The results of the first coherent investigation of the absorption and magnetic circular dichroism spectra of a series of monosubstituted free-base porphyrins possessing vinyl, cyano, ethoxycarbonyl, acetyl, and formyl substituents are reported.Dipole strengths and B values are obtained by curve-fitting and moment analysis of the lowest energy Qox, Q1*, QoY, and Q I Y transitions. These values for the Qox and QJ transitions along with dipole strength and magnetic molar ellipticity data for the Soret transitions are used to compare the responses of the porphyrin and benzene T systems to the electronic demands of the substituents. Marked differences are found, and these are related to steric interactions and N-H proton tautomerism which are the structural features distinguishing the two ?r systems. Absorption spectral changes are treated within the context of Gouterman's four-orbital model, and the Occurrences of MCD band sign variation throughout the series are related to structural perturbations using Michl's perimeter model. Additional theoretical insight into, and the tracking of, the orbital energy shifts attending the interaction of conformational and tautomerism effects is gained from INDO/S calculations. Finally, since the structural effects are manifest only at relatively weak absorption and magnetic circular dichroism signal strengths, careful and explicit consideration is taken of the extent to which low-frequency vibrational bands within and near the Qox envelopes may obscure the elicitation of structurally significant information. (14) Wallace, S. L.; Castellan, A,; Muller, D.; Michl, J. J. Am. Chem. Soc. (15) (a) Michl, J. Stolzenberg, A. M.; Lu, Y . C.; Linder, R. E.; Barth, G.; Moscowitz, A.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. SOC. 1982,104, 4305. (b) Keegan, J. D.; Stolzenberg, A. M.; Lu, Y. C.; Linder, R. E.; Barth, G.; Moscowitz, A.; Bunnenberg, E.; Djerassi, C.
