Reactions of the Hydrated Electron - Advances in Chemistry (ACS

6 Reactions of the Hydrated Electron MICHAEL ANBAR

Downloaded by SUNY STONY BROOK on October 23, 2014 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch006

The Weizmann Institute of Science, Rehovoth, and the Soreq Nuclear Research Center, Yavneh, Israel

Many reactants, both organic and inorganic, react with e at specific rates higher than that of e + H O but slower than diffusion-controlledrates. For these, correlations have been found between the reactivity toward e and the availability of a vacant orbital on the reactant. The first product of the reaction with e , which contains an additional electron, may be of limited stability, but it is always formed as an intermediate. It was suggested that the electron transfer from e to any reactant is an extremely fast process which is never rate determining. Consequently, those reactions which are not diffusion controlled involve pre-equilibria with reactants which have an electron configuration that allows the incorporation of an additional electron. 2

J h e development of chemistry i n the 20th century has been dominated and motivated b y the electronic theory of the chemical bond and the role of electrons i n chemical reactivity. T h e electronic structure of the chemical bond could be deduced by more or less direct methods, such as electronic excitation spectra, dipole moments, or paramagnetism; but there was no direct indication for the transfer of electrons in chemical reactions. Using isotopic techniques it has been possible to demonstrate bond cleavage and atom transfer reactions, but it is impossible to label an electron and trace its transfer from one molecule to another. It was not until the discovery of the radiolytically produced solvated electron that electron transfer processes could be examined directly and unambiguously. 55

In Solvated Electron; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

SOLVATED ELECTRON


56

B y studying the reactions of solvated electrons, which are b y definition electron transfer processes, it has become possible to analyze basic postulates i n modern chemistry. These include the contribution of molecular properties to electron transfer reactions, such as intramolecular electron distribution, electron affinity, and redox potentials. Since electron transfer reactions are, by definition, the most elementary reduction processes, studying these reactions tests the validity of theories of redox processes. T h e investigations have resulted i n a gratifying agreement with the predictions of the prevailing theories. T h e sensational finding from studies of solvated electron reactivity is just the nonsensational fact that the unit negative charge, bound to its medium only b y its energy of solvation, behaves as expected for an electron, thus confirming our basic views on molecular structure and chemical reactivity. A n additional product of these studies was the characterization of many reduced species which have never before been produced or investigated. It may be concluded that the discovery of the solvated electron with its characteristic spectrum of absorption is one of the great achievements i n chemistry of the last few decades—not only because it demonstrated the existence of a most elementary chemical species, but also because it firmly establishes the role of electron transfer i n chemical reactivity. T h e following review will summarize and systematize the available knowledge on the chemical reactivity of solvated electrons and the products of their reactions. Since most of the work was carried out with solvated electrons i n aqueous solutions, we shall confine ourselves mainly to hydrated electrons. W e do not intend to discuss the interaction of solvated electrons with their solvents since this will be covered i n other chapters. Hydrated

Electrons

and their Kinetic

Behavior

T h e F o r m a t i o n of Solvated Electrons a n d the Methodology o f I n v e s t i g a t i n g t h e i r K i n e t i c s . Solvated electrons have been produced in polar, nonreactive solvents like water, ammonia, alcohols, and aliphatic amines by various processes: (1) T h e y may be formed by the solvation of radiolytically generated secondary electrons after these undergo thermalization (106, 123). T h i s has been demonstrated i n water (66, 83, 84) i n alcohols (117) and i n ethylenediamine (12). T h e presence of long-lived trapped electrons has been demonstrated i n radiolyzed alkaline ice (72, 86), and their behavior was found to be analogous to el . (2) Hydrated electrons were suggested as being formed photochemically from species which possess an absorption band corresponding to charge transfer to the solvent (80). T h i s has been confirmed by demonstrating that the spectrum of hydrated electrons formed on photolysis of a n extensive series of anions is similar to that of el produced by radiolysis (61, 94). T h e photolysis of halide ions i n ethanol gave analogous results, yielding solvated electrons with an absorption spectrum similar to that obtained b y radiolysis (62). q

q


6.

