The Cl-nuclear quadrupole coupling tensors and the internal rotation barrier of 1,1-dichloroethane by Fourier transform microwave spectroscopy Original Research Article

The rotational spectra of 1,1-dichloroethane in the ground state of the CH3CH35Cl2, CH3CH35Cl37Cl and CH3CH37Cl2 isotopomers and in four low frequency excited states of the main species have been investigated in the range 6–250 GHz. Measurements have been carried out using molecular beam and waveguide Fourier transform microwave spectrometers as well as Stark and source modulation spectrometers. Accurate rotational and centrifugal distortion constants have been obtained. The complete Cl-nuclear quadrupole coupling tensors have been determined from the analysis of the resolved hyperfine structure for the three observed isotopomers. From the small A–E splittings observed in the first torsional excited state, the barrier to internal rotation of the methyl group has been determined to be 4.260(9) kcal mol−1.