Quartet Spin State Research Articles

1H chemical shifts in nonaxial, paramagnetic chromium(III) complexes — Application of novel pNMR shift theory

We present the first chemical application of the recent, general theory of the nuclear magnetic resonance shielding and chemical shift in paramagnetic compounds, to a set of nonaxial high-spin metallo-organic complexes. The theory is for the first time rigorous for systems of arbitrary spatial and spin symmetry, and introduces new structure to the isotropic, anisotropic but symmetric, and anisotropic and antisymmetric parts of the shielding tensor. We apply the theory using density functional calculations of the proton chemical shift in a family of nonaxial chromium(III) complexes possessing a quartet ground electronic spin state. We discuss the various contributions to the isotropic chemical shift, and compare the full theory to approximate forms appropriate to the doublet case on the one hand, and to the doublet case at the nonrelativistic limit, on the other hand. The performance of various exchange-correlation functionals in reproducing the recently measured experimental chemical shifts is evaluated.

Imino-oxy Acetic Acid Dealkylation as Evidence for an Inner-Sphere Alcohol Intermediate in the Reaction Catalyzed by Peptidylglycine α-Hydroxylating Monooxygenase

Peptidylglycine alpha-hydroxylating monooxygenase (PHM, EC 1.14.17.3) catalyzes the stereospecific hydroxylation of a glycyl alpha-carbon in a reaction that requires O(2) and ascorbate. Subsequent dealkylation of the alpha-hydroxyglycine by another enzyme, peptidylamidoglycolate lyase (PAL. EC 4.3.2.5), yields a bioactive amide and glyoxylate. PHM is a noncoupled, type II dicopper monooxygenase which activates O(2) at only a single copper atom, Cu(M). In this study, the PHM mechanism was probed using a non-natural substrate, benzaldehyde imino-oxy acetic acid (BIAA). PHM catalyzes the O-oxidative dealkylation of BIAA to benzaldoxime and glyoxylate with no involvement of PAL. The minimal kinetic mechanism for BIAA was shown to be steady-state ordered using primary deuterium kinetic isotope effects. The (D)(V/K)(APPARENT, BIAA) decreased from 14.7 +/- 1.0 as [O(2)] --> 0 to 1.0 +/- 0.2 as [O(2)] --> infinity suggesting the dissociation rate constant from the PHM x BIAA complex decreases as [O(2)] increases; thereby, reducing the steady-state concentration of [PHM](free). BIAA was further used to differentiate between potential oxidative Cu/O species using a QM/MM reaction coordinate simulation to determine which species could yield product O-dealkylation that matched our experimental data. The results of this study provided compelling evidence for the presence of a covalently linked Cu(II)-alkoxide intermediate with a quartet spin state responsible BIAA oxidation.

Quantum Mechanics/Molecular Mechanics Studies on the Sulfoxidation of Dimethyl Sulfide by Compound I and Compound 0 of Cytochrome P450: Which Is the Better Oxidant?

The cytochromes P450 are ubiquitous enzymes that are involved in key metabolizing processes in the body through the monoxygenation of substrates; however, their active oxidant is elusive. There have been reports that implicate that two oxidants, namely, the iron(IV)-oxo porphyrin cation radical (compound I) and the iron(III)-hydroperoxo complex (compound 0), both act as oxidants of sulfoxidation reactions, which contrasts theoretical studies on alkene epoxidation by compounds I and 0 that implicated compound 0 as a sluggish oxidant. To resolve this controversy and to establish the potency of compound I and compound 0 in sulfoxidation reactions, we have studied dimethyl sulfide sulfoxidation by both oxidants using the quantum mechanics/molecular mechanics (QM/MM) technique on cytochrome P450 enzymes and have set up a model of two P450 isozymes: P450(cam) and P450(BM3). The calculations support earlier gas-phase density functional theory modeling and show that compound 0 is a sluggish oxidant that is unable to compete with compound I. Furthermore, compound I is shown to react with dimethyl sulfide via single-state reactivity on a dominant quartet spin state surface.

