ONE-BOND DEUTERIUM-ISOTOPE EFFECTS ON N-15 CHEMICAL-SHIFTS IN SCHIFF-BASES

A series of N-15 enriched Schiff bases have been investigated by means of N-15 NMR at variable temperature (300-200 K). Most realiable resul ts are obtained below 250 K, as the exchange of the chelate proton is sufficiently slow on the NMR time scale at this temperature. The N-15 chemical shifts are found to vary linearly with the one bond N-15-H co upling constants. Deuterium isotope effects on N-15 chemical shifts, ( 1) Delta(15)N(D), show a characteristic S-shape, when plotted vs. the one-bond N-15-H coupling constants. From this graph the intrinsic isot ope effects of the two forms can be estimated as (5) Delta(15)N(D)(int )similar to-1.5 ppm and (1) Delta(15)N(D)(int)similar to 3 ppm. The in trinsic isotope effects for the OH-form is shown to vary only little f or the various substituted compounds investigated. The difference in N -15 chemical shifts of the OH-form minus the NH-form is estimated to 1 00-140 ppm. The dependence of (1) Delta(15)N(D) on the mole fraction i s similar to that found for (2) Delta C(OD) for other intramolecularly hydrogen bonded cases, beta-diketones and Mannich bases and looks ver y similar to that determined for (1) Delta(15)N(D) of the organic acid -pyridine complex.