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[1

Abstract

The deuterium-labelled 2,4,6,8-substituted barbaralanes [D5]-1a and b, and the two model barbaralanes 15 und 19 for the estimation of 13C chemical shifts 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 method bridges the gap between the limiting cases “perturbation of shift equivalence in delocalised systems” and “perturbation of degeneracy” by considering the simultaneous presence of delocalised species of higher symmetry and 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 13C spectra with three parameters, viz. the isotopic perturbation parameters ΔHP and ΔSP of the skewed equilibria and the enthalpy difference ΔH0 between the delocalised and localised species. – Relative isotopic splittings ΔδP/Δδ are calculated from estimated chemical shifts in the slow-exchange limit (Δδ) and isotopic splittings (ΔδP) of signals in variable-temperature 151-MHz 13C NMR spectra recorded for solutions of [D5]-1a and b in [D8]toluene and N,N′-dimethylpropylene urea. The results obtained from [D5]-1a in both solvents and from [D5]-1b in the former are compatible with either a skewed equilibrium between localised valence tautomers alone or the simultaneous presence of localised and small amounts of delocalised valence tautomers. In striking contrast, the small isotopic splittings themselves and their small temperature dependence, observed for solutions of [D5]-1b in N,N′-dimethylpropylene urea, demonstrate that one half of the solvated compound exists in the delocalised state [D5]-1b*, which is more stable by 2 kJ mol–1 than the equilibrating localised species [D5]-1b ⇌ [D5]-1b′.