A range of compounds forming intramolecular hydrogen bonds are studied by means of deuterium isotope effects, primarily on 13C chemical shifts but also on i9F chemical shifts. Deuterium isotope effects on chemical shifts are shown to be useful descriptors of intramolecular hydrogen bonded systems, (and not only a parameter proportional to the chemical shifts). It is shown that deuterium isotope effects on 13C chemical shifts can reveal weakening of an intramolecular hydrogen bond due to steric hindrance and strengthening due to steric compression. Two donors competing for one acceptor are shown to weaken the intramolecular hydrogen bond. Exchanging hydrogen for deuterium in an intramolecular hydrogen bond is seen to perturb the equilibrium. This is observed for both tautomeric and conformational equilibria. Solving structural problems like the conformations of the minor form of 3,5- diacetyltetrahydropyran-2,4,6-trione, and the dynamic of l-acetyl-2-hydroxy-3-&o-6- methoxybenzene is greatly helped by means of deuterium isotope effects on r3C chemical shifts. Deuterium isotope effects on i3C chemical shifts is shown to be a probe for tautomerism. For enolic P-diketones, the perturbations of the tautomeric equilibrium are shown to depend on the equilibrium constant. Enolic P-ketoamides, enolic 1,3-indandiones thioamides and enolic 5-acyl 1,3-dimethylbarbituric acids are here by shown to be tautomeric. The isotope effects on ‘jC chemical shifts measured for some enolic 5-acyl meldrum acids suggest that these are tautomeric, which is unusual as the ester moiety in enolic P-ketoesters are not on the enolic form
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