A Theoretical Study on Pyrolysis and Sensitivity of Energetic Compounds(Ⅴ)--Nitro Derivatives of Met

Vol. 16No.12000Chemical Research in Chinese Universities21~30A Theoretical Study on Pyrolysis andSensitivity of Energetic Compounds *( V )Nitro Derivatives of MethylbenzeneGU Zhi-ming' , FAN Jian- fen? , XIAO He-ming' ** and DONG Hai-shan'( 1. Department of Chenmistry , Nanjing University of Science and Technology , Nanjing 210094 ,P. R. China ;2. Department of Chemistry , Suzhou University , Suzhou 215006 ,P. R. China ;3. Institute of Chermical Materials , China Academy of Engineering Physics ,Mianyang 610003 ,P. R. China )( Received Feb. 6 , 1999 )UHF-SCF-AM1 MO calculations were performed for two kinds of pyrolysis reactions of sev-en nitro derivatives of methylbenzene( homolysis reactions by the rupture of bond C- -NO2 intoradicals and isomerization reactions in the way of methyl H transferring to the ortho nitro Oatom ). The molecular geometries of the reactants , the transition states and the products of thefourteen reactions were fully optimized. The activation energies for these two kinds of reactionshave been obtained. The results indicate that this category of compounds is more easily pyrolyzedby being initiated via isomerization reactions. A parallel relationship exists between the activationenergy of the isomerization reactions and the impact sensitivity of the seven titled compounds.Keywords Nitro derivatives of toluene , Pyrolysis reaction , UHF SCF-AM1 MO calculationArticle ID 1005- 9040( 2000 )-01 -021-10IntroductionThe nitro derivatives of methylbenzene are an important category of explosives , in whichTNI(2 A 6-trinitrotoluene ) is widely-used. It is widely accepted that methyl hydrogen in ni-troaromatics is quite active , which influences the impact sensitivity of an explosive. Gonzalezet al.t11 found methyl substitution increases the decomposition rate over that of nitrobenzeneby an order of magnitude. Much effort has been devoted to relating electronic structure andimpact sensitivity in order to develop new energetic systems with high -performance. As forthis category of compounds , the principle of the smallest bond order( PSBO{2-5] was firstlyapplied to assessing their relative magnitudes of impact sensitivity. According to the principle ,for a series of compounds with similar molecular structures , the smaller the bond order of thepyrolysis initiation bond , the more sensitive become中國煤化工s impact and shock ,and the smallest bond order of the initiation bond is:YHC N M H Glargest sensitivity ofthe series compounds.* Supported by the National Natural Science Foundation of China Academy of Engineering Physic( Grant : 960539 ).* *orrespondence should be addressed.22Chemical Research in Chinese UniversitiesVol.16Exploring microscopic pyrolysis mechanism，which addresses the question of how an im-portant impulse can initiate rapid exothermic reactions leading to the detonation of explosivesolids , is always the target of both of theoretical and experimental chemists. It is prerequisiteto understand the facts that determine the impact sensitivity. The pyrolysis mechanisms of thenitro derivatives of methylbenzene have received considerable attention from chemists. Experi-mental studies on the pyrolysis mechanism of this category of compounds touch upon many as-pects and there are many viewpoints on this subject. Fields et al .