The double-isotope-label technique and inorganic reaction mechanisms

THE DOUBLE-ISOTOPE-LABEL T E C H N I O U E AND

I N O R G A N I C REACTION MECHANISMS

53

J. O. Edwards and P. D. F le ischauer

Metcal/ Chemical Laboratories, o/Brown University, Providence, Rhode Island 02912, U.S.A.

CONTENTS

I. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 53 II. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 53

IlI. Advantages and Disadvantages . . . . . . . . . . . . . . . . . 54 A. Advantages . . . . . . . . . . . . . . . . . . . . . . 54 B. Disadvantages . . . . . . . . . . . . . . . . . . . . . . 54

IV. Decompositions of Peroxyacids . . . . . . . . . . . . . . . . . 55 V. Other Peroxide Decompositions . . . . . . . . . . . . . . . . . 56

A. Hydrogen Peroxide . . . . . . . . . . . . . . . . . . . . 56 B. Tetraperoxychromate(V) Ion . . . . . . . . . . . . . . . . . 57 C. Effervescent Magnetic Peroxyborates . . . . . . . . . . . . . . 57

VI. Oxidation of Hydrazine . . . . . . . . . . . . . . . . . . . . 58 VII. The Cage Effect . . . . . . . . . . . . . . . . . . . . . . . 59

VIII. Scrambling in Gaseous Diatomic Molecules . . . . . . . . . . . . . 60 A. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . 60 B. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . 61

IX. Potential Application to Specific Problems . . . . . . . . . . . . . 62 X. References . . . . . . . . . . . . . . . . . . . . . . . . 62

I. SUMMARY

The double-isotope-tracer technique provides a way o/ investigating a number o/ aspects concerning reaction mechanisms. In the case o] the peroxyacid decomposition it is seen that the structures o/ two transition states of identical constitution were clarifi- ed. In the oxidation o/ hydrazine the isotope data showed what intermediates were present and also showed something about the timing o! bond breaking o] these intermediates. Also, in the case o/ the hydrazine oxidations and peroxide decompositions the positions o! bond breaking were discovered. The technique will certainly be used in /urther studies o/ the cage effect. Finally, it is possible to study reactions which occur when there is no net chemical change.

II. INTRODUCTION

The use of the double-isotope-label technique for the study of reaction mechanisms is important in that it gives significant information as to the structures of transition states and intermediates and as to the position of bond breakage in the course of a reaction. It is, however, limited in that only certain types of reactions can be studied. It is our intent in this review to discuss the use of this technique in the investigation of inorganic reaction mechanisms and to give some of the interesting results which have been obtained. We also use some organic examples where it is appropriate for understanding the applications of this technique.

Reviews 1968

The types of reactions to which the technique is applicable involve molecules of reactant with two similar groups and a product (or products) which also has two similar groups. For example, one can write the general reactions appropriate to this technique in schematic equations (where X and X refer to the same element but different isotopes) as follows:

X--M--X q- X--M---X ~ 2X--M--X (1)

X--N--X + X--N--X--~2X--P--X (and other products) (2)

X---Q--X + X--R--X ~ X--S--X + X--T--X (and other products) (3)

In case I , there is no net chamical change but only exchange of ends occurs; in case 2, the scrambling occurs during the course of a reaction wherein the products are different from the reactants; and in case 3 a general equation to cover a variety of situations is given. We use a double arrow--s-to denote a reaction for which a measurable equilibrium position is known to exist under the stated conditions. The single arrow--->denotes a reaction which is essentially unidirectional under the conditions of the experiment.

The reactions of these types, wherein different ends of one molecule become separate parts of two molecules each having two ends, are herein termed ~scrambling~ reactions. (Other names such as isotope randomization and isotope distribution equilibrium are possible.) Scrambling is a process which is favored by the entropy of mixing, even when there is no net chemical reaction. We will not discuss reactions in which the scrambling apparently takes

54" I.O. EDWARDS AND P. D. FLEISCI-IAUER

place either on the walls of the container or on heterogeneous catalysts, although these are obviously important systems.

Two examples are useful here for clarification of the type of reaction to which the technique is appropriate. A very simple example involves the reaction

H,+D, ~.~ 2HD (I)

The ortho-para conversion in hydrogen molecules, as in the equation

Hz(ortho) ~ H,(para) (11)

is a related reaction which gives information equivalent to the isotope-labeling technique.

A second reaction to which the technique is applicable is the decomposition of hydrogen peroxide, which proceeds according to the equation

2H,O, --~ O,+2H20

The oxygen molecule which is evolved may have both atoms from the same molecule of hydrogen peroxide or the oxygen molecule may obtain one oxygen atom from each of two hydrogen peroxide molecules. This can follows:

r--~ / --*--I H O,, O~ H

I I H oi.o: H

and

be shown schematically as

O6+H~O+Hz6 (lid

H [~-~]H / "4' O'-O+2H20

H O O H J OO+2H20 (w)

where a bar over an atom indicates an isotopic label and where the dotted lines enclose the two atoms which will end up in the product oxygen molecule. Scrambling has occured in reaction (Ill), whereas in reaction (IV) the integrity of the oxygen-oxygen bond has been maintained. It is also possible, of course, that in case (IV) the two oxygen atoms could come from a non-labeled molecule.

