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, Delta = I-cc - I-bb - I-aa = -12.65 u Angstrom(2), and the observation of only a and b-type transitions. Here, I-alpha alpha is the moment of inertia of the conformer about its alpha-principal axis. The inertial defects determined for the seven conformers range from -12.65 to -51.29 u Angstrom(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.