1,721,106 research outputs found
Determination of Potential Energy Surfaces for Ground and Excited States by Density Functional Methods
No full textMetal-CO Photodissociation in Transition-MetalComplexes: the Role of Ligand-Field and Charge-Transfer Excited States in the PhotochemicalDissociation of Metal-Ligand Bonds
No full textMolecular Distortions and Spectroscopy of Porphyrin Diacids: A Quantum Chemical Survey
No full textPlenary Lectur
Electronic Structure Analysis of Methane Hydroxylation by Electron-deficient oxoiron(IV) porphyrins
No full textIn heme iron enzymes, oxoiron(IV) porphyrin π-cation radical (Cpd I) and oxoiron(IV) porphyrin (Cpd II) species are proposed as reactive intermediates in dioxygen activation and oxygen-atom transfer reactions. Because of its biological significance, the reactivity of Cpd I has been widely investigated with in situ-generated oxoiron(IV) porphyrin π-cation radical complexes in various types of oxidation reactions. Dissimilar from Cpd I mimics, Cpd II mimics have been considered to be very poor oxidants and hence have received less attention. Several experimental evidences have now accumulated proving that oxoiron(IV) porphyrins are competent oxidants of a variety of substrates. The relationship between the nature of the porphyrin ring and the reactivity of Cpd II mimics is, however, still unclear. As a matter of fact, the experimental data reported for (TPFPP)Fe(IV)=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphyrinate) seem to indicate that electron-deficient porphyrin rings greatly enhance the reactivity of Cpd II mimics.1 On the other hand, a Cpd II mimic also bearing an electron-deficient porphyrin ligand, [(4-TMPyP)Fe(IV)=O]4+ (4-TMPyP = 5,10,15,20-tetrakis(N-methyl-4-pyridinium) porphyrinate), has been recently found to be almost unreactive toward C–H hydroxylation.2 With the aim to understand why synthetic Cpd II mimics have such a great variability in their reactivity, we have theoretically investigated by DFT (Density Functional Theory) methods the methane hydroxylation reaction by the electron-deficient Cpd II mimics above mentioned, (TPFPP)Fe(IV)=O and [(4-TMPyP)Fe(IV)=O]4+. The hydroxylation reaction has been studied on the ground triplet and excited quintet spin-state surfaces within the rebound mechanism scheme. On each spin surface both the σ- and π-channels have been explored. It is found that crossover from the ground state triplet surface to the highly reactive quintet state surface is a plausible scenario for C–H bond activation by these complexes. The efficiency of this TSR (two state reactivity) mechanism depends on the nature of the macrocycle, the presence and nature of an axial ligand and the solvent. A detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state reveals that, just as found in the unsubstituted PorFeIV=O Cpd II model,3 the electron transfer from the substrate σCH into the acceptor orbital of the catalyst, the Fe–O σ* or π*, occurs through a complex mechanism that is initiated by a two-orbital four-electron interaction between the σCH and the low-lying, oxygen-rich Fe–O σ-bonding and/or Fe–O π-bonding orbitals of the catalyst.
REFERENCES
1. Fukuzumi, S.; Kotani, H.; Lee, Y.-M.; Nam, W. J. Am. Chem. Soc. 2008, 130, 15134–15142.
2. Bell, S. R.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 9640–9641.
3. Rosa, A.; Ricciardi, G. Inorg. Chem. 2012, 51, 9833–9845
A Time-dependent Density Functional Theory (TDDFT) Interpretation of the Optical Spectra of Zinc Phthalocyanine π-cation and π-anion Radicals
No full textThe UV–visible and near-IR spectra of the zinc phthalocyanine π-cation and π-anion radicals, [ZnPc(–1)]•+ and [ZnPc(–3)]•–, are investigated by time-dependent density functional theory (TDDFT) calculations using the pure, asymptotically correct, statistical average of (model) orbital potentials (SAOP) functional. The nature and intensity of the main spectral features are highlighted and interpreted on the basis of the ground-state electronic structure of the complexes. Similarities and differences with previous TDDFT/B3LYP results are discussed. TDDFT/SAOP results for the π-anion radical prove to be in excellent agreement with the solution spectra and generally in line with deconvolution analyses of solution absorption and magnetic circular dichroism (MCD) spectra. On the basis of these results a novel interpretation of the Q-band system is proposed. For the π-cation radical TDDFT/SAOP calculations provide a satisfactory description of the UV region of the spectrum. However, they do not reproduce accurately the energy and intensity of the Q band observed at 825 nm. The description of the Q-band region appears to be complicated by the presence of spurious non-Gouterman transitions. Furthermore, the calculations, either in the gas phase or in solution, do not account for the broad absorption near 500 nm that has been suggested to arise from a nondegenerate, z-polarized 2A2g excited state. Theory and experiment can be reconciled if the presence of an axial ligand such as CN– is explicitly considered in the calculations. TDDFT/SAOP results for the axially ligated [ZnPc(–1)(CN)]• species indicate that the 500 nm feature is related to the axial ligation induced symmetry lowering of the π-cation radical and this band is assigned to a z-polarized transition associated with the hole in the 2a1u
