A STUDY OF SOLVENT POLARITY AND HYDROGEN-BONDING EFFECTS ON THE NITROGEN NMR SHIELDING OF ISOMERIC TETRAZOLES AND AB-INITIO CALCULATION OF THE NITROGEN SHIELDING OF AZOLE SYSTEMS

High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for the N-methyl derivatives of the two existing isomeri c tetrazoles (I, II) in a variety of solvents which represent a wide r ange of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of so lvent-induced nitrogen shielding variations of I and II is significant for the pyrrole-type nitrogens (N-Me), up to 9 ppm, and even more so for pyridine-type nitrogen atoms, where it can attain a value of 20 pp m. There is a clear distinction between the two types of nitrogen atom s in that the former exhibit a deshielding effect with increasing pola rity of the medium while the latter experience an increase in the magn etic shielding of their nuclei. The latter effect is significantly aug mented by solvent-to-solute hydrogen-bond formation where the pyridine -type nitrogens are involved directly. It is also quite diversified th roughout the pyridine-type nitrogen atoms and seems to constitute a me asure of relative basicity with respect to hydrogen-bond formation of the nitrogens concerned. This basicity seems to parallel that with res pect to a full transfer of a proton, as can be reckoned from ab initio calculations of the relevant protonation energies reported in the pre sent study. The experimental data for the tetrazoles in cyclohexane so lutions are combined with those obtained in our earlier extensive stud ies on azole, diazole, and triazole ring systems, for a comparison wit h ab initio calculations of the nitrogen shieldings concerned. The lat ter were carried out using the coupled Hartree-Fock/GIAO/6-31++G* app roach and geometry optimizations employing the same basis set; they sh ow a good linear correlation with the experimental data and reproduce not only major changes but also most of the subtle variations in the e xperimental nitrogen shieldings of the azole systems as a whole. (C) 1 998 Academic Press.