Abstract
The role of conjugative interaction in stable carbenes of the 1,2,4-triazol-5-ylidene type was studied at different levels of ab initio theory employing model compounds. In the case of parent 4,5-dihydro-1 H-1,2,4-triazol-5-ylidene this type of interaction was found to be of similar importance as in 1,2,4-triazole and pyridine.
Five stationary points were located on the hypersurface of the dimer (C2N3H3)2 . ΔE for all considered dimerization reactions was found to be negative, and the change of energy associated with the formation of the most stable isomer is about -9.5 kcal/mol. In spite of a high value for the singlet-triplet gap of about 90 kcal/mol for the carbene, the heavy atom skeleton of the most stable dimer is close to planarity. This finding is in perfect accordance with the predictions of other authors regarding the geometry of dimers from singlet carbenes stabilized by π-conjugation.
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