Microwave spectrum and conformational composition of 2-fluoroethylisocyanide

The microwave spectrum of 2-fluoroethylisocyanide, FCH(2)CH(2)N≡C, has been investigated in the whole 50-120 GHz spectral region. Selected portions of the spectrum in the range of 18-50 GHz have also been recorded. The microwave spectra of the ground state and vibrationally excited states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of microwave transitions. The F-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the F-C-C-N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and erroneously predict the antiperiplanar rotamer to be 1.3-1.6 kJ/mol more stable than the synclinal conformer.