Theoretical Stud ies on Thermal D ecom position of Benzoyl Perox ide in Ground State 3

CH EM. R ES. CH IN ESE U. 2003, 19 (3), 355 361 Theoretical Stud ies on Thermal D ecom position of Benzoyl Perox ide in Ground State 3 SUN Cheng2ke 1, 2, YAN G Si2ya 2, L IN Xue2fei 2, M A Si2yu 1 and L I Zong2he 13 3 1. D ep a rtm en t of Chem istry, B eij ing N orm a l U n iversity, B eij ing 100875, P. R. Ch ina; 2. D ep a rtm en t of Chem istry, Q uj ing N orm a l Colleg e, Q uj ing 655000, P. R. Ch ina R eceived O cṫ 21, 2002 System atic studies of the therm al decompo sition m echanism of benzoyl peroxide (BPO ) in ground state, leading to various interm ediates, p roducts and the po tential energy surface (PES) of po ssible dissociation re2 actions w ere m ade com putationally. T he structures of the transition states and the activation energies fo r all the path s causing the fo rm ation of the reaction p roducts m entioned above w ere calculated by the AM 1 sem i2 emp irical m ethod. T h is m ethod is show n to to be one p redict co rrectly the p referred pathw ay fo r the title re2 action. It has been found that in ground state, the therm al decompo sition of benzoyl peroxide has tw o k inds of path s. T he first pathw ay PhC (O )O OC (O ) Ph PhC (O )O Ph + CO 2 p roduces finally phenyl radicals and carbon dioxide. A nd the second pathw ay PhC (O ) OO C (O ) Ph PhC (O ) OO + PhC (O ) PhC (O ) + O 2 Ph + CO + O 2, v ia w h ich the reaction takes p lace only in tw o step s, p roduces oxygen and PhC (O ) radicals, and the further therm al dissociation of PhC (O ) is quite difficult because of the h igh acti2 vation energy in ground state. T he calculated activation energies and reaction enthalp ies are in good agree2 m ent w ith the experim ental valueṡ T he research results also show that also the therm al dissociation p rocess of the tw o bonds o r the th ree bonds fo r the benzoyl peroxide doesn t take p lace in ground state. Ke yw o rds Benzoyl peroxide, T herm al decompo sition, R adical, UAM 1 A rtic le ID 100529040 (2003) 2032355207 In troduction T he chem istry of diacyl perox ide radicals has a long h isto ry, w h ich can be traced back to the early days of the 20th cen tu ry [1, 2 ]. Benzoyl perox ide is the mo st rep resen tative one ex ten sively studied ex2 perim en tally [3 6 ] becau se of its w idely comm ercial u se as a po lym erization in itiato r, especially the u s2 age in m ed icine, food stuffs, co sm et ics and rubber indu stries [7 11 ]. In comparison w ith its w idesp read u sage and experim en tal study, its theo retics has re2 ceived relatively little atten tion. To ou r know l2 edge, no theo ret ica l system a t ic stud ies have been repo rted on the therm al dissociation m echan ism of benzoyl p erox ide in g round sta te. T herefo re, the m echan ism studied fo r the title thermo lysis reaction is p resen ted here w ith the view s of settling ground2 w o rk fo r a fu rther study of radical s p ro tection and app lication, and also help ing the experim en ts and the relevan t indu stries. Com puta tiona l M ethods A ll the calcu lation s described in th is w o rk w ere imp lem en ted on an Pen tium g 2GH z computer w ith the Guassian 98 package [12 ]. T he po ten tial en2 ergy su rface (PES) [13 ] of the variou s dissociation s, the geom et ries and the energ ies of the reactan t s, p roducts, in term ediates and tran sition states w ere fu lly op tim ized by m ean s of the AM 1 m ethod [14 ], w h ich appears to be the best availab le sem iemp iri2 cal p rocedu re fo r the study of the thermo lysis reac2 tion s becau se the m ethod leads to reasonab le ge2 om etries, en thalp ies of fo rm ation and activation en2 ergies to compare w ith experim en tal values [15, 16 ]. A ll the structu res w ith the m in im um energy w ere found v ia the Berny algo rithm [17 ]. A ll the station2 ary po in ts w ere fu rther confirm ed by the analytical computation of the fo rce con stan ts and character2 ized by the num ber of im aginary vib rational fre2 quencies [18 ]. Results and D iscuss ion T he traditional view po in t ho lds that the disso2 ciation of benzoyl perox ide (BPO ) based on the ex2 perim en t is a symm etrical homo lytic cleavage reac2 tion, the in term ediate dissociated is free radical 3 Suppo rted by N ationaln atural Science Foundation of Ch ina (N o. 29773007) and the N atural Science Foundation of Q u2 jing N o rm al Co llege (N o. 200009). 3 3 To w hom co rrespondence should be addressed.

