Values for Hammett pyridine 3-aza and 4-aza replacement constants were obtained by correlation of the amide proton chemical shifts (in DMSO) of some N-(3-pyridyl)-and N-(4-pyridyl)-dichloronicotinamides, benzamides and acetamides with those of their N-(substituted phenyl) amide analogs using standard Hammett a values
Setliff, Frank L.; Soman, Nikhil G.; and Toland, Alan D.
"Determination of Hammett Pyridine 3-Aza and 4-Aza Replacement Constants by 1H NMR Studies of Amide Systems,"
Journal of the Arkansas Academy of Science: Vol. 47
, Article 27.
Available at: http://scholarworks.uark.edu/jaas/vol47/iss1/27