Kinetic Evidence that "Excited Water" is Precursor of Intraspur H

19 Kinetic Evidence that "Excited Water" is Precursor of Intraspur H in the Radiolysis of Water 2

THOMAS J. SWORSKI

Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch019

Chemistry Division, 37831

Oak Ridge National

Laboratory,

Oak Ridge,

Tenn.

Homogeneous kinetics is used instead of dif fusion kinetics to express the dependence of intraspur

G

H2

on solute concentration.

The

rate-determining step for H formation is not 2

the combination of reducing species, but first— order disappearance of "excited water."

Two

physical models of "excited water" are con sidered.

In one model, the H O + O H radical 3

pair is assumed to undergo geminate recom bination in a first-order process with H 0 com 3

bination to form H as a concomitant process. 2

In this model, solute decreases G

H2

with H O. 3

by reaction

In the other model, "excited water"

yields freely diffusing H O + O H radicals in a 3

first-order process and solute decreases by reaction with "excited water." pendence of intraspur G

H2

tion indicates

τ *= H2O

10

G

H2

The de

on solute concentra - 9

—

10

¯-10

sec.

|t was believed that G , and G , o , were independent of solute concentra tion until it was demonstrated (40, 41) experimentally for C o 7-radiation that G o , was markedly decreased b y B r ~ and C l ~ . Furthermore, G H J O , was shown (40, 41) to decrease linearly with the cube root of [Br~] and [Cl~] within experimental error in the concentration range from 10 ~ to 1 0 M. T h i s empirical relationship stimulated theoretical effort to explain quantitatively the dependence of G H , and G ^ o , on solute concen tration b y diffusion kinetics using the Samuel-Magee (34) model. T h e numerical and approximate analytical treatments using the Samuel-Magee model and an intercomparison of the results have been presented i n an excellent review of diffusion kinetics b y Kuppermann (19). H e has pointed out that there is a quantitative inconsistency between the model and experiment: although the agreement can be H

H

60

H l

5

- 1

263

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

SOLVATED ELECTRON

264

made reasonable, the curvature of the experimental a n d theoretical curves for the dependence of yields on solute concentration is different and invariant with respect to most of the pertinent parameters. It was suggested (42) in a preliminary communication that this quantitative inconsistency could be attributed to two factors: (a) H i n water irradi ated with C o 7-radiation is formed b y two processes—intraspur a n d interspur—in contrast with only intraspur processes i n the isolated spur of the Samuel-Magee model, a n d (b) the precursor of H disappears b y a pseudo-unimolecular process. T h e objective of this paper is to present the results of a detailed analysis of the pertinent literature which sup ports this suggestion. 2

60


2

Results Effect o f N O 3 - o n G ( C e ) . There is a striking effect (43) of Ν 0 ~ on C e reduction i n sulfuric acid solutions. T l increases (44) G ( C e + ) from 2 G H , O , + G — G H t o 2GH,O, + G + G H. N O s " at concentra tions greater than 0 . 0 1 M markedly enhances G ( C e ) both i n the presence and absence of T1+, the effect being approximately equal i n both cases. M a h l m a n (24) confirmed this effect and extended the study from 0 . 5 M to 5 . 0 M . T h e enhancement, A G ( C e + ) , for N 0 ~ concentrations from 0.1 Μ to 5 . 0 M is quantitatively expressed b y E q u a t i o n 1 as shown i n Figure 1. + 3

8

+ 4

+

H

H

0

8

0

+ 3

8

l/AG(Ce+ ) 8

8

= 0.185 + 0.0804/[NO3-]

(1)

E q u a t i o n 1, with constants obtained b y the method of least squares as for all subsequent equations, is based on the data of M a h l m a n (24) for Ce + 4

4

40

6 1/[N0-]

Figure 1.

Dependence of l / A G ( C e ) on l/[NO ~\ +s

%

for Co

m

y-Radiation.

solutions containing Ί Ί + a n d on the value of 7.92 (44) for G ( C e ) i n + 8

the absence of N O s ~ .



19.