Quantum Tunneling in Testosterone 6β-Hydroxylation by Cytochrome P450: Reaction Dynamics Calculations Employing Multiconfiguration Molecular−Mechanical Potential Energy Surfaces

Testosterone hydroxylation is a prototypical reaction of human cytochrome P450 3A4, which metabolizes about 50% of oral drugs on the market. Reaction dynamics calculations were carried out for the testosterone 6beta-hydrogen abstraction and the 6beta-d(1)-testosterone 6beta-duterium abstraction employing a model that consists of the substrate and the active oxidant compound I. The calculations were performed at the level of canonical variational transition state theory with multidimensional tunneling and were based on a semiglobal full-dimensional potential energy surface generated by the multiconfiguration molecular mechanics technique. The tunneling coefficients were found to be around 3, indicating substantial contributions by quantum tunneling. However, the tunneling made only modest contributions to the kinetic isotope effects. The kinetic isotope effects were computed to be about 2 in the doublet spin state and about 5 in the quartet spin state.

Methane C−H Bond Activation by Gas-Phase Th+and U+: Reaction Mechanisms and Bonding Analysis

Th+ is the only actinide ion that activates exothermically the strong C−H bonds of methane in the gas phase. In contrast, U+, as well as all of the rest of the experimentally studied An+ cations, is inert in the reaction with CH4. In this work, the activation of methane by thorium and uranium monocations was investigated using two different density functional theory approaches. The reaction mechanisms and the corresponding potential energy profiles were analyzed in detail. From the formation of the initial ion−molecule adduct, the Th++CH4 reaction pathway evolves completely along the doublet spin surface, whereas that of U++CH4 involves solely the quartet bare cation ground spin state. The bonding properties of all of the species involved in the reaction pathways were investigated in terms of diverse analyses including natural bond orbital, atoms in molecules, and electron localization function. The dehydrogenation products (Th+═CH2 and U+═CH2) as well as the last insertion intermediates are characterized by the presence of α-agostic geometries.

Sulfoxide, Sulfur, and Nitrogen Oxidation and Dealkylation by Cytochrome P450.

The oxidation and dealkylation of dimethylsulfoxide (DMSO), dimethylsulfide (DMS), and trimethylamine (TMA) by cytochrome P450 has been studied with density functional theory calculations. The results show that the oxidation reactions always occur on the doublet spin surface, whereas dealkylations can take place for both the doublet and quartet spin states. Moreover, DMS is more reactive than DMSO, and S-oxidation is more favorable than S-dealkylation, whereas N-dealkylation is more favorable than N-oxidation. This is in perfect agreement with experimental results, showing that density functional activation energies are reliable and comparable for widely different reactions with cytochrome P450.

How Do Azoles Inhibit Cytochrome P450 Enzymes? A Density Functional Study

To examine how azole inhibitors interact with the heme active site of the cytochrome P450 enzymes, we have performed a series of density functional theory studies on azole binding. These are the first density functional studies on azole interactions with a heme center and give fundamental insight into how azoles inhibit the catalytic function of P450 enzymes. Since azoles come in many varieties, we tested three typical azole motifs representing a broad range of azole and azole-type inhibitors: methylimidazolate, methyltriazolate, and pyridine. These structural motifs represent typical azoles, such as econazole, fluconazole, and metyrapone. The calculations show that azole binding is a stepwise mechanism whereby first the water molecule from the resting state of P450 is released from the sixth binding site of the heme to create a pentacoordinated active site followed by coordination of the azole nitrogen to the heme iron. This process leads to the breaking of a hydrogen bond between the resting state water molecule and the approaching inhibitor molecule. Although, formally, the water molecule is released in the first step of the reaction mechanism and a pentacoordinated heme is created, this does not lead to an observed spin state crossing. Thus, we show that release of a water molecule from the resting state of P450 enzymes to create a pentacoordinated heme will lead to a doublet to quartet spin state crossing at an Fe-OH(2) distance of approximately 3.0 A, while the azole substitution process takes place at shorter distances. Azoles bind heme with significantly stronger binding energies than a water molecule, so that these inhibitors block the catalytic cycle of the enzyme and prevent oxygen binding and the catalysis of substrate oxidation. Perturbations within the active site (e.g., a polarized environment) have little effect on the relative energies of azole binding. Studies with an extra hydrogen-bonded ethanol molecule in the model, mimicking the active site of the CYP121 P450, show that the resting state and azole binding structures are close in energy, which may lead to chemical equilibrium between the two structures, as indeed observed with recent protein structural studies that have demonstrated two distinct azole binding mechanisms to P450 heme.

