Molecular Orbital Calculations on Porphine and Tetrahydroporphine

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.