Semiexperimental equilibrium structure for the C6 backbone of cis-1,3,5-hexatriene; structural evidence for greater pi-electron delocalization with increasing chain length in polyenes.

Twenty-five microwave lines were observed for cis-1,3,5-hexatriene (0.05 D dipole moment) and a smaller number for its three (13)C isotopomers in natural abundance. Ground-state rotational constants were fitted for all four species to a Watson-type rotational Hamiltonian for an asymmetric top (kappa = -0.