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 ³⁵ Cl and ³⁷ Cl nuclei in both conformers, and in all cases the full Cl nuclear quadrupole coupling tensors have been determined for the first time.