It can be seen that the technique enables a chemist to study mechanisms of scrambling reactions and of reactions where there is a net chemical change and scrambling could occur during the mechanism of the chemical reaction. The technique is basically a sophisticated variant of the common technique employed by organic chemists in forming compounds with different substituents at the two ends of an organic molecule. For example, the following decomposition

CHr-N = N---C2Hs --~ Na + hydrocarbons

proceeds by a unimolecular, radical-forming rate step of the type

CHr-N = N---C2Hs --~CH3"+ N2 +.Calls

If the methyl group remains in the vicinity of the

ethyl group during the decomposition step the hydro- carbon which is obtained will be propane. On the other hand, if the alkyl radicals depart as separate entities then a mixture of ethane, propane and butane will be obtained. The results of an isotope label study of the same reaction give more meaningful information since there is no marked change in the electronic and steric behavior of the two ends of a molecule, as is necessarily the case when different ends have different substituents.

III. ADVANTAGES AND DISADVANTAGES

A. ADVANTAGES

The advantages of this technique are as follows:

(1) Stable isotopes are employed, therefore little hazard is involved. One could conceivably use radioactive isotopes of very long life-time, however, it is preferable to use stable isotopes of different masses if such are available.

(2) In the analysis of the product, use is made of the mass spectrometer which is a sensitive analytical instrument. Good discrimination between particles with small changes in mass is obtained. Precise ratios of the concentration of these species of different mass are possible and it is not necessary to calibrate the instrument for total concentration of product since the ratios are the relevant quantities. For most of the reactions discussed in this review it is seen that only very small quantities of product are necessary.

(3) Since the only difference between the two non- identical reactants is that due to isotopic mass, the problem of variability of reactivity of the two ends is usually negligible; an important exception is replac- ement of hydrogen by deuterium, wherein an isotope effect on relative rate of bond cleavage can easily be as much as power of 10.

(4) It is not necessary to have a high fraction of the molecules of reactant containing double isotope label. For example, we have found that peroxide reactions can be studied successfully with only 1.4% HOOH in the mixture with 98.6% normal HOOH."

(5) The technique can give information about the structures of transition states and intermediates during the course of the reaction sequence; such information may be unavailable by any other technique.

B. DISADVANTAGES

The disadvantages of this technique are as follows:

(1) If there is no more than one stable isotope of an element it is obvious that the technique cannot be applied, as in the elements Be, F, Na, AI, P, Co, As, and I. Some elements to which this technique can be applied are the following: H, B, C, N, O, Mg, Si, S, CI, Ge, Se, and Br.

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Double-Isotope-Label Technique and Inorganic Reaction Mechanisms 55

(2) The cost of separation of two stable isotopes of the same element can be considerable, however, the feasibility of using small quantities of doubly- labeled material makes the cost less serious. For example, in the work on peroxide decomposition it was found that the cost of the isotopically-labeled raw material only amounted to about four dollars per experiment .4

(3) A severe difficulty which can be encountered is the problem of synthesis of an exactly-labeled starting compound, since one must make small amounts of reactant with the isotopes in proper positions.

(4) A potentially serious disadvantage is that adventitious scrambling of isotopes between the labeled reactant and its non-labeled analog may proceed more rapidly than the reaction under investigation. Also, exchange of the labeled material with non-labeled surroundings must be minimized if satisfactory results are to be obtained. The possibility of a non-desired path for changing isotopic positions must be con- tinually considered.

(5) The quantitative mass analysis of the product(s) is more difficult if other atoms in the reactants have several stable isotopes. Application of the oxygen eighteen isotope technique to compounds containing chlorine is hampered by the fact that there are two stable chlorine isotopes.

IV. DECOMPOSITIONS OF PEROXYACIDS

The rate law for the general decomposition reaction

2ROOH --+ 2ROH + O2

where R is either an inorganic or organic electron- withdrawing group has been found to be of the form of equation (V). The first peroxyacid studied which

v = k [ R O O H ] [ R O 0 - ] (v)

showed this kinetic behavior had the monoanion HSOs- of peroxymonosulfuric acid as ROOH. 2 Other peroxyacids which have been found to show similar kinetic behavior are the following: peroxybenzoic

acid and its ring-substituted derivatives, 16 peroxyacetic acid and peroxymonochloroacetic acid, 21 peroxymonophosphoric acid, 21 peroxymonophthalic acid, 3 and peroxypivalic acid. ~ At constant pH, the rate is second-order in peroxide concentration. The rate is dependent on pH in a manner which at first seems surprising; a plot of log k2 against pH shows a maximum at pH=plC~ for the peroxidic proton. On the low-pH side of this maximum the rate is first- order in hydroxide ion concentration, and on the high-pH side of the maximum the rate is first-order in hydrogen ion concentration.

The rate constants for these peroxide decompositions are given in Table I. Since the rate laws are identical and the rate constants show so little variation it could be expected (although incorrectly) that the mechanism does not depend significantly on the nature of the R group. The activation parameters for the decomposition of Caro's acid were found to be 12.4 kcal mole -~ for AH" and - 2 0 cal mole -~ deg -~ for AS*. The similarity of these values to those found for nucleophilic displacements on peroxide oxygen and the rate law suggest a nucleophilic displacement mechanism for the rate-determining step.

The data can be explained by two types of displacement mechanisms. The first type is a displacement on oxygen by oxygen.

HSOs- +SO, 2- .-* HSO~- +SO, z-

HSO,- --+ H++SO,2- +Oz

The initial step is postulated" to be rate-determining and the transition state is presumed to be

1 R o _j

By simultaneous bond breaking and bond formation, an intermediate

H O-- O

Table I. The Uncatalyzed Decomposition of Peroxyacids

Peroxyacids Conditions 1~ × ( 10 ~) • AH" AS s Ref.

