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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

Richard D. Suenram^†, Brooks H. Pate^†, Alberto Lesarri^‡, Justin L. Neill^†, Steven Shipman^†, Robin A. Holmes^§, Matthew C. Leyden^§ and Norman C. Craig*^§

Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, Departmento de Qu

mica F

sica y Qu

mica Inorg

nica, Universidad de Valladolid, Valladolid, Spain 47005, and Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074

J. Phys. Chem. A, 2009, 113 (9), pp 1864–1868

DOI: 10.1021/jp8106777

Publication Date (Web): February 5, 2009

†

University of Virginia.

, ‡

Universidad de Valladolid.

, §

Oberlin College.

Abstract

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 (κ = −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.

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