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 isomeric tetrazoles (I, II) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-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 ppm. There is a clear distinction between the two types of nitrogen atoms in that the former exhibit a deshielding effect with increasing polarity of the medium while the latter experience an increase in the magnetic shielding of their nuclei. The latter effect is significantly augmented by solvent-to-solute hydrogen-bond formation where the pyridine-type nitrogens are involved directly. It is also quite diversified throughout the pyridine-type nitrogen atoms and seems to constitute a measure of relative basicity with respect to hydrogen-bond formation of the nitrogens concerned. This basicity seems to parallel that with respect to a full transfer of a proton, as can be reckoned from ab initio calculations of the relevant protonation energies reported in the present study. The experimental data for the tetrazoles in cyclohexane solutions are combined with those obtained in our earlier extensive studies on azole, diazole, and triazole ring systems, for a comparison with ab initio calculations of the nitrogen shieldings concerned. The latter were carried out using the coupled Hartree-Fock/GIAO/6-31++G** approach and geometry optimizations employing the same basis set; they show a good linear correlation with the experimental data and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the azole systems as a whole. Copyright 1998 Academic Press.
Witanowski
M
Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, 01-224, Poland.
Biedrzycka
Z
Sicinska
W
Grabowski
Z