呂玲玲 朱元成 左國防 袁焜 王永成

(1天水師范學(xué)院化學(xué)工程與技術(shù)學(xué)院，天水741001)(2西北師范大學(xué)化學(xué)化工學(xué)院，蘭州730070)

非血紅素鐵超氧化物活化丙烯分子多態(tài)反應(yīng)機(jī)理的理論研究

呂玲玲＊,1朱元成1左國防1袁焜1王永成2

(1天水師范學(xué)院化學(xué)工程與技術(shù)學(xué)院，天水741001)
(2西北師范大學(xué)化學(xué)化工學(xué)院，蘭州730070)

采用密度泛函DFT-B3LYP理論對非血紅素鐵超氧化物活化丙烯分子多態(tài)反應(yīng)機(jī)理進(jìn)行了探討.研究結(jié)果表明氫原子抽取過程遵守單態(tài)反應(yīng)機(jī)制，主要在基態(tài)高自旋七重態(tài)勢能面進(jìn)行，且具有較低活化能(ΔG≠=65.6 kJ·mol-1)，非血紅素鐵超氧化物可以作為有效氧化劑抽取氫原子。單態(tài)反應(yīng)機(jī)制可能歸因于近來建議的交換-加強(qiáng)反應(yīng)原則(EER，鐵中心具有較大交換穩(wěn)定作用)。對于O-O鍵的活化，在CASSCF(10,8)/6-31+G(d)//TZVP水平下，勢能面交叉區(qū)內(nèi)，高自旋七重態(tài)(S1)和五重態(tài)(Q0)的自旋-軌道耦合(SOC)常數(shù)分別為2.26和2.19 cm-1。軌道分析表明兩條發(fā)生翻轉(zhuǎn)自旋軌道具有相同空間組成(π*sub)，SOC禁阻，因此通過SOC作用反應(yīng)體系不可能有效地從七重態(tài)(S=3)勢能面系間穿越到五重態(tài)(S=2)勢能面，系間穿越可能發(fā)生在反應(yīng)最后的退出階段。

非血紅素鐵超氧化物；多態(tài)反應(yīng)機(jī)理；系間竄越；自旋軌道耦合

0 Introduction

Mononuclear non-heme Fe enzymes catalyze a diverserangeofoxidationreactions,including hydroxylation,halogenation,ring closure,desaturation and electrophilic aromatic substrate that are important inmedical,pharmaceutical,andenvironmental applications[1-2].Several species including ferryl-oxo, ferric-superoxo,and ferric-peroxy have been proposed or found to act as oxidants in these enzymes[2]. However,our understanding of the non-heme ferricsuperoxo complexes is rather scant,as opposed to the well studied oxy-heme species.Thus,mononuclear non-hemecomplexesinenzymesandsynthetic analogueshaveattractedconsiderableinterest recently.Thelower-valentferric-superoxospecies havebeendirectlyobservedinnaphthalene dioxygenase(NDO)and homo-protoctaechute 2,3-dioxygenase(HPCD)[3].Furthermore,synthetic ferricsuperoxoandothermetal-superoxospecieswere recently reported to be capable of catalyzing oxidation, includingC-Hbondactivation[4].Interestingly, comparedwithhemeenzymes,manynon-heme enzymes can use ferric-superoxo species as an oxidant but only a few heme enzymes(tryptophan 2,3-dioxygenase(TDO),and indoleamine 2,3-dioxygenase (IDO)so far)use ferric-superoxo species[5-6],which has also attracted our attention as a candidate for the active oxidant in the non-heme enzymes catalysis. Morokuma and co-workers compared reactivity of several vital ferryl-oxo and ferric-superoxo model complexes including title non-heme complex 1 model through DFT calculations to provide clues for rational design of ferric-superoxo oxidants[2],where it has been shown that a dominant feature of these reactions is the two-state reactivity(TSR)and multistate reactivity (MSR)that transpires due to the close proximity of the different multi-spin states in the ground state[7].

Scheme 1A model reaction for the propene catalyzed by non-heme ferric-superoxo species

For ferric-superoxo complexes,the findings show that ferric-superoxo species can be converted to a ferryl-oxo complex via O-O bond cleavage,thus these species seem to shareonecommonfundamental feature of the TSR/MSR mechanisms,they involve energy profiles of at least two spin states that either crossing or remaininproximity.Thus,thetitle reaction possibly occurs on two or more potential energy surfaces(PESs)under thermal conditions.

