An LCAO MO Study of the Structure of Chlorophyll Prototypes

Abstract

By means of Wheland's method of treating systems with hetero‐atoms three possible isomers of porphyrin and five of dihydroporphyrin are studied from the viewpoint of the energies of the ground states and the transition energies to the lowest lying excited states. In the case of porphyrin it is found that, within the limits of accuracy of the method used, two of the structures, PA (porphyrin‐adjacently bonded hydrogens) and PO (porphyrin‐oppositely bonded hydrogens), have identical ground state energies. In the case of dihydroporphyrin the DHP‐AI (DHP‐A—dihydroporphyrin‐adjacently bonded hydrogens) isomer is most stable and the DHP‐AII and DHP‐OII (DHP‐O—dihydroporphyrin‐oppositely bonded hydrogens) isomers are more energetic by 1.06 and 1.20 kcal/mole, respectively. With respect to the transition energies and the resulting spectra it is possible that the peaks in the spectrum of porphyrin are not due mainly to vibrational fine structure of a single electronic transition, as is proposed by Rabinowitch, but rather are the combination of the electronic spectra of the PA and PO isomers. The dihydroporphyrin spectral analysis, however, indicates that several of the peaks are probably vibrational fine structure of a single electronic transition. The calculated bond orders are given for the various structures discussed.