A conformational study of butyryl chloride using chirped pulse Fourier transform microwave spectroscopy and quantum chemical calculations

The pure rotational spectrum of butyryl chloride has been recorded between 8 and 16 GHz using a chirped pulse, Fourier transform microwave spectrometer. The transitions recorded were assigned to two different conformers which have previously been designated syn-gauche and syn-anti. Quantum chemical calculations have produced rotational constants in good agreement with those obtained from the observed spectra. For the syn-gauche conformer the τ 1 (CCCO) dihedral angle is calculated to be 10° and the τ 2 (CCCC) dihedral angle is calculated to be 70°. For the syn-anti conformer τ 1 (CCCO) is calculated to be 0° and τ 2 (CCCC) is calculated to be 180°. For both MP2 and M06-2X methods with 6-311G∗∗ basis sets the syn-gauche conformer is calculated to be marginally lower in energy than the syn-anti conformer. The resolution of the experiments was sufficient to resolve hyperfine structure from the 35Cl and 37Cl nuclei in both conformers, and in all cases the full Cl nuclear quadrupole coupling tensors have been determined for the first time.