NMR spectrum of 2-methylamino-4-phenylbenzothiazole

Abstract

The abnormal chemical shift of the methyl group in the NMR spectrum of 2-methylamino-4-phenylbenzothiazole (V) in concentrated CDC1₃ solution (12%, vvt/vol.) is attributed to its existence, in part, as the hydrogen bonded dimer (XIV). Further support is provided by the fact that the shift is solvent dependent. In a nonpolar solvent like CS₂, V largely exists as a dimer. The marked high field shifts for V in benzene may be a composite of collision complex and self-association effects. In polar solvent like dioxane and DMSO-d₆ the dimerization by self-association is suppressed due; to competition from the solvent molecules, resulting in the almost normal values of methyl shifts. On the above analogy,' compounds 2-amino-4-phenylbenzo-thiazole (IV), 2-methylaminobenzothiazole (X) and 2-aminobenzothiazole (XI) have the possibility of dimeric association by hydrogen bonding, but the absence of methyl in IV, phenyl in X and both in XI makes it impossible to detect the phenomenon in these cases by NMR spectroscopy. The NMR spectra of mixtures of V and X at different concentrations also show shifts in methyl group, thereby indicating that X effectively takes part in hydrogen bonding with V to produce hetero dimer XV.