A pulse radiolysis study of the reactions of platinum(II) glycine complexes in aqueous media

Radiat. Phys. Chem. 1977, Vol. 10. pp. 77-87. Pergamon Press.

Printed in Great Britain

A PULSE RADIOLYSIS STUDY OF THE REACTIONS OF PLATINUM(II) GLYCINE COMPLEXES IN AQUEOUS MEDIA W. L. WALTZ, J. C. BRODOVlTCH an d K. P . KUNDU Department of Chemistry and Chemical Engineering and the Saskatchewan Accelerator Laboratory, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0 Canada

(Received 16 August 1976; in revised form 4 October 1976) Abstract--An investigation into the reactions of the hydrated electron, hydrogen atom and hydroxyl radical with the two isomers of bis(glycinato)platinum(II) complex, cis- and trans-Pt(gly)._, has been carried out using the technique of pulse radiolysis. The reactions of these radicals with both complexes occur with rate constants near 10'° dm' tool-t s-'. The electron and hydrogen atom products absorb intensely below 300 run; however, their spectral and kinetic behaviors indicate that the reactions of e~ and H atom lead to different platinum transients. The interaction of OH with cis-Pt(gly): yields five transients whereas that of trans-Pt(gly),, generates six intermediates in addition to long-lived products. These species absorb in the near U.V.-visible, and are interrelated through acid-base reactions. These transients are reactive towards Cu(II) and ferrocyanide ion. No detectable amounts of ammonia or of free glycine were found as final products. It is concluded that the intermediates are platinum(III) entities and the nature of these species is discussed.

1. I N T R O D U C T I O N IN EARLIER communications "-'~,we presented the results of investigationsinto the reactions of eL, H atom and hydroxyl radical with Pt(II) and Pt(IV) complex ions containing coordinated ethylene-

characterized geometrical forms, cis- and transPt(gly):, which are square planar in structure. The general behavior of these systems is in the main similar to that of the platinum amine compounds although the complexities of the OH reactions have precluded detailed kinetic analysis. These observations do, however, complement and extend the previous studies as well as providing further insight into the nature of Pt(I) and Pt(III) systems.

diamine and related organic amines. While it was suggested that the products of the reactions of el, and the Pt(II) systems could be designated as Pt(I) entities, it was observed that the corresponding H atom products differed from those of the electron reactions. In addition, a series of transients, characterized as Pt(III) entities, were encountered on the single-electron reduction of Pt(IV) and upon oxidation of Pt(II) by OH. In the case of the platinum ethylenediamine systems, these transients were interrelated by acid-base and substitution reactions; however, it was proposed that the species may also differ in their structures. Of particular note was the observation that the reaction of OH with Pt(en)::" did not lead to release of free ethylenediamine nor to the degradation of the coordinated ethylenediamine, whereas the free ligand is degraded by this reaction ~3). The general purpose of this present work was to study the reactions of eL, H and OH with bis(glycine) platinum(II), Pt(glyh, and in so doing to further characterize Pt(I) and Pt(III) materials and the fate of the coordinated organic ligand. Glycine was choosen as being representative of amino acids and also because Pt(gly): exists in two well RPC Vol. 10, No. 2--A

2. E X P E R I M E N T A L 2.1 Analyses, materials and solutions The preparation of cis- and trans-Pt(gly~ were carried out by published procedures ('~, and their identities were established through elemental analysis and by comparison of the U.V.-visible and LR. spectra with previously reported values '6m.Aqueous solutions of these compounds were observed to be stable in the pH range of ca. 2-10 over the experimental time intervals used here. Nitrous oxide (Matheson high purity, 98% rain.) was used as received, and other materials were of reagent quality. The preparation and handling of deaerated solutions is descn-bed elsewhere"-". Tests for the possible formation of NH, and free glycine arising from the reactions of hydroxyl radical with the platinum compounds were made using the colorimetric phenate method ('), and were carried out in the same manner as that presented in an earlier paper `:). The tests were performed on irradiated solutions where the initial pH values were 5.6-5.8 (natural pH) and 3.5 for cis-Pt(gly): and 5.6-5.8 (natural pH), 8.0, and 9.1 for trans-Pt(glyh.

