1972 Volume 45 Issue 7 Pages 1979-1988
Stilbene and some of its alkyl-substituted derivatives were reduced with alkali metals to dianions in 1,2-dimethoxyethane. From comparison of the electronic absorption spectra of these dianions with one another and with the results of semiempirical SCF-MO-CI calculation of the π-electronic state, the geometries of the dianions were inferred to be as follows: (a) the preferred geometry of a sterically unhindered dianion, such as the dianion of stilbene, is probably planar or nearly planar; (b) when alkyl substituents are introduced into the central ethylenic carbon atoms, the steric interference caused by the substituents is relieved chiefly by the rotation about the central ethylenic bond; (c) the steric interference caused by ortho methyl substituents seems to be comparatively small or to be relieved by the rotation about the bonds connecting benzene rings to the ethylenic carbon atoms as well as the ethylenic bond. From the influence of alkali-metal cations on the spectra of the dianions, it was inferred that the dianions exist mainly as contact ion aggregates in which the counter-ions are located near the central ethylenic bond, one probably almost above an ethylenic carbon atom and the other almost below the other ethylenic carbon atom.
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