Instability of the molecular structure of monobenzoporphin to the alternation of the macrocycle bond lengths and its manifestation in the electronic spectra

Quantum-chemical calculations of the geometric structure of the molecules of monobenzoporphin (H₂ MBP) and monobenzoporphin with methyl and ethyl substituents in the five-member rings (H₂MBPm) have been carried out by the restricted and unrestricted Hartree-Fock methods with the AM1 Hamiltonian (AM1 RHF and AM1 UHF methods). The calculation of the above-indicated molecules by the AM1 RHF method without restrictions on their symmetry has given, for them, a planar structure with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and the symmetry C_1h for their aromatic part. The calculation of the transitions to the excited electron Q states in such a structure by the CNDO/S method has shown that these states are characterized by large hypsochromic shifts (~3000–4000 cm^–1 ) relative to the Q levels of porphin (H₂P), which is in contradiction with the experimental data, according to which these shifts are bathochromic and comprise = –330 cm ^–1 and = –750 cm^–1. Optimization of the geometry of the H₂ MBP and H₂MBPm molecules by the AM1 UHF method gives, for them, a structure with equal lengths of the bonds along the 18-member azacyclopolyene with a symmetry differing insignificantly from the D_2h symmetry; elements of the structure with a lower symmetry and an alternation of the lengths of the bonds are retained in the condensed pyrrolenine and benzene rings. The calculation of the shifts of the Q levels in the H₂MBPm molecule of this geometry relative to the analogous levels in H₂P has shown that they are bathochromic and equal to = –520 cm^–1, and the RHF calculation with optimization of the geometry of the molecule and restrictions on the effective symmetry D_2h of the 18-member azacyclopolyene has given = –350 cm^–1 and = –430 cm^–1. The restrictions imposed on the C₂ symmetry of the H₂MBP molecules by the RHF method are inadequate to equalize the lengths of the bonds along the 18-member azacyclopolyene. The calculations of the energy of the B levels of the monobenzoporphyrins considered also lend credence to their geometric structure with equal lengths of the bonds along the 18-member azacyclopolyene.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 712–721, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.