A comparison of the reaction mechanisms of iron- and manganese-containing 2,3-HPCD: an important spin transition for manganese

Homoprotocatechuate (HPCA) dioxygenases are enzymes that take part in the catabolism of aromatic compounds in the environment. They use molecular oxygen to perform the ring cleavage of ortho-dihydroxylated aromatic compounds. A theoretical investigation of the catalytic cycle for HPCA 2,3-dioxygenase isolated from Brevibacterium fuscum (Bf 2,3-HPCD) was performed using hybrid DFT with the B3LYP functional, and a reaction mechanism is suggested. Models of different sizes were built from the crystal structure of the enzyme and were used in the search for intermediates and transition states. It was found that the enzyme follows a reaction pathway similar to that for other non-heme iron dioxygenases, and for the manganese-dependent analog MndD, although with different energetics. The computational results suggest that the rate-limiting step for the whole reaction of Bf 2,3-HPCD is the protonation of the activated oxygen, with an energy barrier of 17.4 kcal/mol, in good agreement with the experimental value of 16 kcal/mol obtained from the overall rate of the reaction. Surprisingly, a very low barrier was found for the O-O bond cleavage step, 11.3 kcal/mol, as compared to 21.8 kcal/mol for MndD (sextet spin state). This result motivated additional studies of the manganese-dependent enzyme. Different spin coupling between the unpaired electrons on the metal and on the evolving substrate radical was observed for the two enzymes, and therefore the quartet spin state potential energy surface of the MndD reaction was studied. The calculations show a crossing between the sextet and the quartet surfaces, and it was concluded that a spin transition occurs and determines a barrier of 14.4 kcal/mol for the O-O bond cleavage, which is found to be the rate-limiting step in MndD. Thus the two 83% identical enzymes, using different metal ions as co-factors, were found to have similar activation energies (in agreement with experiment), but different rate-limiting steps.

Cobalt cationic sites in ferrierites: QM/MM modeling

Cobalt(II) sites in ferrierites are already well-known from their catalytic activity, their speciation and properties, however, the knowledge are far from completeness. The following paper presents the first in literature combined QM/MM study to elucidate the structure of these sites. With this end force-field parameters describing Co interactions with ionic shells in zeolite have been tested and the cell size for various Al distributions and Co positioning has been determined. Oxide-type Buckingham parameters are shown to perform better than the carbonate ones. Moreover, Co(II) ions stability in α and β sites with various Al distribution indicates at T1T1 Al substitution in β-site as that the preferred Co(II) siting. DFT results show that the quartet spin state of Co(II) is more stable than the doublet one.

Interaction of FeO+ cation with benzene, aniline, and 3‐methylaniline: DFT study of oxygen insertion mechanism

AbstractThe reaction pathways and energetics for oxygen insertion into CH bond in benzene, aniline, and 3‐methylaniline by FeO+ in the gas phase were investigated by means of the DFT methodology with the B3LYP exchange‐correlation functional and 6‐311G** basis set. The main aim of this work was to elucidate the influence of substituents in phenyl ring on stationary points along the energy profile on sextet and quartet surfaces for the reaction of FeO+ with substituted benzenes. The studies show that the amino and methyl groups change the energetics of oxygen insertion by lowering the energy profile along the reaction pathway. The substituents studied in this work facilitate the insertion of oxygen into the aromatic CH bond by stabilizing the intermediate sigma complex (σ‐complex), the amino group being by far more effective. On the other hand, both functional groups increase the activation energy of the rate‐determining step in the gas phase, so that they have unfavorable influence on the kinetics. The comparison of the energy diagrams for the sextet and quartet spin states indicates the dominance of the low‐spin reactivity in oxygen insertion into aromatic CH bond. Aniline and 3‐methylaniline oxidation occurs via electrophilic addition while the conversion of benzene to phenol by FeO+ is mediated by a σ‐complex with mixed radical and cationic character. Present results are also discussed in the context of oxyferryl group reactivity. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Magnetic interactions as well as microscopic origins of the spin-Hamiltonian parameters for 6S(3d5) state ions in cubic symmetry crystal filed