Caro's acid (peroxymonosulfuric) H20, 25 °, pH 9.3 41-210 11.3 --29 2,22 Peroxymonophosphoric H20, 35.8 °, pH 12.5 1.4 - - - - 21 Peroxyacetic H20, 25", pH 8.2 5.4 22.5 b +8 21 Monochloroperoxyacetic HzO, 150, pH 7.2 200 - - __ 21 Peroxybenzoic acids

p-Methoxy H20, 250, pH 8.07 9.8 - - - - 16a p-Methyl H20, 250, pH 7.95 9.4 16 --16 16a p-H H20, 25 °, pH 7.78 9.0 16.7 --14 16a p-Fluoro H20, 250, pH 7.76 10.4 - - - - 16a p--Chloro H20, 250, pH 7.67 15.0 ~ ~ 16a m-Chloro H20, 250, pH 7.60 15.4 16.4 --14 16a p-Nitro H~O, 250, pH 7.29 21.2 - - - - 16a

• Iq is 2k~, at the maximum rate point. The constant has units of liter mole -~ sec-*, b These were calculated from the original data, but still may be in error since the data at higher temperatures are at the pH extremes for uncatalyzed decomposition.

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56 J.O. EBWARBS AND P. D. FLEISClIAUER

is formed, and this breaks down at the designated bonds to give the products of the reaction.

The alternative mechanism t~'.zl differs in that the site of attack by the peroxyanion is postulated to be at that atom of the R group which is attached to the peroxide oxygen. In the transition state, using peroxyacetic acid as an example, the incoming peroxyanion becomes attached to the carbonyl carbon

0.,. / 0 - - 0 ~ CH]/C ""0.~ H~O

O--C~.cH j

forming a cyclic intermediate, which by cleavage at the bonds

C H f \ 0=~0 /C \cH~

designated (arrows) breaks down to form the products 02, CHaCOOH, and CH3COO-. This mechanism is attractive since peroxyanion attack at earbonyl groups is well-established.

A double-isotope-label technique can b e used to determine which of the two proposed mechanisms predominates. From the first mechanism one would predict that for a mixture of doubly-labeled (in peroxide oxygen) peroxyacetie acid and non-labeled peroxyaeid the product oxygen molecules would be predominantly scrambled, to give O, of mass 34. From the second mechanism it would be predicted that the product oxygen gas would be a mixture of normal (mass 32) and doubly-labeled (mass 36) molecules. Measurements of the isotopic distribution of the gas evolved during decomposition of peroxyacetic acid indicate that little scrambling occurs. Since most ( = 8 3 % ) of the evolved oxygen which is labeled has mass 36, the second mechanism is favored. The balance of the isotopic label appears as oxygen of mass 34. This could not have occured by exchange of oxygen atoms with unlal~eled solvent, since the ratio Ot8/OI6 was the same for the product oxygen gas as for the initial peroxide oxygen atoms. It seems probable that the 17% scrambling arose from a transition state wherein oxygen attack at oxygen obtains.

The results from the isotope experiments of the decomposition of Caro's acid" indicate an oxygen attack at oxygen to form the transition state, as the product oxygen is largely scrambled (=91%) . Never- theless, 9% of the oxygen is unscrambled which suggests that oxygen attack at tetrahedral sulfur is also a path for decomposition.

One further test has been carried out. If a large organic group is attached to the earbonyl carbon of a peroxyacid, then attack at carbonyl carbon would be less likely. Little steric effect would be predicted

for oxygen attack at oxygen. Thus a higher percentage (compared to peroxyacetic acid) of scrambling is predicted for a peroxyacid with a sterically- hindered carbonyl. Both monoperoxyphthalic acid ~ and peroxytrimethyl acetic acid ~ have been investigated in this regard. The results are in good agreement with this hypothesis, since for both compounds the percentage of events leading to scrambling increases to about 80%.

Taken all together, the isotope results for the several peroxyearboxylic acids and for Caro's acid indicate that both of the proposed mechanisms are followed in each of the decompositions. Therefore it appears that a peroxyanion nucleophile can attack three different electrophilic centers, namely carbonyl carbon, peroxide oxygen and tetrahedral sulfur. Also, one can conclude on the basis of the percentages for attack at the three centers that the ease of attack follows the order : carbonyl carbon>peroxide oxygen > tetrahedral sulfur.

During these studies, one implicit assumption was made; this is that there is no scrambling between free O2 molecules occuring in solution as the oxygen is released from the peroxyacid. While scrambling in the aqueous solution during (but independent of) the evolution of oxygen is not expected, an experiment to check this point should be carried out. This can be easily done by dissolving doubly-labeled oxygen gas in the solution in which oxygen is being evolved by an unlabeled peroxide which is decomposing. Mass spectra of the final product gas should be O23z and Oz ~ if scrambling is solely due to the decomposition mechanism. Some O2 ~ would be obtained if adventitious scrambling took place in the solution.

V. OTHER PEROXIDE DECOMPOSITIONS

A. HYDROGEN PEROXIDE

Decomposition of hydrogen peroxide is a reaction about which much has been said but surprisingly little is understood; it seems certain that trace metals are powerful catalysts and that the decomposition normally observed in basic solution is due to either these metal catalysts or small particles of suspended solids. The application of the double-isotope-label technique is potentially very useful in the study of hydrogen peroxide decomposition, however, until more is known about the mechanism there would be little likelihood that the isotope technique would be definitive. 13a~

Fortunately hydrogen peroxide does not exchange its oxygens with surrounding water molecules and many oxidants, such as eerie ion, will quantitatively take the peroxide to oxygen without any scrambling. Thus it would seem that the technique will become very important when more is known about the mechanism of hydrogen peroxide decomposition.

