Experimental and theoretical study of stabilization of delocalized forms of semibullvalenes and barbaralanes by dipolar and polarizable solvents. Observation of a delocalized structure that is lower in free energy than the localized form.

[reaction: see text] UV/vis spectra of thermochromic semibullvalenes 1 and barbaralanes 2, which undergo rapid degenerate Cope rearrangements, display temperature-dependent shoulders (1b, 1d, 1e) or absorption maxima (1c, 2c, 2f) at the low-energy side of their strong UV bands. These long-wavelength absorptions are ascribed to Franck-Condon transitions from delocalized structures 1(deloc) and 2(deloc). Gibbs free energy differences, DeltaG*, between delocalized and localized forms were calculated from the temperature dependence of the long-wavelength absorptions.

The search for bishomoaromatic semibullvalenes and barbaralanes: computational evidence of their identification by UV/Vis and IR spectroscopy and prediction of the existence of a blue bishomoaromatic semibullvalene.

Time-dependent B3LYP/6-31G calculations have been performed at the optimized C(2) or C(2v) geometries of several substituted semibullvalenes (1(deloc)) and barbaralanes (2(deloc)), to compare the computed vertical electronic excitation energies with the temperature-dependent, long-wavelength absorptions that have been observed in the UV/vis spectra of some of these compounds by Quast and co-workers. The excellent agreement between the calculated vertical excitation energies and the observed values of lambda(max) provides strong support for the identification of the bishomoaromatic species 1(deloc) and 2(deloc) as the source of these absorptions. Furthermore, the CN stretching frequencies, computed for the C(2) geometry of 1,5-dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene (1f(deloc)), fit the low-frequency absorptions seen in the IR spectrum of 1f, thus furnishing independent evidence that bishomoaromatic geometries of semibullvalenes have, in fact, been observed spectroscopically.