SWORSKI

265

"Excited Water

1

I

ι

0

0.2

ι

0.4

ι

ι 0.8

0.6 [SOLUTE]

I

iO

Figure 2. Dependence of l/G on [Solute] for Co y-Radiation: mH 0 (Ghormley and Hochanadel) ; • H2O2 (Anderson and Hart). 60

Ht

2

T h i s effect of N 0 ~ ion is quantitatively consistent with a reaction mechanism (43) i n which N 0 ~ interacts with an electronically excited water molecule before it undergoes collisional deactivation by a pseudounimolecular process (the N 0 ~ effect is temperature independent (45) and not proportional to Τ/η (37)). Equation 1, according to this mechanism, yields a lifetime for H 0 * of 4 X 10 ~ sec., based on a diffusion-controlled rate constant of 6 Χ 10 for reaction with N 0 ~ . D e p e n d e n c e o f G o n S o l u t e C o n c e n t r a t i o n . Another effect of N 0 ~ i n aqueous solutions is a decrease i n G with increase i n N 0 ~ concentration (5, 25, 26, 38, 39). T h i s decrease i n G H is generally believed to result from reaction of N 0 " ~ with reducing species before they combine to form H . These effects of N 0 ~ on G ( C e ) and G , raise the question as to whether or not they are both caused by reaction of N 0 ~ with the same intermediate. 7-Radiation. T h e decrease i n G H for N 0 " " concentrations from 0.01M to 1.0M is quantitatively expressed for C o 7-radiation by E q u a tion 2 as shown in Figure 2. 3

3

3

10

2

9

3

H S

3

H Î

3

2

3

2

+ 8

3

H

3

2

3

60

1/GH

2

=

2.93

+

7.53 [ N O r ]

(2)

Equation 2 is based on the data of Boyle and M a h l m a n (25, 26). These results agree with the ad hoc mechanism considered above for the effect of N 0 ~ on G ( C e ) , with the added assumption that H results from the 3

+ 3

2


2

SOLVATED ELECTRON

266

first-order disappearance of H 0 * . E q u a t i o n 2 also yields a lifetime for H 0 * of 4 χ 1 0 - sec. i n excellent agreement with Equation 1. There fore, the N 0 ~ effect on G ( C e + ) is caused b y reaction of N 0 " with the precursor of H . T h e decrease in G , for H 0 and UO2SO4 concentrations from 0 . 0 1 M to 1 . 0 M is quantitatively expressed for C o γ-radiation by Equations 3 and 4 as also shown b y Figure 2. 2

1 0

2

8

8

3

2

H

2

2

6 0


1/GH,

-

1/GH,

=

2.90 3.24

+ +

2.72 [ H 0 ]

(3)

12.2 [ U O ^ O * ]

(4)

2

2

Equation 3 is based on the data of Ghormley and Hochanadel (12) and Anderson and H a r t (3). Equation 4 is based on the data of Boyle, Kieffer, Hochanadel, Sworski, and Ghormley (6). Similarly, though not shown i n Figure 2, the decrease i n G H , for N 0 ~ and C u concentra tions from 0 . 0 1 M to 1 . 0 M is quantitatively expressed for C o γ-radiation by Equations 5 and 6. + 2

2

60

1/GH,

=

2.83

+

4.19 [ N 0 - ]

(5)

1 / G H , = 3 . 0 8 + 2 5 . 8 [Cu+ ]

(6)

2

2

Equations 5 and 6 are based on the data of Schwarz (36) with only G H , at 0 . 3 9 4 M K N 0 showing significant deviation. It is currently believed (I) that G H , = 0 . 4 5 i n neutral and acid solu tions. T h e intercept values of Equations 2 - 6 are significantly higher and correspond to about G H , = 0 . 3 3 ± 0 . 2 . T h e remainder of the "molecular" H yield is more easily scavenged and suggests two processes for H formation. A similar viewpoint has been presented by M a h l m a n (26) and supported b y H a y o n (14) and is based on the departure from linearity of G H , on [ N 0 ~ ~ ] at concentrations greater than l.OAf (38). F i s s i o n - R e c o i l s . G H , for ΌΟ&Ο4, U 0 F , and U ( S 0 ) solutions is quantitatively expressed for fission-recoil radiation by Equations 7 - 9 as shown by Figure 3. 2

2

2

1 / 3

3

2

1/GH,

=

1/GH, 1/GH,

= =

0.571

+

2

4

2

0.645 [ U O £ 0 ]

(?)