3 56 CH EM. R ES. CH IN ESE U. V o l. 19 PhC (O )O, and it is fu rther dissociated in to free radical Ph and CO 2 [19 ]. How ever, w hether o ther decompo sition w ays ex ist, too, such as asymm etri2 cal homo lytic cleavage, ion ic hetero lytic cleavage, the tw o bonds dissociation o r the tree bonds disso2 ciation etc. is unknow n. To understand fu lly tho se p rob lem s, the variou s p robab le dissociation m echa2 n ism s o r the cu tting m ethods of benzoyl perox ide are detailed in Schem es 1 5. Scheme 1 Pathway 1. Scheme 2 Pathway 2. Scheme 3 Pathway 3. Scheme 4 Pathway 4. Schem es 1 3 show th ree k inds of reaction pathw ays w ith single bond dissociation respective2 ly, and Schem e 4 is the pathw ay of the tw o bonds d issocia t ion and Schem e 5 is hypo thet ica lly reac2 tion of the th ree bonds dissociation. 1 Com par ison of PES for D ifferen t K inds of D issoc ia tion W ays in the Ground Sta te To iden t ify w ha t k ind of react ion invo lved above can take p lace likely and its reaction m echa2 n ism s w e firstly m ade an investigation in detail, all the PES of Schem es 1 4 dissociation u sing AM 1 w ith op tim ization, as show n in F igs11 4. A ll the F igu res, draw n directly by Excel 2000, are derived from the real values w ith the AM 1 calcu lation ṡ F irst ly, under the cond it ion of keep ing the sp in con servation ( 2s + 1) and ground state, the PES of dissociation s fo r 1a cleavage w ay and 2a cleavage w ay w ere calcu lated by u sing restricted AM 1 (RAM 1 ) and un restricted AM 1 (UAM 1 ) m ethods respectively (F ig. 1 and T ab le 1). T he re2 Scheme 5 Pathway 5. su lts show clearly that the energy increases w ith e2 longating the bond distance fo r the PES (ion ic het2 ero lytic cleavage) of RAM 1 calcu lating w hether 1a F ig. 1 Compar ison of PES prof iles of RAM 1 and UAM 1 scanned for 1a and 2a decom position ways in ground state. o r 2a cleavages ex ist o r no ṫ But the energy cu rve of PES (free radical homo lytic cleavage) w h ich has been calcu lated w ith UAM 1 p resen ts the m ax im um value, and the to tal energy of PES (w ith UAM 1) is low er than that of PES (w ith RAM 1). T he conclu2

N o. 3 SUN Cheng2ke et al. 357 sion indicates that the therm al dissociation of BPO in g round sta te takes p lace v ia free rad ica l ho2 m o lyt ic cleavage, bu t the react ion of ion ic het2 ero lytic cleavage is impo ssib le. T he resu lt is in rea2 sonab le agreem en t w ith the experim en tal m echa2 n ism. Table 1 Change of energ ies (e igenvalues) with bond length E longated bond length for 1a and 2a dissoc iation ways in ground state by mean s of RAM 1 and UAM 1 scan s respectively 3 RAM 1 UAM 1 1a 2a 1a 2a 0. 10 0. 078788 0. 295700 0. 078790 0. 295700 0. 11-0. 003079 0. 112300-0. 003073 0. 112300 0. 12-0. 044157 0. 007950-0. 044151-0. 002270 0. 13-0. 053964-0. 040750-0. 053961-0. 042900 0. 14-0. 045966-0. 053980-0. 045956-0. 053990 0. 15-0. 033189-0. 047720-0. 033184-0. 048810 0. 16-0. 021695-0. 032740-0. 021692-0. 036590 0. 17-0. 011677-0. 014970-0. 013418-0. 022380 0. 