Extension of Saunders' isotopic perturbation method as probe for the structures in solution of 2,4,6,8-substituted barbaralanes - NMR-spectroscopic evidence for the coexistence of localised and delocalised states

The deuterium-labelled 2,4,6,8-substituted barbaralanes [D-5]-1a and b, and the two model barbaralanes 15 und 19 for the estimation of C-13 chemical s hifts in the slow-exchange limit are synthesised from bicyclo[3.3.1]nona-3, 7-diene-2,6-dione (5). - The extension of Saunders' isotopic perturbation m ethod bridges the gap between the limiting cases "perturbation of shift equ ivalence in delocalised systems" and "perturbation of degeneracy" by consid ering the simultaneous presence of delocalised species of higher symmetry a nd skewed equilibria between localised molecules that are degenerate in the absence of the isotopic perturbation. An equation (Equation 7) is derived for such multi-component systems which describes the temperature dependence of the relative isotopic splittings in C-13 spectra with three parameters, viz, the isotopic perturbation parameters Delta H-P and Delta S-P of the s kewed equilibria and the enthalpy difference Delta H-0 between the delocali sed and localised species. - Relative isotopic splittings Delta delta(P)/De lta delta are calculated from estimated chemical shifts in the slow-exchang e limit (Delta delta) and isotopic splittings (Delta delta(P)) of signals i n variable-temperature 151-MHz C-13 NMR spectra recorded for solutions of [ D-5]-1a and b in [D-8]toluene and N,N'-dimethylpropylene urea. The results obtained from [D-5]-1a in both solvents and from [D-5]-1b in the former are compatible with either a skewed equilibrium between localised valence taut omers alone or the simultaneous presence of localised and small amounts of delocalised valence tautomers. In striking contrast, the small isotopic spl ittings themselves and their small temperature dependence, observed for sol utions of [D-5]-1b in N,N'-dimethylpropylene urea, demonstrate that one hal f of the solvated compound exists in the delocalised state [D-5]-1b*, which is more stable by 2 kJ mol(-1) than the equilibrating localised species [D -5]-1b reversible arrow [D-5]-1b'.