CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL ROTATION, AB INITIO CALCULATIONS AND VIBRATIONAL ASSIGNMENT OF FLUOROACETYL FLUORIDE

Article abstract of DOI:10.1016/0022-2860(89)80050-X

The far infrared spectrum (350 to 35 cm^-1) of gaseous fluoroacetyl fluoride has been recorded at a resolution of 0.10 cm^-1.The fundamental asymmetric torsion of the more stable trans conformer where the two fluorine atoms are trans to one another has been abserved at 118.90 cm^-1 with twelve upper state transitions falling to lower frequency.Additionally, the fundamental torsional transition of the cis conformer has been observed at 52.06 cm^-1 with five excited states falling to lower frequency.From these data, the asymmetric torsional potential coefficients have been determined to be: V₁ = 86 +/- 11; V₂ = 946 +/- 33; V₃ = 407 +/- 4; V₄ = 138 +/- 20 and V₆ = -14 +/- 7 cm^-1.The trans to cis and cis to trans barriers were calculated to be 1297 +/- 26 (3.71 +/- 0.07 kcal mol^-1) and 803 +/- 26 cm^-1 (2.30 +/- 0.07 kcal mol^-1), respectively, with the energy difference being 494 +/- 48 cm^-1 (1.41 +/- 0.14 kcal mol^-1).From the temperature dependence of the Raman spectra the enthalpy difference between the conformations has been determined to be 456 +/- 32 cm^-1 (1.30 +/- 0.09 kcal mol^-1) and 695 +/- 8 cm^-1 (1.99 +/- 0.02 kcal mol^-1) for the gas and liquid, respectively.The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally, are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3-21G and 6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.