18-0. 000630 0. 003010-0. 015878-0. 008310 0. 19 0. 013320 0. 020250-0. 021702 0. 004440 0. 20 0. 029806 0. 036440-0. 026052 0. 014370 0. 21 0. 046959 0. 051490-0. 028211 0. 018430 0. 22 0. 062800 0. 065430-0. 028970 0. 019090 0. 23 0. 078312 0. 078280-0. 029140 0. 018610 0. 24 0. 088923 0. 090020-0. 029163 0. 017770 0. 25 0. 097247 0. 100560-0. 029194 0. 016770 0. 26 0. 103690 0. 109840-0. 029247 0. 019670 0. 27 0. 108679 0. 117890-0. 030750 0. 019450 0. 28 0. 112584 0. 124810-0. 030794 0. 019190 0. 29 0. 115693 0. 130740-0. 030830 0. 018900 0. 30 0. 118225 0. 135850-0. 031160 0. 018610 3 1a: PhC (O ) O OC (O ) Ph; 2a: PhC (O ) OO C (O ) Ph. Bond lengthgnm; energygh artree. Second ly, the deta iled stud ies of the d issocia2 tion s of BPO v ia the w ays of 1a to 4a w ere carried ou t w ith the computational model of the free radi2 ca l hom o lyt ic cleavage in g round sta te by un re2 stricted UAM 1, as show n in F ig. 2 ( the detailed da ta a re listed in T ab le 2). Com p a ring the PES p rofiles of the fou r differen t k inds of dissociation F ig. 2 Compar ison of PES prof iles for four differen t k inds of decom position ways in ground state (UAM 1). w ays in ground state show s that the PES of 1a w ay of the sing le bond hom o lyt ic cleavage is the low 2 est, and that of 4a w ay of the tw o bonds decompo2 sition is the h ighesṫ T he PES p rofile cu rves of 1a and 2a w ays show the located po in t of ex trem um, and their to tal energies are relatively low er, w h ich show s that reaction s 1a and 2a cou ld take p lace v ia the tran sition states w h ich have low er po ten tial barriers. But the PES p rofiles of reaction s 3a and 4a do no t have the located po in t of ex trem um, and they are relatively h igher than tho se of 1a and 2a in to tal energy, w h ich increase as the bond length in2 creases. T herefo re, the dissociation s v ia 3a and 4a w ays are qu ite difficu lt in ground state. Table 2 Change of energ ies (e igenvalues) with the change of bond length for the four differen t k inds of dissoc iation ways in ground state 3 E longated bond length 3 UAM 1 1a 2a 3a 4a 0. 10 0. 10466 0. 29570 0. 37044 0. 79642 0. 11 0. 00584 0. 11230 0. 17165 0. 39811 0. 12-0. 04249-0. 00227 0. 04786 0. 15007 0. 13-0. 05395-0. 04290-0. 02052 0. 01307 0. 14-0. 04447-0. 05399-0. 04969-0. 04535 0. 15-0. 02835-0. 04881-0. 05292-0. 05181 0. 16-0. 01354-0. 03659-0. 04077-0. 02751 0. 17-0. 00101-0. 02238-0. 02113 0. 01180 0. 18-0. 00702-0. 00831 0. 00082 0. 05568 0. 19-0. 01294 0. 00444 0. 02231 0. 09872 0. 20-0. 01720 0. 01437 0. 04240 0. 13878 0. 21-0. 01907 0. 01843 0. 06072 0. 17534 0. 22-0. 01944 0. 01909 0. 07456 0. 20298 0. 23-0. 01925 0. 01861 0. 08386 0. 22175 0. 24-0. 01900 0. 01777 0. 09031 0. 23489 0. 25-0. 01887 0. 01677 0. 09491 0. 24434 0. 26-0. 01884 0. 01967 0. 09820 0. 25111 0. 27-0. 01888 0. 01945 0. 10047 0. 25581 0. 28-0. 01895 0. 01919 0. 10197 0. 25949 0. 29-0. 01902 0. 01890 0. 10293 0. 26149 0. 30-0. 01908 0. 01861 0. 10350 0. 