Rotational spectra and conformational analysis of 2-bromobutane

The rotational spectra of 2-bromobutane were obtained by using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer in the frequency range 8–18 GHz. Hyperfine structures of both bromine isotopologues (I = 3/2) for the three conformers (G+, A, and G−) have been investigated to determine the rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants of bromine (Br). The conformational compositions for G+, A, and G− in the supersonic jet, estimated by relative intensity measurements, are 55%, 28%, 17%, respectively, for the ⁷⁹Br species, and 52%, 33%, 15%, respectively, for the ⁸¹Br species. However, the small zero-point energy differences of isotopomers can be negligible and can be considered as an average value instead of individual values, which is 54%, 30%, 16% for G+, A, and G− species. The nuclear quadrupole coupling tensors (χ_xx, χ_yy, and χ_zz) of Br and the quadrupolar angles were obtained, and the χ_zz value of 2-bromobutane is compared with those of other alkyl bromides. The spectroscopic constants, r_s coordinates of Br and the dipole moments of the most stable G+ conformer are predicted well from the ab initio calculations at the MP2/6–311++G(2d,2p) level.