Racemizations of diazacycloheptatetraenes through singlet diazacycloheptatrienylidenes at theoretical levels

The topology of nitrogen atoms is found to play an important role in the racemization of non-planar diazacycloheptatetraenes through their corresponding singlet diazacycloheptatrienylidene transition states, at B3LYP/6-311++G∗∗, MP2/6-31G∗, MP4(SDTQ)/6-31G∗//B3LYP/6-311++G∗∗, CCSD(T)/6-31G∗//B3LYP/6-311++G∗∗, and QCISD(T)/6-31G∗//B3LYP/6-311++G∗∗ levels of theory. Two types of singlet carbenes are encountered: σ2 and p2. The σ2 (-1A) carbenes are aromatic and include 2,3-; 2,4-; 2,5-, and 3,4-diazacycloheptatrienylidenes, while p2 (-1A′) carbenes turn out antiaromatic and include 1,6-; 1,2-; 1,3-; 1,4-; and 1,5-diazacycloheptatrienylidenes. All singlet carbenes showed a negative force constant and except for the 1,6-isomer the rest appear of higher energy than their corresponding triplet states. Nevertheless, all allenes emerged more stable than their corresponding singlet and triplet carbenes. Due to higher energy barriers, racemization is unlikely for allenic structures including 2,3-; 2,4-; 2,5-; and 3,4-diazacyclohepta-1,2,4,6-tetraenes. In contrast, carbene-like reactivity is expected from ketenimine and carbodiimide (1,2-; 1,3-; 1,4-; 1,5-; 1,6-diazacyclohepta-1,2,4,6-tetraenes) which go through N-heterocyclic transition states that are mostly of rather lower energy.