FAR-INFRARED SPECTRUM, BARRIERS TO INTERNAL-ROTATION, STRUCTURAL PARAMETERS, AND VIBRATIONAL ASSIGNMENT OF CYCLOPROPYLCARBONYL FLUORIDE

The far-infrared (400 to 50 cm-1) spectrum of gaseous cyclopropylcarbonyl fluoride, c-C3H5CFO, 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: V1 = 459 +/- 10, V2 = 2131 +/- 47, V3 = 320 +/- 18, V4 = 16 +/- 15 cm-1 and V5 = 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-C3H5CFO from the ab initio values, with adjustments, and the microwave data. The results obtained are compared with the corresponding results for some similar molecules.