans barrier has been determined to be 2180±100 cm −1 (6.23±0.29 kcal mol −1). 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. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311++G**, from which structural parameters and conformational stabilities have been determined. Irrespective of the size of the basis sets, the cis conformer is predicted to be the more stable conformer, which is consistent with the experimental results. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules. © 1997 Elsevier Science B.V.