26217 1a: PhC (O )O OC (O ) Ph; 2a: PhC (O )OO C (O ) Ph; 3a: PhC (O )OOC (O ) Ph; 4a: Ph C (O )OOC (O ) Ph. Bond lengthgnm; energygh artree. To confirm fu rther tha t the d issocia t ion s of the tw o bonds of 4a do no t take p lace sim u ltane2 ou sly, w e also computed its tw o2dim en sional po2 ten tial energy su rface TD 2PES w ith UAM 1 (F ig. 3). A cco rding to the TD 2PES, it indicates clearly that the dissociating energy of the tw o bonds at the sam e tim e is on the saddle po in t diagonal of h igher po ten t ia l energy. M o reover, the doub le2bond cleavage reaction w ill p roduce tw o Ph free radi2 ca ls and a doub le2free2rad ica l C (O )OOC (O ), w h ich is in a h igher energy state ( the calcu lated

3 58 CH EM. R ES. CH IN ESE U. V o l. 19 value is 592164 kj gmo l) and leads to disadvan ta2 geou s dissociation. So the 4a w ay doesn t take p lace. F rom the TD 2PES, o therw ise, the 3a w ay w ith the single2bond dissociation does no t have any ex trem um value and also is no t the tran sition sate. Indeed, in the computing w e didn t find the tran si2 tion state fo r 3a w ay. But as a w ho le, the energy of 3a w ay isn t h igher, w h ich is in the energy val2 ley. So m ay be the single2bond dissociation by on ly heat vib rating. been fu rther studied. T he computed data are listed in T ab le 3 and show n in F ig. 4. F ig. 3 TD -PES of the two bonds decomposition (PhC C2O 4 CPh) of benzoyl perox ide in ground state (UAM 1). Table 3 Change of Energ ies with bond length for the three E longated bond length differen t k inds of in termediates dissoc iation ways in ground state UAM 1 1b 2b 2c 0. 10 0. 35583 0. 40050 0. 35114 0. 11 0. 18757 0. 20812 0. 19628 0. 12 0. 08165 0. 07357 0. 10784 0. 13 0. 02244 0. 01338 0. 06659 0. 14-0. 00315-0. 00950 0. 03295 0. 15-0. 00610-0. 01149 0. 03549 0. 16 0. 00461-0. 00374 0. 04916 0. 17 0. 02210 0. 00739 0. 06687 0. 18 0. 01710 0. 01677 0. 08335 0. 19 0. 01542 0. 02022 0. 09527 0. 20 0. 01054 0. 01965 0. 10185 0. 21 0. 00447 0. 01743 0. 10451 0. 22-0. 00101 0. 01500 0. 10495 0. 23-0. 00504 0. 01302 0. 10541 0. 24-0. 00754 0. 01168 0. 10568 0. 25-0. 00887 0. 01086 0. 10613 0. 26-0. 00950 0. 01038 0. 10663 0. 27-0. 00983 0. 01005 0. 10703 0. 28-0. 01004 0. 00980 0. 10730 0. 29-0. 01023 0. 00957 0. 10737 0. 30-0. 01043 0. 00937 0. 10731 3 1b: Ph CO ; 2b: PhOC O 2 ; 2c: Ph CO. Bond lengthgnm; energygh artree. F inally the PES of the decom po sit ion reac2 tion s of the in term ediates in the ground state have F ig. 4 Compar ison of PES prof ile for the three differen t k inds of in term ed iates decom position in ground state (UAM 1). T he resu lts show that the PES have a m ax i2 m um va lue, and the d issocia t ion energ ies of 1b (Ph CO 2 ) and 2b (PhOC O 2 ) a re low er than that of 2c (Ph CO ), w h ich m ean s that the de2 compo sition s of 1b and 2b take p lace v ia the tran si2 tion state, bu t the therm al decompo sition of 2c that has a h igher energy is mo re difficu lt than tho se of 1b and 2b. If the decom po sit ion of 2c cou ld p roceed, it m u st be heat vib ration dissocia2 tion becau se of no t being the tran