[6,7] reported that the ther-mal decomposition of o-nitrotoluene occurs with the initial formation of anthranilic acid , animportant intermediate 891. Gonzalez and co worker' s early experimental result from the verylow-pressure pyrolysis( VLPP )of 2 A-dinitrotoluene provided a low A factor and low activa-tion energy , not indicative of the thermolysis of the Ar- -NO2 bond. Later this researchgroup 10] reported a new result from laser powered homogenous pyrolysis( LPHP ) that in allthe cases of m- , p- and o-dinitrotoluenes , and 2 4-dinitrotoluene，the rate limiting step isthe homolysis of the Ar- -NO2 bond 11. Tsang and co workers reported that the cleavage ofthe C- -NO2 bond is the primary process in the decomposition of p-nitrotoluene. For the de-composition of o -nitrotoluene ,He et al.[111 reported that there are three channels , and themost important one involves anthranil formation and is specific with respect to the presence ofthe methyl group adjacent to the NO2 moiety. Recently , Miniter et al .[121 reported that theinitial rate-controlling step for p- nitrotoluene decomposition is probably the intermolecular hy-drogen abstraction to form an ArNO2H radical intermediate , a current pathway for o-nitro-toluene is an intramolecular reaction in which anthranil is an intermediate , and 2 ,4-dinitro-toluene and 2 ,6- dinitrotoluene decompose in the same manner as o-nitrotoluene. However ,some authors59,13]J think that 2 A- and 2 6-dinitrotoluene decompose by C- -NO2 homolysis.Modern experimental and theoretical results show that the pyrolysis of most nitroaromat-ics such as the nitro derivatives of benzene and aminobenzene may be initiated by the ruptureof the C- -NO2 bond214-16]. However , for the nitro derivatives of phenols , the pyrolyses areusually initiated via isomerization reactions 217. As for this category of compounds ,it seemsthat the two processe( homolysis by the rupture of the bond C- -NO2 and isomerization by hy-drogen- transferring ) may both occur. The question is which one is the primary initiation reac-tion controlling the reaction rate. Our early M( ) calculation for the titled compounds was proneto supporting the isomerization mechanism breaking the ac- H bond in a methyl group21.Before this work , no theoretical calculations of the pyrolysis reaction of the titled com-pounds has been reported. Early calculations for the pryolysis reactions of some other nitrocompounds and nitroamines based on RHF( restricted Hartree Fock ) MO method could onlyprovide potential curves on which molecular energie中國煤化工ble of locating tran-sition states and giving the corresponding activa:MYHCN MHG)ur papers 1-17222showed that it is necessary to employ UHF ( Unrestricted Hartree- Fock ) MO method to studythe reactions if radicals are involved in. The UHF calculations have been successfully applied tofive model energetic molecules22J , the nitro derivatives of benzene , aminobenzene and phe-nols 15一No.1GU Zhi-ming ,FAN Jian-fen , XIAO He-ming et al.23This is the fifth section of the series theoretical study on the pyrolysis mechanisms and theimpact sensitivity of energetic compounds. UHF-SCF-AM1 method was applied to studyingthe two kinds of pyrolysis reactions of seven nitro derivatives of methylbenzene ，i. e. ,2 A-dinitromethyl- benzene( 1 ), 4 ,6-dinitro-1 ,3-dimethylbenzene( 2 ), 2 ,4 -dinitro-1 ,3-dimethylbenzend 3),2 ,4 dinitro-1 ,3 ,5- trimethylbenzene 4 ),2 ,4 ,6-trinitromethylbenzene(5),2 A 6- trinitro-1 ,3-dimethylbenzend 6 ), s-trinitrotrimethylbenzend 7 ). The moleculargeometries of the reactants , the transition states and the products of the fourteen reactionswere fully optimized. The activation energies for the two kinds of reactions have been ob-tained. The results show that the nitro derivatives of methylbenzene are more easily initiatedvia isomerization reactions. Meanwhile ,it was found that there is a parallel relationship be-tween the activation energy of the isomerization reactions of the seven titled compounds andtheir impact sensitivity. The different effectsof 一NO2 and - -CH3 groups on the heat of theformation of a molecule are discussed.Calculation Method and ResultsUHF SCF-AM1 MO method 23] including the Mopac 6. 