2.2 Pulse radiolysis apparatus and procedures The pulse radiolysis apparatus and the procedures used for dosimetry and for the analysis of the kinetic data were 77

78

W. L.

X*VALTZ

et al. o

the same as described previously ':'. The path length of the irradiation cell was 4 cm unless otherwise specified. The optical detection system measured changes in absorption (AA) relative to the unirradiated solutions. The spectra presented here (..L4 vs wavelength) have not been corrected for loss of platinum starting materials, which absorb to a minor extent in the U.V. region except in the case of trans-Pt(gly): below ca, 260nm, and for absorption contributions in some instances due to H atom products or due to 2-propanol radical '"'. Such corrections ~usually less than 20%) have been attempted where necessary in estimating the extinction coefficients for the absorption peaks of the transients. Where standard deviations have been assigned to numerical data, these were generally based upon at least three independent measurements. All experiments were performed at room temperature (ca. 25°C).

:: )

6

5; 5~ e~q-.z,s-=,

% < <~

( C)

--

,2.

;, ~ /

:,-i ~ C,S - :' t

3. R E S U L T S 220

3.1 Reactions of hydrated electron The r e a c t i o n s o f eL with cis- and trans-Pt(gly): w e r e studied at natural p H (ca. 5.7) in the p r e s e n c e

Complex

cis-Pt(gly): trans-Pt(gly),_

I.

KINETIC

DATA ON THE

380

300

380

Wavelength, nm

of 1 tool d m -3 2-propanol, used to s c a v e n g e O H and H atom "°'"'. U n d e r t h e s e c o n d i t i o n s , the d e c a y of the h y d r a t e d e l e c t r o n was a s s o c i a t e d with the f o r m a t i o n of i n t e n s e l y a b s o r b i n g p r o d u c t s as s h o w n in Figs. la and lb, w h e r e e:,0 = 510 m: mol-' for cis-Pt(gly): and ~:~,---550m'-mol -~ for transPt(gly):. The f o r m a t i o n rate c o n s t a n t s for the p r o d u c t s w e r e f o u n d to be equal to t h o s e of the d e c a y of e2q o b s e r v e d at 5 7 8 n m (Table 1). Additional confirmation of these p r o d u c t s arising f r o m the r e a c t i o n s of eL with the platinum c o m p l e x e s was the o b s e r v a t i o n that s a t u r a t i o n of the solutions with N:O, a good e l e c t r o n s c a v a n get":% eliminated the t r a n s i e n t s a l t h o u g h t h e r e was n o w s o m e m i n o r residual a b s o r p t i o n , having the kinetic and s p e c t r a l p r o p e r t i e s o f 2-propanol radical ~9'. The d e c a y kinetics of the t r a n s i e n t platinum s p e c i e s o b e y e d simple s e c o n d - o r d e r rate TABLE

300

FIG. l. Absorption changes arising from the reaction of eL and H atom with Pt(II) complexes. (a) Reaction of eL with cis-Pt(gly): at -15 ~mol dm -~, [e2q]=,~ 2.4~z moldm-'. 1 moldm-~ 2-propanol. ( ) Igs,( .... / 3/.Ls, ( - - - - ) 5 p.s. (b) Reaction of eL,with trans-Pt(gly),_ at 71 gmol dm -J. [e2,]~,, = 2.2 ~.mol dm -~. 1 mol dm -~ 2propanol. ( ......) 0.6~zs. (. . . . ) 3 ~xs. and light path of 3.93 cm. (c) Reaction of H atom with cis-Pt(gly): at 90 ~mol dm-', [H]m,, = 3.0 t~mol dm-L 0.5 tool dm-L tert-butyl alcohol, ca. 0.01 mol dm -~ HC10, (pH = 2). ( - - ) 1 ~s, (. . . . ) 5 u,s, ( - - - - ) 20~s, ( - - . - - ) 1 ms. (d) Reaction of H atom with trans-Pt(gly),, at 84 tzmol dm -3. [H]~,, 2.5 lzmoldm~', 0.5 moldm -~ tert-butyl alcohol, ca. 0.01 moldm -~ HCIO, (pH=2). ( ) l~zs, (. . . . ) 81xs. l - - - - ) 34 ~zs, ( - - . - - ) I ms.

laws, and the values of the rate c o n s t a n t s are p r e s e n t e d in Table I. On multiple pulsing of these materials, s u s p e n s i o n s of p r e s u m a b l y metallic platinum were formed.