sition state. T he PES of the th ree2bond d issocia t ion of BPO has also been scanned. U sing the resu lts can p redict that the energy p rofile is h igher than that of the tw o2bonds dissociation. T herefo re th is dis2 sociation reaction by 5a w ay cou ldn t take p lace. 2 AM 1 Optim ized Geom etr ies, Energy and Imaginary Frequenc ies A cco rd ing to the above invest iga ted conclu2 sion s on the po ten tial energy su rface of the decom 2 po sition reaction by variou s cu tting m ethods, the m echan ism s of the therm al decompo sition reaction of BPO in ground state v ia Schem es 1 and 2 have been investigated system atically. Becau se all the react ion s a re free rad ica l react ion s, so the un re2 stricted UAM 1 is u sed as the computational algo2 rithm. T he op tim ized geom etry configu ration s, the p a ram eters, the energ ies and the im ag ina ry fre2 quencies of the tran sition states, and som e selected geom etries of the in term ediates and the p roducts are listed in T ab le 4, w here the special geom etry configu ration s tran sfered directly by Gau ssview 211 [20 ], are derived from the computational real values w ith UAM 1. A ll the stationary po in ts w ere fu rther confirm ed by the computation s of the fo rce con stan t s ana lyt ica lly and cha racterized by the num ber of im aginary vib rational frequencies. T he

N o. 3 SUN Cheng2ke et al. 359 system s w ere characterized as the tran sition states by the p resence of one and on ly one negative eigen2 value. T he resu lts of the op tim ized structu res w ith UAM 1 indicate that the reactan t show ed a spacial geom etry, that is, the tw o benzoyl group s w ere perpendicalar to each o ther, bu t the tran sition state of its 1a dissociation is a p lanar structu re, and the tran sition state of its 2a dissociation is a cro ssbeding structu re (T ab le 4). T he distances R s of the dissociated bonds fo r tran sition states T S (1a), T S (1b), T S (2a) and T S (2b) are 011719, 011811, 012176 and 011906 nm resp ect ively. T he o ther selected geom etry param eters are also listed in T ab le 4. Excep t that the bond length of the per2 oxyl bond R (O 13 O 15 ) = 011292 nm is sligh tly sho rter than that of the ob served value, comparing tho se calcu lated param eters w ith their experim en t va lues, w e found tha t the o ther bond leng th s, bond ang les and d ihed ra l ang les w ere in good agreem en t w ith the experim en tal values [15, 16 ]. So Table 4 UAM 1 optim ized geometr ies and parameters, energ ies and imag inary frequenc ies of the tran sition states for the two k inds of homolytic cleavages of benzoyl perox ide System s of Param eters of geom etries Spacial geom etry configuration b decompo sition a [Bond lengthgnm, angleg( ) ] Reactant PhC (O )OOC (O ) Ph (2s+ 1= 1) TS (1a) PhC (O )O OC (O ) Ph (2s+ 1= 1) TS (1b) Ph C (O )O TS (2a) PhC (O )OO C (O ) Ph (2s+ 1= 1) TS (2b) PhC (O ) OO IM (1b) PhC (O )O IM (2b) PhC (O )OO IM (2c) PhCO R (C 12 O 13 ) = 0. 1228 R (C 12 O 14 ) = 0. 1400 R (O 13 O 15 ) = 0. 1290 C 12 O 13 O 15 = 114. 