Theoretical Investigation on Photoionization and D issociative Photoionization of Toluene

Yu-jie ZhaoYou-sheng ZhanLi LiXin LiXiang-yu LianPeiHuangLiu-siShengJun Chen

a.Engineering Research Center of Nuclear Technology Application(East China University of Technology),M inistry ofEducation,Nanchang 330013 China

b.School ofNuclear Science and Engineering,East China University of Technology,Nanchang 330013, China

c.National Synchrotron Radiation Laboratory,School ofNuclear Science and Technology,University of Science and Technology ofChina,Hefei230029,China

Theoretical Investigation on Photoionization and D issociative Photoionization of Toluene

Yu-jie Zhaoa,b,c??,You-sheng Zhana,b?,Li Lia,b,Xin Lia,b,Xiang-yu Liana,b,PeiHuanga,b,Liu-siShengc?,Jun Chenc

a.Engineering Research Center of Nuclear Technology Application(East China University of Technology),M inistry ofEducation,Nanchang 330013 China

b.School ofNuclear Science and Engineering,East China University of Technology,Nanchang 330013, China

c.National Synchrotron Radiation Laboratory,School ofNuclear Science and Technology,University of Science and Technology ofChina,Hefei230029,China

The photoionization and dissociation photoionization of toluene have been studied using quantum chem istry methods.The geometries and frequencies of the reactants,transition states and products have been performed at B3LYP/6-311++G(d,p)level,and single-point energy calculations for all the stationary pointswere carried out at DFT calculations of the optim ized structures w ith the G3B3 level.The ionization energies of toluene and the appearance energies formajor fragment ions,C7H7+,C6H5+,C5H6+,C5H5+,are determ ined to be 8.90,11.15 or 11.03,12.72,13.69,16.28 eV,respectively,which are all in good agreem ent w ith published experim ental data.W ith the help of availab le published experim ental data and theoretical results,four dissociative photoionization channels have been proposed: C7H7++H,C6H5++CH3,C5H6++C2H2,C5H5++C2H2+H.Transition structures and intermediates for those isomerization processes are determ ined in this work.Especially,the structures of C5H6+and C5H5+produced by dissociative photoionization of toluene have been defined as chain structure in thiswork w ith theoretical calculations.

Quantum chem ical calculations,Toluene,Dissociative photoionization m echanism,Density functional theory,Transition states

I.INTRODUCTION

Aromatichydrocarbonssuch asbenzene,toluene,and xylene are major com ponents of volatile organic compounds(VOCs)in urban areas,w ith toluene being themost abundant aromatic hydrocarbon am ong them, which play an im portant role in the formation of secondary organic aerosols[1?4].Moreover,toluene has strong stimulation to eyes,skin,mucousmembrane and respiratory system.Toluene also takes a part in reactions to promote photochem ical smog and other local atmospheric effects,which may contribute significantly to ozone formation in the troposphere[5?9].Therefore,the understanding of the process for dissociative photoionization of toluene is needed for evaluating the risks involved in aromatic compounds.

The photoionization of toluene has been studied by various experimentalmethods.By using electron impact techniques,M cloughlinet al.[10,11]obtained theionization energy(IE)of toluene and the appearance energy(AE)of C7H7+,which are 8.82 and 10.71 eV, respectively.Lifshitzet al.[12,13]investigated two dissociative photoionization channels of C7H7+, through time resolved photoionizationmass spectrometry(TPIMS)by combining w ithab initiocalculations, in which the AEs of C7H7+being determ ined to be 11.1 and 10.7 eV atT=0 andT=298 K,respectively.Shawetal.[14]used threeexperimental techniques(photoabsorption,photoelectron and photoion spectroscopy),togetherw ithmany-body Green’s function calculations to investigate the spectroscopic and therm odynam ic properties of toluene and obtained the IE of the parent ion to be 8.845 eV and the AEs of major fragment ions C7H7+,C6H5+,C5H5+arem easured to be 11.8,14.4, 16.5 eV,respectively.Especially,the obtained AE of C5H6+was 13.5 eV.