The magnetic interactions as well as the microscopic origins of the spin-Hamiltonian(SH) parameters including a, g, and Δg for 6S(3d5) state ion in cubic symmetry crystal field, taking into account the spin-spin (SS), the spin-other-orbit (SOO), the orbit-orbit (OO) magnetic interactions besides the well-known spin-orbit (SO) magnetic interaction, have been investigated using the complete diagonalization method (CDM). It is shown that the SH parameters arise from the five microscopic mechanisms of SO coupling, SS coupling, SOO coupling, OO coupling, and SO-SS-SOO-OO combined coupling. The relative importance of the contributions of the mechanisms to the SH parameters are investigated. It is shown that the SO coupling mechanism and the SO-SS-SOO-OO combined coupling mechanism are the most important of these coupling mechanisms. The contributions from SS coupling mechanism, SOO coupling mechanism, OO coupling mechanism to the SH parameter are quite small but their combined effects (SO-SS-SOO-OO combined coupling mechanism) is appreciable. The zero-field splitting (ZFS) arises from the net spin quartet states as well as the combined effects between the spin doublet states and the spin quartets states whereas the Zeeman g (or Δg)factor arises from the net spin quartet states. The contributions to the SH parameters from the net spin doublet states are zero. It is found that the relations: a>0，a(-|Dq|)a(|Dq|)，g(-Dq)=g(Dq)，a(-Dq,-ξd, B, C)=a(Dq,ξd, B, C) and Δg(-Dq, -ξd, B, C)=Δg(Dq, ξd, B, C) always hold for the selected crystal field area.Illustrative evaluations are performed for the four typical materials. Mn2+: KZnF3, Mn2+: RbCdF3, Mn2+: MgO and Mn2+: CaO. Good agreement between the theoretical values and the experimental results are obtained.

How does the push/pull effect of the axial ligand influence the catalytic properties of Compound I of catalase and cytochrome P450?

Density functional theory (DFT) calculations on the chemoselective epoxidation versus hydroxylation reactions of propene by oxoiron porphyrin models mimicking the active sites of catalase, cytochrome P450 (P450) and horseradish peroxidase Compound I (CpdI) are presented. The catalase reactions are concerted and proceed via two-state reactivity patterns on competing doublet and quartet spin state surfaces, but the lowest barrier is the one leading to epoxide products on the doublet spin surface. The results are compared with earlier DFT studies of models of cytochrome P450, horseradish peroxide (HRP), taurine/α-ketoglutarate dioxygenase and some synthetic oxoiron catalysts. The catalase barriers are midway in between those obtained for HRP and P450 models, so that tyrosinate ligated heme systems should be able to catalyze C–H hydroxylation and C C epoxidation reactions. We show that for heme systems the barrier height of epoxidation linearly correlates with the electron affinity of Compound I as expected from the electron transfer mechanism of the rate determining step. Our studies show that the axial ligand does not influence the chemoselectivity of a reaction but that it does regulate the barrier heights and rate constants. Finally, we estimated the effect of the axial ligand on the oxoiron group and derived that it contributes from a field effect due to the charge of the ligand and a quantum mechanical effect as a result of orbital mixing. In catalase, the major component is the field effect, while the quantum mechanical effect is negligible. This is in contrast to P450 CpdI, where both effects are of similar order of magnitude.

Theoretical Study of Low-Spin, High-Spin, and Intermediate-Spin States of [FeIII(pap)2]+(pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of Light-Induced Excited Spin State Trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.

