Comprehensive analysis of the rotational spectrum of 2,2-dichloropropane

Abstract 2,2-Dichloropropane, ( CH 3 ) 2 CCl 2 , is a globular, plastic crystal molecule characterised by an apparently featureless b -type rotational spectrum. The spectrum is complicated by relatively intense isotopic species, nuclear quadrupole hyperfine splitting from two chlorine nuclei, and clustering of the seven lowest normal modes of ( CH 3 ) 2 C 35 Cl 2 in the wavenumber range 250–390  cm - 1 . A comprehensive investigation of this spectrum was carried out on the basis of the room-temperature spectrum recorded up to 338 GHz and of the supersonic expansion spectrum at cm-wave frequencies. The ground states of the ( CH 3 ) 2 C 37 Cl 2 and 13 CH 3 12 CH 3 C 35 Cl 2 species were assigned for the first time and allowed redetermination of the molecular geometry. The double nucleus hyperfine structure was measured and analysed for both 35 Cl 2 and 35 Cl 37 Cl species of 2,2-dichloropropane, and differences between orientations of the CCl bond axis and the hyperfine z -axis were determined and discussed. Rotational transitions in first excited states of all seven normal modes were found to be strongly perturbed requiring Coriolis type coupled fits for the ( ν 9 , ν 14 ) and ( ν 13 , ν 27 ) dyads, and the ( ν 26 , ν 8 , ν 21 ) triad. The precise vibrational energy differences lead to new values for the lowest normal mode frequencies.