A STUDY OF NITROGEN-15 NUCLEAR MAGNETIC RESONANCE SHIFTS IN LIQUID 15NH3-SOLVENT MIXTURES

In nmr studies of hydrogen bonding, relatively little has been done to determine the influence of hydrogen bonding on the nmr shift of the atom which donates the electron pair in forming the hydrogen bond. As an approach to this problem, we have chosen to investigate the 15N nmr shift for the 15NH3 molecule in various environments. The 15N nmr shift relative to gaseous 15NH3 was measured as a function of concentration for 15NH3 in 13 solvents. Utilizing high-resolution techniques, the 15N shifts could be measured to ±0.2 ppmat 15NH3 concentrations as low as 2 mole %. Extrapolation to zero 15NH3 concentration yields a set of “infinite-dilution” shifts which are well interpreted on the basis of a simple model describing the interactions between the 15NH3 molecule and solvent molecules. The model assumes that the observed infinite-dilution shifts are the sum of contributions due to two general types of interactions: (1) the interaction of the ammonia nitrogen lone-pair electrons with solvent-molecule groups and (2) hydrogen bonding between solvent-molecule unshared electron pairs and 15NH3 protons. The results indicate that interactions involving the nitrogen lone-pair electrons make a rather large paramagnetic contribution to the 15N shift in 15NH3 while the hydrogen bonding of 15NH3 protons by solvent electron pairs makes a much smaller diamagnetic contribution. These results are discussed in terms of the Ramsey chemical-shift formulation, and possible implications with regard to electronic displacements produced by the various types of interactions are noted. © 1969, American Chemical Society. All rights reserved.