0 program packagt 24] was ap-plied to studying two kinds of pyrolysis mechanisms of seven nitro derivatives of methylben-zene , shown as equations( 1 )and( 2 ) , respectively , with 2 A-dinitromethylbenzene as an ex-ample. The initial geometries of reactants were based on the Pople' s standard datd 25] , and .subsequently optimized with an energy gradient method. a and β electronic orbitals and ener-gies were obtained from UHF MO calculations , respectively.HCHC+ NO2(1)OH '((2)In the homolysis reaction shown in equatior( 1中國煤化工1 c atom on benzenering and N atom on NO2 group( corresponding to th:YHC N M H Gand with the sanllestbond order among all the C- -NO2 bonds in a molecule ) was chosen as the reaction coordinate.It is corresponding to the bond C2- -NO2 in the molecules of 2 A-dinitro-1 3-dimethylbenzene(3),2 A 6-trinitromethylbenzene(5 )and 2 A 6-trinitro-1 3-dimnethylbenzen( 6 ), the bond .C4- -NO疽數據molecules of 2 A-dinitromethylbenzend 1 ),2 A -dinitro-1 3 ,5-trimethylben-24Chemical Research in Chinese UniversitiesVol.16zend( 4 ) and s-trinitrotrimethylbenzene( 7 ),and the bond C6- -NO2 in the molecule of 4 ,6-dinitro-1 3- dimethylbenzend 2 ) , respectively( seen Fig. 1 ). The calculations were carried outfor the seven homolysis reactions , respectively , by changing the reaction coordinate in steps of0.01 nm from0.14 to 0.35 nm. The molecular geometries were optimized at different valuesof reaction coordinate. The transition state of each reaction corresponding to the point with thehighest value of heat of formation was optimized with NLLSQ methot 261.H8Hl.121,H8is F1.201|.1.479 110.3110.311.479 ON119.8 CH、Ci、119.91 201o1.490c2REN 1. 202C61.488 0H/H-c-C3、H118.9| 1.488HE1.200 N、1.201~o0-4,6-Dinitro-1,3- dimethylbenzene(2)2,4- Dinitromethylbenzene(1)HoicH1. 201.4811. 202]1. 200 N118.6 Cl.1.201 118.C10" 1.496 C01.493C2~c110.6 , HH- 110.1 c3H1.484C31.123 C7110.7H-|1. 490119.11.494 H H120.4N.1.203N、 1.20101.200o02,4-Dinitro-1 ,3-dimethylbenzene(3)?4-Dinitro-1 ,3,5-trimnethylbenzene(4)s H81. 198 109.9 1,123 1. 199110. 0了.124 01. 199|480 11. 1991.200 N119.5 ci、 1.495 N、1. 201N118.5 Cl、 1.493 N1.201o 1/498 cc6 117.7001. 501C2c6 120.005、.cC3\H118. 41i.491旨~H1.199 iN、1. 199C2.46-Trinitromethylbenzene(5)2,4,6-Trinitro-1 ,3- dimethylbenzene(6)1.123 ,H81. 199,101YiC7RA118.7 .C.4840- 1.499~C2~CC3Cs.jHC4~0中國煤化工2,4,6-Trnitro-1 ,3 ,-trimethTYHCNMHG.Fig.1 The opimized geometries of seven nitro derivatives of methylbenzenes.Bond length in 0.1 nm , bond angle in deree.No.1GU Zhi-ming ,FAN Jian-fen , XIAO He-ming et al.25The isomerization reactions shown in equation( 2 ) involve the methyl H transferring tothe ortho nitro 0 atom. According to the bond order in each reactant molecule ,H8- -C7 is theweakest ac- -H bond with the samllest bond order among all the ac- H bonds in each reactantmolecule( seen Fig. 1 ). In the process of methyl hydrogen-transferring , the other two H atomsin this methyl group turn around and in the product they are almost in the benzene ring plane( seen Fig.2 ). The saddel calculation was performed for each isomerization reaction to obtainthe transition state , which was optimized further with NLL .SQ method 27].All the transition states in the seven homolysis reactions and the seven isomerization reac-tions were confirmed by FORCE calculation( using FORCE and LARGE keywords ) with onlyone negative force constant. All the calculations were done on a HP-9000-842 computer.+0.13 。H+0.23H-0.195 H、0.9811H-0. 15,H H 0.9823H4CH-0.3408.61-0.21 .1.4782109.9 01.3872、1. 