After a general description of the configuration, of the conformation and of the fast and reversible dynamic processes characteristic of the annulenes, results ofan experimental study of [8], [12], [14], [16] and [18]annulenes by N.M . . R. spectroscopy are presented. Theoretical analysis of the N.M.R. line shape has allowed a determinati9n of the structures of these compounds and of the thermodynamic and kinetic parameters of their dynamic processes. Study of bond shift in cyclooctatetraene (COT) and of bond shift and ring inversion in alkoxy-COT's point to a small negative resonance energy in the planar conformation of COT. Two configurations of [16Jannulene in 3:1 ratio, coded 85 and 91, have been found in solution. Both of them undergo isodynamical conformational mobility and bond shift and a fast exchange between both is also occurring. The newly synthesized [12]annulene of configuration 21 undergoes extremely fast conformational mobility but no bond shift. Conformational mobility is rather slow in [18]

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SummaryThe UV./VIS. absorption spectrum of [ 18lannulene has been remeasured in 3-methylpentane at room and at liquid nitrogen (glass) temperature and interpreted by the CNDO/S-CI-method. The confrontation of the experimental electronic transitions with the CNDO/S-CI-calculated ones favors a structure with D6h-symmetry, i.e. a structure with delocalized z-bonds.

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(1 9.W 1.85)Using the 'permutation of indices' method proposed by Kupian and Fruenkel, we could formulate the density-matrix equations required to lit the temperature-dependent I3C-NMR spectra observed with the title compounds. For 6Li'3CHBr2 (1) and 6Li'3CH2SC,H, (2) an exchange mechanism is proposed by which monomers interchange C-and Li-atoms via a non-observed dimeric intermediate; the activation parameters of these intermolecular dynamic processes have been found to be A H # = 10.2 kcal/mol, AS' = 13.7 cal/mol.K for 1 and A H # = 11.1 kcal/mol, A S # = 20.6 cal/mol.K for 2 ((D,)THF as solvent). In the case of ('Li)butyllithium (3), the observed low-temperature spectra indicate that dimeric (3b) and tetrameric (3a) species are in dynamic equilibrium interchanging the C3H,"CH2 groups (and THF molecules) bonded to the 6Li-atonis. The relative concentrations of the dimer and of the tetramer have been determined by peak integration or by line-shape fitting; the 'pseudo'-equilibrium constant, defined by KLq = [3bI2/[3a], was found to be 2.6 -mol/l (at -88") and corresponds to dG, (-88") = 2 AGi(3b) -dGi(3a) = 1.34 kcdl/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as A H # = 3.78 kcal/mol and A S z = -31.3 cal/mol.K. Tentative interpretation of thc thermodynamic and kinetic parameters is given.