Theoretical Study of the Switching between Hückel and Möbius Topologies for Expanded Porphyrins

The expanded porphyrins have become a useful tool to
synthesize
new Hückel-to-Möbius topological switches. Only applying
small changes in the external conditions (temperature, solvent, redox
potential) or in the structure of the ring allows the expanded porphyrins
to achieve Hückel and Möbius topologies with distinct
aromaticities and magnetic and electric properties. In this work,
we report a theoretical investigation of the reaction mechanism of
the conformational switch between the Hückel planar and the
singly twisted Möbius structures for two different meso-substituted
[28]­hexaphyrins(1.1.1.1.1.1); that is, the meso-substituents are the
hydrogen atoms and pentafluorophenyls. Calculations are performed
with the B3LYP, BH&HLYP, CAM-B3LYP, M05-2X, and MP2 methodologies
along with the 6-31G and 6-311G­(d,p) basis sets. For benchmarking
purposes, single-point energies are calculated at the CCSD­(T)/6-31G
level. Our results conclude that, in overall, CAM-B3LYP and M05-2X
are the methods that provide the most equilibrated results for the
different steps of the topological switch. In addition, we point out
that the ring strain and the steric effect of the meso-substituents
play a relevant role in the thermochemistry and kinetics of the Hückel-to-Möbius
topological switches.