A Density Functional Study of the Factors That Influence the Regioselectivity of Toluene Hydroxylation by Cytochrome P450 Enzymes

AbstractDensity functional theory calculations have been performed to elucidate the factors that influence the regioselectivity of toluene hydroxylation by a model of the active species of cytochrome P450 enzymes, so‐called Compound I (Cpd I). Cpd I can hydroxylate the benzylic C–H and generate benzyl alcohol, or it can activate the phenyl group and produce p‐cresol and p‐methylcyclohexanone products. The reactions take place via two‐state reactivity on competing doublet and quartet spin state surfaces. In the gas phase, the benzyl alcohol is preferred over p‐cresol by more than three orders of magnitude. Environmental perturbations, namely, NH···S hydrogen bonding to the thiolate ligand and bulk polarity of the protein, lower this preference to roughly 10:1 in favour of benzyl alcohol. Substitution of methyl hydrogen atoms by deuterium atoms raises the barriers that lead to benzyl alcohol without affecting those leading to p‐cresol. Therefore combining toluene deuteration with the effects of NH···S hydrogen bonding and bulk polarity lowers the free energy difference between the two processes to only 0.4 kcal mol–1, and the two processes become competitive. These results as well as the calculated kinetic isotope effects are in good general agreement with experimental data. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Formation of the Active Species of Cytochrome P450 by Using Iodosylbenzene: A Case for Spin‐Selective Reactivity

The generation of the active species for the enzyme cytochrome P450 by using the highly versatile oxygen surrogate iodosylbenzene (PhIO) often produces different results compared with the native route, in which the active species is generated through O(2) uptake and reduction by NADPH. One of these differences that is addressed here is the deuterium kinetic isotope effect (KIE) jump observed during N-dealkylation of N,N-dimethylaniline (DMA) by P450, when the reaction conditions change from the native to the PhIO route. The paper presents a theoretical analysis targeted to elucidate the mechanism of the reaction of PhIO with heme, to form the high-valent iron-oxo species Compound I (Cpd I), and define the origins of the KIE jump in the reaction of Cpd I with DMA. It is concluded that the likely origin of the KIE jump is the spin-selective chemistry of the enzyme cytochrome P450 under different preparation procedures. In the native route, the reaction proceeds via the doublet spin state of Cpd I and leads to a low KIE value. PhIO, however, diverts the reaction to the quartet spin state of Cpd I, which leads to the observed high KIE values. The KIE jump is reproduced here experimentally for the dealkylation of N,N-dimethyl-4-(methylthio)aniline, by using intra-molecular KIE measurements that avoid kinetic complexities. The effect of PhIO is compared with N,N-dimethylaniline-N-oxide (DMAO), which acts both as the oxygen donor and the substrate and leads to the same KIE values as the native route.

The spectral fine structure, zero-field splitting parameters and Jahn-Teller effect of LiNbO3∶Fe3+crystals

The 252 rank completely diagonalized Hamiltonian matrix of the 3d5 configuration with trigonal symmetry have been established by group theory, crystal field theory and irreducible tensor operator method. The spectral fine structure, zero-field splitting parameters, and Jahn-Teller effect, as well s the influence of spin doublet state and spin quartet state on the ground-state energy levels in LiNbO3∶Fe3+ crystals were studied with this matrix. The results show that the contribution of spin quartet state on the ground- state energy levels is the most important, the contribution of spin doublet state is weak but can not be neglected. The calculated results are in good agreement with the experiments. On their basis, the influence of spin-orbit interaction and spin-spin interaction on the spectra fine structure and zero-field splitting parameters were further studied, and we found that the influence of spin-orbit interaction is dominant, but the influence of spin-spin interaction can not be neglected. The research shows this substance has J-T effect in the spectral structure of spin quartet state. The reason is the combined action of spin-orbit interaction and trigonal distortion.

What External Perturbations Influence the Electronic Properties of Catalase Compound I?