REACTIONS

OF

CiS- A N D trans-Pt(gly): wrrH e2."

k (din 3 tool-' s-')

[Pt(II)] (~mol din-')

h (nm)

1.3 ~ 0.1 x 10~

t5-45

4.2 -. 1.4 x 109~" 1.4 = 0.3 × 10~°

45 15-45

4.2 -'- 0.9 x 109+

30, 45

240-270, 578 250-310 250-270 578 250-270

Remarks Growth of Product and Decay of eL Decay of Product Growth of Product and Decay of eL Decay of Product

*Solutions were at natural pH and contained 1 mol dm -3 2-propanol. +The values of the second-order rate constant were calculated from simple second-order kinetic plots, using the relationship of slope: intercept equals kC, where C is the maximum concentration of the platinum transients, which was taken as that of the maximum e,q concentration. The values of k have been corrected for the presence of decay of 2-propanol radical using spectral and kinetic data which were determined here and which were in close agreement with that of Ref. (9).

A pulse radiolysis study of the reactions of platinum(II) glycine c o m p l e x e s in a q u e o u s media

3.2 Reactions of H atom On pulse-irradiation of solutions at low concentrations of platinum complexes containing 0.01 mol dm -3 HC10, and 0.5 tool dm -3 ten-butyl alcohol, it is expected that e~ will react rapidly with H" to yield H atom "'-~, and that OH radical (but not H atom) will be scavenged by ten -butyl alcohol (t°'"). Under these conditions, there were rapid increases in absorption at lower wavelengths near the end of the pulse as shown in Figs. lc and ld. The addition of 0 . 5 m o l d m -3 2-propanol, a scavenger of H atom"", eliminated the formation of the transients although a small amount of absorption characteristic of 2-propanol radical occurred~gL Absorption by ten-butyl radical alcohol is anticipated to be negligible at these wavelengths(9L In the case of trans-Pt(gly).., the rate of growth in absorption with a peak at 260 nm (~ --- 580 m" tool -t) increased linearly with the concentration of complex (Table II). The subsequent slight decay in absorption was concomitant with a further development in absorption at wavelengths above 300nm (Fig. ld). This latter change obeyed first-order kinetics and the rate was independent of complex concentration (Table II). These features suggested that the initial product rapidly interconverted to give a new species. With the cis-Pt(gly):, system, the rate of growth in absorption with a peak near 250nm (E = 4 1 0 m : tool -~) was dependent upon the complex concentration (Table II). The subsequent behavior of the product (Fig. lc) appeared to be similar to that of the trans-Pt(gly),, case (Fig. ld); however, the changes in absorption were too small for meaningful kinetic analysis. For both systems, the subsequent decays in absorption associated with the secondary products were described by first-order kinetics (Table II), and led to the formation of additional absorbing transients on the 500 ~s-1 ms *At p H = 2.9, the a m o u n t of H atom m a y have been as high as 20% owing to the competition between H- and

N20 for eL. The pH in basic media was restricted to 10 or less in order to avoid the formation of 0-"°). TABLE II. KINETIC D A T A

cis-Pt(gtyh trans-Pt(gly)._

scale. The kinetics of disappearance for these species (1-2 ms scale) were complex. On multiple pulsing of solutions, suspensions of fine particle presumably metallic platinum were observed. 3.3 Reactions of hydroxyl radical Pulse-irradiation of aqueous solutions of cisand trans-Pt(gly),_ saturated with nitrous oxide at pHs between 2.9 and 10.0 gave rise to intensely absorbing transients as shown in Figs. 2--4. Under these conditions, the major radical species present near the end of the pulse is hydroxyl radical"°~ along with a minor amount of H atom"3). * The occurrence (a)

pH :2.9

2.0 ~- 0.2 x 7.3=1.3x 2.2 = 0.3 × 2.5 + 0.3 x 6.4 ~- 1.7×

10 '° llYs-' 10 '° l0 s s - ' 103 s-'

