Conformational studies of cyclopropylcarbonyl fluoride 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 fluoride, c-C 3H 5CFO, dissolved in liquid xenon have been recorded. Utilizing four well separated pairs of skeletal bending modes due to the cis and trans conformers, the enthalpy difference has been determined to be 97 ± 11 cm −1 (277 ± 31 cal/mol) with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From this ΔH value, along with assigned torsional transitions of 89.6 and 65.0 cm −1 for the trans and cis conformers, respectively, the potential function governing the conformational interchange has been calculated. The cis to trans and trans to cis barriers have been determined to be 2100 ± 200 and 2000 ± 200 cm −1, respectively. Utilizing the new infrared data from the xenon solution, Raman data for the liquid, 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 MP2/6–311G ∗∗ and MP2/6–31 + + G ∗∗ from which structural parameters and conformational stabilities have been determined. With the two largest basis sets the cis conformer is predicted to be the more stable conformer consistent with the experimental results. However, in the liquid and solid states the trans conformer is the more stable form. These results are compared to the corresponding quantities for some similar molecules.