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RALPHK. BIRDWHISTELL
University of West Florida Pensacola, FL 32504
Just What Is a Transition State? Keith J. Laidler University of Ottawa, Ottawa, ON, Canada KIN 984
Nowhere more than in science is it necessary for words to he used in a universallv consistent manner. The hi lo sop her John Stuart Mill made a very good point in his A system of Logic (1843) when he wrote that Hardly any original thoughts on mental or social subjects ever make their way among mankind, or assume their proper importance in the mind even of their inventors, until aptly selected words or phrases have as it were nailed them down and held them fast. Scientists cannot communicate effectively with each other unless they are in agreement as to the language they use. Moreover, as all teachers realize, the precise use of language is hound to clarifv our thouehts. , the careless use of words is inevitably associ&ed with careless thinking and with a difficultv in understandine difficult scientific concents. In discussions of chemical reactions, particularly of their rates, a number of expressions have come to he used for the various intermediate species that are formed during the course of chemical chance. Unfortunatelv. different writers have used terms in different senses, with resulting confusion. Certain definitions are used bv the maioritv . . of those who have devoted the most attention to the fundamentals of
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kinetics, and it is to be hoped that these definitions can be used universallv, so as to avoid further confusion. I t is important to distinguish between an elementary reaction, which occurs in one step, and a composite (complex or stepwise) reaction, which involves more than one elementary reaction. Sometimes there is experimental evidence that indicates that a reaction must occur in more than one step; for example, the kinetic equation may not correspond to the stoichiometry of the reaction, or intermediates may he detected of such a character that more than one elementary reaction must be occurring. In the absence of such evidence it may be tentatively concluded that a reaction is elementary, but i t must always be borne in mind that further investigations may lead to the conclusion that the mechanism is composite. Suggested Definitions - First will he given concise suggested definitions of some of the terms, which will Inter be discussed in more detail.
transition state or activated comnlex. (These terms. now used synonymously,mustonly b ~ a p p l l ~ d f ~ ~ ~ n ~ I ~ m r n t a r ) .Ar e a ~ ~ t i ~ n . ) aprrics defined ns corresponding t o an nrlurrary infinitrsimal r w o n near the col (saddle point, of a putential.eneqy sortace.
transition species. Any intermediate species in an elementary reaction. An activated complex (transition state) is an example of a transition species, but the latter term also includes other intermediate species (see Fig. I), to he discussed later.
ACTIVATED COMPLEX, or TRANSITION STATE
I
The above two expressions apply only to an elementary reaction; the following apply only to reactions occurring by composite mechanisms: transient intermediate. A short-lived intermediate occurring in a composite reaction. It is a product of one elementary reaction and a reactant in another. free-radical intermediate. A transient intermediate that is a free radical, i.e., that has one or more unpaired electrons.An atom is a special ease of a free radical. high-energy intermediate. A transient species of high energy. It could be an atom or free radical, not necessarily excited, or a molecule that is excited in some wav-ex.. . .. electronicallv,vihratilmnily, rotntiunnlly,aw tmn4nrimally.The term docs nut hnvea prrrise mennmg,and it might heappl~edt u n n y intrrmed~atethat is formed from the rrdrtnnrr hg an endothermic proress.
TRANSITION
REACTANTS
Activated Complexes or Transition States
The theorv that has come to he called transition-state theory1 applies to elementary reactions only and focuses attention on species that relate to the rol or saddle point of a potential-energy surface. A potential-energy surface (or hypersurface) results from a plot of potential energy against appropriate parameters, such as interatomic distances. The course of a reaction corresponds to motion over a potentialenergy surface, and a schematic potential-energy profile, which is a sertion along the reaction path, isshown in Figure 1.7'0 identifv the activated romolex. or transition state, one considers a n arbitrary and vanishingly small region in the neighborhood of the maximum in the profile, which corresponds to the col or saddle point in the potential-energy surface. Only those species that are represented as lying within this small region should be called activated complexes, or transition states. Eyring's pioneering paper of 1935 was entitled "The activated complex in chemical reactions" (3),and the term "activated complex" was used throughout. A few months later there appeared, quite independently, Evans and Polanyi's paper "Some applications of the transition state method to the calculation of reaction velocities, especially in solution" (4). The ideas in this Evans-Polanyi paper are exactly the same as in Eyring's, although the basic rate equation was not eiven in exolicit form. and the main . ourpose . of the paper . . was to interpret the effects of pressure on the ratesof reactions in solution. In that oaoer the tcrm "transition state" is used in two senses. In thk first part of the paper the term is used in its abstract, thermodynamic sense; a reaction is considered topass from the initialstate through the transition state into the final state. In later parts of the paper the term "transition state" sometimes refers to the species itself; for example, there is a reference to "forces hetween the transition state and the solvent".
'
"Transition-state theory" is the expression now recommended by IUPAC ( I , 2). The.reason for s somewhat comolicated. and in .-. - this - areference I theaurhor s opinon is as follows:Eyring referred to hiitheory as .'the theory of absoldte reaction rates", meaning that it was a nonernpirical way of treating reaction rates. Unfortunatelythis expression became condensed to "absolute rate theory" and sometimes corrupted to "the absolute theory of reaction rates". Such expressions were objectionable, and "transition-state theory" was obviously preferable. This expression is now recommended by IUPAC, which recommends both "activated comolex" ~~-~~ .~ and "transition state" for the 50% cies concerned. The author's preference is for the former for ine reason that it is less likely to be misused. Transition species have sometimes been called transition slates (@, but this usage is liable to cause confusion 7~~~
~~
~
~~
~
PRODUCTS
Figure 1. Schematic potential-energy profile for a reaction, showing the activated complex (transitionstate) and transition species.
