Aromatic character of heptafulvene and its complexes with halogen atoms.

Geometry optimization of heptafulvene-halogen complexes (halogens: F, Cl, Br, I, and At) carried out at the B3LYP/6-311+G(d,p) level of theory allowed us to estimate the geometry-based aromaticity index HOMA, the magnetism-based indices NICS, NICS(1), and NICS(1)(ZZ), as well as the energy of complex formation. Application of the NBO method allowed us to estimate the pEDA characteristics of the π-electron distribution in complexes (i.e., the electron excess/deficiency of the π-electron system in the ring). All of the characteristics of the complexes were found to be mutually interrelated, exhibiting good or at least acceptable correlation coefficients. It was also noted that halogen atoms with greater radii yielded weaker complexes and lower aromaticities, as shown by the HOMA, NICS, and pEDA indices. The energy of complex formation was observed to be linearly correlated with the degree of aromatization of the heptafulvene ring, as expressed by these indices.