Rotational Spectra of Seven Conformational Isomers of 1-Hexene

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 C 1 symmetry, as verified by the observation of a-, b-, and c -type electric-dipole transitions. The remaining conformer has C s symmetry, consistent with its small inertial defect, Δ ≡ I cc − I bb − I aa = −12.65 uÅ 2, and the observation of only a and b -type transitions. Here, I αα 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 uÅ 2 . Both the molecular mechanics and ab initio calculations indicate the observed conformers are associated with the seven lowest-energy conformational minima of 1-hexene. The ab initio calculated energy difference between the lowest and highest energy conformers observed is 326 cm -1, including vibrational zero-point contributions, indicating that the conformational temperature is not equilibrated with the <2 K rotational temperature of the molecular-beam expansion.