Two studies relevant to hydrogen peroxide are reported. Anbar ~ studied the isotope distribution in Oz evolved and in H202 residue when mixtures of HOOH and Hi3OH were irradiated. The amount of



scrambling in the O2 evolved was significant (from 10 to 28 percent)'over a wide range of pH, however, bromide ion completely inhibited scrambling except at pH=14 where 15 percent was observed, either in the absence or in the presence of bromide ion. Again, however, the results at pH----14 were unique for scrambled peroxide was observed in lesser percentage when bromide ion was present. Suffice it to say at this time that the mechanisms of scrambling are tied in with the very complicated behavior of peroxide solutions on radiolysis, that bromide ion is a scavenger for the precursor to scrambled oxygen and that the mechanism at pH= 14 must differ from that at p H = 9 or lower.

The second investigation concerned the mechanism of the catalytic decomposition of hydrogen peroxide by catalase and other ferric compoundsfl In every case which was studied the oxygen evolved originated from oxygen-oxygen bonds in hydrogen peroxide, that is to say there was no scrambling observed.

B. TETRAPEROXYCHROMATE(V) ION

Recent results 7 with more complicated inorganic peroxides have given interesting and definitive results. The decomposition of tetraperoxychromate(V) ion proceeds at a measurable rate in the pH range from 8 to 10.5 with stoichiometry

4Cr(OD, 3- +4H ÷ ~ 4CrO& +702+2H~O

This complicated stoichiometry combined with simple rate law

v=k [Cr(ODZ-] [H ÷]

the

indicates that a number of more-or-less stable intermediates are involved in the rapid steps which follow the rate-determining step. A double-label study of the decomposition of this anion (which has eight equivalent chromium-oxygen bonds in four equivalent peroxide groups) has given some results that lead to definite mechanistic conclusions. The exchange

Cr(OD,'- + H)O2 .~ Cr(6,),'- + H202

is complete within one minute under conditions wherein the decomposition takes hours. There is little scrambling (about 9%) during decomposition. The oxygen atoms in the product chromate seem to come from both peroxide groups and solvent molecules in about a two-to-one ratio. The most interesting observation is that there is very little scrambling when ammonia is present; this is no doubt related to the fact that ammonia cuts the rate of decomposition of Cr(OD, 3- down by a factor of two. The details of this complicated system are discussed elsewhere.'

C. EFFERVESCENT MAGNETIC PEROXVBORATES

The fact that sodium peroxyborate compounds, when heated carefully, could be modified in such a

way as to release significant quantities of gaseous oxygen when mixed with water was discovered by Foerster in 1921) ~ The phenomenon of rapid oxygen release is not characteristic of peroxyborates, and it cannot be attributed to simple decomposition of hydrogen peroxide in basic solution. Some hydrogen peroxide is simultaneously formed, and this only slowly decomposes. The discovery was extended to a variety of materials by Menze125 and others) s The effervescent behavior, which does not appear to depend on the nature of the cation, but has only been reported for peroxyborates, is reminiscent of the superoxides) ° Recently the fact that these unusual compounds are strongly paramagnetic was communi- cated.a

The structure of the anion which exists in the starting material sodium peroxyborate tetrahydrate (NaBO3.4HzO) has been shown by x-ray investigation ~7 to be the following

I HONB/O--O\B/OH HO / NO--O / XOHJ

2-

A rather complete study of the chemistry of these materials is available ~ and therefore it seems pertinent to discuss here only the aspects of the chemistry which are relevant to this review.

At temperatures between I00 ° and 130°C an irre- versible transformation of the peroxyborate occurs. A similar transformation can be induced by radiolysis at lower temperatures. The products, oxygen molecule and hydrogen peroxide, formed on dissolution in water of the transformed materials make possible the use of the double-isotope-label technique to under- stand better the formation and decomposition of these very unusual compounds. The appropriate isotope data are presented in Table II. The first horizontal row shows the isotope distribution in the hydrogen peroxide starting material. The second horizontal row shows the isotope distribution in the sodium peroxyborate tetrahydrate made from the hydrogen peroxide. As expected, from the known rapid rate of cleavage of boron-oxygen links as compared to oxygen-oxygen links, there is no change in the isotope distribution among the peroxide links.

From this peroxyborate sample several efferveseem materials were prepared. The products formed then by the dissolution in water,

EMPB H20 02 + HOOH + other products

where EMPB refers to the effervescent magnetic peroxyborate materials, gave results as follows. The oxygen gas evolved shows two things: (1) there has been nc exchange of the peroxide oxygens with non- peroxide oxygens during any part of the formation and decomposition reactions (this is clear from the fact that the total percentage of O ~8 in the oxygen gas is the same as in the hydrogen peroxide) and (2) a large amount of scrambling has occured and this is seen from the fact that most of the O t8 atoms

Reviews 1968

58

TId~le II. Oxygen Tracer Isotope Data

l- O. EDWARDS AND P. D. Ft~Isc~um~

% ~02 % ~02 % ~02 Total % "O

H~O2 NaBO~ " 4H20 EMPB No. 1 reacted with water EMPB No. 2 reacted with water EMPB No. 3 oxidized in solution by CC v EMPB No. 4 oxidized in solution by Ce w Gas in contact with EMPB No. 5 during its preparation Gas in contact with EMPB No. 6 during its preparation

