Binding Affinities of Metalloporphyrins

Theoretical studies on the complexes formed by metalloporphyrins with heterocyclic compounds are performed using the semiempirical AMI method. These complexes are predicted to be formed because of their negative binding energies. The binding affinity of the Zn-porphyrin to imidazole is stronger than the other guest molecules. The interactions between the metalloporphyrin and heterocyclic molecules are ascribed to the coordinate bonding and hydrogen bonding. Based on the AM I optimized geometries, the electronic spectra and IR spectra of the complexes are calculated by the INDO/CIS and AMI methods, respectively. The first and main absorptions in the electronic spectra of the Zn-porphyrin complexes compared with those of the host are· blue-shifted due to the coordinate interaction. The stretching vibration frequencies of the C-H bonds in the IR spectra of the Zn-porphyrin complexes relative to those of the host are red-shifted owing to the formation of the hydrogen bonds.