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A series of N-methyl-2-R1,R2-5-R3,R4- 1,3-oxazines (where R are either H or p-nitro phenyl group and R3,4 = H or CH3- group) have been theoretically studied by ab initio methods in the frame of MO theory. Optimized geometries were obtained at HF and B3LYP/6-31G" levels, whereas energies were derived at MP2 and B3LYP levels employing the same basis set. Geometries obtained at DFT level are needed to obtain consistent values and good correlations between the experimental and theoretical 1H-NMR data. The nitrogen lone pair would be axial orientated in I and II, since the N-methyl group lies in the equatorial position in the gas phase, whereas in compounds III and IV the methyl axial conformers increase their concentration, implying that inversion of the nitrogen lone pair takes place in an appreciable extent. PCM method predicts that solvents (CCl4, CHCl3, CH2Cl2) produce a significant effect on conformers preference, but just a modest influence on the equilibrium geometries and 1H-NMR properties.

THE EFFECT OF C2 AND C5 SUBSTITUTIONS ON THE NITROGEN LONE PAIR INVERSION IN 1,3-OXAZINES"