3323. +o.221.3904 1. 3361) -0.271.1989|+0. 57c1N+0.28C61.494902、C61. 41930oC:C61. 4503^0-0. 35r C3H-、H;-C3~H~C4C0~oTrmsition stateProductReectantFig.2 The optimized geometries of the reactant , the tansition state and the productin the isomerization reaction of TNT.Bond legth in0.1 nm , bond angles in degree. Also shown are the net charges on someatoms in the optimized species.Fig. 1 shows the optimized geometries of the seven nitro derivatives of methylbenzene andthe numberings of some atoms in the molecules. Fig. 2 depicts the optimized geometries of thereactant , the transition statd TS ] and the product in the isomerization reactionof2 A 6-trini-tromethylbenzend TNT ). Also shown is the result of net charges on some atoms in the opti-mized species. Tables 1 and 2 list the heats of the formation of optimized species and the acti-vation energies of the seven homolysis reactions and the seven isomerization reactions ,Table 1 The heats of the formation of optimized species and the activation energie( Eh ) of thehomolysis reactions of the seven nitro derivatives of methylbenzen( kJ mol~ 1 )Heat of formationReaction“Eh,bReactantTS109. 600260.312 228. 559134. 076 .173.054303. 775262. 927114. 50480. 952228. 342196. 112129. 691VI155. 440283. 554234. 300111.516.III97.328234. 078193.995120. 068142. 068.271.215 228. 568112.81371. 228209. 798169.921121.650中國煤化工a. I-V[ represent the numberings of the homolysis reac.DHCN MH Gne( 1),4 ,6 dnito1,3.dimethylbenzene( 2 ), 2 ,4-dinitro-1 ,3-dimethylbenzene( 3 ), 2 ,4-dinutro-1 13 ,5-trmethylbenzene( 4 ), 2 ,4 ,6-trini-tromethylbenzend 5 ),2 A 6- trinitro-1 3- dimethylbenzend 6 ) , s- trinitrotrimethylbenzend 7 ) , respectively. b. The activationenergy has been modifed with zero point energy,i. e. ,E:=( Ers+ Es)-( Er+ E跟),where ER and ETS are zero point en-ergies of the reactant and the transition state for a homolysis reaction ,and ER and Ers are their heats of formation , respective-y.26Chemical Research in Chinese UniversitiesVol.16Table 2 The heats of the formation of the optimized species and the activation energie( EI ) of theisomerization reactions of the seven nitro derivatives of methylbenzend( kJ/mol )Heat of formationReaction"EiReactionEibReactantTSProductI109. 600203.162 163. 65479. 174V173. 054250. 684 218. 72764.446II80.952170. 046137. 33674. 760VI155. 440232. 643209.04161. 76897.328179. 987 148. 50368.572142.068229. 501196. 56869.626IV71. 228156.243 124. 59568. 387a. I一VI represent the numberings of the isomerization reactions of compounds 1- -X see the notein Table 1 ). b. Theactivation energy has also been modified with zero point energy,i. e. ,E"=( Ers+ Eis)-( ER+ ER), where ER and Eisare the zero point energies of the reactant and the transition state for an isomerization reaction ,and ER and ETS are their heatsof formation , respectively.respectively. Table 3 contains the impact sensitivity of the seven titled compounds , their acti-vation energies of the two kinds of reactions.Table 3 The impact sensitivities( impact energy ) of the seven nitro derivatives of methylbenzeneand the activation energies of the two kinds of reaction( E and E' )Compound10*Eemeg"/(kg m~2)E的(k} mol~1) E(kJ mol~ 1 )2 A4-Dinitromethyl-benzen( 1)18. 9134. 07679. 1944 6-Dinitro- 13-dimnethylbenzend 2)14. 6°129. 691 .2 A Dinitror 1 3 dimethylbenzen( 3)120. 6502 A4-Dinitro-1 3 ,5-trimethylbenzend 4 )13.8121. 6502 A 6-Trinitromethylbenzend 5 )114.504.64. 4462 A 6-Trinitro-1 3-dimethylbenzen( 6)5.7111.516 .s- Trinitrotrimethylbenzend 7 )5.9112.81369. 626a. The data representing the energy must be dropped for a compound to produce detonation. Thus , the smaller the valuegiven , the more sensitive is the compound towards impact. The data were taken from ref.