0.338 [ U 0 F ]

(8)

0.891 [ U ( S 0 ) ]

(9)

4

0.6(H) + 0.605 +

2

2

4

2

Equations 7 - 9 are based on the data of Boyle, Kieffer, Hochanadel, Sworski, and Ghormley (6). In sharp contrast with the dependence of G H , on solute concentration for C o γ-radiation, G H , is quantitatively expressed by Equations 7 - 9 over the entire concentration range investi gated. For U 0 S 0 solutions, this corresponded to concentrations ranging from 1.1 X 1 0 ~ to 2 . 2 M . T h u s we have the surprising result that H formation appears to occur by two processes for C o γ-radiation, but by only one process for fission-recoil radiation. Limiting values for G H , indicated by intercept values of Equations 7, 8, and 9 are 1.75, 1.67. and 1.65. 60

2

4

8

2

6 0



1.0

2.0

[SOLUTE]

Figure 3. Dependence of l/G [Solute] for Fission Recoils. Ht

Figure 4.

Dependence of 1/G Recoils. H%

on

on [Solute] for

Fission

G H , for T h ( N 0 ) and C a ( N 0 ) solutions is quantitatively expressed for fission-recoil radiation by Equations 10 and 11 as shown i n Figure 4. 8

4

3

2


SOLVATED ELECTRON

268

1 / G H , = 0.634 + 0.864 [ T h ( N 0 ) ] 3

1 / G H , = 0.493 +

(10)

4

(11)

0.343[NO ~] 3

Equation 10 is based on the data of Boyle and M a h l m a n (5). Equation 11 is based on the data of Sowden (39). Although excellent agreement between the results from T h ( N 0 ) and C a ( N 0 ) solutions is indicated (39) in a plot of G H , against [ N 0 ~ ] assuming total dissociation of both solutes, Equations 10 and 11 markedly disagree i n the limiting value of G , for infinitely dilute solutions: Equation 10 indicates G H , = 1.58, and Equation 11 indicates G H , = 2.03. R e a c t o r R a d i a t i o n . G H , for C a ( N 0 ~ ) solutions from 0 . 0 1 M to 1.0M is quantitatively expressed for reactor radiation b y E q u a t i o n 12. 3

4

3

2

1 / 3

3


H

3

2

(12)

1 / G H , = 1.49 + 3.51 [ N O . - ]

Equation 12 is based on the data of Sowden (38) for the Harwell experi mental reactor Β Ε Ρ Ο . Equation 12 indicates two processes for H forma tion similar to the results from γ-radiation: Equation 12 indicates G H , = 0.67 for infinitely dilute solutions while the measured value is 0.82. 18.9 M.E.v. D + . G H , for H 0 solutions from 0 . 0 1 M to 1.0M is quantitatively expressed for 18.9 M.E.v. D radiation b y Equation 13. 2

2

2

+

(13)

1 / G H , = 2.10 + 2.05 [ H 0 ] 2

2

Equation 13 is based on the data of Anderson and H a r t (3). Again, two processes are indicated for H formation: Equation 13 indicates G H , = 0.476 for infinitely dilute solutions, while the measured value at 3 X 10" MH O is0.675. Dependence o f G ( H + C I " ) o n C H C l C O O H Concentration. T h e sum of G ( H ) and G(C1 ~) at moderately low concentrations of chlori nated compounds indicates (17) that a l l reducing species react to yield either H or C l ~ . T h e enhancement, A G ( H + C I " ) , at concentrations of C H C l C O O H from 0.1 M to 2 . 5 M is quantitatively expressed for C o 7-radiation b y Equation 14 as shown in Figure 5. 2

5

2

2

2

2

2

2

2

60

2

1 / A G ( H , + C I " ) = 0.545 + 0 . 2 8 0 / [ C H C l C O O H ] 2

(14)

Equation 14 is based on the data of H a y o n and Allen (17) at p H 1.0. A G ( H + C I " ) was taken as the increase over G ( H + C l ~ ) of 3.65 for 0.001MCH C1COOH. 2

2

2

Discussion H a r t and Boag's discovery (13) of the broad optical absorption band of the eâq i n irradiated water confirmed the conclusion (7, 8) that the eâq, instead of the H atom, is the principal reducing intermediate i n the bulk of the solution. Understandably, the Lea-Platzman (31) viewpoint of the primary physical processes of energy absorption gained currency over the Samuel-Magee (34) viewpoint. T h e i r disagreement concerned the fate of the electron from primary ionization of water: Platzman (31)



19.

