S. Biernat

Solvent effects on nitrogen NMR shieldings in azines

Abstract The ranges of solvent effects on the nitrogen shielding are reported for pyridine, 1,2-1,3-, and 1,4-diazine, and 1,3,5-triazine. These ranges are both significant and variable from about 48 ppm for 1,2-diazine to about 1 I ppm for the triazine. The former is the largest solvent-induced range so far observed in nitrogen NMR. Contributions to the nitrogen shielding ranges from hydrogen bonding and solvent polarity effects are estimated. The former effect usually provides a dominant contribution and can be associated with the strength of the hydrogen bond formed from the solvent to the solute. The contribution to the change in nitrogen shielding from a change in solvent polarity is also significant but usually less than that due to hydrogen bonding. Both effects operate to produce changes in the nitrogen shielding in the same direction. The polarity effects are unusually large in the case of 1,2-diazine and this is corroborated by INDO/S-SOS shielding calculations using the solvaton model.

A NMR study of solute-solvent interactions as a function of the nitrogen shielding of pyridine N-oxide

Abstract The range of solvent effects on the nitrogen shielding of pyridine N-oxide is found to be comparable to that of pyridine, amounting to about 30 ppm. The largest contribution to this range arises from hydrogen bonding from the solvents to the oxygen atom of pyridine N-oxide. For the first time it is demonstrated that hydrogen bonding to a neighboring atom can have a comparable effect on nitrogen shielding as would direct hydrogen bonding to the nitrogen atom. Medium polarity effects on the nitrogen shielding of pyridine and its N-oxide are of the same sign and order of magnitude. This is corroborated by INDO/S-SOS shielding calculations using the solvaton model. These medium polarity effects amount to a nitrogen shielding variation not exceeding 6 ppm while those due to hydrogen bonding exceed 20 ppm.

Influence of some paramagnetic relaxation reagents on nitrogen nuclear shielding

Abstract 13 N NMR nuclear screening data are presented for a number of molecules containing commonly encountered functions, both in the absence and presence of Cr( acac ) 3 and Gd( dpm )3. The results are obtained by means of an improved differential saturation technique. Bulk susceptibility effects are removed by the use of concentric spherical sample and standard containers. Although Cr( acac ) 3 does not produce large nitrogen screening changes, at concentrations in excess of 10 −4 M , Gd( dpm )3 induces significant nitrogen screening increases. The induced screening changes are particularly large for basic nitrogen environments.

A quantitative 14N NMR study of the dimerization equilibria of 2-methyl-2-nitrosopropane

Abstract High-precision 14 N NMR data are reported for equilibria involving 2-methyl-2-nitrosopropane and its dimer, in a variety of solvents and at several concentrations. It is found that the dimer dissociation constant is dependent upon both solvent and concentration at 34.7 ± 0.2°C. The dissociation constant of the dimeric form decreases as the polarity of the solvent increases. This observation is reproduced by means of INDO/S calculations incorporating the solvaton model.