Deuterium isotope effects on 13C chemical shifts of intramolecularly hydrogen-bonded Schiff bases

Abstract

The proton transfer equilibrium in a series of Schiff bases derived from substituted salicylic aldehydes and aliphatic amines has been investigated by means of variable temperature multinuclear magnetic resonance and the deuterium isotope effect on 13C nuclear shielding. Most of the compounds exist in two tautomeric forms in CDCl3. The populations of the tautomeric forms have been estimated using 3J(NH,H) and 1J(15N,H) coupling constants. One goal is to describe the characteristics of both tautomers. Deuterium isotope effects on 13C chemical shifts (nΔC(XD)) are shown to depend on the position of the tautomeric equilibrium. nΔC-2(XD) and nΔC-1′(XD) are found to be a sensitive non-monotonic function (S-shape) of the mole fraction. The pattern of the dependence seems also to be general for other tautomeric compounds with intramolecular hydrogen bonds, but with much lower barriers to interconversion like β-diketones and piroxicam as well as organic acid–pyridine complexes with intermolecular hydrogen bonds. This new feature is a valuable tool in the characterisation of tautomeric and other equilibrium systems.