[2] b. E and E: correspond to theactivation energies of the homolysis and isomerization reactions , respectively. c. The value is given for the mixture of com-pounds 2 and 3.Discussion1 The Molecular Geometries , the Electronic Structures and the Formation Heats of the Re-actantsFig. 1 depicts the optimized geometries of the seven nitro derivatives of methylbenzene.The calculation results show that N atom on NO2 group in each reactant molecule exists in sp2hybrid state with bond angle∠CONO about 120° and bond length N- -O about 0.12 nm. Catom in methyl group exists in sp' hybrid state with bond angle∠HCC as about 110.0° andthe bond length ac- H about 0.112 nm. Our calculations show that the potential barrier for theinternal rotation of the methyl hydrogen is quite low , and that for C3 conformation is the low-est.中國煤化工The optimized geometries of 2 A-dinitromethyMHC N M H Gnitro-1 3-dimethyI-benzend( 2 )show that all the NO2 groups in these two molecules are almost in the benzene ringplane. However , in the molecule of 2 A -dinitro- 13 -dimethylbenzen( 3 ) , NO2 group attachedto C2 atom leaves away from the benzene ring plane with the dihedral angle between the ben-zene ring有教瑪elf about 55.5° , resulting from the steric hindrance , while that attached to C4No.1GU Zhi-ming ,FAN Jian-fen , XIAO He-ming et al.27is nearly in the benzene ring plane. The similarity was observed in the moleculeof2 A 6-trini-tro-1 3-dimethylbenzene( 6 ), in which NO2 group attached to C2 is out of the benzene ringplane with the corresponding dihedral angle 65.2° , and the other two NO2 groups are almostin the plane. The two NO2 groups in the molecule of 2 A -dinitro-1 3 5-trimethylbenzene( 4 )are both out of the benzene ring plane with the dihedral angles 39. 4° and 50. 6° , respectively.The optimized geometry of 2 A 6-trinitromethylbenzen( 5 ) shows that the NO2 group attchedto C2 atom is away from the benzene ring plane with the corresponding dihedral angle 37.3° ,while other two NO2 groups are nearly in the benzene ring plane. The difference between NO2groups attached respectively to C2 and C6 atoms is resulted from the conformation of menthylgroup. The optimized geometry of 2 A 6-trinitro-1 3 ,5-trimethylbenzene( 7 ) is nearly sym-metric with the dihedral angle between NO2 and benzene ring plane about 58.3° .From UHF SCF-AM1 calculations , the bond orders of the weakest C- -NO2 bonds withthe smallest bond order among all the C- -NO2 bonds in the seven reactant molecules were ob-tained. They were 0.854 ,0. 859 ,0.840 ,0.844 ,0. 840 ,0. 832 and 0.835 in the moleculesof compounds 1-7 , respectively , corresponding to the bonds C4- -NO2 , C6-NO2 ,C2-NO2 ,C4-NO2 ,C2- -NO2 , C2-NO2 and C4- -NO2( seen Fig. 1 ). Their optimized bondlengths were0.1488 ,0.1488 ,0.1496 ,0.1494 ,0.1498 ,0.150 1 and0.1499 nm ,re-spectively.The isomerization reactions via methyl hydrogen transferring to the oxygen on the orthoNO2 group involve the rupture of bond ac- H. The bond orders of the weakest ac- H bondswith the smallest bond order among all the ac- H bonds in the reactant molecules were ob-tained. They were 0.953 ,0.954 ,0. 950 ,0. 950 ,0.944 ,0. 