SWORSKI

269

"Excited Water

1

I/[CH CICOOH] 2

Figure 5. Dependence of 1/AG(H Cl~) on l/[CH ClCOOH] for Co radiation. 2

2

60

+ 7-

has accepted Lea's viewpoint (20) that the electron escapes from the parent positive ion, while Samuel and Magee (34) have deduced that the electron does not escape. Plausible arguments have been presented (14, 29, 35) for the e' , not the H atom, as the main precursor of H . Assume that it is the reaction of N 0 ~ with the e~ which causes both enhancement of G ( C e + ) and decrease i n G . T h e n , Equations 1 and 2 would yield a lifetime for e~aq of 5 X 10 ~ sec, based on a rate constant (4) for reaction of N 0 - with e' of 8.15 X l O ' M " s e c . " T h i s is much longer than the lifetime of 8 X 1 0 - sec. to be expected for the e' i n 0.8iV sulfuric acid ([K ] ~ 0 . 6 M (46)) owing to reaction with H % alone. This, together with the absence of any marked influence of p H on the effect of N 0 ~ on either G ( C e + ) [0.08ΛΓ to O.SN rlSO (45) ] or G H , (27) forces the con clusion that the N 0 - effects are not attributable to reaction of N 0 " with e~ . Therefore, the presursor of H cannot be the e~ . T h i s agrees with conclusions (2, 21) based on the absence of any effect of H on isotopic enrichment of hydrogen with light hydrogen from H O - D 0 mixtures. aq

2

3

aq

3

H 2

10

1

aQ

8

1

1 1

aq

+

aq

8

3

A

3

3

aq

aq

2

+

2

m

2

Magee (22) proposed that three consecutive elementary processes, occurring i n about 10 ~ sec., yield O H and H 0 as the primary chemical intermediates i n the radiolysis of water 13

3

H 0

H 0

2

2

H 0+ + H 0 2

+

+ e~

H 0+ + O H

2

3

H 0 + + e- - * H 0

(a) (b)

3

(c)

aq

(d)

3

and that H 0 is the precursor of e . aq

3

H O^H+» 3

q

+e


270

SOLVATED ELECTRON

T h e results summarized b y Equations 1-14 are interpreted i n this paper by reaction mechanisms which include reaction of solute with H 0 . There is sufficient recent theoretical and experimental evidence to justify serious consideration of H 0 as a n intermediate i n the radiolysis of water. Bernstein (15) has presented thermochemical evidence for the stability of H atom adducts such as H 0 and N H and estimated (16) the stability of H 0 with respect to H 0 + H i n the gas phase as about 5 kcal./mole. C . E. Melton and P . S. Rudolph of this Laboratory have demonstrated to H . A . M a h l m a n and the author, b y mass spectrometric techniques, the existence of H 0 , D 0 , and D H O i n the gas phase. These species were presumably desorbed from the instrument walls. T h e ratio [ H 0 ] / [ H 0 ] was 1/200 at a background pressure of 10 ~ torr in an ion source designed to eliminate ion-molecule reactions. 3

8

8

3

3


4

2

8

3

2

8

2

Effect o f L E T . It is commonly assumed i n comparative studies using radiations which lose energy at different rates, E.v./A. denoted as linear energy transfer or L E T , that the chemical effects differ only as a result of spatial distribution of the primary chemical intermediates. I n the Samuel-Magee model, these intermediates are assumed to be i n isolated spheres for C o y-radiation and i n a cylinder—the length of the track—for particles of higher L E T for which higher G H , values are ob served. T h e variations i n G , with variations in L E T are correlated i n the Samuel-Magee model with variations i n the initial density of inter mediates i n the cylinder. A different model to explain the variations in G H , with L E T is induced here. T h e failure of Equations 2-6 for C o y-radiation, Equation 12 for reactor radiation, and Equation 13 for 18.9M.E.v. D to represent quan titatively the dependence of G H , on solute concentrations less than 0.01M is interpreted as evidence that H formation for these radiations results from two reactions: intraspur and interspur reactions. I n interspur reactions, intermediates from one spur react with intermediates from an adjacent spur before they escape into the bulk of the solution. If we denote the G-value for H formed i n intraspur reactions b y G H , ° , then the G-value for H formed i n interspur reactions is equal to G H , — G H , ° . T h e variations i n G „ G , ° and G , — G , ° with increase in L E T , shown i n Table I, result from a concomitant decrease in average distance between spurs i n a particle track. G , — G H , ° increases from 6 0

