Ab initio calculations on Zinc Porphirins Complexec to Amines. Geometrical Details and NMR Chemical Shifts

Abstract

[eng] 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 1H NMR spectra of Zn-porphyrin±DABCO 1:1 and 2:1 complexes show an up®eld shift for the DABCO methylene signal compared to the free ligand (Dd) of 25.90 and 28.00 ppm, respectively, and the Zn-porphyrin±quinuclidine complex shows a Dd value of 26.34 ppm for the methylene signal alpha to the nitrogen atom. These shifts are caused by the proximity of the porphyrin p 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/6-311Gp//HF/3-21G level of theory and the values obtained agree satisfactorily with experimental results