Rotational Spectra of Seven Conformational Isomers of 1-Hexene

Abstract

The rotational spectra of 7 of the expected 13 conformational isomers of 1-hexene have been measured and assigned at a rotational temperature of <2 K using a pulsed-molecular-beam Fourier transform microwave spectrometer. The rotational assignments were guided by predictions from the MM3 molecular mechanics force field of Allinger et al. and by ab initio electronic-structure calculations (MP2/6-31G*). Six of the seven observed conformers have C1 symmetry, as verified by the observation of a-, b-, and c-type electric-dipole transitions. The remaining conformer has Cs symmetry, consistent with its small inertial defect, Δ ≡ Icc − Ibb − Iaa = −12.65 uA2, and the observation of only a and b-type transitions. Here, Iαα is the moment of inertia of the conformer about its α-principal axis. The inertial defects determined for the seven conformers range from −12.65 to −51.29 uA2. Both the molecular mechanics and ab initio calculations indicate the observed conformers are associated with the seven lowest-energy con...