3oc

/ ,o

pH % x

<~

(c) pH = 8.O

/\ I

/f]'

4OO 500 6O0 WQvelength, nm FIG. 2. Absorption changes on kradiation of cis-Pt(glyh solutions. (a) p H = 2.9, cis-Pt(glyh at 235 ~ . m o l d m -3, NO

3OO

[OH]~.=4.6 ~.moldm -3, light path of 3.93cm. (, ,- ) 0.5 ~ s , ( . . . . ) 4 ~,s, ( ~ - - ) 1 m s , ( . . . . ) 5 m s . (b) Effect of pH on the apparent values of the extinction coefficients at 360nm (0) and at 520nm (©). (c) pH= I0.0, cisPt(gly)2 at 188 tzmol dm -3, [OH],~ ~ 5.3 ~mol dm-L (---) 0.5 ~s, (. . . . ) 5/zs, ( - - - - ) ca. 20 #.s. (-----) 40~s, (. . . . ) 4 ms. All solutions were saturated with N:O. cis-

AND

trans-Pt(gly)..

(~mol dm -3)

~. (rim)

15-60 84, 90 15-58 15-102 84, 99

250-310 250-310 250-290 320-370 250-360

[Pt(II)]

(din~ tool-' s-')

T

E

ON THE REACTIONS OF

k

Complex

79

WITH H

ATOM*

Remarks Growth of Initial P r o d u c t Initial Decay of Product(s) Initial Growth Secondary Growth Initial Decay of Product(s)

*Solutions contained 0.5 tool d m -3 tert-butyl alcohol, and about 0.01 mol d m -~ HCIO, (pH = 2).

80

VV'. L .

WALTZ

et al.

F/

'\

,

Dp* =4

3

--' ~,

Not. Si"

q7

i

% ,

I

I

!

t

~3

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I

I

1

I

!

i

q

!

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= iO.O

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240

t t r

.. [ , 300

~

~

;

|0 400 i

nm

FIG. 4. Absorption changes on irradiation of transPt(gly): solutions at intermediate pHs. (a) p H = 4 . 3 . trans-Pt(gly): at 116/xmol dm-', [OH]m,~= 4.3/xmol dm ~. ( ) 0.5/xs. (. . . . ) 4/xs. ( - - - - ) 2 ms, ( - - . - - ) 10 ms, ( . . . . ) 10 s. (b) Natural pH (ca. 5.7), transP t(gly): at 190 and 216/x tool dm -~, [OH]m,, 3.8/xmol dm -3. ( ) 0.5/~s, (. . . . ) 3/xs, ( - - - - ) 100/xs, ( . . . . ) 5 ms. All solutions were saturated with N:O.

'~

I F'-"t\

"~,, ' / . - ' - - ' , \

300

400

Wavelength,

s x ,'-,

i

,

(b',

/

I

\',

"''E X

i'!

,~

<

k~ j

!

"'