ANBAR

(3) Solvated electrons i n ammonia are formed in equilibrium with metal ions dissolved i n this medium (76). Analogous behavior was re ported for ethylenediamine (42). O n mixing ethylenediamine solutions of alkali metals with water, hydrated electrons were claimed to be formed as transients (43). (A) Hydrated electrons are also formed as a product of the inter action of hydroxide ions with hydrogen atoms. T h i s reaction was first established kinetically (4, 6, 81, 82, 99) and then corroborated spectro photometrically using flash radiolysis (95, 96). It should be noted that the rate of the H + O H e~ reaction is only 1.8 Χ 10 M~ sec. (96); thus, this step may become rate determining in many reactions with reactive substrates. T h e participation of hydrated electrons as transients i n certain elec trode processes as well as i n the reaction of alkali metals with water re mains an open question and requires further investigation. T h e kinetic behavior of solvated electrons has been followed directly using flash radiolysis (44, 45, 58) or flash photolysis technique (62, 94, 107). T h e former method is more universally applicable owing to the high absorption coefficient of e~ i i n a spectral region where most re actants contribute little to the overall optical density. Stopped-flow spectrophotometry has also been applied i n the specific case of the eâ + H 0 reaction (43), but it is not applicable to reactions where the e~ i halflife is below 0.1 msec. I n all the direct kinetic measurements β~ ι ΐβ generated i n minute concentrations of 10~ -10~* M, while the other reactants are present i n excess, so that a pseudo first-order disappearance of é~ iv is followed. T h e disappearance of eâ by the reactions eâg + eâ or e~ + radicals follows second-order kinetics. Before direct kinetic techniques were adapted to studying the reactions of e~ , the only experimental approach was competition kinetics, based on determining the yield of products. T h e major drawback of these methods is the presumption that the products analyzed have been formed i n the primary step of the reaction. In many cases however, the final product analyzed is formed i n a secondary or tertiary step, and the effect of additives on its yield does not necessarily reflect their interference with the primary step. T a k i n g even rather recent results obtained by this method, significant discrepancies may be encountered between rate data derived from competition kinetics and those obtained by direct measurement (4, 105, 111). -


57

Hydrated Electron Reactions

so

l

7

aq

- 1

v

Q

so

2

v

80 Ό

8

so

g

q

aq

aq

In many cases excellent agreement has been found between relative rates derived from competition kinetics and from direct measurement—e.g., for the ratio k ΝΟ,-)/*(«ΪΙ + acetone) (112). If, on the other hand, large discrepancies are observed for relative reaction rates, this would imply that secondary reactions contribute to the formation of the products. Competition kinetics may therefore find their justification in the study of the chemical behavior of secondary products. F o r e~ iv + X reactions, this means studying the chemical behavior of X ~. In any case it should be remembered that competition kinetics require (e q

+

so


SOLVATED ELECTRON

58

several cross-checks with competitors of different chemical characteristics before it is established that the given yield originates from a primary reaction only. T h e C h e m i c a l Reactivity of e . T h e chemical behavior of solvated electrons should be different from that of " f r e e " thermalized electrons i n the same medium. Secondary electrons produced under radiolytic conditions will thermalize within 10 ~ s e c , whereas they will not undergo solvation before 10 ~ sec. (106). T h u s , any reaction with electrons of half-life shorter than 10 ~ sec. will take place with nonsolvated electrons (75). Such a fast reaction will obviously not be affected b y the ultimate solvation of the products, since the latter process will be slower than the interaction of the reactant with the thermalized electron. T h i s situation m a y result i n a higher activation energy for these processes compared with a reaction with a solvated electron. N o definite experimental evidence has been produced to date for reactions of thermalized nonsolvated electrons, although systems have been investigated under conditions where electrons may be eliminated before solvation (15). aq

1 3

n


n

Once the electron becomes hydrated, its charge is spread out over a number of water molecules (106,123,124, 85) with a hydration energy of approximately 40 kcal./mole (26). It takes the hydrated negative charge about 3.10 ~ sec. to attain equilibrium with its ionic environment; consequently, any reaction that proceeds faster than this will involve a hydrated electron without its normal gegen-ion sphere. T h i s has been experimentally demonstrated both for radiolytically and for photolytically produced electrons (36,38). Since photolytically produced hydrated electrons were shown to have a normal ionic environment, it must be concluded that the "cages" suggested for these species (80) are a n expression of the nonhomogeneous distribution of the I ~ + el pairs i n the photolyzed solution rather than cages in the classical sense (101). After 3.10 ~ sec. the hydrated electron attains its final configuration in solution, with a n outer sphere of positive ions which affects its reactivity according to the classical Bronsted-Bjerum theory of ionic reactions. T h i s has been demonstrated both i n competition studies (34, 37) and by following the effect of ionic strength on the rate of the e + F e ( C N ) ~ reaction (59). Very high concentrations of electrolytes i n variably were found to diminish the reaction rates of e~ with charged and noncharged reactants (12). If the shifts i n the absorption spectrum of e~ in these media are considered as indicating an increase i n the energy of hydration, the decrease in the reactivity of e~ is not surprising. T h e stabilization of e~ in concentrated solutions may be considered analogous to the behavior of trapped electrons i n ice (85, 86). Alternatively, the rate decrease may be caused b y the fact that the required rearrangement of solvent molecules around the activated complex is partially inhibited i n concentrated solutions of electrolytes (47). It is interesting to note at this point that the reaction rates of eâoiv i n alcohols with different substrates are identical, within the experimental error, with those measured for e (9, 117). T h e energy of solvation of the electron is also comparable in both media. 9

