[Babcock_Hall]

We have been making secondary carboxamide derivatives of isonicotinic acid (a pyridine ring with a carboxylic acid in the 4-position).  We assigned a peak at about 7.7 ppm to the hydrogens of the 3-position in the pyridine ring.  When I compared our assignment with N,N-diethylisonicotinamide as a model, I found that the hydrogens at the 3-position were found below 7.2 ppm.  After a modest amount of further searching, it appears that tertiary amides have signals roughly in the range 7.2-7.5 ppm (Takashi Kumagai and coworkers, Tetrahedron 66 (2010) 8968-8973), whereas secondary amides have signals roughly in the range of 7.6-7.7 ppm (http://onlinelibrary.wiley.com/store/10.1002/cmdc.200700107/asset/1527_ftp.pdf?v=1&t=hvpfyo62&s=e400c92a11518565b3aedcfbbf787ea1dfe78bab).  Does anyone have a good guess as to why there should be this difference between the chemical shift when the amide is secondary or tertiary.  My working hypothesis is that it has something to do with differences in conjugation between the carbonyl group and the pyridine ring.