However,as far as we know,there is few theoretical investigations on dissociative photoionization of toluene.In particular,there is very lim ited information about the geometries of the parent ion and the m ain fragment ions in the literatures up to date.Moreover, the dissociative photoionization mechanism s of toluene are still unclear so far.Therefore,in the present work, DFT m ethod is em p loyed to investigate the dissocia-tive photoionization of toluene.The theoretical IE of toluene,AEs for its fragment ions,transition structures (TS)and intermediates(INT)are determ ined.The m echanism s of the dissociative photoionization pathways are also discussed on the basis of theoretical and experimental data from literature.In addition,the transition and interm ediatesstates involved in the pathways are also obtained by theoretical calculations and described in detail in thiswork.

II.QUANTUM CHEM ICAL CALCULATIONS

In this theoretical study,all the geometry optim izations of the reactants,transition states,intermediates, and other productsare done at the B3LYP levelw ith 6-311++G(d,p)basis sets,and harmonic vibrational frequencies are also com puted analytically at the sam e level in order to characterize the optim ized geometries as potentialm inima or saddle points.The structures of transition states(TS)and interm ediates(INT)for dissociative photoionization channels are also identified in this study.To confi rm that the obtained transition states connect w ith the right reactants and products,the intrinsic reaction coordinate(IRC)calculations were performed at B3LYP/6-311++G(d,p)level. On the basisof theobtained stationary points,moreaccurate energieswere then obtained by single-point calculations at the G3B3 level.

The m ethod of G3B3 has been reported elsewhere [15],and only a brief summary is given here.(i)Step 1:produce an initial equilibrium structure at the Hartree-Fock level using the 6-31G(d)basis set.Verify that it is a m inimum w ith a frequency calculation and predict the zero-point energy(ZPE).This quantity is scaled by 0.8929.(ii)Step 2:beginning w ith the final optim ized structure from step 1,obtain the final equilibrium geometry using the full MP2 method w ith the 6-31G(d)basis set.This geometry is used for all subsequent calculations.(iii)Step 3:a series of single-point energies calculations are carried out at higher levelsof theory.The fi rst higher level calculation is MP4/6-31G(d).This energy is then modified by a series of corrections from additional calculations.(iv) Step 4:the MP4/6-31G(d)energy and four corrections from step 3 are combined in an additivemanner along w ith a spin-orbit correction,?E(SO),for atom ic species only.(v)Step 5:a“higher level correction”(HLC)isadded to take into account rem aining deficiencies in the energy calculations:(vi)Step 6:finally,the total energy at 0 K is obtained adding the zero-point energy,obtained from the frequencies of step 1 to the energy.This energy is referred to as the“G3 energy”. A ll calculations above-mentioned are all performed w ith the Gaussian 03 program.

TABLE I Calculated energies of species(neutral toluene and its cation,p roducts,by-p roduced fragm ents,interm ediates(INT),transition states(TS))involved in the photodissociation of toluene at the G3B3 level.

III.RESULTS

W ith the theoretical calculation,the totalenergies of species involved in the study of dissociative photoionization of toluene areobtained at theG3B3 level,which are listed in Table I.Especially,the imaginary frequenciesof transition states(TS)aregiven in Tables S3?S17 (see supp lementarym aterials).Because thereare lotsof diff erent products,intermediates and transition states in the present work,they are named by using their prefix w ith a unique number,such as P1,INT 1 and TS1, which can make them distinguished easily.In the case of isomers,suffi xes of a,b,c,etc.are app lied(for exam p le,P5a)in the dissociative photoionization channel of C5H5+.

It is well known that the parent ion C7H8+can be generated directly by a single-photon ionization[10, 14].The present calculation using G3B3 m ethod gives an adiabatic IE of8.90 eV,which is in reasonableagreementw ith theavailableexperimentalvalue,8.82 eV[10] and 8.845 eV[14].The IE of C7H8is calculated as follows:IE(C7H8+)=E(C7H8+)?E(C7H8)=8.90 eV.In the case of possible dissociation channel C7H8→C7H7+(P1a)+H,the AE of C7H7+(P1a)is obtained from: AE(C7H7+(P1a))=E(C7H7+(P1a))+E(H)?E(C7H8)= 11.15 eV,which is in good agreement w ith the exper-imental value of 11.1±0.1 eV by Lifshitzet al.[12]. As the dissociation energy(Ed)can be calculated by subtracting the IE of parent molecu les from the AE of the corresponding fragment ion,theEdof C7H7+(P1a) can be expressed in the follow ing form:

TABLE II Theoretical and literature values of the ionization energy(IE),appearance energy(AE),and dissociation energy (Ed,theoretical)of possible dissociative photoionization channels.