q

2

9

aq

e

8

aq

aq

aq

aq

aq


6.

ANBAR

59


Diffusion-Controlled Reactions. T h e specific rates of many of the reactions of e~ exceed 10 M~ sec." , and it has been shown that many of these rates are diffusion controlled (92, 113). T h e parameters used i n these calculations, which were carried out according to Debye's theory (41), were a diffusion coefficient of 10~ s e c . " (78, 113) and an effective radius of 2.5-3.0 A . (77). T h e energies of activation observed i n e~ reactions are also of the order encountered i n diffusion-controlled proc esses (121). A very recent experimental determination of the diffusion coefficient of e q b y electrical conductivity yielded the value 4.7 ± 0 . 7 X 10 ~ c m . s e c . (65). T h i s new value would imply a larger effective crosssection for e'aq and would increase the number of diffusion-controlled re actions. A quantitative examination of the rate data for diffusion-con trolled processes (47) compared with that of e~ q reactions reveals how ever that most of the latter reactions with specific rates of < 10 M s e c . are not diffusion controlled. l

9

aa

1

4

1

a

+ n

Λ

n

—

ΑΒ-


62

SOLVATED ELECTRON

A B < -!> and A B ~< may be stable species or may subsequently undergo chemical changes analogous to those cited above for A B ~. It has been suggested that certain species formed on association with el have no chemical stability at all and may be considered merely as transition states, as i n the dissociative electron capture reactions observed i n the gas phase (49, 51, 72, 90). It should be noted that there is an intrinsic différence between the capture of an electron i n the gas phase and an interaction with elq in aqueous solutions. I n the gas phase one also obtains i n the first stage e~ + A B A B " * . I f the bond energy A — Β - is smaller than the electron affinity of the molecule, the energy gained i n the electron capture may be dissipated either by radiative processes or by dissociation (110). T h e cross-section for capturing a thermalized electron i n the gas phase is very small, even for molecules of the high electron affinity (48, 51, 72). I n solution, A B - is stabilized by solvation, and it m a y dissipate any excess energy b y collisions with its close neighbors i n its hydration shell. These two factors increase the probability of interaction with " t h e r m a l " electrons from negligible values to collision frequencies. Taking, for example, the e" + I reaction i n the gas phase (51), the cross-section for thermal electrons is very small, and furthermore no I ~ is formed since even a slight excess energy induces its dissociation i n the gas phase. T h e e~ + I -*» I + I - as well as the e~ + RC1 R + CI - reactions apparently require an electron with a minimum kinetic energy for completly dissociating the activated complex (48, 110). In aqueous solutions, on the other hand, the el + I reaction is diffusion controlled, and I - is produced quantitatively (122). B r ~* and C l ~* were also shown to be relatively stable species i n solvated form (24, 94), whereas their existence i n the gas phase could not be demonstrated. A s we shall see later i n discussing the experimental data, evidence is accumulating that an A B ~ intermediate is formed i n practically all elq reactions, and no dissociative electron capture takes place i n aqueous solutions. If this intermediate undergoes monomolecular dissociation in less than 10 ~ sec. (which still leaves 10 vibrations of the bond before rupture plus ample time for solvation) it will escape all presently avail able detection techniques based on physical properties, and its scavenging by the most reactive additives requires rather high concentrations (>0.1 M). Such additives are likely to react also with elq or with other primary species and interpreting the results obtained b y competition kinetics may become extremely difficult. Intermediates which persist for 10 sec. or longer are easily detected by competition kinetics, and many of those which persist longer than 10 ~ sec. can be detected directly and characterized by physical methods. +

n

n+ l )


q

2

2

2

q

2

2

2

8

2

e

6

In the following sections we shall systematize and t r y to interpret the information available on the reactivity of different chemical species toward elq and the products of these reactions. Since specific rate con stants of elq reactions have been previously tabulated (21, 45) we do not intend to cite them here. Free Radicals a n d Other Paramagnetic Reagents. T h e first group of reactants to be considered will be atoms and molecules which


6.