97.3±02 0.5±0.1 2.2±0~ 2.45 97.2±0.2 0.6±0.1 2.2±0.2 2.50 95.3±0.2 4.4±0.2 0.3±0.1 2.50 95.7±0~ 3.9±0.2 0.5±0.1 2.45 96.7±0.2 0.9±0.1 2.4±0.2 2.85 97.0±0.2 0.8±0.1 2.2±0.2 2 .~ 96.1±0.2 2.9±0.2 1.0±0.l 2.45 96.5±0.2 2.6±0~ 0.9±0.1 2.~

appear in 02 ~ molecules. Scrambling has not, however, reached equilibrium as the isotope ratios in product 02 are not those predicted for a random distribution.

The next two rows show the isotope distribution in the hydrogen peroxide remaining in aqueous solution after effervescence is finished. There has been some scrambling but the amount is small; again there has been no exchange with non-peroxidic oxygens in the peroxyborate or with oxygen atoms of the solvent water.

In the final two rows isotope data on the small amount of oxygen gas released during the transformation from tetrahydrate to EMPB are shown. Con- siderable scrambling but no exchange with non- peroxidic oxygens is observed.

The utility of the double-isotope-label technique in peroxide decompositions is clearly shown by the fact that peroxide oxygens can be completely scrambled without the complication of exchange with other oxygen atoms in the vicinity. This is of considerable importance in terms of the postulation of mechanisms since the results can be very definitive. Although the details of the mechanisms remain to be elucidated the isotope data put stringent requirements on any suggested mechanism.

VI. OXIDATION OF HYDRAZINE

The mechanisms of the oxidation of hydrazine are of considerable interest, both because of their com- plexity and because of their intrinsic utility. The work up to 1927 was reviewed by Kirk and Browne? ° They pointed out that the nature of the product in some ways reflected the ability of the oxidant to extract either one electron at an encounter or two electrons at an encounter. Two independent research groups ',t8 looked at the study of hydrazine oxidation using the double-isotope-tracer technique in order to test the mechanisms proposed by Kirk and Browne, There are two primary reactions in aqueous solution at room temperature. The stoichiometries are as follows:

N2tL --~ Nz+4H ÷ +4e- (VI)

2N2H~ ~ N2 +2NI-K+ +2e - (VII)

The first stoichiometry is followed fairly well when the oxidant is acid iodate, neutral iodine or alkaline ferricyanide. The second stoichiometry is approached

when the oxidant is eerie ion, manganic ion and under some conditions ferric ion. Some common oxidants such as permanganate ion or chromate ion give a mixture of the two stoichiometries.

Previous to the isotope work by both of the recent groups 9,~ the stoichiometry of the reactions was checked. For those oxidants which form only nitrogen gas as the product, four equivalents of oxidant were used up for each mole of hydrazine. In view of the probable simplicity of this process the integrity of the nitrogen-nitrogen bond is expected to be maintained; indeed with isotopes it was shown that no scrambling occured during the oxidation process.

For those oxidants which produce both nitrogen gas and ammonia, the number of equivalents used up per mole of hydrazine varied between one and four, with the amount of ammonia increasing as the number of equivalents decreased toward one. By a short extrapolation it was shown that one equivalent was used up for each mole of ammonia produced. There- fore, the second stoichiometry obtained in these cases. It seems reasonable to postulate that a one-electron oxidation of a hydrazine molecule will form an intermediate N~H3, as in the equation

NzH, ---* N2Hj+ H* +e" (VIII)

This radical intermediate could presumably dimerize to form a tetrazane.

2N2H~ --* H2N--NH--NH--NH2 (Ix)

The breakdown of this intermediate to give the final products then could occur by several possible mechanisms, the most probable of which would be end group cleavage to give ammonia. It was the nature of this breakdown which was the subject of the isotope experiments.

With one-electron oxidants such as cerium(IV) it was found that one-half of the nitrogen gas is scrambled and. the other half has the original isotope distribution shown by the reactant hydrazine. The proposed mechanism involves the formation of the tetrazane mentioned above (X). The tetrazane then breaks down by cleavage of one end nitrogen to form ammonia and triazene (XI). The two ends of the intermediate triazene apparently become equivalent (Xll) and then a second ammonia molecule splits out (XIII). Using again a bar over the atom to denote isotope label, the mechanism is as follows:

HzN'--NH + HNNH, ---, H2/~--I~H--NH--NH2 (X)

Inorganica Chimica Acta

Double-Isotope-Label Technique and Inorganic Reaction Mechanisms 5 9

_ _ .,,.~rH,N + HN = N--NH, H2N--NH--NH--NH, "~HjN + HN = N--~IH2 (XD

HN=N--NH2 ~.~ H2N--N=NH

HN=NH2 ~-- H2N--N=NH (XlI)

HN = N--N H, "* ~IN + NH,

H3N--N = NH "* N,+NH3 (XHI)

In this mechanism the important feature is the cleavage of the first ammonia, rapidly followed by equilibration of the two ends of the triazene prior to the second cleavage to form N2 and ammonia. From this mechanism then it would be predicted that exactly one half of the product nitrogen gas molecules will be scrambled and the other half will have maintained the identity of the nitrogen-nitrogen bond. If the triazene had been a cyclic structure with three equivalent NH groups, then the percentage of scrambling would have been 66.7%.