Reactivity of Compound II: Electronic Structure Analysis of Methane Hydroxylation by Oxoiron(IV) Porphyrin Complexes
No full textThe methane hydroxylation reaction by a Compound II (Cpd II) mimic PorFeIV=O and its hydrosulfide-ligated derivative [Por(SH)FeIV=O]− is investigated by density functional theory (DFT) calculations on the ground triplet and excited quintet spin-state surfaces. On each spin surface both the σ- and π-channels are explored. H-abstraction is invariably the rate-determining step. In the case of PorFeIV=O the H-abstraction reaction can proceed either through the classic π-channel or through the nonclassical σ-channel on the triplet surface, but only through the classic σ-mechanism on the quintet surface. The barrier on the quintet σ-pathway is much lower than on the triplet channels so the quintet surface cuts through the triplet surfaces and a two state reactivity (TSR) mechanism with crossover from the triplet to the quintet surface becomes a plausible scenario for C–H bond activation by PorFeIV=O. In the case of the hydrosulfide-ligated complex the H-abstraction follows a π-mechanism on the triplet surface: the σ* is too high in energy to make a σ-attack of the substrate favorable. The σ- and π-channels are both feasible on the quintet surface. As the quintet surface lies above the triplet surface in the entrance channel of the oxidative process and is highly destabilized on both the σ- and π-pathways, the reaction can only proceed on the triplet surface. Insights into the electron transfer process accompanying the H-abstraction reaction are achieved through a detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state. It is found that the electron transfer from the substrate σCH into the acceptor orbital of the catalyst, the Fe–O σ* or π*, occurs through a rather complex mechanism that is initiated by a two-orbital four-electron interaction between the σCH and the low-lying, oxygen-rich Fe–O σ-bonding and/or Fe–O π-bonding orbitals of the catalyst
Reactivity of Compound-II and Compound-I Models: Quantum Chemical Studies of C–H Bond Hydroxylation by Electron-deficient Oxoiron(IV) Porphyrin Species
No full textStructural and Charge Transport Properties of Molecular Metals Based on M(dmit)2 (M= Ni, Pd, Pt) Complexes: a Density Functional Investigation
No full textREACTIVITY OF COMPOUND II: ELECTRONIC STRUCTURE ANALYSIS OF C–H HYDROXYLATION BY ELECTRON-DEFICIENT OXOIRON(IV) PORPHYRINS
No full textIn heme iron enzymes, oxoiron(IV) porphyrin π-cation radical (Cpd I) and oxoiron(IV)
porphyrin (Cpd II) species are proposed as reactive intermediates in dioxygen activation and
oxygen-atom transfer reactions. Because of its biological significance, the reactivity of Cpd I
has been widely investigated with in situ-generated oxoiron(IV) porphyrin π-cation radical
complexes in various types of oxidation reactions. Dissimilar from Cpd I mimics, Cpd II
mimics have been considered to be very poor oxidants and hence have received less attention.
Several experimental evidences have now accumulated proving that oxoiron(IV) porphyrins
are competent oxidants of a variety of substrates. The relationship between the nature of the
porphyrin ring and the reactivity of Cpd II mimics is, however, still unclear. As a matter of
fact, the experimental data reported for (TPFPP)Fe(IV)=O (TPFPP = mesotetrakis(
pentafluorophenyl)porphyrinate) seem to indicate that electron-deficient porphyrin
rings greatly enhance the reactivity of Cpd II mimics.1 On the other hand, a Cpd II mimic also
bearing an electron-deficient porphyrin ligand, [(4-TMPyP)Fe(IV)=O]4+ (4-TMPyP =
5,10,15,20-tetrakis(N-methyl-4-pyridinium) porphyrinate), has been recently found to be
almost unreactive toward C–H hydroxylation.2 With the aim to understand why synthetic Cpd
II mimics have such a great variability in their reactivity, we have theoretically investigated
by DFT (Density Functional Theory) methods C–H hydroxylation reactions by the electron deficient
Cpd II mimics above mentioned.
The hydroxylation reactions have been studied on the ground triplet and excited quintet spinstate
surfaces within the rebound mechanism scheme. On each spin surface both the σ- and π-
channels have been explored. It is found that crossover from the ground state triplet surface to
the highly reactive quintet state surface is a plausible scenario for C–H bond activation by
these complexes. The efficiency of this TSR (two state reactivity) mechanism depends on the
nature of the macrocycle, the presence and nature of an axial ligand, and the solvent.
A detailed electronic structure analysis of the transition state species and the reactant
complexes en route to the transition state reveals that, just as found in the unsubstituted
oxoiron(IV) porphyrin, PorFeIV=O,3 the electron transfer from the substrate σCH into the
acceptor orbital of the catalyst, the Fe–O σ* or π*, occurs through a complex mechanism that
is initiated by a two-orbital four-electron interaction between the σCH and the low-lying,
oxygen-rich Fe–O σ-bonding and/or Fe–O π-bonding orbitals of the catalyst.
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References:
1. Fukuzumi, S.; Kotani, H.; Lee, Y.-M.; Nam, W. J. Am. Chem. Soc. 2008, 130, 15134–15142.
2. Bell, S. R.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 9640–9641.
3. Rosa, A.; Ricciardi, G. Inorg. Chem. 2012, 51, 9833–9845
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