Scalar coupling constants across the intramolecular NHN hydrogen bond of symmetrically and non-symmetrically substituted 6-aminofulvene-1-aldimines

Multinuclear liquid state magnetic resonance experiments have been performe d on two seven-membered N-15-labeled H-chelates of the 6-aminofulvene-1-ald imines type in order to characterize the strong intramolecular NHN hydrogen bonds as a function of the molecular symmetry. In particular, the symmetri cally substituted N,N'-diphenyt-6-aminopentafulvene-1-aldimine-N-15(2) (1) and its asymmetric analog N-phenyl-N'-(1,3,4-triazol)- 6-aminopentafulvene- 1-aldimine-N-15(5) (2) have been studied. For 1, an NN coupling constant ac ross the hydrogen bridge of (2h)J(N-15,N-15) = 10.6 Hz was determined indir ectly by C-13 NMR at two different Larmor frequencies, 125.76 and 67.93 MHz ; this coupling constant is characteristically enhanced compared with the v alue of 8.6 Hz obtained previously for 2. Because of a fast degenerate prot on tautomerism the hydrogen bond proton in 1 is coupled with both nitrogen atoms with a coupling constant of -40.8 Hz. {N-15} tickling experiments wer e performed on 2 in order to determine the relative signs of the coupling c onstants of the NHN hydrogen bridge. We find that (2h)J(N-15, (15) N) and ( 1h)J(H-1,(15) N) = +4.4 Hz exhibit the same sign, i.e. the opposite sign co mpared with (1)J(N-15,(1) H) = -88.6 Hz. This finding proves that (1h)J(H-1 ,(15) N) corresponds to an intrinsic coupling, which is not induced by a ta utomerism absent in 2 because of the large difference in basicities of the aniline and the amino-1,3,4-triazole substituents. Therefore, these observa tions indicate a sign change of J(N-15,(1) H) when the proton is transferre d successively from one nitrogen to the other, as observed previously for F HF hydrogen bonds. The relation between the values of the coupling constant s and the hydrogen bond geometries is discussed in terms of the valence bon d order model, as are the implications for obtaining equilibrium constants of tautomerism from coupling constant data. Copyright (C) 2001 John Wiley & Sons, Ltd.