There is interesting chemistry here relevant to the nature of the oxidants. As mentioned above, iodate, iodine and ferricyanide all oxidize the hydrazine to nitrogen. Similar behavior is observed with T1 m ion. For the three cases other than the ferricyanide, a two- electron oxidation step seems reasonable. This would form N2Hz, as in the equation

N2H, ~ N,H, + 2H + + 2e-

The formation of the N2H3 radical is thereby by- passed. For the one-electron oxidants (at least under extreme conditions) the mechanism shown in equations (VIII) and (X) through (XIII) involves the radical intermediate (N2H3) with resultant formation of both nitrogen and ammonia.

Several oxidants give products which are explicable in terms of a combination of the two mechanisms. For example, vanadium(V), permanganate ion, chromate ion, ferric ion and mixtures of ferric ion and cupric ion all produce a greater ratio of nitrogen to ammonia than that expected by equation (VII). In part, this is a consequence of the fact that some of these oxidants can readily undergo either one-electron reduction or two-electron reduction, depending on the circumstances. However, the mixed stoichiometry can also occur if the intermediate N2H3 reacts with an oxidant molecule in competition with the dimerization step. Such seems to be the case with ferric ion wherein the step

Fe'+(.) + NzH3 --~ Fe2+t.) + N~H2 + H+

must be considered. This is demonstrated clearly by the behavior of the mixture of ferric ion and hydrazine when copper(II) is present. In such mixtures little ammonia is obtained even though there is no direct reaction between cupric ion and hydrazine. The two additional rapid steps

Cu3+(~) + Nail) -.~ Cu+(.q) + N,H2 + H +

÷ 34- Cu (.q)+Fe (.q)--) Cu)+(.~)+Fe*+(,4)

must be postulated. Analysis of the stoichiometric data showed 9 that cupric ion reacts about 1200 times as fast as ferric ion does with N21-13.

It is also possible that the oxidants could react with tetrazane and triazene intermediates. The observation of HN3 (hydrazoic acid) in some cases suggests that the triazene can be oxidized as in the equation

N3Hr--*HN~+2H ÷ +2e-

The data, however, are not conclusive on this point.

VII. THE CAGE EFFECT

When a reaction such as a homolytic, unimolecular decomposition occurs in the gas phase the two resultant radicals move apart and have little chance to recombine except by diffusion. This is the case because the mean free path (the average distance a particle travels before it collides with another particle) in the gas phase is about 1000 A. The situation in the liquid phase is quite different. During the act of homolytic scission the two radicals move apart, however, they are prevented from leaving the area next to each other because the solvent molecules which surround the radicals act as a physical barrier and the two radicals are ~reflected~, back towards each other. The mean free path in a liquid is only of the order of 1 A. The influence of the solvent on the chemistry of these radicals can be discussed in terms of two radicals being held in close proximity to each other as in a cage. The way in which the solvent cage influences the chemistry of the radicals is called the ~cage effecb,.

There are three means by which the cage effect is normally studied. The first one deals with the quantum yield for a photolysis. For example, it is usually observed that the quantum yield for a photolytic dissociation is lower in an inert solvent than it is in the gas phase. This is a consequence of the fact that the two particles formed by photolysis are de-excited by the surrounding solvent molecules which at the same time prevent the particles from leaving their vicinity of formation. Thus the chemical act for which the quantum yield is calculated does not occur because the particles recombine with no net chemical change. The experimental observation in such cases is that the two particles recombine to give back the starting material. For example, it is observed that the values of the quantum yield for photolysis of hydrogen peroxide in aqueous solution are a function of both temperature and wavelength o f excitation. The quantum yield decreases as the wavelength increases; the more energetic is the light the less likelihood there is of the solvent cage pre- venting diffusion apart and therefore the radicals become independent of each other and go to products. The quantum yield decreases as you decrease the temperature at constant wavelength of excitation; this is presumably due to the stronger solvent structure which tightens the cage wall at lower temperatures. ~ In the scheme given it can be seen that

Reviews 1968

60 J.O. EDWARDS AND P. D. FLElSCS*UEn

the two hydroxyl

HOOH+hv k,) HOOH*

. o o . . {2o..}

Foil.} k', HOOH

{2OH.} k, > OH.(,~+OH.(,q)

OH'(..) k, :- O,

radicals in the cage (denoted by curly brackets) can recombine as in step k3 or diffuse apart as in step h with only the latter step accomplishing chemical reaction.

The cage effect also can be demonstrated by the use of reagents which react at essentially every collision with radicals; these have been called scavengers. The decrease in the amount of scavenger as a function of time is therefore considered to be due to radicals not in the same solvent cage. The loss of scavenger is less than the production of radicals; the difference is due to the amount of radical recombination in the cage. Discussions bearing on the use of scavengers to evaluate the importance of the cage effect are available, s,~

It is occasionally observed that the stoichiometry of a homolytic scission reaction depends on the proper- ties of the solvent cage. For example, it is known that acetyl peroxide decomposes in the gas phase with the stoichiometry

0 0

CHr--C--O--O---C---CH3 -+ C, H6+2CO,

wherein ethane and carbon dioxide are the important products. In the above reaction two acetoxy radicals decarboxylate to give methyl radicals and carbon dioxide and then the two methyl radicals recombine to give ethane. Recently it has been found ~ that in solution methyl acetate is an important product along with carbon dioxide and ethane. This can be explained as a consequence of the recombination of a methyl radical and an acetoxy radical in a solvent cage.

