A theoretical study of the charge distribution of aminopyridines, aminopyrimidines, and some diazine N-oxides

Abstract

We have carried out STO-3G minimal basis set ab initio calculations to determine the charge distribution, using the YSP population analysis, of mono- and di-substituted aminopyridines, mono-, di-, and trisubstituted aminopyrimidines, and some mono- and bi-cyclic diazines and their corresponding mono-N-oxides. Our results indicate that the nitrogen charge densities do reflect the trend in the corresponding ¹⁵N chemical shifts, provided the nitrogen atoms have similar hybridization and substitution patterns. The amino-substituent effects are fully discussed. We have also shown that there exists a linear correlation between the ring nitrogen charge density of heteroaromatic derivatives and that of the homologous carbon atoms of the corresponding benzene derivatives. We have also studied, using the correlation between 1s orbital energies and gas-phase proton affinities, the intrinsic basicity of amino-pyridines and pyrimidines. Our results indicate that the ring nitrogens are the most basic sites in both families of compounds. In the particular case of aminopyrimidines the intrinsic basicities of both ring nitrogens are practically identical. These results are always in fairly good agreement with the ¹⁵N n.m.r. spectra of these molecules in acidic media. The charge distribution of the mono-N-oxide derivatives of diazine confirm the correlation between charge densities and chemical shifts. Their ¹⁵N n.m.r. spectra can be explained in terms of the contribution of mesomeric forms, which accumulate electronic charge at ortho- and para-positions, relative to the oxidized nitrogen atom.