A DOSY spectroscopy study of proton exchange processes involving inner N-H protons in natural and synthetic porphyrins

Abstract

DOSY spectroscopy was used to assess the involvement of inner N-H protons of porphyrinic compounds in exchange processes. The assessment was made by comparison of increase of self-diffusion coefficients of N-H and C-H protons. The difference in the measured coefficients can serve as a quantitative measure of hydrogen exchange intensity and thus of N-H acidity. Using DOSY allowed to demonstrate that regardless of the strongly different chemical shifts, the N-H bonds in porphyrins and chlorins have similar acidity and the magnetically non-equivalent inner N-H protons in chlorins have a different degree of involvement in exchange processes.