Porphyrins degeneracy lifting

Porphyrins, 21 14, 36, 135 -based manganese complexes, 46 400-402 as cobalt complex ligants, 44 284-290 compared to phthalocyanines, 7 75 complexes, 19 144, 145, 147 complex stability, 42 135-137 degeneracy lifting, 36 206 metalloporphyrins, DNA cleavage and, 45 271-283... 

Scheme 4.2 Frontier orbitals of the porphyrins according to the Gouterman four-orbital model for D4h point group (top). In the presence of a perturbation (substituents or ligand distortion) the degeneracy might be lifted (bottom). With permission from Wiley, ref 7a.

The thermal tautomerism of the unsubstituted solid parent compound porphyrin was discovered by Wehrle et al. [18c[. Again, the degeneracy of the tautomerism was not lifted. The HH, HD, DD rate constants in the liquid and the solid state were determined by Braun et al. [18d[ leading to the Arrhenius diagram of Fig. 6.23(b). Again, no kinetic liquid-solid state effects could be observed. The motivation of these studies was to elucidate the influence of substituents on the tautomerism and to facilitate the comparison with theoretical studies which are generally performed on the non-substituted parent compound. In fact, although the observed isotopic pattern is similar to that of TPP, it is found that the reaction... 

In other words, the Q bands are forbidden and simply shouldn t exist. The fact that they do is because of molecular vibrations within the porphyrin macrocycle. These have the effect of marginally lifting the degeneracy of bj and b2 so that the difference in intensities represented by Qy and is no longer equal to zero, i.e. the Q bands become weakly allowed. Their weakness allows them to show vibrational fine structure. In the case of a porphyrin dication or metal complex, where the x and y directions for the components of the transition dipoles are equivalent, this gives rise to two Q bands. For porphyrin free bases, where the x and y directions are perpendicular to each other, each component has two associated Q bands, so the total number is four. 

Fig. 3.33 Lifting of degeneracy of porphyrin HOMOs on (M) metal chelation and (D) dication formation bi, b2, Cj, and C2 represent the MOs in Figure 3.30. On metal chelation (M), b, c, and C2 are raised in energy. On dication formation (D), h , Ci, and C2 are lowered in energy.

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