Spectra of porphyrins

A model is presented for the excited states of porphin, which are viewed as arising from two configurations which can be mixed to varying degrees. The effects of substituents are then discussed, first the limitations imposed by symmetry requirements and then the requirements imposed if they act as one electron perturbations. The theory is applied to certain experimental problems. First it is shown how the metal porphins can be viewed as a series showing increasing mixing of the two fundamental configurations. Next the effect of free base formation is explained as a cyclic polyene perturbation. This theory predicts that the lowest porphin transition is polarized parallel to the H–H axis. Formulas are derived for intensity changes caused by various external substituents; their effect on transition energies are also discussed. Finally formulas are developed to account for the effects of aza substitution on the methine bridges.

Study of the Effects of Substitution on the Absorption Spectra of Porphin

Spectra of porphyrins: Part II. Four orbital model

1 – Optical Spectra and Electronic Structure of Porphyrins and Related Rings

The ESR spectra of vanadyl acetylacetonate and tetraphenylporphyrin have been studied in various liquid solvents and frozen glasses. The spectra have been analyzed and the components of the g tensor and of the vanadium hyperfine interaction tensors have been determined, and for the porphyrin the nitrogen extrahyperfine interaction tensor has been obtained. The unpaired electron is in the b2g* orbital. The extrahyperfine splitting is very isotropic which indicates that the b2g* orbital is localized on the vanadium in the porphyrin complex and the ``in‐plane π bonding'' is slight. An explanation of the origin of this extrahyperfine splitting is discussed in terms of configuration interaction. An analysis of the vanadium hyperfine interaction indicates that the b2g* orbital is localized on the vanadium in the acetylacetonate also, and that the Fermi contact interaction depends upon a number of excited configurations and varies from complex to complex and with change of solvent. The g‐tensor analysis suggests...

ESR Studies and the Electronic Structure of Vanadyl Ion Complexes

The first three electronic transitions of porphine and the first five electronic transitions of tetrahydroporphine are predicted as simple one‐electron LCAO molecular orbital transitions (with overlap included). The electron densities and bond orders in the ground states are also computed. The observed levels are tentatively assigned to the predicted levels with the help of comparisons with spectra of other aza‐amine derivatives of conjugated hydrocarbons. Probable degeneracies and polarizations are determined. The blue shift and loss of intensity of the first electronic transition, in going from tetrahydroporphine to the larger conjugated system of porphine, is typical of the change from a ``long‐field'' to a ``round‐field'' molecule (see previous paper) and is shown to occur in several other ``long‐field'' systems when additional conjugated atoms are added at the side of the system.

C. W. Rector

Papers

Molecular Orbital Calculations on Porphine and Tetrahydroporphine