Semiexperimental Equilibrium Structure for the C6 Backbone of cis-1,3,5-Hexatriene;Structural Evidence for Greater π-Electron Delocalization with Increasing Chain Length in Polyenes

摘要

Twenty-five microwave lines were observed for cis-1,3,5-hexatriene (0.05 D dipole moment) and a smaller number for its three 13C isotopomers in natural abundance. Ground-state rotational constants were fitted for all four species to a Watson-type rotational Hamiltonian for an asymmetric top (K ) -0.9768).Vibration-rotation (alpha) constants were predicted with a B3LYP/cc-pVTZ model and used to adjust the ground-state rotational constants to equilibrium rotational constants. The small inertial defect for cis-hexatriene shows that the molecule is planar, despite significant H-H repulsion. The substitution method was applied to the equilibrium rotational constants to give a semiexperimental equilibrium structure for the C6 backbone. This structure and one predicted with the B3LYP/cc-pVTZ model show structural evidence for increased π-electron delocalization in comparison with butadiene, the first member of the polyene series.