2-苯基苯并噁唑的光解反应

李会学 王晓峰 李志锋 朱元成

(天水师范学院生命科学与化学学院,新型分子材料设计与功能省教育厅重点实验室,甘肃天水741001)

2-苯基苯并噁唑的光解反应

李会学*王晓峰 李志锋 朱元成

(天水师范学院生命科学与化学学院,新型分子材料设计与功能省教育厅重点实验室,甘肃天水741001)

聚对苯撑苯并二噁唑(PBO)纤维对光较为敏感,在紫外光照射下会发生降解.本文研究了该纤维的单体2-苯基苯并噁唑(PO)的初级光化学反应机理.当PO分子吸收一个光子而跃迁到第一激发态后,克服25.59 kJ· mol-1能垒而越过过渡态,此时噁唑环打开,且两个苯环形成大约90°的二面角而得到产物,该产物可进一步与空气中的水发生次级反应.计算结果表明在第一激发态上噁唑开环反应很容易,但在基态势能面并没有发现噁唑的开环路径.分子中的原子(AIM)的计算结果与上述分析过程相吻合.

理论研究;光解反应;2-苯基苯并噁唑

1 Introduction

Poly[p-phenylene benzobisoxazole](PBO)can be processed into an organic fiber,which exhibits high tensile strength and modulus with a low density,therefore PBO fibers possess outstanding mechanical and thermal properties and can be used as reinforcement materials in advanced composites.Owing to above advantages,this kind of fiber has great potential applications in the fields of aerospace,military industry,and general industry.For example,high performance fibers based on PBO have become prominent in body armor,ropes,cables,and recreational equipment.1-13

In order to improve the interfacial adhesion of PBO fiber and the tensile strength,the modification of PBO fiber is of great importance.Commonly used method is to introduce polar functional groups in the polymer repeat unit.5,14,15Dang16added－OH to PBO and obtained extremely high modulus/high strength fibers,which is thought to have a bidirectional network of hydrogen bonds in its chemical structure17-19when hy-droxyl attaches to the monomer of polymer PBO.Axial compression bending test showed that the introduction of binary hydroxyl groups into macromolecular chains apparently improved the equivalent bending modulus of the fibers.Hodges et al.20used nonpolar and polar solvents to deal with the fibers, only polar ethanol washing allowed to detect changes in the surface characteristics of PBO as-spun fibers.Kumar et al.21synthesized PBO fiber in the presence of single-wall carbon nanotubes(SWNTs),the tensile strength of the PBO/SWNT fiber containing 10%(mass fraction)SWNTs is about 50%higher than that of the control PBO fibers containing no SWNTs. Ran et al.22,23employed in situ solution spinning of PBO in poly(phosphoric acid)(PPA),when the coagulation time exceeded 30 min,the complex PBO/PPAstructure completely disappeared,and well-oriented PBO crystals were formed.

However,the anti-ultraviolet aging stability of PBO fiber was poor and the tensile strength of PBO fiber decreased to half its former size after UV exposure for an hour.24,25According to the experimental observation of Fourier transform infrared(FTIR)techniques,the degradation mechanism of PBO was suggested by Chin,26Jackson,27and Kim28et al.Through acid-catalyzed hydrolysis,the benzoxazole ring is opened and the benzamide will be formed with the action of water molecules in the air.Further the breakdown of benzamide groups could lead to the formation of aminophenol and benzoic acid groups.So et al.29prepared several benzoxazole compounds and investigated their extended configurations characterized by optical absorption and emission spectroscopy,and the molecular configuration permited benzoxazole compounds to undergo photoinduced electron transfer in the solid state and generated superoxide in the presence of oxygen.Zhang et al.30found that under irradiation of 254 nm UV light,phenyloxazolyl compounds underwent photolysis promptly to reproduce the transmonomers and formed cis-isomers by trans-cis isomerization. In addition,benzimidazo[2,1-b]benzoxazole was prepared photolytically at 360 nm from 1-(2-benzoxazolyl)benzotriazole.31However,there were few reference about the theoretical study on the optical degradation about PBO,in this paper we employed time dependent density functional theory(TD-DFT)to study the property of the excited state of PBO monomer and elucidated the primary process of photolytic destruction.