ANBAR

63


contain an unpaired electron. These include the primary and secondary products of radiolysis: el , H , O H , H 0 , O " , and 0 " , organic free radicals such as C H O H or ( C H ) C O H , stable chemical species like N O or N 0 , and biradicals like 0 . Q

2

2

3

2

2

2

2

T h e el + el reaction proceeds at a diffusion-controlled rate (96). A s theoretically predicted (el ) is utterly unstable (77,123), and it might convert into two H atoms, a H ~ ion, or an H molecule. It has been demonstrated unambiguously that the el + el g reaction produces hydrogen, and the hydrogen produced does not originate from the recombination of two H atoms or from e~ + H i n a cage (46, 96). Since it is hard to believe that an H molecule is formed in a single step, it may be suggested that H ~ is formed as an intermediate. It may be speculated that a second electron interacting with the region of negative charge, which constitutes el , results i n detaching an O H " ion, leaving a hydride ion to interact with water and give H . T h e formation of H ~ from el + el + H 0 H " + O H " is thermodynamically feasible, taking AFof el as 57.5 kcal./mole (64) and AF of H " , H 0 , and O H " as 51.9, —56.7 and —37.6 kcal./mole, respectively (88). Therefore, this would be a plausible mechanism for the formation of H . However it would be hard to prove the presence of H ~ as a n intermediate since it reacts with water at a diffusion-controlled rate. T h i s can be deduced from the H / D isotope effects involved i n the reaction of L i H with H 0 i n tetrahydrofuran (23). Q

q

q

2

2

Q


2

Q

2

q

q

2

q

2

2

2

T h e el + H reaction, which also proceeds at a diffusion-controlled rate (58), most probably produces H ~ as a primary product. T h e reaction el + H H " has a negative AF of 54 kcal./mole (neglecting the energy of hydration of H which is small) (26) and the final product will be H . A s already pointed out, it is hard to prove the existence of h y dride ions as intermediates i n aqueous solutions. It should be noted here that although the reactions el + el and e~ + H were shown to produce H , these reactions probably are not the source for the formation of the "molecular" hydrogen i n the radiolysis of water (16,114). T h e el + O H and el + O " reactions are also diffusion controlled (96), and the product is obviously O H " . N o information is available on the reaction rate of H 0 and 0 ~ with el because it is difficult to maintain electrons i n the presence of the oxygen or H 0 necessary for producing 0 ~. q

g

2

g

q

aq

2

q

Q

2

Q

2

2

2

2

Free radicals produced from alcohols by hydrogen abstraction—e.g., C H O H and C H C H O H , show a low reactivity toward el (47, 58). T h i s property m a y be correlated with the fact that C H O H tends to act as an electron donor toward alkyl bromides, inducing their debromination (22); in other words, C H O H tends to convert to a carbonium rather than to a carbanion. Furthermore, C H O H radicals tend to dimerize rather than to disproportionate (50), which again reflects their reluctance to act as electron acceptors. T h i s inertness of C H O H as an electron acceptor makes methanol a n ideal additive for scavenging O H and H radicals, producing a free radical which does not interact with elg. 2

