NMR AND SEMIEMPIRICAL CONFORMATIONAL-ANALYSIS OF THE YL-1,3-DIHYDROXY-4,4,5,5-TETRAMETHYLIMIDAZOLIDINES

yl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidines (I) were synthesize d and their H-1 and C-13 NMR spectra recorded. Quantum mechanical semi empirical calculations were also performed for a better understanding of the signals recorded in the NMR spectra of imidazolidines. The conf ormation of the imidazolidine ring was initially studied for the 2-met hyl-1,3-dihydroxy-4,4,5,5-tetramethyl imidazolidine (2), used as a mod el molecule. The results of the calculations obtained for structure 2 showed that the methyl groups are located in axial and equatorial posi tions. At these positions, the methyl groups are affected by the magne tic anisotropic effects of carbon-nitrogen and carbon-carbon bends of the imidazolinyl ring with different intensities. Semiempirical calcul ations for structure 2, suggested that the effect of the gamma-oxygen on the carbon atoms of methyl groups (gamma-effect) might lead to an a lteration of the electronic charge density and consequently to a chang e in the diamagnetic shielding on these groups. These data were used f or the analysis of the NMR spectra of compound 1. The diamagnetic shie lding effects, estimated from the calculated electronic charge densiti es on the carbon atoms of methyl groups, are in agreement with the sig nals observed in the NMR spectra of compound 1. By combining the contr ibution of the anisotropic and gamma-effects, it appears that the axia l methyl groups are located relatively closer to the gamma-oxygens in compound 1. (C) 1997 Elsevier Science Ltd.