Microwave, structural, conformational, vibrational studies and ab initio calculations of fluoroacetyl chloride

• The most stable trans conformer has been identified from microwave spectra.
• Enthalpy difference has been determined between the three forms.
• Adjusted r0 structures were obtained for trans form.
• Ab initio calculations were performed for the three conformers.

The infrared and Raman spectra (3200–50 cm−1) of the gas, liquid or solution, and solid of fluoroacetyl chloride, FCH2COCl have been recorded. FT-microwave studies have also been carried out and 22 transitions were recorded for the trans conformer. Variable temperature (−50 to −105 °C) studies of the infrared and Raman spectra (3200–50 cm−1) of xenon solutions have been carried out. From these data, the trans, cis and gauche conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 159 ± 11 cm−1 (1.90 ± 0.14 kJ mol−1) with the trans conformer the more stable form than the cis. The energy difference between the cis and gauche form is 222 ± 18 cm−1 (2.66 ± 0.21 kJ/mol) and the energy difference between the trans and gauche forms is 386 ± 13 cm−1 (4.61 ± 0.16 kJ/mol). Vibrational assignments have been made for the observed bands for the three conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, and Raman activities for the three conformers. By utilizing the microwave rotational constants of two isotopomers for trans, combined with the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained for the trans conformer. The results are discussed and compared to the corresponding properties of some related molecules.

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