The deuterium-labelled 2,4,6,8-substituted barbaralanes localised species. -Relative isotopic splittings ∆δ P /∆δ are calculated from estimated chemical shifts in the slow-[D 5 ]-1a and b, and the two model barbaralanes 15 und 19 for the estimation of 13 C chemical shifts in the slow-exchange exchange limit (∆δ) and isotopic splittings (∆δ P ) of signals in variable-temperature 151-MHz 13 C NMR spectra recorded limit are synthesised from bicyclo[3.3.1]nona-3,7-diene-2,6dione (5). -The extension of SaundersЈ isotopic perturbation for solutions of [D 5 ]-1a and b in [D 8 ]toluene and N,NЈdimethylpropylene urea. The results obtained from [D 5 ]-1a method bridges the gap between the limiting cases "perturbation of shift equivalence in delocalised systems" in both solvents and from [D 5 ]-1b in the former are compatible with either a skewed equilibrium between and "perturbation of degeneracy" by considering the simultaneous presence of delocalised species of higher localised valence tautomers alone or the simultaneous presence of localised and small amounts of delocalised symmetry and skewed equilibria between localised molecules that are degenerate in the absence of the isotopic valence tautomers. In striking contrast, the small isotopic splittings themselves and their small temperature perturbation. An equation (Equation 7) is derived for such multi-component systems which describes the temperature dependence, observed for solutions of [D 5 ]-1b in N,NЈdimethylpropylene urea, demonstrate that one half of the dependence of the relative isotopic splittings in 13 C spectra with three parameters, viz. the isotopic perturbation solvated compound exists in the delocalised state [D 5 ]-1b*, which is more stable by 2 kJ mol Ϫ1 than the equilibrating parameters ∆H P and ∆S P of the skewed equilibria and the enthalpy difference ∆H 0 between the delocalised and localised species [D 5 ]-1b v [D 5 ]-1bЈ.
bona fide assumption of very fast Cope rearrrangements cupy the delocalised state 1*. [10] Therefore, the isotopic per- [a]