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 solvent-induced nitrogen-shielding variations by a variety of solvents. A comparison is made with comparable, earlier, results for the other azine systems. The observed range of solvent-induced nitrogen-shielding variations ofIis modest but significant, about 10 ppm. The major factor producing this variation is that due to solvent polarity effects, which is supported by the results of some shielding calculations based upon the Solvaton model. A smaller, but significant, contribution is produced by solvent-to-solute hydrogen bonding effects. This correlates well with the low nitrogen basicity ofIas indicated by publishedab initiomolecular orbital calculations of gas-phase protonation energies. There is found a general correlation between nitrogen basicity with respect to hydrogen bonding and that with respect to protonation. GIAO/CHF nitrogen-shielding calculations are reported for all of the existing monocyclic heteroaromatic azine systems includingIusing a 6-31++G** basis set. These appear to indicate that the inclusion of electron-correlation effects, reported for the SOLO method, does not result in a dramatic improvement in the calculated nitrogen shieldings.