A SEMIEXPERIMENTAL EQUILIBRIUM STRUCTURE OF {\em cis}-HEXATRIENE FROM MICROWAVE SPECTROSCOPY

Author Institution: Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074; Department of Chemistry, University of Ulm, D-89069 Ulm, Germany; Laboratoire de Physique des Lasers, Atomes et Molecules, Universite de Lille I, 59655 Villeneuve d'Ascq Cedex, FranceGround state rotational constants for a full set of carbon and deuterium isotopologues of {\em cis}-hexatriene, which has a dipole moment of only 0.05 D, have been determined by MW spectroscopy. Ground state rotational constants were converted into equilibrium rotational constants with spectroscopic alphas computed from force constants found with an MP2/cc-pVTZ model. Corrections for electronic contributions were applied. Despite the availability of a full set of rotational constants for a suite of isotopologues, an accurate structure was not accessible. The mixed estimation method, in which high-quality bond parameters computed by quantum chemistry with appropriate uncertainties were fit simultaneously with the moments of inertia, was applied. A structure good to 0.001 {\AA} was the result. The structural parameters reflect an increase in pi-electron delocalization compared to butadiene