An Analysis of the Isomerization
Energies of 1,2-/1,3-Diazacyclobutadiene,
Pyrazole/Imidazole, and Pyridazine/Pyrimidine with the Turn-Upside-Down
Approach
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Abstract
The isomerization energies of 1,2- and 1,3-diazacyclobutadiene,
pyrazole and imidazole, and pyridazine and pyrimidine are 10.6, 9.4,
and 20.9 kcal/mol, respectively, at the BP86/TZ2P level of theory.
These energies are analyzed using a Morokuma-like energy decomposition
analysis in conjunction with what we have called turn-upside-down
approach. Our results indicate that, in the three cases, the higher
stability of the 1,3-isomers is not due to lower Pauli repulsions
but because of the more favorable 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 the 1,2-isomers