2 Computational details

It is known that the complete active space self-consistent field(CASSCF)methods are effective for theoretical studies of excited electronic states of molecules and molecular ions,32however,selection of the active space is the crucial step in CASSCF calculations,which is usually a little difficult,in addition,the complete active space is impossible to be selected enough large due to computational cost.For TD-DFT,the linear response approach can be applied to solve the equations. By simply adding the ground state DFT energy to the excitation energy of the selected state,TD-DFT provides a fast and reliable approach to obtain potential energy surfaces for the excited states as a function of the molecular geometry.The work of Scalmani et al.33indicated that the theory of TD-DFT analytical gradients about the excited state energy,not only in the gas phase but also in solution,can be applied to the analysis of UV spectra and to understand photophysical and photochemical pathways.

Fig.1 Molecular structure diagrams of PBO and PO H atoms are omitted.

TD-DFT has rapidly emerged as an extremely useful method for studying the excited electronic states of molecules,for many systems,it yields computational accuracy for the electronic excitation energies within tenths of eV,excited state bond lengths within 1%,dipole moments and vibrational frequencies within 5%,not to mention that the computational cost scales very favorably with the number of electrons.34So we adopted TD-DFT to investigate the photolytic mechanism of PO instead of CASSCF method.

The degradation of PBO occurs in 2-phenylbenzo[d]oxazole (PO)moiety,which is the primary structure of the monomer of PBO.In consideration of calculating difficulty of polymer,especially for excited states,we researched PO instead of PBO, the molecular structures of PO and PBO are shown in Fig.1.In general,fluorescence normally occurs from the zero vibrational level of the first excited state to the ground state and photochemical reaction occurs,so in this paper we only investigated the reaction of PO molecule in the potential energy surface of

Electronic structure calculations were carried out using Gaussian 09 program.36The geometry of the PO molecule in S0was optimized with B3LYP methods at the 6-31G*basis set level,the geometry of the S1was optimized using three different DFT approaches,(i.e.,B3LYP,B3PW91,and MPW1PW91) with the same basis set.All stationary points were positively identified as minima or first-order saddle points by evaluation of the frequencies and normal modes.Since the accuracy of DFT calculation also depends on the number of points used in the numerical integration in addition to the sources of numerical errors in the Hartree-Fock calculations,fine grids should be employed.In the present calculations,we used the default grid. Three-dimensional molecular orbital(MO)plots were obtained with the GaussView software.Each orbital was displayed with the 0.08 isodensity value and oriented to give the best view.

3 Results and discussion

3.1 Structures of ground and excited states

The ground-state structure(S0)of PO molecule and the atomic numbers are shown in Fig.2.All the atoms are at the same plane and it belongs to Cspoint group.The calculated bond pa-rameters using B3LYP/6-31G*are listed at the second column and the experimental values37are at the third column in Table 1.The calculated results are found to be in agreement with the experimental ones.This indicates that the adopted basis set and functional is feasible to the studied system.

The geometry of the lowest-excited state S1was optimized using three different DFT approaches(i.e.,B3LYP,B3PW91, and MPW1PW91)with the same basis set 6-31G*,the selected optimized bonds are listed at Table 1.One can see that the parameters of PO are very close to each other in S1,the calculated largest difference of each bond with different DFT approaches is less than 0.01 nm,all the optimized structures of PO in S1still keep the ground-state characters and possess a symmetry plane,and the above results imply that the optimized geometry of S1using B3LYP/6-31G*is relatively reliable.In addition, the calculated Wiberg bond indices using natural bond orbital (NBO)by B3LYP/6-31G*,which are related to the strength of the bonds,38are shown in Table 1 at the fourth column for the S0and the sixth column for the S1as well.All the changes of the bond lengths by B3LYP/6-31G*from S0to S1are listed simultaneously in parenthesis,thepronounced geometrical changes for PO are 7C－24N and 7C－12C bonds.One can see that C＝C double bonds of benzene ring in the S0including 1C－2C,12C－14C and 12C－13C are 0.1403,0.1404,and 0.1405 nm,respectively,however all those in S1obtained by TD-B3LYP are elongated and the corresponding data are 0.1437,0.1437,and 0.1441 nm,respectively,accordingly all the Wiberg bond orders(WBO)of S1are smaller than the ones of S0.The above suggests that the strength of all the C＝C double bonds are weakened when the S0turns into the S1.Similarly,optical radiation can also elongate the bond lengths of 7C－24N and 7C－23O in oxazole ring,which are 0.1301 and 0.1382 nm in S0,and 0.1365 and 0.1406 nm in S1,respectively, the WBOs of both the bonds decreases after PO molecule absorbs photons and transfers to S1,especially for 7C－24N,the WBO value(1.2652)in S1is 0.3549 less compared with the one(1.6201)in S0,which shows that the change of the electronic density at 7C－24N is the largest before and after the transition.On the contrary,the bond lengths in oxazole ring including 1C－23O,2C－24N,and bridge bond 7C－12C in S0are all larger than ones in S1,relative WBO values increase in S1. The result indicates that the electron distributing in benzene ring and in 7C－24N and 7C－23O of oxazole ring in S0transfers to 1C－23O,2C－24N,and 7C－12C in S1.