Therefore,detailed analyses of crossing seam between the different PESs are important in order to better understand the TSR/MSR mechanism of the propenecatalyzedbynon-hemeferric-superoxo species(Scheme 1).This kind of knowledge is essential for understanding the whole reaction mechanism and is useful for establishing an appropriate model for the O-O bond cleavage processes.To our knowledge,a deep theoretical study for the propene catalyzed by non-hemeferric-superoxospecieshasnotbeen reported.However,since an experimental proof of mechanism is not a simple matter,in this sense, theoretical chemistry,specifically density functionaltheory(DFT)has been playing an essential role in role inprovidingmechanisticdataandstructuresof unstableintermediatesandinderivinguseful concepts.In the present paper we have performed hybrid DFT calculations on the reactions of the propenecatalyzedbynon-hemeferric-superoxo compound 1 models(Fig.1)to paint global pictures and discussed crossing seams,spin-orbit coupling (SOC)and possible spin-inversion processes in the OO bond cleavage step.

1 Computational details

1.1 Geometrical optimization

Energiesandgeometriesofthereaction intermediates and the transition states were calculated using the Gaussian 09 program package[8]and the unrestricted hybrid density functional UB3LYP with the 6-31+G(d)basis set[9].The basis set used in DFT calculationforsinglepointenergiesonfinal geometries is LACVP+*[10],which has been widely used for transition-metal-containing systems and has an effective potential that accounts for the scalar relativistic effects in iron.At the non-local functional UBP86 level,single-point energy calculations were performed using the LACVP+*basis set for all the atoms.The PCM approach for accounting solvent effects(single points with CH3CN as solvent)was applied in the UBP86/LACVP+*level.However, UBP86 tended to overstabilize the low-spin ground state resulting in a large energy splitting between spin states,and in some cases this lead to an incorrect ground state(see Supporting Information).In addition, previous investigations of transition metal compounds employing the B3LYP functional by other groups[11]and us[12]indicated that this approach shows a very promising performance to predict properties such as bond dissociation energies,geometries,and harmonic frequencies with an accuracy comparable to that obtained from highly correlated wave function based ab initio methods.

1.2 Treatment of spin-orbit coupling

The SOC matrix elements are treated by an accuratemulticentermean-field(RI-SOMF) approximation[13-15]with the reasonable complete active space self-consistent field,CASSCF(10,8)(ten activateelectronsoccupytheeightmetal-ligand activate orbitals).An efficient implementation of the SOMF concept was explained,which is based on the following formulation of the effective one-electron operator[16]:

2 Results and discussion

2.1 Electronicstructuresofferric-superoxo species

The optimized geometries and energetic data for the septet,quintet,and triplet electronic states aredepictedinFig.1andTableS1(Supporting information),respectively.Inordertokeepthe discussion more simple,the goal complex,denoted as7(5)1side-onor7(5)[3]1end-on,is initially formed as Fe center and O2collide side-on or end-on with each other, where the superscripts denote the spin multiplicities. Comparedtothereactionmechanism,side-on complex,7(5)1side-onis not an important point discussed.

We obtain a septet71end-oncomplex,wherein O2is bound end-on and is an Fe-superoxo complex.The electronic structure of71end-onis in detailed shown in Fig.2.From Fig.2,O2here is a superoxide,having a singly occupied π*⊥,which is perpendicular to the Fe-O-O plane,while the other doubly occupied π* orbital,π*∥in the Fe-O-O plane,forms a 3-e bond with the Fe dz2orbital.In other words,in the plane π*∥orbital of the superoxo interacts with the dz2orbital of the Fe in a π-type fashion,which leads to forming two new orbitals dz2±π*,as shown in Fig.2. Thus,the septet71end-onwill inv o lve ferromagnetic coupling of S=5/2 Fewith the S=1/2 superoxo anionO2-.

Fig.1Optimized geometries of the different spin states non-heme Ferric-superoxo complexes at the UB3LYP/6-31+G(d)level

Fig.2Electronic configurations of septet,quintet and triplet states of the end-on complex 1