H

6 0

+

2

2

2

H

H

H

H

H

Table I.

Dependence of

Radiation

G H , , and G H , - G H , ° on LET 0

G ,°

G H ,

Fission Recoil Harwell Reactor 18.9 M.E.v. D C o y'B "Isolated Spur" +

80

GH„

H

1.67» 0.67 0.476 0.33 ^0.33

1.67 0.84 0.675 0.45 ^0.33

GH*

-

G H ,

0

0 0..17 0..199 0..12 0

β Intercept values for Equations 7 - 9 and 11 indicate G H , for fission recoils is about 1.67 db 0 . 0 9 . 8

zero for an "isolated s p u r " to a maximum value for some intermediate value of L E T and then decreases to zero for fission recoils since essentially all spurs initially overlap adjacent spurs. G , ° increases continuously H


19.

SWORSKI

271

"Excited Water

1

with increase i n L E T since the probability for adjacent spurs to overlap each other initially, increases. Pucheault and Ferradini (33) have also presented arguments for spurs in an α-particle track which do not overlap adjacent spurs initially. T h e y quantitatively interpreted a wide variety of experimental results assuming that H and H 0 are formed i n intraspur reactions and destroyed i n interspur reactions. A p p l i c a b i l i t y o f H o m o g e n e o u s K i n e t i c s . T h e applicability of homogeneous kinetics to the reaction of solute with H 0 i n the spur raises the question as to whether Equations 1-14 are only an approxima tion of the dependence on solute concentration which would result from treatment of intraspur reactions by diffusion kinetics. A n answer is pro vided by Figure 6. T h e slope and intercept values in Equation 15 2

2

2


8

= 1 + 4.42 [X]

1/GBS

(15)

were selected to yield the same value for G ( R ) / G ( R ) o of 0.43 at [X] = 0.3 as shown in Figure 1 of Schwarz's (36) paper. There is a large dis agreement i n the curvature of the dependence of G ( R ) on solute concen tration between the results from Schwarz's treatment and Equation 17. 2

2

2

Figure 6.

Dependence of G(R )/G(R ) on [X]: II. Equation 15. 2

2

0

I. Schwarz's treatment;

Since the shape of the curves i n diffusion kinetics is invariant with respect to most of the pertinent parameters (19), the conclusion is reached that diffusion kinetics can not quantitatively express the dependence of G H , ° on solute concentration. Therefore, the question arises as to whether a


SOLVATED ELECTRON

272

fundamental and simple reason exists for the applicability of homogeneous kinetics. Equations 1-14 demonstrate that some effects of solute which are commonly attributed to reaction with e can be quantitatively ex pressed by a reaction mechanism i n which bimolecular reaction of the precursor with solute is i n competition with precursor disappearance by a first-order process. T h e first observation that such a reaction mechanism may be applicable i n water was made b y Dainton and Peterson (9) who proposed H 0 * and its reaction with H to explain the increase i n radical pair yield for C o 7-radiation with increase i n sulfuric acid con centration. Dainton and W a t t (10) proposed the alternative hypothesis that H + a q makes available to solutes isolated radical pairs [H + O H ] which would otherwise revert to water. H a y o n (18), however, has normalized the increase i n radical pah* yield by a wide variety of solutes with reference to H on the basis of their reactivity towards e and concluded that the increase i n radical pair yield is caused b y reduction of the back reaction to form water i n the spur. A strong correlation exists in the Magee proposal (22,23). Consider primary ionization to occur i n one of two molecules of water with a hydrogen bond: aq

+

2

a q


6 0

+

aq

a q

H

Η"

\

+

Η—Ο

e-

(e)

