13C, 15N CP-MAS, FT–IR and PM5 studies of some Schiff bases of gossypol in solid

The Schiff bases of racemic gossypol with l-arginine methyl ester (GSB1), 2-(aminomethyl)-18-crown-6 (GSB2) and 5-hydroxy-3-oxapentylamine (GSB3) were studied by FT–IR, 13C and 15N CP-MAS spectroscopy as well as by PM5 semiempirical calculations. The spectroscopic methods give clear evidence that all Schiff bases exist in the solid state as enamine–enamine tautomers. The structures of GSB1–GSB3 and the hydrogen bonds within these structures are discussed.