Substituent effects on deuterium isotope effects on nuclear shielding of intramolecularly hydrogen‐bonded aromatic ketones, aldehydes and esters

Deuterium isotope effects on 13C nuclear shielding, nΔC(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. 2ΔC‐2(OD) shows a linear dependence on ΔOH, 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 13C chemical shifts. Isotope effects in o‐hydroxyacetophenones can be predicted from the δOH and nΔC(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. 2ΔC(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.