Microwave spectrum, ab initio calculations, and structural parameters of bromocyclopentane,

Abstract The microwave spectrum of bromocyclopentane, c-C 5 H 9 Br, has been recorded from 19.7 to 38.3 GHz. The a-type R-branch transitions have been observed and assigned for the ground state for both the axial and equatorial conformations of the 79 Br and 81 Br isotopic species. One vibrationally excited state of both isotopic species of the axial conformation have been identified and assigned. Additionally, some c-type Q-branches of both isotopic species of the axial conformer have been assigned. For most of the transitions partly resolved hyperfine structure could be assigned. The rotational constants, quartic centrifugal distortion constants and the quadrupole coupling constants have been determined by an iterative least-squares fit of the more than seventy assigned frequencies of the hyperfine components for each of the isotopic species in the ground vibrational state. Ab initio calculations have been carried out at MP2/6-31+G ∗∗ and MP2/6-311++G ∗∗ levels with full electron correlation. The calculation with the larger basis set predict the rotational constants within 19 MHz (1.3%) of the experimental values and the structural parameters from this calculation were adjusted to fit the experimental rotational constants. The maxima adjustments for bond lengths are 0.005 A and for the bond angles 1.1°. The ring-puckering angle is 3.8° smaller in the axial conformer compared to that of the equatorial conformer. These results are compared to the corresponding quantities for some similar molecules.