FIG.1 The optim ized geom etries of the neutral toluene and itscation.(a)C7H8,(b)C7H8+.Bond length is in unit of?A.

In addition,the AEs andEds of other p roducts are also obtained in the sameway,which are listed in Table II.

The optim ized geometries of neutral toluene and its parent cation are obtained at the B3LYP/6-311++G(d,p)level,and all their C?C bond lengthsare also calculated,which areallshown in the FIG.1.From FIG.1,we can know that some C?C bonds of parent cation becom e shorter,while someothersbecom e longer in comparison w ith those of neutralmolecule,which indicates that dissociation of parent cation w ill undergo diff erent pathwaysw ith the photon energy increase.

IV.DISCUSSION

FIG.2 The dissociation channels for toluene cation to produce fragment ions,C7H7+(P1)calculated at the G3B3 level.The energy of neutral toluene is defined to be zero.

FIG.3 The dissociation channel for toluene cation to produce fragm ent ion,C7H7+(P2)calculated at the G 3B 3 level. The energy of neutral toluene is defined to be zero.

The fragm ent ions of toluene in the dissociative photoionization have already been discussed elsewhere [10,12,16,17,18],wherein the main fragmentation channels have been proposed as follows:C7H7++H, C6H5++CH3,C5H6++C2H2,C5H5++C2H2+H.However,the detailed dissociative photoionization mechanisms of the fragmentation pathways have not been clarified.In this work,the dissociativem echanism s of C7H8+are discussed based on our theoretical results and available experimental data[10?30].These disso-ciativephotoionization channelsareshown in FIG.2?4, respectively.In addition,the detailed information on the geom etries of the optim ized reactants,transition states,intermediates and products are also shown in FIG.5?7,where the main bond angles and distances are indicated.

FIG.4 The dissociation channels for toluene cation to produce fragm ent ions C6H5+(P3)and C5H6+(P4a),C5H6+(P4b) and C5H5+,calculated at the G 3B 3 level.The energy of neutral toluene is defined to be zero.

FIG.5 The geom etries of the transition states at the B3LYP/6-311++G(d,p)level.Bond length is in unit of?A and bond angel in unit of(?).

As the photon energy increases,the parent ion w illundergo different dissociative photoionization pathways.Generally,there are two types of mechanism s for dissociation:direct sim p le bond cleavage or indirect bond cleavage via transition states and interm ediates.For instance,the C6H5+ion are formed by loss of CH3from the parent ion(C7H8+).This is one-step dissociation process w ith no distinct transition states as reported previously[19].AE of the C6H5+ion is predicted to be 12.72 eV,which is in good agreement w ith available theoretical value(12.72 eV) [16].By com paring w ith the structure of parent cation and neutralmolecule(see FIG.1),we found that the C3?C12 and C2?C1 bond lengths are increased by about 0.0350 and 0.0257?A,respectively,while the C2?C3 and C5?C6 bond lengths are decreased by about 0.0395 and 0.0287?A,respectively.Especially,the C3?C12 bond is the longest in the parent ion.Therefore,excited by higher photon energy,the C3?C12 bond can be cleaved easily and dissociated to form C6H5++CH3,which agreesw ith the results in Ref.[14].

A.C7H7++H

The structure of C7H7+ion has been investigated by various methods[20?22].Two im portant isom ers of C7H7+:benzyl(six-membered ring)and tropylium cation(seven-membered ring),are themost im portant dissociation products of parent ion(C7H8+)near the threshold[12,18,23,24].Therefore,there are two possible formation pathwayswhich can produce C7H7++H (shown in FIG.2 and 3).

The benzyl ion is form ed by direct loss of H from the parent ion.It is obvious that there are four types of hydrogen atom s in the parent ion.We calculated the AEs of four possible fragment ion isomers,and the outcom es are shown in Table Iand FIG.2.

FIG.6 The geom etries of the reactant interm ediates at the B3LYP/6-311++G(d,p)level.Bond length is in unit of?A and bond angel in unit of(?).

