Effects of Protonation and Hydrogen Bonding on Nitrogen-15 Chemical Shifts of Compounds Containing the >C:N-Group

The effects of solvent changes from chloroform to trifluoroethanol to trifluoroacetic acid on the chemical shifts of nitrogen-15 resonances have been determined for fourteen imines, four oximes, and two pyridines. Upfield shifts were observed for all of the compounds in trifluoroethanol and trifluoroacetic acid, ranging from 8 to 28 ppm in the first and from 110 to 150 ppm in the second. These shift changes can be attributed to hydrogen bonding and protonation, respectively. The hydrogen-bonding shifts can be generally rationalized through consideration of the basicities of the nitrogens involved while the protonation shifts seem mostly influenced by the degree of substitution by phenyl groups, as expected from changes in the substantial contributions to the >C=N- type nitrogen shifts from the second-order paramagnetic effect