We have performed density functional theory calculations on an active-site model of catalase compound I and studied the responses of the catalytic center to external perturbations. Thus, in the gas phase, compound I has close-lying doublet and quartet spin states with three unpaired electrons: two residing in pi(FeO) orbitals and the third on the heme. The addition of a dielectric constant to the model changes the doublet-quartet energy ordering but keeps the same electronic configuration. By contrast, the addition of an external electric field along one of the principal axes of the system can change the doublet-quartet energy splitting by as much as 6 kcal mol(-1) in favor of either the quartet or the doublet spin state. This sensitivity is much stronger than the effect obtained for iron heme models with thiolate or imidazole axial ligands. Moreover, an external electric field is able to change the electronic system from a heme-based radical [Fe=O(Por*+)OTyr-] to a tyrosinate radical [Fe=O(Por)OTyr*]. This again shows that oxo-iron heme systems are chameleonic species that are influenced by external perturbations and change their character and catalytic properties depending on the local environment.

Propene Activation by the Oxo-Iron Active Species of Taurine/α-Ketoglutarate Dioxygenase (TauD) Enzyme. How Does the Catalysis Compare to Heme-Enzymes?

Density functional calculations on the oxygenation reaction of propene by a model for taurine/alpha-ketoglutarate dioxygenase (TauD) enzyme are presented. The oxo-iron active species of TauD is shown to be a powerful and aggressive oxidant, which is able to hydroxylate C-H bonds and epoxidize C=C bonds with low barriers. In the case of propene oxygenation, the hydroxylation and epoxidation mechanisms are competitive on a dominant quintet spin state surface. We have compared the mechanism and thermodynamics of TauD with oxo-iron heme catalysts, such as the cytochromes P450, and found some critical differences. The TauD model is found to be much more reactive toward oxygenation of substrates than oxo-iron complexes in a heme environment with much lower reaction barriers. We have analyzed this and assigned this to the strength of the O-H bond formed after hydrogen abstraction from a substrate, which is at least 10 kcal mol(-)(1) stronger in five-coordinated oxo-iron nonheme complexes than in six-coordinated oxo-iron heme complexes. Since, the metal in TauD enzymes is five-coordinated, whereas in heme-enzymes it is six-coordinated there are some critical differences in the valence molecular orbitals. Thus, in oxo-iron heme catalysts one of the antibonding pi orbitals is replaced by a low-lying nonbonding delta orbital resulting in a lower overall spin state. Moreover, heme-enzymes have an extra oxidation equivalent located on the heme, which is missing in non-heme oxo-iron catalysts. As a result, the oxo-iron species of TauD reacts via single-state reactivity on a dominant quintet spin state surface, whereas oxo-iron heme catalysts react via two-state reactivity on competing doublet and quartet spin states.

Theoretical Study of the Mechanism of Acetaldehyde Hydroxylation by Compound I of CYP2E1

Recent experimental studies revealed that cytochrome P450 2E1 (CYP2E1) could metabolize not only ethanol but also its primary product, acetaldehyde, accompanying the well-known acetaldehyde dehydrogenases (ALDH) in the metabolism of acetaldehyde. Mechanistic aspects of acetaldehyde hydroxylation by Compound I model active species of CYP2E1 were investigated by means of B3LYP DFT calculations in the present paper. Our study results demonstrate that acetaldehyde hydroxylation by CYP2E1 is in accord with the effectively concerted mechanisms both on the high quartet spin state (HS) and on the low doublet spin state (LS). The rate-limiting step is H-abstraction, and the activation energy is about 11.7 approximately 14.0 kcal/mol on the quartet (doublet) reaction route, which is about one-half to one-third of that needed by methane hydroxylation. The phenomenon that the HS and LS reaction routes are both effectively concerted was shown for the first time to occur in trans-2-phenyl-iso-propylcyclopropane hydroxylation by Kumar et al. (see Figure 7 in the paper of Kumar, D.; de Visser, S. P.; Sharma, P. K.; Cohen, S.; Shaik, S. J. Am. Chem. Soc. 2004, 126, 1907) and was confirmed in our work of acetaldehyde hydroxylation by cytochrome P450. Theoretical exploration of the HS O-rebound barrier degradation is also presented in the present paper on the basis of Shaik's valence bond (VB) model.

Spin Crossover in Cobalt(II) Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.