P1a((C7H7+)in Table I)is formed from the cleavageofC12?H15 bond,which is1.1064?A in length.The other three isomers are the H elim ination from C1 for P1b,C2 for P1c and C6 for P1d.These C?H bond lengths are 1.0831,1.0838 and 1.0847?A correspondingly.And the theoretical AEs for P1a,P1b,P1c and P1d are 11.15,12.89,12.95,and 13.04 eV,respectively. It is not surprising to find that the P1a channel is the lowest energy required.There are also possible TS for the other three channels,but their AEs are apparently much higher than the experimentalvalue(11.1±0.1 eV) [12].Ow ing to the former theoretical result is in good accordance w ith the experimental value,11.1±0.1 eV [12],we tend to consider the H elim inates from the C12. Thegeometry of the fragment ion P1a at the B3LYP/6-311++G(d,p)level is shown in FIG.7.

The process for Tropylium cation(seven-m embered ring,P2)is somewhat com p licated,which need to be carried out via TSand INT,asshown in FIG.3.Firstly, the hydrogen atom H15 of themethyl group is transferred to the C3 of the benzene ring to form INT 1 via TS1.Then,because of the steric effect[29]from the ortho C4 of benzene ring,C12?C4 bond reconstructs to form INT 2.Third ly,INT 3 is form ed through C3?C2 bond cleavage via TS3.Finally,the INT 3 tends to produce P2 by further loss of the H radical via TS4. Calculated AE of this channel is 11.03 eV,which is also in agreement w ith available experimental value (11.1±0.1 eV)[12].

B.C5H6++C2H2

Further dissociation of C7H8+can produce C5H6+and C2H2when the photon energy rises[14,25].Flammang and Meyrantet al.[25]have studied the structures of gas phase C5H6+ions,which are generated by direct electron ionization of C5H6isomers or dissociative ionization of other precursormolecules(phenol, thiophenol and so on).They have concluded the geometry of C5H6+ion can be identified as the ring structure (P4a),which are from phenol and thiophenol.

FIG.7 The geom etries of the neutral toluene,its cation and its fragm ents(ions and neutrals)at the B3LYP/6-311++G(d,p) level.Bond length is in unit of?A and bond angel in unit of(?).

Sim ilarly,the geometry of C5H6+ion from C7H8+can also be fi rstly proposed as the ring structure(P4a), which is shown in FIG.7.The detailed form ation pathway of C5H6+w ith potential energy is depicted in FIG.4(a)and(b).For the ring structure of C5H6+(P4a),the decom position reaction is initiated by methyl H-m igration to an ortho carbon,followed by the decom position to C5H6+and C2H2from C7H8+.To make it more feasible,the dissociation mechanism is described as follows.(i)First,a TS7 is proposed and the H 15 transfers from C12 to C4 w ith an energy barrier of 1.52 eV,giving rise to INT6.(ii)Second,one H atom on the C4 transfers to C5 via TS8 to produce INT7.(iii)Third,Because of ortho effect,INT8 can be formed via TS9w ith an energy barrier of3.14 eV.Then, onehydrogen atom on C12 is transferred to C3,form ing INT9 before C2H2elim ination.(iv)Finally,the C5H6+is form ed by C3?C4 bond cleavage.The theoretical AE in this dissociation pathway is 12.32 eV,which ismuch lower than the experimental value of 13.50 eV[14].On the other hand,Liet al.[26]has performed experimentaland theoretical studies on the dissociative photoionization oftrans-2-methyl-2-butenal.In their study,the structure of C5H6+ion can be identified as chain structure.According to the above situation,the geometry of C5H6+ion from C7H8+is tentatively identified as the chain structure(P4b)in the present work,which is also shown in FIG.7.For the chain structure of C5H6+(P4b),the calculated AE of C5H6+is determ ined to be 13.69 eV,which is in agreem ent w ith the previous value 13.50 eV[14].FIG.4(b)shows that the parent cation undergoesa hydrogen atom shift to C6 from C5 to yield the INT12 via TS13.Next,thehydrogen atom m igrates from the C4 to its neighboring C3 through TS14 to produce INT13 by overcom ing an energy barrier of3.17 eV. Then,INT14 is formed through C3?C2 bond cleavage via TS15.Finally,C5H6+(P4b)is yielded w ith loss of C2H2from INT 14,and no barrier for this elim ination reaction is found at the B3LYP/6-311++G(d,p)level. The reaction barrier for the formation of C5H6+(P4b) is 4.79 eV(relative to parent ion),which is in agreement w ith the experim ental value,13.50 eV,obtained by Shawet al.[14].

