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Experimental and Theoretical Study of Stabilization of Delocalized Forms of Semibullvalenes and Barbaralanes by Dipolar and Polarizable Solvents. First Observation of a Delocalized Structure that Is Lower in Energy than the Localized Form

Seefelder, M.; Heubes, M.; Quast, H.; Edwards, W. D.; Armantrout, J. R.; Williams, R. V.; Cramer, C. J.; Goren, A. C.; Hrovat, D. A.; Borden, W. T.
J. Org. Chem. 2005, 70, 3437.

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 longwavelength absorptions are ascribed to Franck-Condon transitions from delocalized structures 1deloc and 2deloc. Gibbs free energy differences, DG*, between delocalized and localized forms were calculated from the temperature dependence of the long-wavelength absorptions. Dipolar and polarizable solvents strongly affect and even may reverse the relative stabilities of the localized and delocalized forms of 1c, 2c, and 2f. For example, DG*(2c) = 8 kJ mol-1 in cyclohexane, 2 kJ mol-1 in dimethylformamide, and -3 kJ mol-1 in N,N'-dimethylpropylene urea (DMPU), so that (2cdeloc)DMPU becomes the global minimum. In contrast to the case for 2c, the intensities of the long-wavelength shoulders of the yellow semibullvalenes 1b, 1d, and 1e are only moderately influenced by solvents, and the rates of Cope rearrangements of the non-thermochromic, colorless barbaralanes 2a and 2b, determined by NMR methods, are almost solvent-invariant. In search of the solute properties that are decisive in determining the influence of solvent upon DG*, electrical dipole and quadrupole moments and molecular polarizabilities have been 3 calculated, using the B3LYP/6-31G* method; and solvation energies have been computed with the Conductor-like Polarized Continuum Model (CPCM). The results of these calculations indicate that the solvent effects are due to the greater polarity and polarizability of the delocalized, relative to the localized structures.

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