Knowledge Bank

The theory of spectroscopic moments has been applied to the prediction of intensity changes in the visible bands of porphyrins, chlorins, and bacteriochlorins under different substitutions. Bands I and III (numbering from the red end) are shown to be probably 0-0 bands of two different forbidden electronic transitions (like 'A-'Bzu and 'A-'Blu of benzene). From their intensity changes under 1256-tetraalkyl and 12345678-octaalkyl substitution, we infer that 1. Both transitions are even-odd; 2. Their polarizations are mutually perpendicular; 3. The molecule has $D2h$ symmetry, with an axis through rings I and III ($D4h$ in strong acids---the two transitions become degenerate); 4. The central hydrogens are on opposed nitrogens, with less than 6 percent of the molecules at room temperature having them on adjacent nitrogens; 5. The alkyl spectroscopic moments for both transitions are almost the same. 6. And with the help of a tentative theory about the directions of the moment vectors, we also infer that band I, the longest-wavelength band, is probably polarized perpendicular to the line joining the central hydrogens. From the first 84 compounds studied by Stern, Wenderlein, and Molvig, we can determine the moment vector directions at every position of substitution for bands I and III, and the moment values for 13 substituents (including dihydro addition treated as a substitution). Moments are proportional to the benzene moments determined earlier, but are larger, possibly because of the greater polarizability of the porphin ring. Most intensities can be predicted from the vector sum of the moments within about 20 percent; but there are several unexplained discrepancies.