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

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 (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 = -0.9768). 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 C(6) backbone. This structure and one predicted with the B3LYP/cc-pVTZ model show structural evidence for increased pi-electron delocalization in comparison with butadiene, the first member of the polyene series.

Gas phase infrared spectra and corresponding DFT calculations of , -diphenylpolyenes

“…A possible explanation for this finding could be the increased p-electron delocalization between double and single bonds in DPH. This greater p-electron delocalization with increasing chain length has just recently been confirmed using microwave spectroscopy and high level calculations [54,5,55] for the unsubstituted analogues and is also reflected in the bond lengths as shown in Table 3. During the c(C-H) out-ofplane vibration the planar character of the orbitals located at the carbon atoms is decreased.…”

A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

“…An isomeric mixture of the HTEs was prepared by passing 1,5-hexadiene-3-ol (Aldrich) through a phosphorus pentoxide-packed tube on a vacuum system and isolating the pure isomers by preparative gas chromatography, as described before [4]. Infrared spectra of the isomers agreed with published spectra [5,6].…”

“…From the ground state rotational constants and the quantum chemical calculation of the vibration-rotation constants (spectroscopic alphas), equilibrium rotational constants were derived for the normal species and the three 13 C species. A Kraitchman substitution analysis (r s ) with these equilibrium rotational constants yielded an r e structure for the C 6 backbone of cis-HTE [4]. This partial ''r s /r e " structure shows the expected lengthening of the C@C bonds and shortening of the C-C bonds in comparison with butadiene.…”

“…Although cis-HTE has a small dipole moment (0.05 D with the B3LYP/cc-pVTZ model), microwave (MW) spectra have been measured for the normal species and the three 13 C isotopomers in natural abundance [4]. From the ground state rotational constants and the quantum chemical calculation of the vibration-rotation constants (spectroscopic alphas), equilibrium rotational constants were derived for the normal species and the three 13 C species.…”

From the Hubbard to the PPP Model

“…This value apparently traces back to the usually reliable experimental work of Haugen and Traetteberg 27 in 1966. However recent (though still preliminary) work on cis 28 and trans 1 This yields a model based on the MN form for the long-range integrals which gives excellent agreement with both excitation energies and ground-state bond lengths, but with a significantly smaller value of the one center electron repulsion U than is usually assumed. By contrast the Ohno form for the long-range integrals agrees poorly with several experimental excitation energies.…”