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

Abstract

Abstract A series of 15 N enriched Schiff bases have been investigated by means of 15 N 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 15 N chemical shifts are found to vary linearly with the one bond 15 N‐H coupling constants. Deuterium isotope effects on 15 N chemical shifts, 1 Δ 15 N(D), show a characteristic S‐shape, when plotted vs. the one‐bond 15 N‐H coupling constants. From this graph the intrinsic isotope effects of the two forms can be estimated as 5 Δ 15 N(D) int ˜‐1.5 ppm and 1 Δ 15 N(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 15 N chemical shifts of the OH‐form minus the NH‐form is estimated to 100‐140 ppm. The dependence of 1 Δ 15 N(D) on the mole fraction is similar to that found for 2 ΔC(OD) for other intramolecularly hydrogen bonded cases, β‐diketones and Mannich bases and looks very similar to that determined for 1 Δ 15 N(D) of the organic acid‐pyridine complex.