One‐bond deuterium isotope effects on 15N chemical shifts in Schiff bases

A series of 15N enriched Schiff bases have been investigated by means of 15N NMR at variable temperature (300‐200 K). Most reliable results are obtained below 250 K, as the exchange of the chelate proton is sufficiently slow on the NMR time scale at this temperature. The 15N chemical shifts are found to vary linearly with the one bond 15N‐H coupling constants. Deuterium isotope effects on 15N chemical shifts, 1Δ15N(D), show a characteristic S‐shape, when plotted vs. the one‐bond 15N‐H coupling constants. From this graph the intrinsic isotope effects of the two forms can be estimated as 5Δ15N(D)int˜‐1.5 ppm and 1Δ15N(D)int˜3 ppm. The intrinsic isotope effects for the OH‐form is shown to vary only little for the various substituted compounds investigated. The difference in 15N chemical shifts of the OH‐form minus the NH‐form is estimated to 100‐140 ppm. The dependence of 1Δ15N(D) on the mole fraction is similar to that found for 2ΔC(OD) for other intramolecularly hydrogen bonded cases, β‐diketones and Mannich bases and looks very similar to that determined for 1Δ15N(D) of the organic acid‐pyridine complex.