For the quintet51end-oncomplex,one character of51end-onis that its formal iron oxidation state can be assigned as Fe-peroxo.The reason is that the π*⊥orbital of the O2moiety in51end-onis doubly occupied. Formally,there are four unpaired α electrons in51end-on, spin density on Fe is 4.09.To identify some main atomic orbital interactions,the main antiferromagnetic orbital interactions were also inferred from overlaps calculated from the broken symmetry wave function (UB3LYP/6-31+G(d)),theresultscalculatedare plotted in Fig.3.Calculation results show that dz2±π*∥electronsareactuallyhighlypronetospinpolarization,i.e.,partial separation of α-and β-spin electrons in the dz2±π*∥orbital into spatially different regions,since electrons paired in orbital repel each other electrostatically.Restricted open-shell B3LYP calculations indeed show instability relative to brokensymmetry(BS)solutions.The overlap between dz2and π*∥is considerably better than the overlap of π*⊥and any Fe3d orbital,a nd the overlap is T=＜dz2| π*∥＞=0.64.The singlet coupling between dxzand π*∥electron pair is therefore strong enough to lead to a short of the Fe-O distances(0.201 8 nm)in51end-on,as compared with that(0.213 2 nm)of71end-on.

As for31end-on,O2is bound end-on and is an Fe-superoxo complex,having a singly occupied π*⊥orbital and a doubly occupied π*∥orbital.Then31end-oninvolves ferromagnetic coupling of S=1/2 Fewith the S=1/2 superoxo anion O2-.Relative to71end-on,the DFT-calculated relative free energy of31end-onis 48.9 and 51.8 kJ·mol-1at the B3LYP/6-31+G(d)and B3LYP/LACVP+*levels,respectively.Compared with the coupling of51end-on,the singlet coupling between π*∥and dz2in31end-onis much stronger,the overlap T=＜dz2

|π*∥＞≈1 with the covalent interaction,which will lead to decrease the distance of Fe-O bond (0.192 1 nm).

Fig.3Spin natural orbitals(SNO)and natural orbitals(NO)obtained with the symmetry broken method in51end-on

2.2 Hydrogen-atom abstraction

The optimized geometries and relative energies in the triplet,quintet,and septet electronic states are shown in Fig.4 and Table S2(Supporting information), respectively.The calculated potential energy profiles forthedifferentspinstatesareshownin Fig.5.Initially,the three reactive states of7(5)[3]1end-onform reactant complexes,7(5)[3]R1,in which7(5)[3]1end-onis weakly bound to propene.An electrophilic attack by a7(5)[3]1end-onspecies is enabled through a σ-attack of the superoxo π*⊥orbital.This leads to the transfer of a H-atom along with a spin-down electron from the C-H bond of the substrate into the π*⊥orbital of O2to generate aferrichydroperoxoproductandaradicalon substrate.Thus,a strong π(O2-)bond is broken.Since the electron is transferred into the superoxo π*⊥orbital, this requires an end-on approach of the C-H bond of the substrate relative to the Fe-O-O plane to ensure good orbital overlap.

As can be seen from Fig.5,the lower energy pathway of the H-abstraction process was occurred on the high-spin(HS)S=3 state PES.The transition state7TSHhas calculated barrier heights of ΔE≠=78.6 kJ· mol-1and ΔG≠=65.6 kJ·mol-1relative to7R1.If electronic energies and free energies in the gas-phase are compared,the spin state ordering in the reactants and transition states remains the same.This indicates that the reaction will take place through single-state reactivity on the HS S=3 state potential surface only, which is compared with the behavior of nonheme and heme iron-oxo complexes where generally two-or multi-state reactivity modes are obtained on competing spin state surfaces.This difference is possibly due to the exchange stabilization of the Fe center during the H-abstraction.

Fig.4UB3LYP/6-31+G(d)optimized structures for the key species for the 2-propenol reactions of7(5)[3]1end-onwith propene

Fig.5Energy profiles(in kJ·mol-1)for the 2-propenol reactions of7(5)[3]1end-onwith propene.All energy values are at the UB3LYP/LACVP+*level

Basedontherecentlyproposedexchangeenhanced reactivity(EER)principle by Shaik et al[19], which states that if the number of identical-spin unpaired electrons on the metal center increases in the transition state(or the orbitals get more localizedon the metal center),this will maximize the exchange stabilization of the transition state.For the S=3,S=2, and S=1 spin states,during the H-abstraction,an electron shifts from the C-H bond to the O2π*⊥orbital andtherebythedelectronisfreedfromits antiferromagneticcoupling.i.e.,thenumberof unpaired d electrons of the Fe center is the same from reactantstoferrichydroperoxointermediate. Therefore,the condition of the smaller deformation energy of the reactants on the spin state,the HS S=5/ 2 state has a lower barrier as compared with the lowspin(LS)states,leading to single-state reactivity.The suggestion that the HS S=5/2 iron center of all these electronic structures has a high reactivity due EER is consistent with the experimental results of the key role of the HS non-heme iron center in O2activation.