34 12 O 13 O 15 C 16 = - 90. 0 R (O 13 O 15 ) = 0. 1719 R (C 12 CO 13 ) = 0. 1350 R (C 12 O 14 ) = 0. 1230 O 13 C 12 O 14 = 118. 12 12 O 13 O 15 C 16 = 180. 0 R (C 3 C 12 ) = 0. 1811 R (C 12 O 13 ) = 0. 1217 R (C 12 O 14 ) = 0. 1217 O 13 C 12 O 14 = 147. 59 C 2 C 3 C 12 O 13 = 133. 2 R (O 15 C 16 ) = 0. 2176 R (C 12 O 14 ) = 0. 1220 R (C 12 O 14 ) = 0. 1190 C 12 O 13 O 15 = 116. 74 C 12 O 13 O 15 C 16 = - 90. 2 R (C 12 O 13 ) = 0. 1906 R (C 12 O 15 ) = 0. 1200 R (O 13 O 14 ) = 0. 1100 C 3 C 12 O 15 = 140. 93 C 3 C 12 O 13 O 14 = 180. 0 R (C 3 C 12 ) = 0. 1474 R (C 12 O 13 ) = 0. 1330 R (C 12 O 14 ) = 0. 1230 O 3 C 12 O 14 = 116. 52 C 2 C 3 O 12 O 13 = - 180. 0 R (C 12 O 15 ) = 0. 1221 R (C 12 O 13 ) = 0. 1460 R (O 13 O 14 ) = 0. 1170 C 12 O 13 O 14 = 117. 03 C 3 C 12 O 13 O 14 = - 180. 0 R (C 2 C 3 ) = 0. 1405 R (C 3 C 12 ) = 0. 1430 R (C 12 O 13 ) = 0. 1190 C 3 C 12 O 13 = 142. 81 C 2 C 3 C 12 O 13 = 0. 00 EnergiesgH artree; im aginary frequenciesgcm - 1 H f= - 0. 054025 E ZPE= 0. 216175 H f= - 0. 013201 E ZPE= 0. 215217 Im a. freq. = 2949. 4 i Rel. inten. = 0. 10 H f= 0. 017130 E ZPE= 0. 100763 Im a. freq. = 655. 0 i Rel. inten. = 0. 64 H f= 0. 019163 E ZPE= 0. 210164 Im a. freq. = 228. 8 i Rel. inten. = 0. 65 H f= 0. 018377 E ZPE= 0. 106296 Im a. freq. = 537. 7 i Rel. inten. = 0. 19 H f= 0. 014547 E ZPE= 0. 105451 H f= 0. 012364 E ZPE= 0. 109765 H f= 0. 030094 E ZPE= 0. 100308 a. T S rep resents transition states, IM rep resents interm ediates; b. all geom etry configurations are directly cited from Gaussview conver2 sion.

3 60 CH EM. R ES. CH IN ESE U. V o l. 19 the UAM 1 m ethod is show n to be one to p redict co rrectly the reasonab le configu ration s fo r the de2 compo sition reaction of benzoyl perox ide, w h ich is very impo rtan t to the reliab ility of calcu lating ener2 gy. 3 The M echan ism s of Therma l D ecom position of BPO in Ground Sta te Based on the analysis of computed PES and the system atical calcu lation s of 1a and 2a pathw ays of BPO therm al dissociation in ground state, the energ ies of the reactan t, the in term ed ia tes, the p roducts and the variou s tran sition states w ere computed, and the energy p rofiles fo r the reaction pathw ays w ere draw n, as show n in F ig. 5. A cco rd ing to the energy p rofiles w e can see clearly that the activation energies E a (1a), E a (1b), and H (1a) of the dissociation reaction s of 1a and 1b by p a thw ay 1 a re 107107, 83108 and 65140 kj gmo l respectively, and are in good agree2 m en t w ith the experim en tal values ( 8316 2019, 66188 12154 and 62170 12154 kj gmo l ) [21 ], w h ich indiacates that UAM 1 leads to reasonab le resu lts. In F ig. 5, the activation energies E a (2a) and E a ( 2b ) of 2a and 2b dissociation reaction s are 191196 and 80163 kj gmo l respectively, w h ich are reasonab le resu lts [22 ]. T he fact that the activation energies of 1a and 1b dissociation s are low er than tho se of 2a and 2b indicates that the reaction pathw ay of Schem e 1 takes p lace mo re easily than pathw ay 2 in Schem e 2. How ever, as a