3.2 Frontier molecular orbitals and electronic spectrum

A sketch of the highest occupied molecular orbital(HOMO), lowest unoccupied molecular orbital(LUMO),and other occupied molecular orbitals involved electron transition are shown in Fig.3.In spite of the HOMO and LUMO locating on the whole molecule,the HOMO mainly spreads on 1C－2C－3C, 24N－7C－23O,and 14C－12C－13C moieties but the LOMO on 2C－24N,23O,and 7C－12C moieties,the HOMO-2 and HOMO-1 are located on benzene ring moiety.

Fig.2 Structure of PO molecule and the atomic numbers in S0

Fig.3 Molecular orbitals involved in the photolysis reaction in S0

Table 1 Selected optimized geometrical parameters and the Wiberg bond orders

Table 2 Calculated maximum absorption peaks,oscillator strength(f),main contribution,and the dipole moments of the first three excited state

The maximum absorption peaks,oscillator strength,main contribution of the first three low excited states are calculated by TD-DFT based on the optimized ground state configuration and all the data are listed in Table 2,we can draw the following conclusion from Table 2:firstly these orbitals are π symmetry characters,and so the main UV-visible absorption features are well described as π→π*transitions.Secondly the oscillator strength of S1is the largest,which is proportional to the transition moment and it reflects the transition probability from S0to the excited state,this means that the process of S0→S1is allowed by optical selection rule,the transition process is characterized by electron promotion from the HOMO to the LUMO and are polarized along the molecular plane.The S0dipole moment of PO is 3.803×10-30C·m,the ones of S1and S2are 1.256×10-29and 2.687×10-29C·m,respectively,it means that the charge transfers significantly in the excited state.The calculated S1transition energy is 289 nm which implies that ultraviolet light will damage the molecular structure.The other two excited states are transition forbidden due to the small oscillator strength,the involved molecular orbitals mainly are the HOMO-1 and the HOMO-2,of which both the orbitals are located on the benzene rings.

3.3 Potential energy surface of S1of PO molecule

At the B3LYP/6-31G*level,we established the potential energy surface of the photolytic destruction of PO molecule in S1. One elementary channel was explored and the ring-opening reaction takes place at C－O bond in oxazole ring to form a zwitterion structure(Scheme 1).

3.3.1 Structural parameters ofthe involved species in the S1potential energy surface

The geometries in S1including reactant,transition and product state are presented in Fig.4.

跑步，是人生美丽的主题，是心灵快乐的涅槃。人生也就像跑步一样，需要不断地去踏足、磨炼、超越，然后勇往直前、坚持到底，这样，才能遥遥领先他人，奔向成功。

Fig.4 Geometries of the reactant(a),the transition state(b),and the product(c)in S1

