Conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of fluoroacetyl chloride.

The far infrared spectrum (375 to 35 cm-1) of gaseous fluoroacetyl chloride, CH2FC(O)C1, has been recorded at a resolution of 0.10 cm-1. The fundamental asymmetric torsions of the more stable trans (halogen atoms are trans) and the high energy cis conformations have been observed at 116.18 and 49.42 cm-1, respectively, each with several upper state transitions falling to lower frequency. From these spectral data, an asymmetric potential function has been calculated and the potential coefficients are: V1=43±6, V2=1039±36, V3=498±3, V4=149±21, and V6=-10±7 cm-1. The trans to cis and cis to trans barriers are 1455±25 cm-1 (4.16±0.07 kcal/mol) and 914±24 cm-1 (2.61±0.07 kcal/mol), respectively, with an enthalpy difference of 541±45 cm-1 (1.55±0.13 kcal/mol). From studies of the Raman spectra at variable temperatures, values of 509±37 cm-1 (1.46±0.10 kcal/mol) and 310±8 cm-1 (0.89±0.02 kcal/mol) have been determined for the enthalpy difference for the gas and liquid, respectively. The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally, are compared to those obtained from ab initio Hartree–Fock calculations employing both the 3-21G* and 6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules. [ABSTRACT FROM AUTHOR]