THE PERTURBED MICROWAVE SPECTRUM OF CYCLOPROFYL BROMIDE

Author Institution: Department of Chemistry, The University of British Columbia; Department of Chemistry and Biochemistry, Southern Illinois UniversityThe microwave spectra of two isotopic species of cyclopropyl bromide, $C_{3}H_{3} ^{78}Br$ and $C_{3}H_{5} ^{81}Br$, have been measured in the frequency range 15-90 GHz. The molecule is a near-symmetric prolate rotor. The spectrum is dominated by strong a-type R branches: the anticipated c-type transitions are weak and difficult to identify. Usually in this situation accurate values of only two rotational constants, B and C, are available. However, by taking advantage of perturbations in the Br quadrupole hyperfine structure, we have evaluated accurately all three rotational constants, using exclusively a-type transitions. This was done using a computer program for global least squares fitting to all spectroscopic constants. As a result of these fits some c-type transitions could be identified, and accurate values for all rotational constants, quartic centrifugal distortion constants and Br quadrupole coupling constants have been obtained. The Br quadrupole coupling tensors have been diagonalized, and their structural implications, along with those of the rotational constants, have been deduced