1. Stability pyridazine > pyrazine > pyrimidine
They are strongly influenced by the NN bond weakness arising from genimal N lone pair-NN
σ bond repulsion.1
No substituents on the ring. B3LYP/6-311++G**
2. “ Mean values obtained from G4 and W1 calculations are considered as the recommended
best ‘ theoretical ‘ values for the diazines studied, G3 and CBS-APNO are also accurate
methods, among these four methods G3 is the cheapest.2
3. “ Pyrimidine is 20.0 kcal/mol more stable than pyridazine at the BP86/TZ2P level”. 1,3-forms
are more stable because the more involved in the favorable electrostatics and σ-orbital
interactions involved in the formation of two C-N bonds in comparison with the generation of
C-C and N-N bonds in 1,2-isomers. The configuration with C-C,N-N, and two C=N bonds is
5.2 kcal/mol more stable than the arrangement with C=C,N=N, and two C-N bonds.3
Only pyrimidine and pyridazine, no substituents.
4. Aromaticity order pyrimidine > pyrazine > pyridazine, according to NICS and magnetic
susceptibility.4
B3LYP/6-311+G(d, p)
62c : pyrazine, 62b : pyrimidine, 62a : pyridazine
5. “ΣΔEV-πp arising from π-delocalization is destabilizing, playing important role in controlling
the delocalization ofπ-delocalization. Theπ-system of an electron-releasing group ids always
strongly destabilized due to theπ-delocalization. The space interaction between the π- and
σ-systems has a great effect on the molecular behaviors. Aromaticity has comprehended the
thermodynamic, kinetic, and magnetic behaviors of aromatic compounds. Now it should
include not only the effect of the π-delocalization on theπ-system itself but also its influences
on the σ-framework and various σ-bonds.” Energy are calculated at STO-3G level.5
6. Structural non-rigidity in the ring is promoted by an increase on the number of nitrogen
atoms within the ring.6 MP2/6-31G(d, p) level
7. Neural network is used to quantify aromaticity of one substituted pyrimidine, all substituents
decrease the aromaticity of the ring. “electronic properties, such as the HOMO and LUMO
gaps, are strongly affected by the substituents, which is important in the non-linear optical
applications of these derivatives. The presence of only one substituent attached to the
pyrimidine ring influences strongly the aromaticity of the molecule. The poor correlations
between the different aromaticity indices (HOMA, Λ, NICS, NICS(1), NICS(1)zz and ASE)
indicate that the use of a single aromaticity descriptor should be avoided.” The author
consider Self Organized maps methodology can be a useful tool to identify the vest
aromaticity criteria, not only the three types used herein but also of other kinds. 7
B3LYP/6-311+G(d, p)
g03 package
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