Investigation of structure, stability and racemization of triazacycloheptatetraene isomers: A theoretical approach

Regarding to important role of azacycloheptatetraenes and their relative carbenes in organic chemistry, structure and stability of ten triazacycloheptatetraenes and their corresponding carbenes as well as their racemization are studied at ab initio and DFT levels. The species of 7a is introduced as the most stable triazacycloheptatetraenes with the largest di-imine angle and the lowest twist angle. The species of 1a and 9a are found as the most instable allenes through lone pairs’ repulsion of their adjacent nitrogens. Their non-planar symmetry results to their optical activity. Racemization of triazacycloheptatetraenes performs through their singlet triazacycloheptatrienylidenes and depends on the topology of nitrogen atoms. Species with one of their nitrogen atoms in the 1 position of the ring, including 1,2,4-; 1,2,5-; 1,3,4-; 1,3,5; and 1,4,5-isomers, show racemization through their corresponding anti-aromatic singlet triazacycloheptatrienylidenes as TS. In contrast, aromatic singlet carbenes appear as TS for 2,3,4-; 2,3,5-triazacycloheptatetraenes. In addition species with a nitrogen atom in the 1 position of the ring specially 2a have smaller ΔE#, ΔE#, ΔH#, and ΔG#.