946 and 0.950 , respectively ,corresponding to the bond H8一-C7( seen Fig. 1 ). Their optimized bond lengths in themolecules of compounds 1- -7 were0.112 1 ,0.112 1 ,0.1123 ,0.1123 ,0.1123 ,0.1124and 0.112 3 nm ,respectively.Also obtained were the net charges on some atoms in optimized species. Here we only il-lustrate the results for 2 A 6-trinitromethylbenzend( TNT )in Fig.2 for brevity. In each reac-tant molecule , the net charges on CH3 groups all exhibit positive , while those on NO2 groupsshow negative , which indicates that NG )2 is an electron- attractive group and CH3 is an electron-pushing one.Tables 1 or 2 shows that the formation heats of the reactant molcules are in the order of 4<2<3<1<7< 6< 5. The seven compounds can be divided into two groups with two andthree NO2 groups , respectively. The differences among the formation heats of compounds5 ,6and 7，which contain three NO2 groups , show that the more the CH3 groups in a molecule ,the lower the heat of the formation of the molecul中國煤化工observed among theformation heats of compounds 1 ,2 ,3 and 4 whicl:MYHCN M H Gups. The formationheats of compounds 5 ,6 and 7 with three NO2 groups are much higher than those of com-pounds 1- -4 with two NO2 groups , showing that the molecule with more NO2 groups is ac-companied with a higher formation heat of the molecule. This meaningful regularity also existsin the o萬(wàn)布贅握es compounds 17].28Chemical Research in Chinese UniversitiesVol.162 Pyrolysis Initiation MechanismFor a homolysis reaction shown in equation( 1 ), the reaction proceeds towards formingradical products with the increase of the distance between C atom on benzene ring and N atomon NO2 group( corresponding to the weakest C- -NO2 bond ). The calculations performed bychanging the reaction coordinatd( the distance between C and N atoms ) show that the molecu-lar energies first rise then decrease slowly. At about 0.21 nm of the distance between C and Natoms , the transition state was formed. The heats of the formation of the reactants , the tran-sition states and the products are listed in Table 1. The molecular energies of the products arehigher than those of the reactants , which is consistent with the experimental fact that the pro-cess breaking a bond is endothermic. The activation energies for the homolysis reactions ofcompounds 1- -7 are 134.076 ,129.691 , 120.068 ,121.650 , 114.504 ,111.516 , 112.813kJ/ mol , respectively.For the isomerization reaction shown in equation( 2 ), with the progression of the reac-tion , methyl H transfers to the ortho nitro O atom , meanwhile ,the other two H atoms in thismethyl group turn around towards the benzene ring plane. The transition states of theisomerization reactions of the seven nitro derivatives of methylbenzene all located at about0.232 to 0.297 nm of the distance between methyl H8 and C7 atoms. The formation heats ofthe reactants , the transition states and the products are listed in Table 2. The molecular ener-gies of the products are higher than those of the reactants , showing that this process is also en-dothermic. The activation energies for the isomerization reactions of compounds 1- -7 are79.174 ,74.760 , 68.572 , 68.387 ,64.446 ,61 .768 and 69.626 kJ/mol , respectively.Comparing the activation energies of the two kinds of the pyrolysis reactions of each titledcompound seen Table 3 ), it is found that the activation energy of the homolysis reaction is .higher than that of the isomerization reaction by at least 40 kJ/mol , which means that themolecules of the seven nitro derivatives of methylbenzene are more likely initiated via isomer-ization reactions taking place by methyl hydrogen transferring to the ortho nitro O atom.3 Geometric and Electronic Structural Variations in the Isomerization ReactionsAs an example ，the optimized geometries of the reactant，the transition state and theproduct in the isomerization reaction of TNT( 5 ) are ilustrated in Fig.2.Our saddle calculations for the isomerization reactions show that in the transition states ,the methyl H atom bonds with the ortho nitro O atom ,and the distance between H8 atom andC7 atom is about 0.232- -0. 