Stereoisomerism and spectroscopic properties of α, ω-diphenylpolyenes. Part 2.—Theoretical study on conformations, electronic transitions, and band shape of butadiene-, hexatriene-, and octatetraene-derivatives

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Abstract
Conformations of sterically hindered cis-isomers of the α,ω-diphenylpolyenes series are studied in terms of internal rotations around exocyclic single and double bonds. Maps of the potential energy were drawn which show that the twist of the bonds connecting the phenyl rings to the polyene fragment is the preferred deformation to relieve steric strain. The most probable configurations of the molecules in the ground-state were used to calculate energies and intensities of the electronic transitions; the u.-v. bands were assigned according to Platt's selection rules. In order to explain the lack of fine structure in the “conjugation” band of the hindered cis-isomers, Franck-Condon distributions corresponding to excitation of the twisting mode of the phenyl-polyene bonds were calculated for cis-trans- and cis-cis-1,4-diphenylbutadiene. The effect brought about on the vibrational structure by excitation of more complex vibrational modes is also qualitatively analyzed.

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