Far-infrared spectrum, barriers to internal rotation, structural parameters, and vibrational assignment of cyclopropylcarbonyl fluoride

Abstract

The far-infrared (400 to 50 cm −1) spectrum of gaseous cyclopropylcarbonyl fluoride, c-C 3H 5CFO, was recorded at 0.1 cm −1 resolution. The fundamental asymmetric torsion for the most stable conformer, which has the carbonyl group oriented cis to the ring, is observed at 83.51 cm −1 with five torsionally excited-state transitions proceeding to lower frequencies. For the high energy trans conformer the fundamental asymmetric torsional mode was assigned to a series of C-type Q branches in the region of 94 to 85 cm −1. From these data the potential function for internal rotation of the asymmetric top was determined and the potential coefficients are: V 1 =459±10, V 2=2131±47, V 3 = −320±18, V 4 = 16 ± 15 cm −1 and V 5 = 57±9. The cis to trans and trans to cis barriers were determined to be 2307 ± 28 and 2111 ± 26 cm −1, respectively, with an enthalpy difference between the conformers of 196 ± 75 cm −1 (560 ± 214 cal mol −1). The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree-Fock gradient calculations at the 3-21G and 6-31G* basis set levels. The structural parameters were determined for c-C 3H 5CFO from the ab initio values, with adjustments, and the microwave data. The results obtained are compared with the corresponding results for some similar molecules.