C.C5H5++C2H2+H

It iswell known that the C7H7+(P1a)ion produced from parent ion decom poses into C5H5+ion by losing C2H2[27].However,the structure for C5H5+is controversial.On the one hand,Occolow itz and W hite have concluded the geometry of C5H5+ion was identified as the chain structure rather than the ring structure by m easuring the heat of formation[28].On the other hand,in the studies on the dissociative photoionization ofp-nitrotoluene by Zhanget al.in 2012,the structure of C5H5+ion was identified as ring structure[29].

In this work,both structures are calculated,and we get the TSsand INTs involved in the processusing DFT theory.Detailed pathway is described in FIG.4(c).For the ring structure of C5H5+(P5a),fi rst,in the benzyl ion(P1a),C2?C4 bond reconstructs via TS5 to form INT 4.Next,INT 4 undergoes a hydrogen atom shift to C12 from C3 to yield INT5 via TS5.The barrier of this step is calculated to be 1.48 eV.Finally,C5H5+(P5a)is produced by the fission of C3?C4 bond w ithout any apparent TS.AE of C5H5+(P5a)obtained from our theoretical(15.47 eV)ismuch lower than the previous value (16.4±0.2 eV)reported by Tajimaet al.[30].

For the chain structure of C5H5+(P5b),the benzyl ion(P1a)undergoes a hydrogen atom shift to C4 from C1 to yield INT10 via TS11.The barrier of this step is calculated to be 5.13 eV.The energies of TS11 and INT 10 are higher than that of parent ion by 7.38 and 5.16 eV,respectively.The breaking and form ing C?H bond lengths at TS11 are 1.3686 and 1.2801?A,respectively.Next process is from INT 10 to INT 11,in which the hydrogen atom m igrates from C12 to its neighboring C3 through TS12 by overcom ing an energy barrier of 1.77 eV.Finally,P5b is generated by the C3?C4 bond broken in INT 11,coup led w ith a C2H2loss.The overall barrier for the form ation of C5H5+(P5b)is 16.28 eV (relative to neutral toluene),which is in good agreement w ith the experimental value(16.4±0.2 eV)by Tajimaet al.[30].

V.CONCLUSION

In thiswork,quantum chem istry methods have been used to study the photoionization and dissociative photoionization of toluene.The present theoretical results provide several new insights into the dissociative photoionization m echanism s of toluene.The energies and possible dissociative channels for fragm ent ions from toluene have been estimated on the basis of the quantum chem ical calculations.Specific inform ation of fragmentation pathways are discussed in detail.Generally speaking,the dissociative photoionization processes of toluene are somewhat comp licated, m any of them undergo diff erent dissociative photoionization pathways,such as transition structures,intermediates,H-m igration and/or H-elim ination,except for the channel(C6H5++CH3),which undergoes direct simp le bond cleavage.And some dissociative products have diff erent isom ers,which are all distinguished in the present work.In particular,according to calculation and comparison,the C5H5+and C5H6+can be identified as the chain structure in the dissociative photoionization of toluene.The m echanistic study of dissociative photoionization of toluene w ill be help ful in understanding the fragmentation.

Supp lem entary m aterials:The imaginary frequencies of transition states pertinent to thiswork are given in Tables S3?S17.

VI.ACKNOW LEDGM ENTS

This work was supported by the National Natural Science Foundation of China(No.11275006, No.U1232209,No.U1232130,No.41275127, No.11575178,No.U1532137),Nuclear Technology App lication Engineering Research Center Open Foundation of M inistry of Education(No.HJSJYB2015-6), the Chinese Scholarship Council(No.201608360053), the G raduate Students High-Quality Course Construction Program of Jiangxi Province(No.JXYYK 2016-12),the China Postdoctoral Science Foundation (No.2013M 531530),the Doctoral Foundation of East China University of Technology(No.DHBK 201401) and the Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (No.KJ2012B086).

[1]Y.J.Zhang,Y.J.Mu,J.F.Liu,and A.Mellouki,J. Environ.Sci.24,124(2012).

[2]H.J.Avens,K.M.Unice,J.Sahm el,S.A.G ross,J.J. Keenan,and D.J.Paustenbach,Environ.Sci.Technol. 45,7372(2011).

