New version page

The intramolecular hydrogen atom

Upgrade to remove ads

This preview shows page 1-2 out of 6 pages.

Save
View Full Document
Premium Document
Do you want full access? Go Premium and unlock all 6 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 6 pages.
Access to all documents
Download any document
Ad free experience

Upgrade to remove ads
Unformatted text preview:

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


Download The intramolecular hydrogen atom
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view The intramolecular hydrogen atom and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view The intramolecular hydrogen atom 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?