Ab initio and semiempirical (PM3) calculations were carried out on zinc porphyrins complexed to 1,4-diazabicyclo[2.2.2]octane (DABCO) and 1-azabicyclo[2.2.2]octane (quinuclidine). The experimental H-1 NMR spectra of Zn-porphyrin-DABCO 1:1 and 2:1 complexes show an upfield shift for the DABCO methylene signal compared to the free ligand (as) of -5.90 and -8.00 ppm, respectively, and the Zn-porphyrin-quinuclidine complex shows a Delta delta value of -6.34 ppm for the methylene signal alpha to the nitrogen atom. These shifts are caused by the proximity of the porphyrin pi system. The geometries were optimized at HF/3-21G and PM3 levels of theory and selected parameters were compared with X-ray data of 19 Zn-porphyrin-ligand 1:1 complexes taken from the Cambridge Structural Database. The proton chemical shifts of these complexes were calculated by ab initio/GIAO-SCF/6-311G*//HF/3-21G level of theory and the values obtained agree satisfactorily with experimental results. (C) 2000 Elsevier Science B.V. All rights reserved.