Initially PO will turn into S1after absorbing a photon at 289 nm,it quickly comes to its optimized geometry,like the S0configuration,the geometrical structures of S1are planar as well. When the dihedral angle between the benzene ring and the phenylbenzo[d]-oxazole increases from 0°to 9.19°with elongating 7C－23O bond in oxazole ring to 0.1812 nm,the transition state(TS)structure will be obtained.When the dihedral angle continues to increase to 91.02°,the oxazole ring thoroughly opens and 7C－23O bond length turns into 0.2865 nm,thus the ring-opening product is formed.In this process,the 7C－23O bond order also changes due to the transformation of the PO molecule,the WBO value of 7C－23O shows that the strength of the bond gradually decreases from the reactant to TS,and finally to ring-opening product,all the data are listed in Table 3.The WBO values of 7C－23O of the reactant is 0.9952,which suggests that it is close to a single bond.For transition state,the WBO value of 7C－23O is 0.4895 which implies that the bond has been weakened greatly.However that in the ring-opening product is only 0.0091,it indicates that the bond has been broken completely,the change tendency of WBO value is consistent with the change of the bond strengths along the ring-opening path.Similarly,the WBO values of the 1C－2C,2C－24N,12C－14C,and 12C－13C are also decreased from the reactant to TS,and finally to ring-opening product.Comparatively these bond orders have few changes, which indicate that these bonds are essentially single bonds. On the contrary,the WBO values of 7C－24N are increased along the reacting path,it is 1.5214 for the reactant,1.7106 for TS,and 2.0884 for the product,obviously it ultimately forms double bond.Both the 1C－23O and 7C－12C are also increased,the WBO values are 1.6012 and 1.4078 in S1,respectively,thisisduetotheformationof πbonds inthefinalproduct.

Scheme 1 Reaction routes for the ring-opening channel in S1

Table 3 Selected optimized geometrical parameters and the Wiberg bond orders of the reactant,the transition state,and the product in S1

Fig.5 Charge distribution(e)of ring-opening product of PO The charges on O and N atoms are-0.524e and-0.551e,respectively.

On the other hand,one can obtain useful information based on the distribution of the charge.The each atomic charge distribution of ring-opening product of PO is shown in Fig.5.The negative charge within the moiety of benzene with oxygen and nitrogen atom is-0.68e.Most negative charges distribute on oxygen and nitrogen atoms.The other moiety has 0.68e positive charge and almost the half concentrates on bridge C atom. The above indicates the product is a zwitterion structure.

3.3.2 Potential energy surface of the ring-opening

reaction in S1

The calculated total energy and relative energy values of the involved species in S1are listed in Table 4.The first excited energy(FEE/eV)refers to the transition energy from the ground state to the first excited state in the same geometry.

For the ring-opening reaction,we selected the optimized reactant in S1as the standard of relative energy,thus the TS is only 25.59 kJ·mol-1higher than the reactant,in other words,the energy barrier of the process is considerably low,which suggests that the ring-opening reaction is easy to process to process after PO absorbs one photon.The relative potential energysurface for the reaction in S1is shown in Fig.6.A saddle point is found and possesses an imaginary frequence(352i cm-1),the imaginary vibration model of the transition state corresponds to 7C－23O stretching vibration and points to the reactant and the product,this illuminates that stationary points locate the right reaction channel.

Table 4 Total energies(ET),the first excited energy(FEE),relative energies(ER),and dipole moment for various species

In addition,we attempted to find a reactively ring-opening path in S0of PO molecule,however,it can not been hit.In order to investigate the structural change in ground state,we have done a relaxed scan about the bond length of 7C－23O, which perform the optimization of the geometry at each point, the curve graph is shown in Fig.7.With the increase of the bond length from 0.138 to 0.348 nm,the energy of the respective geometry gradually increases,too.The inflection point does not appear which indicates the ring-opening reaction is very difficult in ground state.

