Theoretical study of stability, structures, and aromaticity of multiply N-confused porphyrins

The total electronic energy and nucleus-independent chemical shift (NICS) of 95 isomers of N-confused porphyrin (NCP: normal porphyrin (N₀CP), singly N-confused porphyrin (N₁CP), doubly N-confused porphyrin (N₂CP), triply N-confused porphyrin (N₃CP), and fully N-confused porphyrin (N₄CP)) have been calculated by the density functional theory (DFT) method. The stability of NCP decreased by increasing the number of confused pyrrole rings. Namely, the relative energies of the most stable isomers in each confusion level increased in a stepwise manner approximately by +18 kcal/mol: 0 (N₀CP1), +17.147 (N₁CP2), +37.461 (N₂CPb3), +54.031 (N₃CPd6), and +65.636 kcal/mol (N₄CPc8). In this order, the mean plane deviation of these isomers increased from 0.000 to 0.123, 0.170, 0.215, and 0.251 Å, respectively. The unusual tautomeric forms of pyrrole ring with an sp³-carbon were found in the stable forms of N₃CP and N₄CP. The NICS values at the mean position of the 24 core atoms were nearly the same for the most aromatic isomers regardless of the confusion level: -15.1280 (N₀CP1), -13.8493 (N₁CP2), -13.7267 (N₂CPd1), -11.7723 (N₃CPb5), and -13.6224 ppm (N₄CPa6). The positive correlation between aromaticity and stability was inferred from the plots of NICS and the relative energy of NCP for N₀CP, N₁CP, and trans-N₂CP. On the other hand, the correlation was negative for cis-N₂CP, N₃CP, and N₄CP isomers.