MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2001; 39: S67–S80 NMR study of proton transfer equilibrium in Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone. Deuterium isotope effects on 13 C and 15 N chemical shifts T. Dziembowska, 1 Z. Rozwadowski, 1 A. Filarowski 2 and P. E. Hansen 2* 1 Institute of Chemistry, Technical University of Szczecin, Al. Piast ´ ow 42, 71-065 Szczecin, Poland 2 Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260, 4000 Roskilde, Denmark Received 1 June 2001; Revised 17 July 2001; Accepted 18 July 2001 The proton transfer equilibrium in series of Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone were measured by means of 1 H, 13 C and 15 N NMR spectra and deuterium isotope effects on 13 C and 15 N chemical shifts at variable temperature and in different solvents. The latter made it possible to investigate a broad range of equilibrium positions. All compounds exist mainly as the NH tautomer over the full range of temperatures in CDCl 3 solution. The position of the equilibrium was estimated using 1 J. 15 N, 1 H/ and 3 J. 15 NH, 1 H/ coupling constants. The correlation between deuterium isotope effects n 1 13 C-2(XD) or n 1 15 N(XD), where X = O or N, and the mole fraction of the NH form show the characteristic, non-monotonic function as for other intramolecular hydrogen bonded systems. The position of the minimum depends on substituents and the maximal negative values of n 1 13 C-2(XD) varies with substituents and solvents. Copyright  2001 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H NMR; 13 C NMR; 15 N NMR; Schiff bases; proton transfer; H/D isotope effects on chemical shifts INTRODUCTION Schiff bases, derivatives of aromatic o-hydroxyaldehydes, are a class of compounds which have received attention owing to their interesting properties, biological activity and technical applications. 1–3 Proton transfer and intramolecular hydrogen bridge are the major factors controlling properties of these Schiff bases. Recently, we have studied the proton transfer equilibrium in N-(R-salicylidene)alkylamines by measuring the deuterium isotope effect on 13 C 4,5 and 15 N NMR 6 chemical shifts. This method is known to be very useful in studies of intramolecularly hydrogen bonded systems 4–15 and also for systems taking part in tautomeric equilibria. 4–9,13 – 15 A number of features have been discussed in this context. The main question is whether these compounds with a relatively low potential barrier can be treated as a simple di-species equilibrium? Related to this was the determination of the characteristic values for the individual tautomers. For Schiff bases the key parameters were the coupling constants 1 J⊲ 15 N, 1 H⊳ and 3 J⊲N 1 H, 1 H⊳. 4–6 The characteristic S-shaped correlation between deuterium isotope effects and mole fraction has been seen for several equilibrium systems. 5,10,11,13,14 The minima and maxima were close to K D 0.25 and 4, respectively. 10,13 Ł Correspondence to: P. E. Hansen, Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260, 4000 Roskilde, Denmark. Continuing our investigation of the proton transfer equilibria in Schiff bases, we have synthesized deriva- tives of 2-hydroxynaphthaldehyde (1 – 7) and 1-hydroxy-2- acetonaphthone (8) with aliphatic and heterocyclic amines (Scheme 1). N O H 1 2 3 4 5 6 7 8 9 10 1' 10 9 8 7 6 5 4 3 2 1 N R O H R 1 1" (8) R 1 = CH 3 R = CH 3 (7) R 1 = H R = CH 2 -C 6 H 5 (6) R 1 = NO 2 R = t-C 4 H 9 (5) R 1 = H R = 8-Quinoline (4) R 1 = H R = Adamantane (3) R 1 = H R = t-C 4 H 9 (2) R 1 = H R = n-C 4 H 9 (1) R 1 = H R = CH 3 R 1 1' R α α Scheme 1. Compounds studied. Some of these Schiff bases have been studied before by different methods. 16 – 21 Extension of the -electron delocal- ization by substituting the benzene ring by a naphthalene moiety and introduction of more aliphatic or heterocyclic DOI: 10.1002/mrc.949 Copyright  2001 John Wiley & Sons, Ltd.