A THEORETICAL INVESTIGATION OF THE STRUCTURE AND ELECTRONIC-SPECTRA OF PORPHYRIN HOMOLOG MACROCYCLES - HEXAAZACYCLOPHANE AND ITS NICKEL ANDCOPPER-COMPLEXES

A quantum chemical study of the geometry, electronic structure, and el ectronic spectra of hexaazacyclophane and its Ni and Cu complexes, of major interest in the electrochemical reduction of CO2, has been carri ed out res, using the INDO rbklhda, in its INDO/1 and INDO/S parametri zations. The electronic spectra were calculated using a CI-S, generati ng the configurations by means of a Rumer diagram technique. The calcu lations confirm the singlet ground state for the ligand and Ni complex , as well as the doublet ground state for the Cu one. They are planar structures of D-2h Symmetry Although the structures are considered por phyrin homologs, the calculated spectra cannot be described in terms o f Q and B (Soret) bands as in porphyrins, and no band in the visible d evelops for the base. Low-energy transitions are associated with d --> d excitations (around 450 nn) in the Ni complex and with metal --> li gand charge transfer from the HOMO d(xy) (in the 900-450 nm region) in the Cu complex, characterized by an open shell structure.