3.4 Potential energy surface of triplet state(T1)of PO molecule

Fig.6 Schematic potential energy surface of the reaction in S1

Fig.7 Schematic scan potential energy surface of the 7C－23O bond length in S0

3.5 Atoms in Molecules(AIM)analysis

In AIM analysis,the topological properties of the scalar field electron density(ρ(rC))can be described by the numbers and the categories of the critical points.A critical point is the spatial position where the first derivative of the ρ(rC)is zero, according to the critical pointʹs curvature obtained by calculating the second derivative of the ρ(rC),the type of the critical point can be defined.The Hessian matrix of electron density is composed by nine secondary derivatives of ρ(rC)in three dimensions.The three eigenvalues(λ1,λ2,and λ3)can be acquired by performing a diagonalized operator on Hessian matrix.The sum of the three eigenvalues is equal to Laplacian of the electron density(∇2ρ(rC)=λ1+λ2+λ3).Among the three eigenvalues, if two of them are negative and the other is positive,the corresponding critical point is designated as the bond critical point (BCP)and marked as(3,-1),indicating the linkage between the two atoms.If two of them are positive and the other is negative, the corresponding critical point is designated as the ring critical point(RCP),and marked as(3,+1),indicating the existence of the ring structure.In general,the ρ(rC)of a BCP is related to the strength of the bond:the larger the ρ(rC)is,the stronger the bond will be;the smaller the ρ(rC)is,the weaker the bond will be.The∇2ρ(rC)of a BCP reflects the characteristic of the bond.If∇2ρ (rC)＜0,BCPcharges will be concentrated,and the more negative the∇2ρ(rC)is,the more covalent the property is;if∇2ρ(rC)＞0, BCPcharges will be dispersed,and the more positive the∇2ρ(rC) is,themoreionicthepropertyis.39-41

Fig.8 Schematic potential energy surface of the reaction in T1

Table 5 Electron densities and laplacians of various complexes in S1at bond critical points along the reaction coordinate calculated at the B3LYP/6-31G＊level within theAIM theory

The electronic densities of the 7C－23O,7C－24N,7C－12C,and 1C－23O bonds in the potential energy surface of S1at the bond critical point and their Laplacian are given in Table 5.The ρ(rC)of 7C－23O is reduced from the reactant(0.2654 e·a0-3,a0is the atomic unit of length which is called Bohr and equals 0.529×10-10m)to TS state(0.1099 e·a0-3),however,the ρ(rC)of the product can not be obtained by default of the program,it indicates that the strength of the bond is so weak that hardly existence of electron between the bond;on the contrary, the ρ(rC)of 7C－24N,and 1C－23O are increased in turn along reaction path(i.e.,from the reactant to TS state,finally to the product),it shows that these bonds are to be strengthened,the results is corresponding to those from WBOs mentioned above;the ρ(rC)of 7C－12C bond in TS state is slight smaller than those of the reactant and the product,however,the change is so small that it can almost be ignored.All the Laplacian of the electron density,∇2ρ(rC),are negative,it implies that these bonds belong to covalent property;the∇2ρ(rC)of the 1C－23O,and 7C－24N bonds are decreased along the reacting path,indicating that the electronic charge is from concentrating in the bond to distributing on two atomic nucleus. The∇2ρ(rC)of the 7C－12C bond is increased along the reacting path,indicating that the electronic charge is concentrated in the internuclear region,resulting in the more strong covalent bonds.In addition,there are the three calculated RCP(3,+1) for the reactant,which correspond to two benzene rings and one oxazole ring,however,for the transition state and the product,there are only the two calculated(3,+1)points corresponding to two benzene rings,the topological properties of these compounds are in good agreement with those discussed in WBOs and the photolytic mechanism.

4 Conclusions

The primary photochemistry process of PBO was investigated at the B3LYP/6-31G*level.After PO molecule absorbs a photon and turns into S1,firstly the molecule overcomes the en-ergy barrier of 25.59 kJ·mol-1to get to the transition state,then the oxazole ring is opened at C－O bond and both the benzene rings form about 90°angle to obtain the product,which is the base for further addition reaction with water.In order to improve the light stability of PBO fiber,it is necessary to add some anti-ultraviolet radiation material.

Acknowledgments: We thank Prof.SHI Qiang for the support and guidance in this work.Parts of the calculations were performed on the computer workstation of SHI Qiang group in Institute of Chemistry, ChineseAcademy of Sciences.

(1)Fu,Q.;Zhang,H.;Song,B.;Liu,X.;Zhuang,Q.;Han,Z. J.Appl.Polym.Sci.2011,121,1734.

(2)Hu,X.D.;Jenkins,S.E.;Min,B.G.;Polk,M.B.;Kumar,S. Macromol.Mater.Eng.2003,288,823.

(3) Jiang,J.M.;Zhu,H.J.;Li,G.;Jin,J.H.;Yang,S.L.J.Appl. Polym.Sci.2008,109,3133.