Η Proton transfer occurs which is effectively an ion-molecule reaction: +

^H Ο—Η

Η /

Ο + β'

(f)

Η Charge neutralization yields H 0 and O H with a hydrogen bond: 3

H

H Ο—Η

-Ο

(g)

H Reactions e, f, and g more clearly present the Magee proposal represented above b y Reactions a, b, and c. T h e applicability of homogeneous kinetics is attributed to first-order disappearance of H 0 * , "excited water," as the rate-determining step for H formation, instead of the combination of reducing species as commonly assumed when using the Samuel-Magee model. T w o alternative phys ical models of H 0 * are proposed. In one, H 0 * is the H 0 + O H radical pair which is assumed to undergo geminate recombination with 2

2

2

2

8


19.

SWORSKI

273

"Excited Water"

HgO combination to form H as a concomitant process. I n this model, solute decreases G , ° b y reaction with H 0 . I n the other model, H 0 * yields freely diffusing H 0 + O H radicals i n a first-order process, and solute decreases G ° by reaction with H 2 0*. G e m i n a t e R e c o m b i n a t i o n o f H 0 + O H . Noyes (30) has dis cussed the kinetics of competitive processes when reactive fragments are produced i n pairs. Geminate recombination [original partners such as H 0 + OH] occurs within about 10~ s e c , and significant competition with this combination is not expected unless solute concentration is 0.01M or greater, even for diffusion-controlled reactions. Therefore, the kinetic dependence expressed b y Equations 1-14 occurs at concentrations when competition with geminate recombination becomes possible. I f such competition does occur, Noyes (30) has pointed out that the amount of additional reaction varies approximately as the square root of the con centration of scavenger. T h i s is just the concentration dependence originally reported (6) for G H i n the decomposition of water b y fission recoils. Since G = G H ° for fission recoils, the conclusion is drawn that solutes decrease G ° b y reaction with H 0 i n competition with H 0 dis appearance by geminate recombination. Let geminate recombination volume denote that volume within which the H 0 + O H radical pair can diffuse before geminate recombina tion occurs. T h e spur is then a number of H 0 + O H radical pairs whose geminate recombination volumes initially overlap each other. Intraspur formation of H results from combination of H 0 radicals from pairs whose geminate recombination volumes overlap each other. W e assume, just as Dainton and W a t t (10) have assumed, that an isolated radical pair disappears approximately b y a first-order process to reform water. T h e n for first-order kinetics t o be approximately valid for geminate recombination i n the spur, the concentration of foreign radicals in each geminate recombination volume must vary only slowly with time during the lifetime, r o * , of the H 0 + O H radical pair. T h e R o l e o f E x c i t a t i o n . It would be surprising if excitation played no role i n the radiolysis of water. T h i s is implied, however, by attributing all the effects of radiation either to charge separation which yields O H and elq, or to charge neutralization which yields H 0 + O H . Therefore, the suggestion is made here that excitation may also result i n forming the HsO + O H radical pair. 2

H

8

2

3

H 2

3

9


3

2

Bi

8

H 2

3

3

3

3

2

3

H2

8

3

H

\

0--

H

Η

Η \

Η—Ο

Η

Ο—Η-

/ Ο

(h)

Η

Superexcited states of water (32) would certainly provide as much energy for Reaction h as charge neutralization. Superexcited states are not required, however, for Reaction h to proceed, since charge neutralization undoubtedly produces H 0*. Assume Reaction h t o be reversible. 3


SOLVATED ELECTRON

274

T h e n ionization which yields H 0 + O H b y the sequence of Reactions e, f, and g would result in formation of "excited water" by the reverse of Reaction h. T h u s , the effect of both ionization and excitation would be 8

formation of H 0 * . 2

Assume that the lifetime of H 0 * is longer than the lifetime of the radicals which disappear in intraspur reactions. T h e n the rate-determining step for inhibition of intraspur H 0 formation would be reaction of solute with H 0 *. L e t us assume that the rate constant A R . R for intraspur combination of freely diffusing radicals is about 10 sec.~\ and that the lifetime of H 0 * is about 4 X 10 ~ sec. T h e n the concentration of radicals during intraspur formation of H must be greater than 0.25M which is not unreasonable. L i f e t i m e for " E x c i t e d W a t e r . " r o*fe.H o*, where & . H , O , * is the rate constant reaction of solute with "excited water" (reaction of solute with H 0 in one model and with H 0 * in the other model), is a constant which can be derived from results summarized b y Equations 1-14 as discussed below and which is independent of any constant errors i n absolute dosimetry. L e t G H O * denote the yield of H 2 0 * which disappears intraspur b y a first-order process with resultant H , H 0 , and H 0 formation. L e t a denote the number of H molecules formed for each H 0 * which disappears intraspur. 2

