1-PHENYL-3-METHYL-5-N-BENZYLIDENEAMINOPYRAZOLES - SUBSTITUENT EFFECTSAND PROTONATION SITES STUDIED BY NMR AND AB-INITIO (6-31G-ASTERISK) MO CALCULATIONS

1-Phenyl-3-methyl-5-N-benzylideneaminopyrazole and its derivatives 11 prepared by condensation of 1-phenyl-3-methyl-5-aminopyrazole and arom atic aldehydes have been studied by multinuclear (H-1, C-13, N-14/15 a nd O-17) magnetic resonance spectroscopy. The C-13 NMR chemical shifts and the direct spin-spin coupling constants (1)J(C,H) of the azomethi ne carbon of these Schiff bases (SE) correlate significantly with the Hammett substitueut constants, sigma(p), of the para-substituents in t he aryl ring bound to the azomethine carbon, The assignments of the N- 15 NMR chemicals shifts of SBs in CDCl3 were based on (2)J(N,H)s obser ved for the azomethine nitrogen as well as H-1, N-15 HMBC experiments. Based on the present H-1, C-13 and N-15 NMR data these SBs can be tra nsformed to single and double protonated forms in trifluoroacetic acid (TFA). The protonation sites (the first one at the unsubstituted nitr ogen of the pyrazole ring and the second one at the azomethine nitroge n) deduced from the NMR data are supported by ab initio MO calculation s at HF/6-31G level with a full geometry optimization performed for a model compound, 1,3-dimethyl-5-N-benzylideneaminopyrazole.