Q

3

2

2

2

2


64

SOLVATED ELECTRON

T w o other paramagnetic reactants which react with e~ at close to diffusion-controlled rates are N O and 0 . It has been already pointed out that whereas N O reacts with a diffusion-controlled specific rate of 3.14 ± 0.2 X 10 M sec. - (58), 0 reacts with a rate of 1.88 =fc 0.2 X 10 M " s e c . (92) and may therefore have some geometrical restrictions. T h e product of é~ + N O is most probably N O ~ which on reaction with N O may yield N 0 . T h i s requires experimental verification. T h e formation of 0 ~ from e~ + 0 has been demonstrated unambiguously b y the fact that the rate of appearance of 0 - is identical with the rate of disappearance of e~ i n its reaction with 0 (60). 0 ~ disappears by disproportionation to 0 + H 0 . 0 ~ was shown to accept an electron from H 0 , following the reactions 0 " + H 0 0 ~ + H 0 H0 ~~ + H 0 H 0 + 0 ~ , thus inducing isotopic exchange of oxygen without "scrambling" (8). In conclusion, it may be stated that all the paramagnetic reactants examined, with the exception of alkyl radicals, accommodate the electron from é~ in their orbitale at a diffusion-controlled rate. W a t e r , H 0 a n d Brônsted A c i d s . T h e most important reagent in the chemistry of e~ is obviously the solvent, water. Were it not for the relatively low reactivity of e~ with H 0 most of our information on hydrated electrons would be merely hypothetical. Fortunately the rate of the e~ + H 0 H + O H - reaction is slow enough to enable one to examine the kinetic behavior of any solute reacting with e~ at a rate over 10 M~ s e c . T h e rate of the e' + H 0 reaction has been determined by many investigations. Since this reaction follows first-order kinetics, any i m purity present will add to its apparent rate. W i t h increasing care i n purifying the solvent, the upper limit of the specific rate of the e~ + H 0 reaction has been reduced to 16 ± 1 M s e c . (67), which is in excellent agreement with the value obtained by stopped-flow kinetics when a metal solution i n ethylenediamine was mixed with H 0 (43). These observations suggest that the apparent isotope effect reported i n D 0 (33), was probably caused by a difference i n the purity of the solvents used. T h e new value for k z + H O decreases the calculated redox potential of e~ from 2.7 (26) to 2.5 volts (64). If one assumes that e~ reacts with M g + , although extremely slowly (k < 10 M' sec. ) (12), just as it reacts with M g + i n ice (98), one may conclude that E°eâ > 2.4 volts, which implies k + H O ^ 2 X 1 0 " M~ s e c . " In order to test this experimentally, reactive trace impurities i n water must be k( q + Fe(CN) -»)/ £eâ +acetone, probably because of the significant difference i n salt effect on the reactions of monovalent and divalent anions. T h e formation of A H " and its decomposition without producing H atoms may take place even with a strong acid like H S 0 " ~ ( p K = 1.92) 8

2

4

- 3

4

+

- 3

4

8

3

- 1

2

4


2

-

7

6

-

1

4

9

aq

4

-1

- 1

-

l

8

aq

- 1

8

-

3

l

8

- 1

-

-

aq

ag

4

3

8

8

3

2

8

8

2

2

2

2

8

2

-

2

4

- 2

2

4

4

- 2

2

3

4

-

2

2

2

4

-

- 2

aq

2

4

t

ea

4

ff

4


e

+

6.

ANBAR

67


which was shown (in ice) to form H S 0 ~ , which decomposes to S 0 ~ + O H " rather than to H + S 0 " . It may be concluded that as a rule an electron is accommodated on the acid molecule which subsequently dissociates, i n the absence of other reagents, to give the most stable fragments. In the case of oxygen-con taining acids, the hydration energy of O H ~ is generally greater than that of the anion of the given acid and the free energy of formation of the cor responding free radical is lower than that of a hydrogen atom. It is not surprising, therefore, that A O H ~ dissociates to A + O H ~ rather than to A O ~ + H . A n exception t o this behavior is H P 0 ~ which ultimately dissociates quantitatively to give H P 0 ~ + H . T h i s is to be expected since the energy of dissociation of a Ρ — Ο bond i n phosphate is significantly higher than that of an O H bond (35). Oxygen free acids like ammonium or alkylammonium ions also dissociate to give those fragments whose formation requires the least energy and which are best stabilized by solvation. 2

4

4

3

2

2


4

4

2

2

B i - a n d T r i a t o m i c Molecules. T h e most abundant triatomic molecule, H 0 , and the biatomic paramagnetic 0 and N O have been dis cussed previously. Here we shall examine a number of other simple reactants. 2

2

Hydrogen shows no reactivity toward e (96) which is not sur prising since e + H H Η + H ~ is thermodynamically u n feasible. T h e reactivity of nitrogen toward e~ has not been examined but such a reaction seems rather unlikely i n view of the inertness of the nitrogen molecule as an electron acceptor. C O which is isoelectronic with N has a longer bond length (126*), a finite dipole moment, with the oxygen positively polarized (104), and a much higher electron affinity (110). T h i s results i n a much higher reactivity toward e~ with k = 1.10 M~ sec. - (69). T h e product of the e~ + C O reaction is probably C O - which reacts with water to give H C O . T h e latter species is capable of abstracting an O H from H 0 yielding formic acid; alternatively, it dimerizes to glyoxal (69). T h e chain reaction of C O i n alkaline solution to yield formate ions (125) suggests that C O ~ is capable of abstracting an O H radical from H 0 . C O - was not detected i n the gas phase by elec tron impact (49), although it must be formed as an intermediate. aq

