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.