Porphyrins density functional theory

Huang, Y., Zhong, A., Rong, C., Xiao, X., and Liu, S. 2008. Structure, spectroscopy, and reactivity properties of porphyrin pincers A conceptual density functional theory and time-dependent density functional theory study. J. Phys. Chem. A 112 305-311. 

Oxoiron(V) porphyrins—red in color—an isoelectronic form of oxoiron(IV) porphyrin cation radicals—green—have been proposed (references 55a-c), although the density functional theory calculations have indicated that there are no true oxoiron(V) porphyrins (references 55d-f). Further spectroscopic studies are necessary to confirm the existence of this intermediate. 

The geometries of Mn(3-TRPyP) and Mn(4-TRPyP) species already investigated by density function theory (DPT) and molecular mechanics calculations (270, 271). They are consistent with a planar configuration for the porphyrin ring in both systems. However, while the peripheral ruthenium ions are in the same plane in the Mn(4-TRPyP) species, they are located above and below the porphyrin plane in the Mn(3-TRPyP) complex (Fig. 27), so that the stereochemical positioning of these groups provides local picket fences, or pockets, capable of influencing the accommodation of the substrates near the catalytic site. 

Figure 17 The electronic structure of the P450 Compound I intermediate from density function theory (DFT) calculations. The two unpaired electrons on the Fe(lV) center are in the 3d it orbitals while the other unpaired electron may be in the porphyrin ei2u Jt orbital (a) or sulfur per orbital (b). The solid arrows in the 2u or per orbital show the electron spin parallel to those in the 3d n orbital (quartet state) while the dotted arrows indicate the doublet state...

Numerous investigations have used the Raman-active Fe-His vibration to probe heme protein interactions and protein structural dynamics for five-coordinate heme proteins. Identification of a similar vibrational probe for six-coordinate heme proteins wonld be valnable. Comparison with results on iron porphyrins snggests that the feature near 130 cm in the MbNO data (Figure 6) is a potential candidate for probing the Fe-His bond. We note the absence of this feature in the five-coordinate [Fe(PPIXDME)(NO)]. Further studies, including density functional theory (DFT) calculations, will clarify the character of this mode. However, the relatively small Fe amplitude with respect to the Fe His mode in deoxyMb indicates an altered mode character. 

Key Words Density functional theory, DFT calculations. Porphyrin, Alkene, Styrene, Epoxidation, Reaction pathway. 2008 Elsevier B.v. 

There is also debate about the nature of the intermediates involved in the second step in the catalytic cycle illustrated in Fig. 19.1, that is, the transfer of oxygen to the alkene to form the epoxide. No intermediates have been detected experimentally, but five different possibilities have been proposed in the literature for the alkene complexed to the oxidized porphyrin [11,25-29]. The five proposed intermediates are radical, cation, concerted, metallaoxetane, and pi-radical-cation species. The literature is rather complicated due to the lack of direct experimental observation, and it is not clear that conclusions from, say, iron and chromium porphyrins also apply to manganese porphyrins [28]. Arasasingham et al. claim unequivocal evidence for a radical intermediate being involved in the oxidation of alkenes by manganese porphyrins [28]. They also discuss a charge-transfer complex that is similar to the concerted intermediate. Recently, density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations were applied to styrene epoxidation by Mn-porphyrins ... 

Further, it was shown that porphyrin molecules can be anchored inside the pores of VPI-5, once the water molecules are eliminated. Quantum chemical calculations based on density functional theory was used to calculate the eletrophilic and nucleophilic centers in both porphyrin and VPI-5[6]. By comparing these electronic properties and by an empirical fitting based on molecular graphics, it was observed that the orientation of the molecule is inclined at 45° as shown in Fig. 3. 

Johannson et aO have carried out Density Functional Theory (DFT) calculations of spin density on low-spin (S = 1/2) ferric porphyrins, posing the question... 

Patchkovskii S, Ziegler T (2000) Structural origin of two paramagnetic species in six-coordinated nitrosorron (II) porphyrins revealed by density functional theory analysis of the G tensors. Inorg Chem 39 5354—5364... 

Finally, the recent major evolutions in quantum chemistry (especially the development of DFT [density functional theory]) have made it feasible to compute the EPR parameters of low-spin heme systems, as was demonstrated for simple ferric porphyrin systems [46,47], However, state-of-the-art quantum chemistry is still a long way from an identical match between experimental and computed EPR parameters, and the further development of these quantum-chemical methods form undoubtedly one of the biggest challenges for the next years. 

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