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Deuterium isotope effects on the 13C nuclear shielding of intramolecularly hydrogen‐bonded systems

A large number of intramolecularly hydrogen‐bonded molecules were investigated by means of deuterium isotope effects on 13C nuclear shielding. The compounds illustrate three different types of molecules: compounds with a localized hydrogen bond (strongly asymmetric double minimum potential), and tautomeric molecules of symmetrical or unsymmetrical nature (symmetrical or almost symmetrical double minimum potential). In the localized case the strength of the hydrogen bond reflects itself in the magnitude of 2Δ COD. The presence of a hydrogen bond is, furthemore, demonstrated in an orientational dependence of 3Δ COD, and usually also in the presence of an 3Δ COD. The tautomeric systems can also be characterized by means of deuterium isotope effects. Investigations of β diketones showed the presence of an equilibrium isotope effect. Large long‐range isotope effects of naphthazarin are explained in a similar manner. Tautomerism was established in usnic acid and the type of tautomerism was determined. 2,6‐Dihydroacetophenone shows an interesting type of perturbation of the equilibrium on unsymmetric deuteriation.