2

2

10

10


2

2

H2

8

2

8

2

2

2

2

2

2

2

2

H 0*

aH

2

(i)

2

A reaction mechanism i n which Reaction i competes with reaction of solute with H 2 0 * , £ S , H Î O * [H 0*][S], yields the following kinetic relationship: 2

1 / G H ° = l/aG o* + * . o*[S]/AiaGH o* 2

H2

8 Hs

8

(16)

T h e ratio of slope to intercept from a plot of 1 / G H * against [S] yields TH o*£s.Hto*, where T H O * is l/k\. Equations 2-13 are of the form of Equation 16, and the values of T O * ^ S . H O * obtained from them are listed i n Table II. Equations 1 and 14 are of a different form than Equations 2-13. Let G°(C1 ~) denote the yield of C l ~ formed by intraspur reaction of H 0 * with C H C l C O O H . 0

2

2

H

2

2

2

2

H 0* + CH ClCOOH — CI-

(j)

2

2

We assume that G°(C1~) = 6AG(H + Cl~") for the experimental data expressed by Equation 14 where b is a proportionality constant. A reaction mechanism in which Reaction i competes with Reaction j yields the following kinetic relationship: 2

1/AG(H,

+

C1-)

=

+ 6*i/GH *ifej[CH ClCOOH]

6/GH O*

î0

2

2

(17)

Equations 1 and 14 are of this form, and the ratio of intercept to slope yields the values for T H O * & S , H O * listed in Table II. There is excellent agreement between the three values for ^ Β . Η , Ο · obtained for N 0 8 ~ with C o 7-radiation and reactor radiation, 2

2

Τ Η , Ο * -

6 0


19.

SWORSKI

275

"Excited Water

1

Table II.

Constants Derived From Equations 1-14

Radiation Co

Solute

NO," (1)* NO," (2)*

y

80

H 0 2

2.30 2.57 0.938 3.77 1.48 8.38 1.95 1.13 0.563 1.47 1.36 0.70 2.36 0.976

2

UOtSO*

N0 Cu CH ClCOOH 2

+ 2

2

Fission-Recoils

UOiSO*

U0 F U(S0 ) Th(NO,)4 NO,- (11)* NO," (12) « H 0 2

2


4

Reactor 18.9m.E.v. D

+

2

2

2

Parenthetical notation indicates which Equation in the text was used.

a

and between the two values obtained for H 0 with C o 7-radiation and 18.9 M.E.v. D + . T h e corresponding values obtained for fission recoils are significantly lower, T H J O ^ S . H S O * obtained for N 0 " and 110^04 with C o 7-radiation, reactor radiation, and 18.9 M.E.v. D + is about 3.4 times as large as for fission recoils. T h i s is attributed either to a marked i n crease i n the degree of overlapping of geminate recombination volumes i n one model or to reaction of H 0 * with primary intermediates in the other model. T h e values of T H O * * S , H O * i n Table II indicate a value for r o * ranging from 10 ~ to 10 ~ sec., based on a value for fe.Hto* of 6 Χ 10 M " s e c . - from Debye's equation (11). T h e constants in Table II are also a measure of the relative reactivity of solute with H 0 * for any particular form of radiation for which T H J O * can be considered a constant. T h e relative reactivities of solute with H 0 * and e~ for C o 7-radiation differ slightly but significantly as shown in Table III. Relative reactivi ties with are based on measurements of absolute rate constants b y pulsed-radiolysis techniques (4). 2

60

2

8

6 0

2

2

9

J

Ht

2

10

9

1

2

Table III.

6 0

aq

2

Relative Reactivities of H 0 * and ·ά

Kinetic Evidence that "Excited Water" is Precursor of Intraspur H

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