aq

2

2

aq

2

aqf

l

9

1

aq

2

2

2

N 0 reacts with e~ at a high rate (* = 8.7 ± 0.6 X 10* M~ s e c . - ) (58, 84), yielding nitrogen and O H radicals as the final products (40). N 0 ~ , which is formed as an intermediate, has been shown to have a lifetime long enough to participate i n various chemical reactions. N 0 ~ or N O H show a strong oxidative reactivity i n many reactions. T h i s behavior gave the impression that O H radicals are produced i n stantaneously as a product of a dissociative electron capture by N 0 (38). It was shown however that the O H - l i k e behavior of N 0 " or N O H does not hold quantitatively; thus &(OH+iPrOH)/£(OH+en) > £(Nto-+iPrOH)/ l

aq

2

1

2

2

2

2

2

2

£(N*o-+en) (20), (en = ethylenediamine) £(ΟΗ+ΤΙ+)/£(ΟΗ+ΪΡΓΟΗ)>> AJ(NSO-+TI+)/

£(NjO-+iPrOH) (-20), and that ^(OH + Br-)/A"(OH + RH) > £(N 0- + Br-)/^(NjO- + RH) (22) where the product R is the same i n both reactions. It was further

9

2

found that N 0 ~ reduces I and B r and that £(Ν*Ο- + ΒΓ,) > k(s o- + w (22), 2

2

2

t


68

SOLVATED ELECTRON

whereas *( â +Br,) < *(eâ + i ) (7). T h e increase i n G(I ) under N 0 when the I " concentration is increased from 10 ~ to 10 ~ (18) may be attributed to the competition between N 0~~ + I ~ - * I + N + H 0 and N 0 ~ + I N 0 + I ~. Under the experimental conditions, where the concentration of I d i d not exceed 10 ~ M, the I could not possibly compete with N 0 (1.6 X 10 ~ M) for e~ . N 0 ~ is a rather mild reducing agent, reducing I and B r but not R B r (22). N 0 ~ was also shown to reduce C u (39). It thus appears that N 0 ~ or N O H may persist at least 10 ~~ sec. and may act both as an oxidizing and as a reducing agent. A s an oxidizing agent, N 0 ~ tends toward hydrogen abstraction rather than to electron transfer. T h i s is apparent from all cases where competition kinetics were studied. Regarding the case of i P r O H vs. ethylenediamine (19) it was shown that i P r O H reacts with O H radicals b y electron transfer, whereas the oxidation of ethylenediamine proceeds by hydrogen abstraction^ T h e reaction of N 0 ~ with T l and B r ~ vs. R H corroborate the same conclusion. e

ff

ff

s

2

2

4

3

2

2

2

2

2

2

2

δ

2

2

2

2

2

aq

2

2

2

+ 2

2

7

2


2

+

2

C 0 , which is isoelectronic with N 0 and has a similar linear con figuration (116), reacts with e~ at a surprisingly similar rate k = 7.7 =fc 1.1 Χ 10 M~ s e c (58). T h e presence of the product, C 0 ~ , has been demonstrated i n a number of studies. C 0 " " was shown to add to organic radicals to form carboxylates (111, 112, 125, 126). It has been pointed out that the C 0 ~ + C H O H reaction must proceed extremely fast (> 10 M~ sec."" ) to compete with the recombination reactions 2 C 0 ~ — ( C 0 ) " and 2 C H O H — ( C H O H ) (121). I n studying the competition between C 0 and N 0 for e~ it was shown that N 0 is three times more efficient as a scavenger of e~ (111). T h i s is not i n ac cord with the direct rate measurements for the e~ + C 0 and e~ + N 0 reactions, and may be explained by an efficient electron transfer from C 0 " to N 0 that is, C 0 - + N 0 — C 0 + N 0 - . T h e efficiency of C 0 ~ as a reducing agent has been demonstrated i n many reactions; C 0 ~ was found to reduce H 0 (3, 74), I and B r , as well as R B r , N O 3 - , and C u + (22). It was shown that £ ( C O I - + I ) / ^ ( C O - + H J O I ) 2

2

aq

l

9

1

2

2

2

1

9

2

2

1

2

2

2

2

2

2

2

aq

2

2

aq

aq

aq

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

S

C = C < ) - + H + aq

aq


1 1

C — C H

Reactions of the Hydrated Electron - Advances in Chemistry (ACS

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