Effects of protonation and hydrogen bonding on carbon-13 chemical shifts of compounds containing the group

The effects of hydrogen bonding and protonation produced by changing solvent from chloroform to 2,2,2-trifluoroethanol to trifluoroacetic acid were determined for the 13C chemical shifts of fourteen imines, four oximes, and two pyridines. Downfield shifts relative to chloroform were observed for the C=N carbon of the imines and oximes in both trifluoroethanol and trifluoroacetic acid, ranging from 1.5 to 27.5 ppm, which contrasts with upfield Cα shifts for the pyridines in trifluoroacetic acid. Substantial downfield and upfield shifts of from +10 to −15 ppm were found for the para and ipso carbons, respectively, of the aromatic rings attached to the C=N group in trifluoroacetic acid. Possible explanations of these shift changes are considered as well as their relation to the corresponding changes in 15N shifts with the identical variations in solvent.