CARBON-13 MAGNETIC RESONANCE .X. 6-MEMBERED NITROGEN HETEROCYCLES AND THEIR CATIONS

Carbon-13 chemical shift data on the azines and their protonated cations are presented. Simple additivity parameters were found to account for the effect of nitrogen protonation on the chemical shifts of α-,β-, and γ-carbon atoms. These parameters are highly reproducible with monoprotonation but deviate from additivity quite significantly for diprotonated species. A theoretical analysis of the chemical shift expression indicated that carbon-13 shifts are critically dependent on both charge-transfer features and a variation in the bond-order parameter. The upfield protonation parameter for the α-carbon is explained with a decrease in bonding between N-Cα, while the β and γ parameters arise from charge polarization effects. Extended HMO wave functions were used to estimate the carbon-13 shifts. It was found that a protonated nitrogen must be made more electronegative than corresponding basic nitrogens if reasonable agreement between predicted and experimental results are to be obtained in all of the essential features. © 1968, American Chemical Society. All rights reserved.