How nitrogen modifies the nuclear magnetic shielding in tetraazanaphthalenes.
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 + sigma)-currents associated to the nitrogen nuclei diminish the perpendicular component of shielding, abnormal values of (1)H NMR sigma(zz) are obtained. Therefore, a consistent aromaticity measure must be based only on the pi-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 pi-current contribution to magnetic shielding represents 6-8% of the out-of-plane shielding for nitrogen and 9-12% for carbon.
García Cuesta
Inmaculada
I
Instituto de Ciencia Molecular, Universidad de Valencia, P.O. Box 22085, E-46071, Valencia, Spain. garciain@uv.es
Sánchez Marín
José
J
Sánchez de Merás
Alfredo M J
AM