Substituent conformational effects in the magnetic circular dichroism and absorption spectra of free-base carbonyl porphyrins

The authors 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/sup 0/ for the acetyl moiety of the tert-butyl-, isopropyl-, and ethyl-substituted porphyrins, respectively, are deduced from variations in the observed MCD of the Q/sub 0//sup x/ band. INDO calculations of B terms for acetylporphyrin free base are found to reproduce the angle dependence observed for the MCD.