[3]S.Vardoulakis,E.Solazzo,and J.Lumbreras,Atmos. Environ.45,5069(2011)

[4]L.Fishbein,Sci.Total Environ.40,189(1984).

[5]L.Fishbein,Sci.Total Environ.43,165(1985).

[6]V.Cocheo,P.Sacco,C.Boaretto,E.D.Saeger,P.P Ballesta,H.Skov,E.Goelen,N.Gonzalez,and A.B. Caracena,Nature 404,141(2000).

[7]E.Borras and L.A.Tortajada-Genaro,Int.J.Environ. Anal.Chem.92,110(2012).

[8]E.Durmusoglu,F.Taspinar,and A.Karadem ir,J.Hazard.Mater.176,870(2010).

[9]Y.Zhou,H.F.Zhang,H.M.Parikh,E.H.Chen,W. Rattanavaraha,E.P.Rosen,W.X.Wang,and R.M. Kam ens,A tm os.Environ.45,3382(2011).

[10]R.G.M cloughlin,J.D.M orrison,and J.C.Traeger, Org.Mass Spectrom.14,104(1979).

[11]J.C.Traeger and R.G.M cloughlin,Int.J.M ass Spectrom.Ion Phys.27,319(1978).

[12]C.Lifshitz,Y.Gotkis,A.Ioff e,J.Laskin,and S.Shaik, Int.J.M ass Spectr.Ion Proc.125,196(1993).

[13]C.Lifshitz,Y.Gotkis,J.Laskin,A.Ioff e,and S.Shaik, J.Phys.Chem.97,12291(1993).

[14]D.A.Shaw,D.M.P.Holland,M.A.M acDonald,M.A. Hayes,L.G.Shpinkova,E.E.Rennie,C.A.F.Johnson, J.E.Parker,and W.von Niessen,Chem.Phys.230, 97(1998).

[15]L.A.Curtiss,K.Raghavachari,P.C.Red fern,V.Rassolov,and J.A.Pop le,J.Chem.Phys.109,7764(1998)

[16]L.Tao,Master Thesis,Hefei:Anhui Institute of Optics and Fine M echanics,Chinese Academ y of Sciences, China(2010).

[17]M.Schwell,F.Du lieu,C.G′ee,H.W.Jochim s,J.L. Chotin,H.Baum g¨artel,and S.Leach,Chem.Phys. 260,261(2000).

[18]C.J.Chul,J.Phys.Chem.A 110,7655(2006).

[19]J.R.Majer and C.R.Patrick,J.Chem.Soc.Faraday Trans.58,17(1962).

[20]K.R.Jennings and J.H.Futrell,J.Chem.Phys.44, 4315(1966).

[21]P.N.Rylander,S.M eyerson,and H.M.G rubb,J.Am. Chem.Soc.79,842(2002).

[22]C.Q.Jiao and S.F.Adam s,Chem.Phys.Lett.573, 24(2013).

[23]R.Bombach,J.Dannacher,and J.P.Stadelm ann,J. Am.Chem.Soc.105,4205(2002).

[24]R.Bombach,J.Dannacher,and J.P.Stadelmann, Chem.Phys.Lett.95,259(1983).

[25]R.Flammang,P.M eyrant,A.Maquestiau,E.E. K ingston,and J.H.Beynon,O rg.M ass Spectrom.20, 253(1985).

[26]Y.Li,M.Cao,J.Chen,Y.Song,X.Shan,Y.Zhao,F. Liu,Z.W ang,and L.Sheng,J.M ol.Struct.1068,130 (2014).

[27]S.M eyerson and P.N.Rylander,J.Chem.Phys.27, 901(1957).

[28]J.L.Occolow itz and G.L.W hite,Aust.J.Chem.21, 997(1968).

[29]Q.Zhang,W.Z.Fang,Y.Xie,M.Q.Cao,Y.J.Zhao, X.B.Shan,F.Y.Li,Z.Y.Wang,and L.S.Sheng,J. Mol.Struct.1020,105(2012)

[30]S.Tajim a and T.Tsuchiya,Bu ll.Chem.Soc.Jpn.46, 3291(1973).

ceived on March 20,2017;Accepted on March 27,2017)

?These authors contributed equally to this work.

?Authors to whom correspondence shou ld be add ressed.E-m ail: jackzy j@ustc.edu.cn,lssheng@ustc.edu.cn