2.3 Calculations of O-O bond cleavage process 2.3.1Crossing of the different PESs.

From Fig.5,the ground state product in quintet state,5P,will be formed from the intermediate in septet state,7IM1 via the transition state with the O-O bond broken.Therefore,at least a crossing and spin inversion process may be take place in the O-O cleavage reaction pathway.The geometric structure of the HS S=3 transition state,7TSOHis very different from those of the intermediate spin(IS)S=2,5TSOHand LS S=1,3TSOHtransition states(Fig.4).The7TSOHhas an O-O bond of 0.168 3 nm,which is shorter than those of the5TSOH,and3TSOH(0.1711,and 0.173 4 nm, respectively).These bond lengths indicate the7TSOHoccurs early in the O-O bond cleavage coordinate.As the O-Obond distanceincreases,theHSS=3 potentialenergysurfacesteeplyincreasesin energy and the S=2 potential energy surface gradually increases,which will lead to the crossing of different spin surfaces.

It is noted that the likelihood of such a crossover seems significant in view of the fact that the spin state surfaces are so close and cross from7IM1 to the crossing region(Fig.5).And,the7IM1-5IM1 energy gap is very small(Fig.5).As such,a change in the geometry of the septet complex7IM1 in the direction ofthequintetcomplexgeometry,5IM1,causes crossingbetweenthetwostates.Thereafter,the reactioncanproceedonthequintetsurfaceor bifurcate again to the septet surface.These willdepend on the magnitude of the transition probability. Among the factors that affect the magnitude of the transition probability is the SOC interaction between the states.Let us then discuss the SOC interaction.

Table 1Contributions to the calculated ZFS between SOC and Spin-Spin(SS)(all numbers are in cm-1) in the crossing region

Table 2Calculated SOC matrix elements(cm-1)of septet and quintet states in the crossing region by CASSC (10,8)method

2.2.2 Spin-orbit coulping(SOC)inthecrossingregion. Because of the intricate interplay of the spin-spin (SS)dipolar interaction with the SOC of the quintet state in the crossing seam,here we considered it desirable to include the calculation ofzero-field splitting(ZFS)parameters(D-tensor,D=Dzz-1/2(Dxx+

Dyy))[20].The ZFS and SOC matrix elements were evaluated at the CASSCF(10,8)wave function with 6-31+G(d)and TZVP basis sets using quasi-degenerate perturbationtheory.Thesecalculationswere performed with the program ORCA 2.8[18].The mixing of the S=3 and S=2 levels in the crossing seam by the spin-dependent terms in the Hamiltonian is treated approximately.Only the elements of SOC operator between the lowest HS septet state and the lowest three quintets are considered,where elements between quintets and triplets are ignored.These detailed results of the ZFS calculations are shown in Table 1. From the results in Table 1,the main contribution is from the second-order SOC interaction,while the SS contributions are negligible.The SOC part contains three parts:the SOC of electronic excited states of the same spin(Sexcited=Sground;ΔS=0,D(0))into the ground state;from states differing by one spin flip(Sexcited= Sground±1;ΔS=-1,D(-1)and ΔS=+1,D(+1));and the elements of quintets,S=2→triplets,S=1(S=-1), which are ignored(D(-1)=0.0).The ΔS=0 contribu-tions are found to make significant contributions to Dxx=-0.099 cm-1,Dyy=-0.202 cm-1,and Dzz=-0.193 cm-1, with the main contribution arising from the same spin states(i.e.,the quintet ground state→excited quintet mixing).In addition,it is very small that the SOC contributions come from the spin-raising ΔS=+1 excitations corresponding to the quintet ground state septet mixing,which indicates that the quintet and septetmixingcanbeforbiddenbytheSOC interaction.