w ho le, the activation energies of 2a and 2b are also no t too h igh. So at the h igher temperatu re it can be carried ou t po ssib ly. But the activation energy of 2a reaction is h igher than that of 1a after all. M ak ing u se of the A rrhen iu s fo rm u2 la can lead to the ratio of the rate con stan ts of the tw o reaction s [23, 24 ], i. e., k 1a = exp k 2a H 2a - R T H 1a exp S 1a - R S 2a (1) A s sub stitu ting the calcu lated thermodynam ic data fo r the variab les, w e can ob tain the ratio as fo llow s: k 1agk 2a = 21573 10 13. T herefo re, as a w ho le, the m ain reaction is the pathw ay of 1a. In regard to 2c reaction, the therm al dissocia2 tion of it cou ldn t take p lace in g round sta te be2 cau se of the h igh energy of radical Ph w h ich is in2 stab le, so that the final p roducts can on ly be radi2 cal PhC (O ) and O 2. But the dissociation energy (202159 Jgmo l) of PhC (O ) is in the energy range of visib le ligh t to u ltravio let radiation [25 ], so under the condition of visib le radiation o r u ltravio let radi2 ation illum inating etc., 2c can fu rther dissociates to p roduce Ph and CO. Conclus ion s T he therm al decompo sition m echan ism of ben2 zoyl p erox ide in g round sta te, lead ing to va riou s in term ediates, p roducts and the po ten tial energy su rface (PES) of the po ssib le dissociation reaction have been studied system atically by m ean s of the AM 1 sem i2emp irical m ethod. T h is m ethod is show n to be u sed to p redict co rrectly the p referred pathw ay fo r the reaction of the title compound. T he p resen ted calcu lated resu lts based on the as2 sump tion of five k inds of dissociation pathw ays show that in ground state, the therm al decompo si2 tion of benzoyl perox ide has tw o k inds of path s. N o t on ly ex ists the symm etrical homo lytic cleavage reaction w h ich is held traditionally, e. g., pathw ay

N o. 3 SUN Cheng2ke et al. 361 1, PhC (O )O OC (O ) Ph PhC (O ) O Ph + CO 2 p roduces fina lly the p henyl free rad ica l and carbon diox ide, bu t also the asymm etrical ho2 mo lytic cleavage reaction ex ists, e. g., pathw ay 2, PhC (O )OO C (O ) Ph PhC (O )OO + PhC (O ) PhC (O ) + O 2 Ph + CO + O 2 is carried ou t on2 ly in tw o step s, that is, the dissociation p roduces oxygen and free radical PhC (O ). A nd the fu rther therm al dissociation of PhC (O ) is qu ite difficu lt becau se of the h igh act iva t ion energy in g round state. But in the condition of visib le radiation o r u ltravio let radiation illum inating etc., 2c dissociat2 ing m ay be po ssib le, fu rther p roducing free radical Ph and CO. T he research resu lts also indicate that the therm al dissociation p rocess of the tw o bond s o r the th ree bond s of benzoyl p erox ide doesn t take p lace in ground state. T he calcu lated geom et ries, the therm odynam ic da ta and the act i2 vation energies are in good agreem en t w ith the ex2 perim en tal values, w h ich show s that the AM 1 m ethod is reasonab le fo r BPO decompo sition lead2 ing to the reliab le m echan ism put fo rw ard