MAGNETIC CIRCULAR-DICHROISM STUDIES .68. SUBSTITUENT CONFORMATIONAL EFFECTS IN THE MAGNETIC CIRCULAR-DICHROISM AND ABSORPTION-SPECTRA OF FREE-BASE CARBONYL PORPHYRINS

Our previous MCD and absorption spectral studies of monosubstituted free-base alkylporphyrins found evidence in the sign and magnitude of visible-band MCD for out-of-plane conformers of .pi. substituents, as well as effects due to the electronic influence of alkyl groups and the equilibrium of central-proton tautomers. The present study examines the MCD and absorption spectra of a series of acetylheptaalkylporphyrin free bases with various alkyl groups adjacent to the acetyl, as well as a fused-ring ketocyclohexenoporphyrin. Out-of-plane rotation angles of approximately 85, 65, and 35.degree. for the acetyl moiety of the tert-butyl-, isopropyl-, and ethyl-substituted porphyrins, respectively, are deduced from variations in the observed MCD of the Q0x band. INDO calculations of B terms for acetylporphyrin free base are found to reproduce the angle dependence observed for the MCD.