How nitrogen modifies the nuclear magnetic shielding in tetraazanaphthalenes

Abstract

Although for planar conjugated hydrocarbons the out-of-plane component of proton magnetic shielding is an unquestionable quantitative aromaticity indicator, the same is not true for tetraazanaphthalenes. As in these compounds the (core + σ)-currents associated to the nitrogen nuclei diminish the perpendicular component of shielding, abnormal values of ¹H NMR σ_zz are obtained. Therefore, a consistent aromaticity measure must be based only on the π-contribution to the out-of-plane component of proton magnetic shielding. Otherwise, the behavior of these compounds in presence of an external magnetic field parallels that of naphthalene, with the nitrogen nuclei contributing to the ring current in a comparable amount to carbon nuclei. The π-current contribution to magnetic shielding represents 6–8% of the out-of-plane shielding for nitrogen and 9–12% for carbon.