In order to further understand the mechanism of intersystem crossing from the septet state to quintet state PES,the ROHF orbitals for the construction of the quintet and septet CASCI wave functions to be used in the SOC evaluation have been generated in the crossing region by quintet ROHF calculations, which were performed with the GAMESS program package[21].At least eight active orbitals,as given in Fig.6(in order to save space,the two nonactive doubly occupied orbitals are omitted),are found to be essential to reproduce the qualitative trends of SOC in the O-O bond cleavage step.The SOC matrix elements between the septet state and the quintet states in the crossing region are indicated in Table 2,we computed the SOC constants of the sextet,S1and quintet,Q0state at the crossing region and found it to be 2.26 and 2.19 cm-1at the CASSCF(10,8)/6-31+G(d)//TZVP levels,respectively.These values are very low and provide a first hint that intersystem crossing may be forbidden primarily for an electronic reason.For facile spin flip from the S=3 to S=2 surfaces,the crossing points are required to have similar geometries and energies.Moreover,the electronic configurations must be able to SOC.SOC is effectively a localized,singlecenter,one-electron operator and can be written as

Fig.6Electronic configurations of the SOC interactions of the septet state and quintet states(Q0,Q1and Q2)in the vicinity of thecrossing region for the O-O bond breaking step.The labels S and Q refer to the spin states septet and quintet, respectively.

where L is the orbital angular momentum operator, and S is the spin operator,while L·S=I is the angular momentum of electron(see formulations 2 and 3 in Computational details);the φ is the space part of the molecular orbital,θ the spin of the electron.The L+S-+ L-S+operator in Eq.4 performs a spin-flip and this process is accompanied by achange in the orbital due to the L+/L-raising/lowering operator[22].Therefore, two orbitals of opposite spins in SOC have to different spatial components.In addition,SOC is also feasible only if two microstates differ solely in the occupation of two orbitals with the same spin states or two microstates have the same Msfor the two different spin states and these two orbitals can couple through the Lzoperator.

Orbital analysis on the SOC mechanism are listed in Fig.6,for both spin states,S1and Q0,the Fe center remains HS ferric with strong bonding interaction with the O atom.Hence,the major difference in the electronic structure between the S=3 and S=2 spin states at the crossing point lies in the spin of electron residing in the singly occupied π*sub,with α for S=3 and β for S=2.Obviously,two spin orbitals have the same spatial component in their wave functions(π*sub). Therefore,theS=3surfacecannoteffectively intersystem cross to the S=2 surface through the SOC interactionsastheorbitalangularmomentum operators associated with SOC in Eq.4 require a change in orbital occupation.Thus,thereaction system can still proceed on the S=3 surface.

We also explored the SOC interaction of the septet state and two low lying quintet excited states, Q1and Q2,involving mostly Fe-3d excitations due to a transition metal complex where there are a number of near-degenerate states for close lying metal d-orbials. From Fig.6,because the SOC constant(ζFe)is an order of magnitude greater than the SOC values for oxygen, it is a reasonable approximation to consider only the Fe contribution when discussing spin-orbit mixing with quintet states.Thus for the SOC matrix elements of S1and Q1can be written as[17,23]

where η is the Ms-dependent weighing factor,and θ=α and/or β.In this case,for the septet state,S1,the fundamentalopen-shellconfigurationhasone dominantcoefficient,i.e.C0=0.961,whilethe coefficient for quintet state is CQ1=0.87.Thus,the Q1state is generated from the septet S1state by electron shifts from φ5to φ1,lead to the d-atomic orbital matrix elements,Based on transfer of d orbitals under the operator of Lx,y,zoperators,the former will generate a y component of the SOC,the latter will lead to z component of the angular momentum,which is consistent with the calculated SOC values of＜7φcm-1at CASSCF(10,8)/6-31+G(d)level.Similarly,the Q2state originates from the septet S1state by electron shifts from φ5to φ2,leading thereby to an x,y components of SOC with theelements,respectively.These calculated results show that the Q1and Q2states in crossing point will produce a significant one-center SOC interaction.Therefore,this can enhance the probability of intersystem crossing from the septet to the quintet state.However, these spin-flip pathways(S1→Q1,S1→Q2)are unfeasible because the excited crossing points have signifi-cantly higher in energy than the S1state,Q1and Q2are approximately 56.2 and 64.2 kJ·mol-1higher than the S1state at the CASSCF(10,8)/6-31+G(d),respectively.Thus,the O-O bond homolysis step should remain on the S=3 surface as the reaction proceeds, overcoming an activation free energy of 124.9 kJ·mol-1(Fig.5),while the intersystem crossing is possibly occurred at the exit stage of the reaction.

