HYDROGEN-BONDING AND SOLVENT POLARITY EFFECTS ON THE NITROGEN NMR SHIELDING OF 1,2,4,5-TETRAZINE

Results of the first extensive investigation of the nitrogen shielding of 1,2,4,5-tetrazine (I) are reported. Due attention is paid to solve nt-induced nitrogen-shielding variations by a variety of solvents. A c omparison is made with comparable, earlier, results for the other azin e systems. The observed range of solvent-induced nitrogen-shielding va riations of I is modest but significant, about 10 ppm. The major facto r producing this variation is that due to solvent polarity effects, wh ich is supported by the results of some shielding calculations based u pon the Solvaton model. A smaller, but significant, contribution is pr oduced by solvent-to-solute hydrogen bonding effects. This correlates well with the low nitrogen basicity of I as indicated by published ab initio molecular orbital calculations of gasphase protonation energies . There is found a general correlation between nitrogen basicity with respect to hydrogen bonding and that with respect to protonation. GLAO /CHF nitrogen-shielding calculations are reported for all of the exist ing monocyclic heteroaromatic azine systems including I using a 6-31(+)G* basis set. These appear to indicate that the inclusion of electr on-correlation effects, reported for the SOLO method, does not result in a dramatic improvement in the calculated nitrogen shieldings. (C) 1 997 Academic Press.