EFFECTS OF ACID ON LINE WIDTHS IN THE PROTON NMR-SPECTRA OF PORPHYRINS NOT SUBSTITUTED AT THE MESO-POSITIONS

It was shown in earlier work that the proton NMR spectra of some meso- tetraarylporphyrins in deuterochloroform-trifluoroacetic acid solution s show little dependence of signal line widths upon acid concentration , but chemical shifts (particularly of the N-H signal) are strongly ac id dependent. In contrast, the corresponding NMR spectra of meso-unsub stituted coproporphyrin I tetramethyl ester in deuterochloroform solut ion show weak signals assigned to porphyrin N-H and to O-H (water impu rity 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 bro ad line when the TFA:COPH2 ratio exceeds 2. It gradually narrows and m oves upfield as the TFA concentration increases further. Within the sa me range of TFA:COPH2 concentration ratios, the narrow H(meso) signal also broadens to about 35 Hz and then narrows again while it moves sig nificantly 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 structu ral classes of porphyrins also display characteristic differences in t he optical spectra of their diprotonated dications: COPH4++ and the di cations of other porphyrins unsubstituted on the four meso-positions g ive red-purple solutions with band intensities II, I for the two bands in the visible region. Dications of meso-tetraarylporphyrins give gre en solutions with relative band intensities I < II, as pointed out ove r 25 years ago by Fleischer. The catalytic acid species may be the muc h discussed monoprotonated monocation COPH3+, but the spectra reported here give no information on its structure or identity.