A magnetic resonance study of metal-porphyrin/caffeine complexes

Abstract

The effects are described of MnIII, FeIII, CoII, NiII, CuII and AgII porphyrins on the chemical shifts and linewidths of the proton resonances of caffeine, and of caffeine on the electron paramagnetic resonance spectra of CoII, CuII and AgII porphyrins in chloroform solution. The largest n. m. r. shifts are produced by divalent ions with the largest g tensor anisotropy, and trivalent ions with the largest zero-field splittings; metal porphyrins with the largest effective magnetic moments produce the most broadening of the resonances. The n. m. r. and e. p. r. effects are interpreted as arising from molecular complex formation between caffeine and the metal porphyrins. An attempt is made to relate the observed dipolar n. m. r. shifts and broadenings for CoII porphyrin-caffeine to the structure of the complex in solution. Acceptable structures were of the plane-to-plane type, with the metal situated above N9 of the five membered ring.