Molecular orbital porphyrin ring

Table 6. Calculated n molecular orbital (MO) energies (eV) and symmetries for the fully conjugated corrole and porphyrin rings. Both macrocycles have 26 a-electrons. Their net charges are — 3 and — 2, respectively. According to Ref. [36]...

Most molecular orbital calculations on metalloporphyrins have focussed attention on the porphyrin ji-orbitals, while the metal is regarded simply as a positive charge at the centre of the porphyrin ring. However, Zemer et al. (93) have recently performed calculations based on the Wolfsberg-Hdmholtz model (94) in which the metal orbitals are explicitly included in the basis set. The results of such calculations must be treated with caution (95) but we shall occasionally appeal to this paper where it is necessary to estimate a quantity such as an orbital or transition energy, and where no better evidence is available. An interesting result of this calculation was that the displacement of the iron atom out of the haem plane was an essential condition for a high-spin compound. 

Instead of the usual valence bonds, the metal atoms in chlorophylls and cytochromes should be presented in terms of the six coordinate bonds described by ligand-field (molecular-orbital) theory. Fe has one coordinate bond to each of the four N s in the porphyrin ring (the two solid lines and the two broken lines emanating from Fe in Fig. 5-16 also see Fig. 5-2), one to... 

The above mentioned enzyme systems all contain at the catalytic site transition metal ions specifically coordinated by groups of the apoprotein either directly or indirectly via a porphyrin ring. For the activity the interaction between the c -orbitals of the transition metal ion and the tt-molecular orbitals of the substrates seems to play a central role. 

D4h symmetry, the interaction involving the dz2(Fe) orbital does not very much affect a t-state of the porphyrin. By contrast, the jr-interaction causes a change in the extent of the delocalization of n electrons in the porphyrin plane, because the d (Fe) orbitals belong to the Cg species as well as the LUMO of porphyrin, n (ring). Since they have the same symmetry, there would be a jt-type molecular orbital represented by Eq. 6. 

Figure 3 Molecular orbitals (a) and molecular states (b) obtained by MO calculation on the D4A porphyrin ring [5],...

In addition to the central atom, substituents in the porphyrin ring also affect the absorption spectrum. The number of bromine atoms in the pyrrole position in CoTPP was correlated with the red shift of the Soret band [9], while an effect of distortion of the porphyrin ring should be considered [10] (Fig. 5). As anticipated from the molecular orbital coefficient (see Fig. 3) the aiu orbital would be more dependent on the substitution on the pyrrole position than the a2u orbital. A strong electron-donating substituent was reported to induce an inversion from a2u > aiu to a2 < ai , i.e., the ai of 2-aminoporphyrin was reported to be more destabilized by 0.71 eV than that of 2-nitroporphyrin, while a2 suffers destabili-... 

Most of the model complexes for compound I are the derivatives of Fe(TPP) due to their stability. The HOMO s (highest occupied molecular orbital) of the porphyrin rings of these iron porphyrins are known to be A2u [73-75]. While Aiu and A2u characters of catalases and peroxidases are still controversial [53, 54, 76-82], as shown in Fig. 5, orbital pattern for the A2u and Aiu orbitals is very different [59, 83], thus, the reactivities of these two species are expected to be different. Especially that the pyrrole nitrogens of compound I having Aiu as HOMO is node indicates less interaction between the spin on the porphyrin ring and central oxo-ferryl orbitals. These considerations lead to the prediction that compound I whose HOMO is A2u is much reactive than Aiu lyP species. 

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