Over the years kineticists have come to regard "transition state" and "activated complex" as synonymous and have somewhat preferred the former term to the latter.2 It is important, if confusion is to be avoided, for this usage to be respected. Unfortunately some workers have used the term "transition state" to refer to any intermediate species in a reaction, even to fairly stable intermediates in reactions occurring by composite mechanisms. Transition Species
Since activated complexes (transition states) correspond to a vanishingly small range of atomic configurations, their concentrations are vanishingly small, and there is no possibility of detecting them experimentally. During recent years i t has become oossihle to find exoerimental evidence for intermediate species in elementary reactions. These species, which cover a wider ranee of atomic confieurations (see F i c I), should he distingui&ed carefully from activated COGolexes or transition states. and it has been suggested that ... they should be called trans'itim species rj,;" Transitionspecies were first detected hs J. C. Polanyi and co-workers (firfor the reaction F
+ Na,
-
F-NaNa
-
NaF
+ Na*
The product of Na* is in an electronically excited state and emits the yellow D-line. On both sides of the line there was "wing" emission, attributed t o t h e transition species F - N a N a . Later work has found similar evidence for other reactions, and there is no doubt that further important advances will be made in this area of research. Reaction Intermediates in Composite Reactions
~
When a reaction occurs by a composite (complex or stepwise) mechanism there are formed certain intermediates, which can he of various types. Sometimes they are relatively stable substances that undergo further reaction. For example, the gas-phase reaction between nitric oxide and hydrogen may involve nitrous oxide as an intermediate, the reaction probably occurring a t least in part by the two steps
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Hydrogen peroxide may also be formed in the first step,
and then react with hydrogen H,O,
+ Hz
-
2H,O
Nitrous oxide and hydrogen peroxide may be called reaction intermediates. or transient intermediates, but the terms "transition state" and "transition species" must be avoided. Sometimes the intermediates in reartions occurring hy composite mechanisms are much less stable species, which, although they do have an independent existence, are very reat.tivp and have half-lives of a small fraction of a second. Such intermediates of short life may be referred to as highenerev intermediates and can be of various tvoes. In the pho&hemical reaction between hydrogen andyhlorine, for examole. i t was first realized bv Nernst (7) that the mechanism invdves the following steps: C1,
+ h"
-
2C1
The atoms H and C1, even though not electronically excited, are thus high-energy intermediates. Prior ro Nernst's work it had been suggested by Hodenstein 18) that the first step is theionizationofthechlorine mdecule, with the formationof Cl,+. - . and he later sueeested that excited chlorine molecules. Clz*, are involved. I t is now known that intermediates such as Ch+ and Clq* are not oresent when, as is usual. the ohotochem&alreaciion between hydrogen and chlorink is hiought about bv visible radiation. However, thev do occur as hiahenergy intermediates when the reaction is brought abouchy electromagnetic radiation of high frequency (e.g., X-rays or y rays) or by high-energy particle radiation. The meaning of the ex~ression"hiah-energy intermediate"is not precise, and care should be taken in ;singit. Atoms and free radicals, even if not excited in any way, can reasonably be called high-energy intermediates, as can otherwise stable molecules if they are in excited states in which they have substantial amounts of excitational energy. With other intermediates the situation is not clear cut. If an intermediate is formed from the reactants in a nrocess that is endothermicit might reasonably he called a high-energy intermediate: otherwise it should s i m.~.l vbe called an intermediate. ~ n z b e - s u b s t r a t e complexes are sometimes formed endothermallv and, if so, could be called high-energv -.intermediates. When organic reactions take d a c e there is sometimes evidence for-intermediate spec& having distributions of charges. It is misleadina to refer to these as transition states unless one wishes to po&date that they really are transition statesin the senseused in this article. Otherwise, a term such as "highly polar intermediate" should be employed. The acid-catalyzed iodination of acetone presents exam~ l e of s reaction intermediates in an oreanic reaction. The essential process is the conversion of the keto form of acetone into the enol form. which is verv raoidlv . .iodinated. The first step is believed to'be the transfer of a proton from the catalyzing acid HA to the acetone:
F f g ~ r2. e Schamatic potentla -energydiagramfor areactmn o c c w ng n more than one elementary step. me acid-cata.yzed tod nation of acetone The activated complexes (transition states) for the elementary processes are indicated by the usual symbol %.Twointermediates are shown.
--
-
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Journal of Chemical Education
The protonated species is an intermediate of fairly high energy since the process is endothermic; it must not, of course, be called a transitioh state. In a second stage the protonated species transfers one of its other protons to some other base B present (which may he water), and the enol form of acetone is formed: H
H
This enolic species is another intermediate: its energy is higher than that of the keto form and could also be called a high-energy intermediate. The diagram shown as Figure 2 in a purely schematic way indicates the activated complexes and reaction intermediates involved in this process. Literature Clted Gdd. V. Pure Aool. Chom. l>83..5I. 1281-1371. Laid1er.K. J . d r p A p p 1 . Chem. 1981.53. 753-771. Eyring, H. J. Chrm.Phys. 1935,3.107-115. Evans, M. G.: Polanyi,M. Troru. Foroday Sac. 1935.31,37>394. Laidler. K. J. Chemieol Kinetics. 3rd ed.; Harper & Row: New York. 1987 6. Arrousmith.R.;Bartoszak.F. E.;Bly,S. H.P.;Carrington.T.:Charteri,P J. C. J Cham Phys. 1980.73. S89M897, and several later papers. 7. Nernat, W. 2 . El~kfrmhem.1918.24,335356. 8. Bodenstein, M.Z.physB.Chem. 1913,85,329-397.
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