Effects of Acid on Line Widths in the Proton NMR Spectra of Porphyrins Not Substituted at themeso-Positions

It was shown in earlier work that the proton NMR spectra of some meso-tetraarylporphyrins in deuterochloroform-trifluoroacetic acid solutions show little dependence of signal line widths upon acid concentration, but chemical shifts (particularly of the N-H signal) are strongly acid dependent. In contrast, the corresponding NMR spectra of meso-unsubstituted coproporphyrin I tetramethyl ester in deuterochloroform solution show weak signals assigned to porphyrin N-H and to O-H (water impurity in solvent). Addition of trifluoroacetic acid at mole ratios TFA:COPH2 as low as 0.05 produces simultaneous broadening of these signals, both of which become too broad to observe at normal instrument gain and room temperature when this mole ratio exceeds 0.3. Additional acid causes the N-H signal to narrow again and then reappear as a very broad line when the TFA:COPH2 ratio exceeds 2. It gradually narrows and moves upfield as the TFA concentration increases further. Within the same range of TFA:COPH2 concentration ratios, the narrow H(meso) signal also broadens to about 35 Hz and then narrows again while it moves significantly downfield. We propose that an acid species catalyzes proton exchange between COPH2 and COPH4++ in the intermediate rate region to produce these line width effects, which are also seen with other meso-unsubstituted porphyrins under the same conditions. These two structural classes of porphyrins also display characteristic differences in the optical spectra of their diprotonated dications: COPH4++ and the dications of other porphyrins unsubstituted on the four meso-positions give red-purple solutions with band intensities II, I for the two bands in the visible region. Dications of meso-tetraarylporphyrins give green solutions with relative band intensities I < II, as pointed out over 25 years ago by Fleischer. The catalytic acid species may be the much discussed monoprotonated monocation COPH3+, but the spectra reported here give no information on its structure or identity.