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

Maximilian Seefelder, Markus Heubes, Helmut Quast, W. Daniel Edwards, John R. Armantrout, Richard Vaughan Williams, Christopher J. Cramer, Alan C. Goren, David A. Hrovat, Weston Thatcher Borden

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Abstract

(Chemical Equation Presented) 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 1deloc and 2deloc. Gibbs free energy differences, ΔG*, 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 deiocaiized forms of 1c, 2c, and 2f. For example, ΔG*(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 (2c deloc)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 nonthermochromic, 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 ΔG*, electrical dipole and quadrupole moments and molecular polarizabilities have been calculated using the B3LYP/6-31G* method and solvation energies have been computed with the conductorlike 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 structures relative to the localized structures.

Original languageEnglish (US)
Pages (from-to)3437-3449
Number of pages13
JournalJournal of Organic Chemistry
Volume70
Issue number9
DOIs
StatePublished - Apr 29 2005

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