SUBSTITUENT EFFECTS ON DEUTERIUM-ISOTOPE EFFECTS ON NUCLEAR SHIELDING OF INTRAMOLECULARLY HYDROGEN-BONDED AROMATIC KETONES, ALDEHYDES AND ESTERS

Deuterium isotope effects on C-13 nuclear shielding, (n)DELTAC(OD), were investigated for a large number of substituted ortho-substituted aromatic hydroxyacyl compounds of the type salicylaldehyde, 2-hydroxyacetophenone, etc. The isotope effects were analysed as a function of substitution patterns. 2DELTAC-2(OD) shows a linear dependence on deltaOH, with one slope for 4- and 6-substituted derivatives and another for 5-substituted derivatives. A similar pattern is also found for the long range isotope effects. In addition, isotope effects also correlate with C-13 chemical shifts. Isotope effects in o-hydroxyacetophenones can be predicted from the deltaOH and (n)DELTAC(OD) values of salicylaldehyde, and vice versa. The isotope effects were analysed in terms of a vibrational term related to the intramolecular hydrogen bond and an electronic term related to the substituent influence on the transmission of the effect along the pathway between the deuterium-substituted oxygen and the carbon in question. 2DELTAC(OD) isotope effects are found to depend on the one-bond bond order of the C=C bond of the unsubstituted hydrocarbons. Long-range isotope effects are observed to the olefinic carbons of 2-hydroxychalcone, whereas no effects beyond C-1' are seen in o-hydroxyketones or o-hydroxybenzophenones. The tautomeric equilibrium of usnic acid is perturbed by the removal of the weak hydrogen bond between 9-OH and the carbonyl group at C-1.