(4)Li,J.;Chen,X.;Li,X.;Cao,H.;Yu,H.;Huang,Y.Polym.Int. 2006,55,456.

(5)Li,X.;Huang,Y.;Cao,H.;Liu,L.J.Appl.Polym.Sci.2007, 105,893.

(6)Lin,H.;Zhuang,Q.;Cheng,J.;Liu,Z.;Han,Z.J.Appl.Polym. Sci.2007,103,3675.

(7) So,Y.H.Polym.Int.2006,55,127.

(8)Wang,L.;Meng,Y.Z.;Wang,S.J.;Hay,A.S.J.Polym.Sci. Part A:Polym.Chem.2004,42,1779.

(9) Jin,J.;Yang,S.;Li,G.;Zhu,H.;Jiang,J.Preparing Ultraviolet-ResistantPBO FiberComprisesHeating4,6Bi-Aminoresorcinol Hydrochloride,TerephthalicAcid,Phosphorus Pentoxide and PolyphosphoricAcid to Dissolve andAdding Nanometer Titanium Dioxide and PolyphosphoricAcid.CN Patent 101215732-A,2008

(10) Qian,J.;Zhang,Y.;Li,X.;Zhuang,Q.;Zhang,P.;Yin,X.High-Pressure Spinning for Preparing Polybenzoxazole(Pbo)Fiber ComprisesAdding PolyphosphoricAcid,Phosphorus Pentoxide, 4,6-Aminoresorcinol Hydrochloride,and TerephthalicAcid into Prepolymerization Kettle and Polymerizing.CN Patent 101824662-A,2010.

(11)Zhang,C.;Huang,Y.;Meng,L.;Lu,X.;Huang,S.High Pressure Nitrogen Storage Bottle Made of PBO Fiber and Carbon Fiber Composite Material and Its Preparation Method. CN Patent 1948816-A,2007.

(12)Zhang,C.H.;Huang,Y.D.;Zhao,Y.D.Mater.Chem.Phys. 2005,92,245.

(13)Zhang,T.;Hu,D.Y.;Jin,J.H.;Yang,S.L.;Li,G.;Jiang,J.M. Eur.Polym.J.2009,45,302.

(14)Fukumaru,T.;Fujigaya,T.;Nakashima,N.Polym.Chem.2012, 3,369.

(15) Park,S.J.;Seo,M.K.;Lee,J.R.J.Colloid Interface Sci.2003, 268,127.

(16)Dang,T.D.Polym.Mater.Sci.Eng.1990,62,86.

(17) Takahashi,Y.Macromolecules 2002,35,3942.

(18) Takahashi,Y.Macromolecules 2003,36,8652.

(19) Zhang,T.;Yang,S.L.;Hu,D.Y.;Jin,J.H.;Li,G.;Jiang,J.M. Polym.Bull.2009,62,247.

(20)Hodges,C.S.;Neville,F.;Konovalov,O.;Hammond,R.B.; Gidalevitz,D.;Hamley,I.W.Langmuir 2006,22,8821.

(21) Kumar,S.;Dang,T.D.;Arnold,F.E.;Bhattacharyya,A.R.; Min,B.G.;Zhang,X.;Vaia,R.A.;Park,C.;Adams,W.W.; Hauge,R.H.;Smalley,R.E.;Ramesh,S.;Willis,P.A. Macromolecules 2002,35,9039.

(22) Ran,S.;Burger,C.;Fang,D.;Zong,X.;Chu,B.;Hsiao,B.S.; Ohta,Y.;Yabuki,K.;Cunniff,P.M.Macromolecules 2002,35, 9851.

(23) Ran,S.;Burger,C.;Fang,D.;Zong,X.;Cruz,S.;Chu,B.; Hsiao,B.S.;Bubeck,R.A.;Yabuki,K.;Teramoto,Y.;Martin, D.C.;Johnson,M.A.;Cunniff,P.M.Macromolecules 2001, 35,433.

(24) Walsh,P.J.;Hu,X.;Cunniff,P.;Lesser,A.J.J.Appl.Polym. Sci.2006,102,3517.

(25) Walsh,P.J.;Hu,X.;Cunniff,P.;Lesser,A.J.J.Appl.Polym. Sci.2006,102,3819.

