Theoretical calculations of proton affinities of azines. Prediction of the relative basicities and preferred protonation sites

Theoretical calculations at the 3-21G and 3-21 + G ab initio levels and at the MNDO and AM1 semiempirical levels of several six-membered nitrogenated heterocycles and their protonated species have been carried out. The 3–21G calculated proton affinities are systematically too high, in relation to the available experimental data, and it is estimated that inclusion of electron correlation and zero-point corrections is not sufficient to reach the desired agreement; however, additional inclusion of diffuse functions (3-21 + G/3-21G calculations) lowers the calculated proton affinities by 5.4–6.8 kcal/mol, a good agreement being thus obtained, at least for 1–7 . On the other hand, semiempirical methods underestimate the repulsion between each pair of vicinal nitrogens; however, if a correction of ?9 kcal/mol is added to the AM1 results for each pair of neighboring nitrogens containing lone pairs of electrons, the corresponding proton affinities match fairly well the available exoerimental data and corrected 3-21 + G results. As expected, all methods predict that the introduction of additional nitrogens decreases the overall absolute basicity. Futhermore, comparison of the relative basicity of the isomers and of the preferred protonation site for each isomer indicates that nitrogen atoms with (only) one α-nitrogen and without a γ-nitrogen are more basic than any others. In benzazines, MNDO and AM1 suggest that the 2,3-diaza arrangement has a higher intrinsic basicity than the 1,2-diaza arrangement.