Revealing Low-Barrier Conformational Flexibility in Perfluoropropanoyl Halides: A Structural Gas Phase Study of CF 3 CF 2 C(O)X (X = Cl, Br, I)

In contrast to a previous investigation of perfluoropropanoyl chloride, CF3CF2C(O)Cl, by microwave spectrometry which detected only the anti-conformer, the present structural investigation of CF3CF2C(O)Cl by gas electron diffraction (GED) allowed the detection of an additional gauche-conformer. Its double degeneracy by symmetry makes it more populated than the lower-energy anti-conformer. The conformational distribution as a function of temperature was studied by infrared spectroscopy. The enthalpy difference between the gauche- and anti-forms was determined by applying the Van’t Hoff equation to resolved conformer bands in the vibrational spectrum. Additional gas phase conformational studies of perfluoropropanoyl bromide and iodide, CF3CF2C(O)Br and CF3CF2C(O)I, provide a comparison to the structures of CF3CF2C(O)F, CF3CF2C(O)OH and CF3CF2C(O)Cl, determined in this and previous work. The vibrational spectra of the title compounds are governed by the low anti-gauche rotational barriers. The magnitude and consequences of these barriers are determined by a detailed analysis of the spectroscopic data.