Pyrimidine electron densities, experimental

Even though at present various obstacles prevent a more extensive application of NMR spectroscopy to the study of 77-electron distribution, it seems that it will be possible to make use of the interrelation between the magnitude of the chemical shift of a proton attached at a certain position and the 77-electron density at that position. Among the studies so far published attention should be drawn to the papers dealing with pyridine,111-112 substituted pyridines,113 and pyrimidine and its derivatives.114 Table VII presents data (taken from ref. 112) on experimental and theoretical 77-electron densities in pyridine and the pyridinium cation. The NMR spectra of some azines are briefly discussed by White.114"... 

Triazine is a very electron-deficient system. The data from quantum-chemistry calculations, as well as experimental data on electron density distribution obtained by nuclear magnetic resonance (NMR) (see Section 9.02.3.3.2), indicate that 1,2,4-triazines are more electron-deficient compounds than pyridines, pyrimidines, pyra-zines, or pyridazines (Table 1, Figure 1) <1989AHC(46)73, 1996CHEC-II(6)507, 2001JHC901, 2003CHE616>. 

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