in the article. In addition, the studied resu lts show that w hen the temperatu re is low er, pathw ay 1 is the m a in react ion to p roduce the p henyl free rad ica l and CO 2, bu t w hen the temperatu re is h igher, pathw ay 2 takes p lace sim u ltaneou sly to fo rm free radical PhC (O ) and O 2, even fu rther to p roduce the phenyl free radical and CO. So in the experi2 m en ts and indu strial p rocesses, w hen BPO is u sed as the in itiato r of po lym erization reaction s, it shou ld operate at a low er temperatu re, to fo rm in2 ert CO 2 and phenyl free radicals rather than active O 2 and CO leading to danger. Re fe re nce s [ 1 ] C rich D., Kom atsu M., Ryu İ, Chem. R ev., 1999, 99, 1991 [ 2 ] M ekarbane P. G., T abner B. J., J. Chem. S oc., Perk in T rans, 2000, 2, 1465 [ 3 ] H ammond G. S., J. A m. Chem. S oc., 1950, 72, 3737 [ 4 ] H ammond G. S., Soffer L. M., J. A m. Chem. S oc., 1950, 72, 4711 [ 5 ] B raun W., Rajbenback L., E irich F. R., J. P hy ṡ Chem., 1962, 66, 1591 [ 6 ] Edge D. J., Koch i J. K., J. A m. Chem. S oc., 1973, 95, 2635 [ 7 ] Sanchez J., M yers T. N., K irk2o thm er E ncy clop ed ia of Chem ical T echnology, V o l. 18, 4th Edn., John W iley & Sons Inc., N ew Yo rk, 1996, 230 [ 8 ] W alling C., F ree R ad icals in S olu tion, John W iley & Sons Inc., N ew Yo rk, 1957, 273 [ 9 ] Sanchez J., M yer T. N., K irk2o thm er E ncy clop ed ia of Chem ical T echnology, V o l. 14, 4th edn., John W iley & Sons Inc., N ew Yo rk, 1995, 431 [ 10 ] M oad G., So lomon D. H., Comp rehensive P olym er S cience, Pergamon P ress, O xfo rd, 1989, 3, 97 [ 11 ] Chateauneuf J., L usztyk J., Ingo ld K. U., J. A m. Chem. S oc., 1988, 110, 2877 [ 12 ] F risch M. J., T ruck s G. W., Sch legel H. B., et al., Gaussian 98, P ittsburgh PA, Gaussian Inc., 1998 [ 13 ] Sch legel H. B. ; Ed., Yarkony D. R., M od ern E lectronic S tructure T heory, W o rld Scientific Publishing, 1995 Singapo re, [ 14 ] D ew ar M. J. S., Zoebisch E. G., H ealy E. F., J. A m. Chem. S oc., 1985, 107, 3902 [ 15 ] L ide D. R., H and book of Chem istry and P hy sics, 73rd Edn., CRC P ress Inc., L ondon, 1992 1993, 921 [ 16 ] Karch N. J., Koh E. T., W h itsel B. L., et al., J. A m. Chem. S oc., 1975, 97, 6729 [ 17 ] Sch legel H. B., J. Comp. Chem., 1982, 3, 214 [ 18 ] Fo resm an J. B., F risch M. J., E xp loring Chem istry w ith E lectronic S tructure M ethods, 2nd Edn., Gaussian Inc., P ittsburgh, PA, 1996, 61 [ 19 ] H uang R. L., Goh S. H., O ng S. H., T he Chem istry of F ree R ad icals, Edw ard A rno ld, L ondon, 1974, 61 [ 20 ] Gaussv iew 2. 1, Sem ichem Inc., Gaussian, P ittsburgh PA, 2000 [ 21 ] V an Sick le D. E., J. O rg. Chem., 1969, 34, 3446 [ 22 ] W alling C., Savas E. S., J. A m. Chem. S oc., 1960, 82, 1738 [ 23 ] Sun C. K., Gong S. Y., L i Z. H., et al., Chem. J. Ch i2 nese U niversities, 2000, 21 (6), 912 [ 24 ] Sun C. K., Yang S. Y., M a S. Y., et al., Chem. J. Ch i2 nese U niversities, 2002, 23 (1), 109 [ 25 ] H uang R. L., Goh S. H., O ng S. H., T he Chem istry of F ree R ad icals, Edw ard A rno ld, London, 1974, 27, 58, 62