Theoretical insight into pyrrole inversion and planarity in 5,10,15,20-tetraphenylsapphyrin and 5,10,15,20-tetraphenyl-26,28-diheterosapphyrins with two O, S, or Se atoms.

Density functional calculations at the B3LYP/6-31+G(d) (LACVP(D) for Se) theory level have been carried out on 5,10,15,20-tetraphenylsapphyrin ( TPS), 5,10,15,20-tetraphenyl-26,28-dioxasapphyrin ( TP2OS), 5,10,15,20-tetraphenyl-26,28-dithiasapphyrin ( TP2SS), and 5,10,15,20-tetraphenyl-26,28-diselenasapphyrin ( TP2SeS). In agreement with experimental findings, our theoretical results show that TPS and TP2OS present an inverted conformation, whereas TP2SS and TP2SeS are more stable in the normal one. It was found that the relative stability of the normal and inverted conformers of the just mentioned sapphyrins correlates positevily with their degree of planarity and aromaticity, which depends on the size of the heteroatom, the steric repulsions produced by phenyl rings at the meso C atoms, and the network and nature of the bond critical points (BCPs) inside the macrocycle. These BCPs have been characterized by means of the AIM analysis and, some selected ones, by the changes in the total energy of significant fragments when distorted to avoid them.