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 (N(0)CP1), +17.147 (N(1)CP2), +37.461 (N(2)CPb3), +54.031 (N(3)CPd6), and +65.636 kcal/mol (N(4)CPc8). In this order, the mean plane deviation of these isomers increased from 0.000 to 0.123, 0.170, 0.215, and 0.251 Angstrom, respectively. The unusual tautomeric forms of pyrrole ring with an spa-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 (N(0)CP1), -13.8493 (N(1)CP2), -13.7267 (N(2)CPd1), -11.7723 (N(3)CPb5), and -13.6224 ppm (N(4)CPa6). 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.