CONFORMATIONAL STUDIES OF CYCLOPROPYLCARBONYL FLUORIDE FROM TEMPERATURE-DEPENDENT FT-IR SPECTRA OF XENON SOLUTIONS

Variable temperature (-60 to -100 degrees C) studies of the infrared s pectra (3500 to 400 cm(-1)) of cyclopropylcarbonyl fluoride, c-C3H5CFO , dissolved in liquid xenon have been recorded. Utilizing four well se parated pairs of skeletal bending modes due to the cis and trans confo rmers, the enthalpy difference has been determined to be 97 +/- 11 cm( -1) (277 +/- 31 cal/mol) with the cis conformer (oxygen atom cis to th e three-membered ring) the more stable rotamer. From this Delta H valu e, along with assigned torsional transitions of 89.6 and 65.0 cm(-1) f or 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 initi o predictions from MP2/6-31G calculations several reassignments of th e fundamentals have been made. Ab initio calculations have been carrie d out with several different basis sets up to MP2/6-311G* and MP2/6-3 1++G* from which structural parameters and conformational stabilities have been determined. With the two largest basis sets the cis conform er is predicted to be the more stable conformer consistent with the ex perimental results. However, in the liquid and solid states the trans conformer is the more stable form. These results are compared to the c orresponding quantities for some similar molecules.