It can be seen that the double-isotope-tracer technique can be employed to study the cage effect. Scrambling is to be expected if the two radicals from a doubly-labeled molecule can diffuse apart, but the cage effect would prevent diffusion apart allowing recombination of the two particles to give the new product without scrambling. Seltzer and Hamilton 3t studied the cage effect using this technique. They decomposed (thermally) nearly equal quantities of azobis~-phenylethane and its hexadeuterated analog in ethyl benzene and studied the isotope distribution in the product 2,3-diphenylbutane. Using a computer to analyze the data, they found that 29% of the events in which homolysis (to accomplish loss of nitrogen) occurs, the 1-phenylethyl radicals recombine

before escape from the solvent cage. The technique can be applied, of course, to acetyl peroxide if the hexadeuterated molecule is pyrolyzed in the presence of normal acetyl peroxide; the resultant methyl acetate due to cage recombination should not be scrambled. However, the ethane formed from acetyl peroxide decomposition both in gas phase and liquid phase should be scrambled (provided, of course, that the ethane is not a cage product as well as a product of diffusion together of two separate methyl radicals).

VIII. SCRAMBLING IN GASEOUS DIATOMIC MOLECULES

Many gases and molecules are diatomic, often with both atoms being the same element. In those cases wherein the element has two stable isotopes the possibility of carrying out a scrambling experiment in order to learn about the mechanisms of gas phase reactions is a real one. Data are presented here for the cases of 02 and of H2. It would of course also be possible to do this type of experiment with N2 and C12, however, we do not know of any such

• investigations to date.

A. OXYGEN

A mixture of oxygen gas with non-equilibrium distribution of isotopes does not proceed towards equilibrium over a period of years, provided of course that there is no catalyst for scrambling. Equilibrium is readily established if the mixture of species is sparked with a Tesla coil. Ogg has carried out a number of experiments in which scrambling is achieved by reactive intermediates in the gas phase. His data suggest that wall effects were not important under the conditions of his experiments. However, it seems almost certain that under some conditions surface catalysis of scrambling will be important.

Ogg and Sutphen ~.~ investigated the scrambling of oxygen isotopes in the presence of ozone. They started with the species 02 ~ and with the approach to equilibrium there followed an increase in O232 and O2 z concentrations. They observed that little of the oxygen-18 isotope was transferred to the ozone molecule. This eliminates any bimolecular collisional transfer of an oxygen atom from ozone to oxygen to form oxygen and ozone as in the equation

D,+O, ~ O,+O,

Also this eliminates the known equilibrium

03 ~___ O+O,

from being the prime way of scrambling, since scrambling is much faster than exchange. Never- theless, the rate of scrambling is proportional to the ozone concentration. They proposed the mechanism

O) --~ 0,+0 k,

0 + O, -+ O, k,

O+O,-~O,+O k)



and it is obvious from their results that k3 must be a relatively fast reaction. They observed that scrambling was many times faster than exchange, thus k3 must be larger than kz. Using thermodynamic values they calculated the concentration of oxygen a t o m s present in the mixture of ozone and oxygen molecules. From this concentration it was then possible to calculate a rough value for k3. This value was of the same magnitude as the value calculated for number of collisions. One must conclude that the O3 species formed in such a collision is highly excited and that the species breaks down randomly before de-excitation occurs.

Ogg ~ found that chlorine atoms formed in the photolytic dissociation of chlorine molecules could also induce scrambling in the oxygen. To explain his results Ogg postulated two intermediates, CIO and ClOO with the latter species being a peroxy isomer of the normal CIOz. His mechanism is

c12 + hv ~ 2C1.

CI + O, --~ CIOO

CI + CIOO ~ 2C10

2CIO ---* CI + CIOO

CIOO --'* Cl + O2

2Cl +M'-* CI2+M

where M is any species in the gas phase which can aid the recombination step by drawing off the bond energy of the newly formed C12 molecule. Analysis of this scheme shows that oxygen scrambling dan occur by means of the third and fourth steps since the O2 molecule is produced by random recombination of CIO. Admittedly the scheme is only a first postulation, however, it does fit the results well.

In another study Ogg ~ investigated the scrambling of the oxygen isotopes by nitrogen-oxygen compounds. For example, he observed scrambling to occur while dinitrogen-pentoxide is slowly decomposing. His postulated steps towards scrambling involve the known species NO2 and nitrate-like NO3. The steps postulated are the following:

o N _ _ o \ o / N + O~O ---, O/N--O--.O

O , , . _ _ / O O \ / O

O/~N--O--O + O--N~,,. O-'~ O/N--O + O--O--N\ O

o•N•--O--O -+ + O--O O~ N O /

o,, +O_N(O o / N - - O + O - - O - - N ~ o ~ O""j.N__O__ O O /

Scrambling was also observed during the oxidation of nitric oxide (NO) by oxygen. As before, a peroxy species is postulated. The steps are as follows:

O-.--N + O---O --., O.--N---O--O

O---N---O---O + O---N--O --. O--N---O + O--O--N---O

O---N--O--O ~ O--N + O--O

O---N~O + O--O--N--O -.-* O----N~O---O + O-.-N---O

Again there is no strong evidence in support of these mechanisms but the peroxy species NO4 and NO~ seem reasonable and the scrambling of the oxygen is clearly a consequence of the proposed mechanism.