297 nm. In these transition states , no six- membered ring be-tween the methyl H and nitro O atoms was observed , which really occurred in the isomeriza-tion reactions of the nitro derivatives of phenols 17]. The reason is that there is no hydrogenbond existing in the molecules of the titled compoun|中國煤化τethyl H transferringto the ortho nitro O atom , the other two H atomsi:MYHc N M H Gn around slightly to-wards the benzene ring plane , and are still out of the plane in the transition states. In the pro-cess from the transition states to the products , these two H atoms continue to turn around andfinally go into the benzene ring plane in the products. Meanwhile , H8 atom turns around awayfrom C7A高熬據reach a stable state( product ).No.1GU Zhi-ming ,FAN Jian-fen , XIAO He-ming et al.29The net charges on some atoms in the optimized species in the isomerization reactionof2 ,4 6-trinitromethylbenzene are also ilustrated in Fig. 2. Large changes of net charges on themethyl C atom and the ortho nitro O atom take place. The former increased from -0.23( R )to -0.19 TS)and to -0. 15(P ), and the latter changes from - 0.34( R)to -0.21( TS)and to - 0.27 P )due to the binding of the methyl H with itself. The net charges on nitro Natom changed from +0.57R)to +0.22( TS)to +0.28( P ). Those on the transferring Hatom varied from +0.13(R)to +0.23(TS)to +0.26 P).4 Activation Energy and Impact SensitivityTable 3 shows the order of the impact sensitivities of the titled compounds ,i.e. ,1<2 ,3<4<5<7~6. From the above calculations for the two kinds of pyrolysis reactions ,it is de-duced that the seven titled compounds are probably more easily initiated by isomerization reac-tions. The activation energies of the isomerization reactions for compounds 1- -7 are 79. 174 ,74.760 ,68.572 ,68.387 ,64.446 ,61. 768 and 69. 626 kJ/mol , respectively , which is inthe same order as that of the impact sensitivities except compound 7. It is reasonable due tothe complexity of impact sensitivity. As we know，the impact sensitivity is a whole propertyfor an explosive solid. However , our calculation was performed only for a single molecule. Onthe other hand as we discussed in the introduction section , there are different channels of thepyrolyses for this category of compounds , homolysis and isomerization reactions probably takeplace simultaneously. In the case of the nitro derivatives of benzene 15 Jwith one channel of py-rolysi( homolysis bond C- -NO2 into radicals ) ,a good parallel relationship was found betweenthe activation energy of the pyrolysis initiation reactions and the impact sensitivity of the com-pounds. Such parallel relationship also exists in the case of the nitro derivatives of aminoben-zenes 16 J with one channel of pyrolysis also. Therefore , we think the main reason for the ex-ception of the parallel relationship between the activation energy of pyrolysis initiation reac-tions and the impact sensitivity of this category of compounds is probably that two channels ofpyrolysis take place simultaneously.References[ 1 ] Gonzalez,A. C. ,Larson,C. W. ,McMillen,D. F. ,etal. J Phys. Chem. , 89 ,4 809 1985)[ 2 ] Xiao,H. M. , Molecular Orbital Theory of Nitro Compounds , National Defense Industry Press , Bejing , 1993[ 3 ] Xiao,H. M. ,Wang,Z. Y. , Yao,J. M. ,Acta Chimica Sinica ,43 , 14( 1985 )[4]Xiao,H.M.,Li,Y.F.,ScienceinChina,SeriesB,38,538(1995)[ 5 ] Xiao,H. M. ,Li,Y. F. ,FengP. L. , In :"Proeeding of the International Symposium on Pyrotechnics and Explo-sives" , Beijing ,434( 1987 )[ 6 ] Fields,E. K. ,Meyerson,S. ,Tetrahedron Lett. , 10,1 201( 1968)[ 7 ] Fields,E. K. ,Meyerson ,S. ,Adv. Free -Radical Chem.中國煤化工[ 8 ] Bakke J , Heikman ,H ,Nystrom ,G. , Acta Chem. ScanTYHCNMHG[ 9 ] Willadsen,P. ,Zermer ,B. ,J. Org. 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