(26) Chin,J.;Forster,A.;Clerici,C.;Sung,L.;Oudina,M.;Rice,K. Polym.Degrad.Stab.2007,92,1234.

(27)Jackson,P.F.;Morgan,K.J.;Turner,A.M.J.Chem.Soc. Perkin Trans.2 1972,1582.

(28)Kim,Y.J.;Einsla,B.R.;Tchatchoua,C.N.;McGrath,J.E. High Perform.Polym.2005,17,377.

(29)So,Y.H.;Zaleski,M.J.;Murlick,C.;Ellaboudy,A. Macromolecules 1996,29,2783.

(30)Zhang,W.;Shen,G.;Zhuang,J.;Zheng,P.;Ran,X. J.Photochem.Photobiol.A 2002,147,25.

(31) Martineau,A.;Dejongh,D.C.J.Anal.Appl.Pyrolysis 1983,5, 39.

(32)Roos,B.O.Adv.Chem.Phys.1987,399.

(33)Scalmani,G.;Frisch,M.J.;Mennucci,B.;Tomasi,J.;Cammi, R.;Barone,V.J.Chem.Phys.2006,124,094107.

(34)Wu,C.;Tretiak,S.;Chernyak,V.Y.Chem.Phys.Lett.2007, 433,305.

(35)Rohatgi-Mukherjee,K.Fundamentals of Photochemistry;Wiley Eastern:New Delhi,1978.

(36) Frisch,M.J.;Trucks,G.W.;Schlegel,H.B.;et al.Gaussian 09, RevisionA.01;Gaussian Inc.:Wallingford,CT,2009.

(37) Starikova,Z.;Obodovskaya,A.;Bolotin,B.J.Struct.Chem. 1982,23,105.

(38) Wiberg,K.Tetrahedron 1968,24,1083.

(39) Sosa,G.L.;Peruchena,N.M.;Contreras,R.H.;Castro,E.A. J.Mol.Struct.-Theochem 2002,577,219.

(40)Li,Z.F.;Zhu,Y.C.;Zuo,G.F.;Tang,H.A.;Li,H.Y.Acta Phys.-Chim.Sin.2010,26,429. [李志锋,朱元成,左国防,唐慧安,李红玉.物理化学学报,2010,26,429.]

(41) Yuan,K.;Liu,Y.Z.;Zhu,Y.C.;Zhang,J.Acta Phys.-Chim. Sin.2008,24,2065. [袁 焜,刘艳芝,朱元成,张 继.物理化学学报,2008,24,2065.]

January 6,2012;Revised:March 5,2012;Published on Web:March 6,2012.

Photolysis Reaction of 2-Phenylbenzo[d]oxazole

LI Hui-Xue*WANG Xiao-Feng LI Zhi-Feng ZHU Yuan-Cheng
(Key Laboratory for New Molecule Design and Function of Gansu Education Department,College of Life Science and Chemistry, Tianshui Normal University,Tianshui 741001,Gansu Province,P.R.China)

The temperature-resilient,high tensile-strength fiber poly[p-phenylene benzobisoxazole] (PBO)is light-unstable and it degrades under ultraviolet radiation.In this paper we study the photolytic mechanism of the PBO monomer,2-phenylbenzo[d]oxazole(PO).Following absorption of a photon and excitation into the first excited state(S1),the molecule overcomes an energy barrier of 25.59 kJ·mol-1to enter the transition state;the oxazole ring is then opened and both benzene rings form a dihedral angle of about 90°to obtain the product,which undergoes further addition reaction with water.Calculations reveal that ring-opening is easily achieved in the potential surface of S1.However,the pathway through which the oxazole ring opens in the ground state remains obscure.The topological properties of these compounds are in good agreement with the expected bond orders and the photolytic mechanism.

Theoretical study;Photolysis reaction;2-Phenylbenzo[d]oxazole

10.3866/PKU.WHXB201203062

O641

∗Corresponding author.Email:li_hx2001@126.com;Tel:+86-15097274526.

The project was supported by the Research Fund of Tianshui Normal University for Young College Teachers,China(TSA1116).天水师范学院中青年教师科研基金资助项目(TSA1116).