DEUTERIUM-ISOTOPE EFFECTS ON C-13 NUCLEAR SHIELDING OF AMINO AND ACETAMIDO COMPOUNDS - TAUTOMERISM AND INTRAMOLECULAR HYDROGEN-BONDING

Deuterium isotope effects on C-13 nuclear shielding were investigated for a series of o-amino and acetamido azo compounds and for amino- and acetamido-9,10-anthra- and leucoanthraquinones. A variety of these compounds form intramolecular hydrogen bonds and are possibly tautomeric in nature. Deuterium isotope effects on C-13 chemical shifts were used to probe both the tautomerism and the nature of the hydrogen bond. Deuterium isotope effects over two bonds reflect the strength of the hydrogen bond. In general, isotope effects do not depend on the solvent and, in this study, are independent of temperature. In the acetamido compounds isotope effects on the internal aromatic carbons, 2-DELTA-C(ar)(ND), vary much more than the 2-DELTA-CO(ND) values of the external carbonyl carbon. Plots of 2-DELTA vs. delta(H) accordingly show different slopes. Investigations of these types of compounds allow a correlation between the hydrogen-bond enthalpy and 2-DELTA-CO(ND) of amides. 2-DELTA-CO(ND) values therefore form a basis for extending the use of deuterium isotope effects to, for example, biomolecules. Diamino-9,10-leucoanthraquinone represents, to our knowledge, the first case of a compound in which isotope effects from both a hydrogen-bonded and a free ND deuterium can be observed. Bis(phenylamino)leucoanthraquinone exhibits interesting equilibrium isotope effects over many bonds. Many of these isotope effects are non-additive. Hydrogen bonding is discussed in general, based on these observations.