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The intramolecular hydrogen atom

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27 March 1998Ž.Chemical Physics Letters 285 1998 404–409The intramolecular hydrogen atom transfer in thephotodissociation of o-chlorotolueneWei-Qiao Deng, Ke-Li Han), Ji-Ping Zhan, Guo-Zhong He, Nan-Quan LouState Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,PR ChinaReceived 7 August 1997; in final form 17 December 1997AbstractBased on ab initio calculated results, a new mechanism is proposed that intramolecular hydrogen atom transfer may occurŽ.during the photodissociation of o-chlorotoluene at 266 nm. This mechanism is different from the general suggestion, i.e. 1Ž. Ž. Ž. Ž.o-chlorotoluene S ™ o-chlorotoluene T ™ Clqo-tolyl™ benzylqCl. Our suggestion is 2 o-chlorotoluene S ™ o-11 1Ž. Ž.Ž.chlorotoluene T , S ™ 5-methylene-6-chloro-1,3-cyclohexadiene MCC S ™ benzylqCl. Our results show that the10 0Ž.energy barrier of the rearrangement from o-tolyl to benzyl in path 1 is too high to be overcome, but the intramolecular HŽ.atom transfer of o-chlorotoluene in path 2 may be more reasonable. q 1998 Elsevier Science B.V.1. IntroductionWith the development of computer techniques, abinitio calculation has become a strong implement forstudying chemical reaction systems and amends somemechanisms proposed by experiment. For example,in the photodissociation of gloxal at 454.5 nm, Logeand Parmenter, measured a mixture of products,wxincluding H and CO 1 . They suggested that the2products come from two steps, i.e. first, H C O ™222HCOqCO and second, H CO™ H q CO. How-222wxever, Schaefer and coworkers 2,3 were sceptical ofthis mechanism based on ab initio calculated results.The energy barrier of the decomposition of H CO is2too high to allow further reaction, therefore Schaefer)Corresponding author.and coworkers suggested that the H product ob-2served by Loge and Parmenter maybe explained bythe direct dissociation of H C O . The proposal has222wxbeen proved by the experiment of Hepburn et al. 4 .This example indicates the importance of ab initiocalculations in studying chemical reactions. In thisLetter, an example of a new mechanism for the Hatom transfer in the photodissociation of o-chloro-toluene is proposed, based on ab initio calculationresults.Benzyl radicals play important roles in photo-chemical reactions. They may be produced not onlyfrom the dissociation of the reactant involving abenzyl group but also from those involving the tolylŽ.wxgroup mainly refer to o-chlorotoluene 5–8 . Chil-wxdress et al. 5 first reported this appearance byCIDNP measurements and found the benzyl radical0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž.PII S0009-2614 97 01447-4()W.-Q. Deng et al.rChemical Physics Letters 285 1998 404–409 405formed by the thermal decomposition of o,oX-di-methylbenzoyl peroxide in the presence of hex-wxachloroacetone. Porter and Wright 6 reported theformation of the benzyl radical by the flash photoly-sis of o-chlorotoluene. Certainly, the traditional sug-gestion for the mechanism of benzyl product forma-tion, observed by experiment, is that the o-tolylradical yielded by the primary decomposition ofo-chlorotoluene is rearranged to the benzyl radicalwx7,8 . Following this train of thought, in order tomeasure the rate constant of the rearrangement fromwxo-tolyl to benzyl, Ichimura and Mori 7 used a 266nm laser to photodissociate the o-chlorotoluene anddetected the increasing absorption spectrum of thebenzyl radicals. They regarded the rate constant ofthe rearrangement of o-tolyl radicals as the rateconstant of the increasing absorption of benzyl.Moreover, the process of the production of benzylradical should be divided into two steps:Ž.1Ž.2wxwith enthalpies D Hs 85 kcalrmol 20 and D Hswx23 kcalrmol 19 , respectively.Ž.The rate constant of reaction 2 equalled the rateconstant of the yield of benzyl, which was measuredas 1.5= 108sy1in the experiment of Ichimura andwxMori 7 .Calculating the energy barrier of this rearrange-ment, we state our scepticism concerning the impor-Ž.tance of reaction 2 in the formation of the benzylradical. Section 3 shows that since the energy barrierof the rearrangement from tolyl to benzyl was 43kcalrmol at the B3LYPrrUHFr6-31G)level andonly 22 kcalrmol of the available energy went intothe internal energy of tolyl radical, it was difficult toyield the benzyl radical from the tolyl radical. There-fore, we assume that the benzyl radical observed bywxIchimura and Mori 7 may come from the reactions:Ž.3Ž.4The purpose of the present work is to theoreticallyŽ. Ž.explore the viability of reactions 3 and 4 . Al-though it cannot be stated unequivocally that reac-Ž.tion 4 is the source of benzyl in the o-chloro-toluene photodissociation, this work provides firmtheoretical support for the plausibility of such amolecular rearrangement.2. A qualitative prediction for theoryHere, we develop in considerably more detail thequalitative prediction made earlier, namely that the Hatom may be transferred during the process of pho-todissociation of o-chlorotoluene. This argument ismade in the light of Fig. 1, which summarizes bothprobable mechanisms.The hinge of our discussion of Fig. 1 is theenergy for the rearrangement of the o-chlorotoluene.In order to elucidate this question, the photochem-istry of the o-chlorotoluene should be studied sys-tematically. However, it has not yet been studied indetail. Thus we cogitate it by referring to the photo-chemistry of some systems similar to this one, forwxexample, the photolysis of C H Cl at 248 nm 9 .65wxNishi et al. 9 detected the translational energydistribution of the recoiling Cl atom, the photofrag-ment of chlorobenzene, and found that the photolysisof C H Cl at 248 nm gave rise to a translational65energy distribution which consisted of two modes,one Boltzmann and the other Gaussian. This resultcan lead to a conclusion that the photo-decomposi-tion of the C–Cl bond in chlorobenzene by the 248nm excitation takes place through two different dis-sociation channels with probabilities of similar mag-nitudes: one channel via vibrationally excited levels()W.-Q. Deng et al.rChemical Physics Letters 285 1998 404–409406Fig. 1. Schematic energy relationships for the decomposition ofo-chlorotoluene. The relative energies in the brackets are calcu-)) )Žlated at the B3LYPr6-31G rrHFr6-31G qZPE HFr6-).Ž31G level. The ZPE is scaled by 0.894 for anharmonicity.correction.of the triplet state and


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