B. HYDROGEN

The scrambling of hydrogen atoms in hydrogen molecules can of course be studied by use of the deuterium isotope, however, for many years it was studied in a different manner. Each hydrogen atom has a nuclear spin. When the two hydrogen atoms in a molecule have parallel nuclear spins this is termed ortho-H2. When the spins are anti-parallel it is termed para-Hv The conversion of ortho-H2 to para-H2 occurs in the gas phase and the kinetics have been studied rather thoroughly." It was observed that the rate of approach to equilibrium was dependent on the three-halves power of the hydrogen molecule concentration, that is to say the kinetic order of the reaction is three-halves. Farkas and Farkas proposed the following mechanism

H2+M K H+H+M (XIV)

p--Hz + H "-'* o'-H2 + H (XV)

where M is any specie which aids the equilibrium formation of hydrogen atoms, and p-Hz and o-H2 represent para-H2 and.ortho-H2 respectively. From the first step it can be seen that the hydrogen atom concentration is given by the equation

[HI ={K[.~]}½ Inserting this into the rate equation appropriate to the second step (equation (XV)) leads directly to the observed dependence on hydrogen molecule concentration.

The same reaction was studied using deuterium isotope by the group at the University of Rome, 6 and further studies to clarify the details have been published) ° ' ' ~ The reaction is

H2+D2 ~_ 2HD

The basis for the reinvestigation by the group at the University of Rome was the desire to check more thoroughly the predictions of the transition state theory. (They were, for example, extra careful to exclude oxygen gas by exhaustive out-gassing at high temperatures, since the oxygen appears to have a small catalytic effect.) The rate law was as given previ- ously and it was found that the hydrogen atom equilibrium was established primarily at the walls of the vessel (which is designated M in equation (XIV)), yet the homogenous concentration of hydrogen atoms was that predicted by thermodynamics.

From the data it is possible to extract rate constants for all of the six possible isotopic situations, as for

Reviews 1968

6 2 I.O." EDWARDS AND P. D. FLEIf~2HAUEit

example,

H + D 2 - - ~ H D + D

The agreement between the observed rate constants and the rate constants predicted by transition state theory is surprisingly good.

iX. P O T E N T I A L A P P L I C A T I O N TO SPECIF IC PROBLEM

The technique is of wider utility than is often realized. The purpose of this section is to suggest some specific chemical problems appropriate to the technique.

When a diatomic molecule is adsorbed on a surface the immediate question is does this molecule get adsorbed as atoms or is the integrity of the bond between the two atoms maintained on the surface of the adsorbent. If the adsorption is due solely to Van der Waals interaction it is expected that the molecule will retain its identity. If, however, the process is a <<chemisorption>, there is the possibility of scission of the bond in the molecule during the act of adsorption. In such case there is a considerable likelihood that recombination of the atoms on desorption will be at least in part to give new pairs of atoms and scrambling would be observed. Some of the possible diatomic molecules for which this technique is applicable are H2, O2, N2, and C12. In similar fashion a variety of surfaces from metals to molecular sieves can be used as adsorbents.

The already available results on hydrazine oxidation are of considerable interest. There are, however, a number of problems which remain in hydrazine chemistry. Further work on oxidation of hydrazine itself should be done. The study of the oxidation of methyl hydrazine by one-electron oxidants would be very interesting. The compound N2F4 is reported to undergo reversible homolytic scission to give the radical NF2; the double-label technique could presumably be employed to investigate the rate of this homolytic scission. The application of the technique to some of the new, interesting transition metal complexes with hydrazine, such as the species

H N II

P h ~ P \ / N N / P P l h

P t P t

Ph3P / N N / \ P P l h | |

N H

(BPh,)2

recently reported v would be informative. Many problems remain in the area of peroxide

chemistry. The unsatisfactory present state of the work on hydrogen peroxide decomposition was discussed above. Some inorganic peroxides such as peroxymonophosphoric acid and the interesting transition metal peroxides remain to be studied. The decomposition of ring-substituted peroxybenzoic acids would

give information on the relative importance of peroxy anion attack at carbonyl carbon and peroxy anion attack at peroxidic oxygen.

The chemistry of homogeneous catalysis by metal ions is an area of current interest. It seems probable that reaction of a diatomic molecule with a metal complex precedes the catalytic step. Scrambling of the atoms of the diatomic molecule might be one consequence of an adsorption and desorption cycle. For example, the reaction of hydrogen gas with a hydride complex of a platinum metal could result in scrambled hydrogen.

To our knowledge the application of the double- isotope-label technique to molecules wherein the two labeled atoms are different elements has never been reported. It is, however, a very. distinct possibility. Two examples are given. In the first example

N-"O + N O ~ N O + N ~

one molecule of nitric acid contains a nitrogen-15 and an oxygen-18, whereas the other molecule has the usual isotope distribution and content. In the second example

H C i + H C I ~_ H C I + H C i

the labels involve hydrogen and chlorine atoms. Scrambling could occur by a variety of mechanisms and a study of the kinetics of scrambling would give information, not otherwise available concerning the interactions of the various particles.

Acknowledgments. The authors wish to thank the University of Rome for kind hospitality and the Consiglio Nazionale di Ricerche (C.N.R.) of Italy for financial aid. One of us (IOE) thanks Brown Univer- sity for a sabbatical leave and the lohn Simon Gug- genheim Memorial Foundation for a Fellowship.

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I n o r g a n i c a C h i m i c a A c t s

Double-lsotopc-l,z~bel Technique and Inorganic Reaction M e c h a n i s m s 6 ]

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Rev iews 1968

Documents

The double-isotope-label technique and inorganic reaction mechanisms