To further understand mechanism of the S1→Q0spin-flip,the corresponding splitting and population distributions of Zeeman sublevels of an S=2 species with an applied field B in the vicinity of the S1/Q0crossing region can be seen in Fig.7.In zero magnetic field,the lowest quintet state Q0is split into three spin states with eigenfunctions|Qx＞,|Qy＞,and|Qz＞, with an energy splitting described by the parameter D. For splitting of Zeeman sublevels,the eigenfunctions of the quintet spin states are given by|Q±2＞,|Q±1＞, and|Q0＞and can be related to those at zero field by mixing coefficients that depend on the strength and direction of the magnetic field.From Fig.7,three zero field sublevels Qx,Qy,and Qzare selectively populated,and their relative populations are carried over to the high field energy levels,Q±2,Q±1,and Q0,Qy,and Qzoverpopulation and some population on the Qxsublevel.The populations on Qy,and Qzlevels are nearly equal,1.17×10-1,whereas that on Qxis somewhat smaller,1.16×10-1.These different populations are mainly attributed to the SOC-ISC interactions(＜7φ-1.57 cm-1),but these populations are very small, which indicate that intersystem crossing from septet to quintet is low efficient in the crossing region.

Fig.7Splitting and population distributions of Zeeman sublevels of an S=2 species with an applied field B in the vicinity of the S1/Q0crossing region z axis of the molecule is defined as its Zeeman axis

3 Conclusions

In this study,the multi-state reaction mechanism for the propene catalyzed by non-heme ferric-superoxo model complex has been investigated using density functional theory calculations.For H-atom abstraction step,an electrophilic attack by a7(5)[3]1end-onspecies is enabled through a σ-attack of the superoxo π*⊥orbital. This leads to the transfer of a H-atom along with a spindown β electron from the C-H bond of the substrate into the π*⊥orbital of O2to generate a ferric hydroperoxo product and a radical on substrate.Thus,a strong π(O2-) bond is broken.The lower energy pathway of the H-abstraction process was occurred on the HS S=3 state potential energy surface(PES).By contrast,the corresponding quintet and triplet H-abstraction barriers are well higher in energy and will not play a role of importance.These are possibly due to the exchange stabilization of the Fe center during the H-abstraction.As for the O-O bond broken step,at least a crossing and spin inversion process may be taken place in the O-O cleavage reaction pathway.In order to quantitatively understand the crossing of the S=3,S=2,and S=1 PESs,we computed the SOC constants(2.26 and 2.19 cm-1at the CASSCF(10,8)/6-31+G(d)//TZVP levels,respectively) of the septet,S1and quintet,Q0state at the crossing re-gion.Orbital analysis show that the S=3 surface cannot effectively intersystem cross to the S=2 surface through the SOC interactions as the orbital angular momentum operators associated with SOC require a change in orbital occupation.Thus,the reaction system can still proceed on the S=3 surface.

Supporting information is available at http://www.wjhxxb.cn

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Theoretical Investigation on the Multi-State Reaction Mechanism for the Propene Catalyzed by Non-Heme Ferric-Superoxo Species

Lü Ling-Ling＊,1ZHU Yuan-Cheng1ZUO Guo-Fang1YUAN Kun1WANG Yong-Cheng2
(1College of Chemical Engineering and Technology,Tianshui Normal University,TianShui,Gansu 741001,China)
(2College of Chemistry and Chemical Engineering,Northwest Normal University,LanZhou,730070,China)

The multi-state reaction mechanism for the propene catalyzed by non-heme ferric-superoxo model complex has been investigated at the DFT-B3LYP level.The calculations show that non-heme ferric-superoxo complex can be considered as effective oxidants in hydrogen atom abstraction reaction(single-state-reactivity),for which we find a lower barrier of ΔG≠=65.6 kJ·mol-1on the septet spin state surface.Single-state-reactivity is possibly due to the recently proposed exchange-enhanced reactivity(EER)principle with larger exchange stabilization of the Fe center.For the O-O bond activated step,we computed the spin-orbit coupling(SOC)constants of the septet,S1and quintet,Q0state at the crossing region and found it to be 2.26 and 2.19 cm-1at the CASSCF (10,8)/6-31+G(d)//TZVP levels,respectively.Orbital analysis show that two spin orbitals have the same spatial component in their wave functions(π*sub),therefore,the S=3 surface cannot effectively intersystem cross to the S=2 surface through the SOC interactions，and the intersystem crossing is possibly occurred at the exit stage of the reaction.

non-heme ferric-superoxo;multi-state reaction mechanism;intersystem crossing;spin-orbit coupling

O641.12+1

A

1001-4861(2017)02-0329-11

10.11862/CJIC.2017.028

2016-02-04。收修改稿日期：2016-12-03。

國家自然基金(No.21263022;21663025;2163024)、甘肅省教育廳導(dǎo)師基金和天水師范學(xué)院“青藍(lán)”人才工程基金資助項目。＊

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