~,(

i6

40o 5o0 Wevelength,nm

600

FiG. 3. Absorption changes on irradiation of transPt(gly): solutions. (a) pH =2.9, trans-Pt(gly), at 129u.moldm-% [ O H ] ~ . . = 4 . 2 / z m o l d m - ' . ( ) 0.5/xs, (. . . . ) 5 ~xs, ( - - - - ) 0.5 ms, ( - - . - - ) 2 ms, ( . . . . ) 150 ms. (. . . . ) 20s. (b) pH = 10.0. trans-Pt(gly), at 102 and l l 0 # m o l d m -~, [OH]~,~=5.2~.moldm -~. ( ) 0.5/xs, (. . . . ) 2 ~ s , ( - - - - ) ca. 5 ~ s , ( - - . - - ) ca. 3 0 ~ s , ( . . . . ) 10 ms. All solutions were saturated with N:O, of t h e s e t r a n s i e n t s c o u l d b e e l i m i n a t e d b y the a d d i t i o n of 2 - p r o p a n o l (ca. 1 tool dm-3), a n efficient s c a v e n g e r of O H (~°~. The initial p r o d u c t s of the r e a c t i o n s of O H w i t h t h e s e c o m p l e x e s a p p e a r e d to a b s o r b in the r e g i o n of 250-280 n m , a n d their r a t e s of f o r m a t i o n in g e n e r a l i n c r e a s e d w i t h i n c r e a s i n g c o n c e n t r a t i o n s of the c o m p o u n d s . While the d e t e r m i n a t i o n s of the rate c o n s t a n t s a s s o c i a t e d

w i t h t h e s e r e a c t i o n s were c o m p l i c a t e d by the rapid, s u b s e q u e n t r e a c t i o n s of the p r o d u c t s , e s t i m a t e s of t h e s e values are g i v e n in T a b l e III. T o s u b s t a n t i a t e t h e s e e s t i m a t e s , the c o m p e t i t i v e r e a c t i o n of HCO3w i t h O H to yield c a r b o n a t e radical '~'''' was studied at 600 n m in the p r e s e n c e of v a r i o u s c o n c e n t r a t i o n s of the c o m p l e x e s . It was o b s e r v e d that c a r b o n a t e radical also r e a c t e d with the p l a t i n u m starting m a t e r i a l s to yield a b s o r b i n g p r o d u c t s (Table III). T h e s e r e a c t i o n s w e r e , h o w e v e r , sufficiently wellr e s o l v e d f r o m t h o s e leading to the f o r m a t i o n of c a r b o n a t e radical so that a s t a n d a r d c o m p e t i t i v e a n a l y s i s of the effect of the p l a t i n u m c o m p l e x o n the yield of c a r b o n a t e radical could be carried out. T h e s e results are p r e s e n t e d in T a b l e III, a n d are in a c c o r d with the values d e t e r m i n e d directly f r o m the

TABLE III, REACTIONS OF cis- AND trans-Pt(gly): w i n 4 HYDROX~'L AND CARBON-kTE RADICALS

Species Hydroxyl Radical

Carbonate Radical

Complex ciscistranstranscistrans-

k (10 -~° dm 3 moV' s -~) ca. 1.5

1.4 0.9-1.9 1.5 0.44 *_ 0.08 0.34 -* 0.06

[Pt(II)] (/xmol din-')

,~ (nm)

6. l-80 1050, 2090 21-190 1230, 2470 52, 105 62, 124

250-320 600 240-310 600 600 600

Method Direct* Competition+ Direct* Competi.tion-

*Direct observation of the formation of the products, c i s - P t ( g l y ) : : p H = 3.0 to 9.5. t r a n s - P t ( N y h : p H = 2.9 to 10.2; the value of the rate constant tended to increase with increasing pH. tThe effects of the platinum complexes on the formation of carbonate radical from the reaction of OH with NaHCO3 (50 mmol dm -~, pH = 8.5) was determined from the slopes of standard competitive plots of the ratio of absorbances vs the concentration ratio of complex to HCO~- after correcting for the absorption by the platinum products at 600 nm. The rate constant used for the reaction of HCO,- with OH was measured as 2.1 x 10' dm 3 tool-' s-', which is in agreement with previously reported values [Refs !14) and (15)].

A pulse radiolysis study of the reactions of platinum(ID glycine complexes in aqueous media reactions of hydroxyl radical with the platinum complexes. For the reactions of both cis- and trans-Pt(gly):, the absorption changes while complex were in general similar, and exhibited a pronounced dependence upon pH (Figs. 2-4). Considering these changes in detail for the cis-system first, the initial absorption change at low pH (Fig. 2a) was marked by the development of an intense band with a peak below 250 nm and bands of lesser intensity at 360 and 520rim. At 250rim, the subsequent decrease in absorption obeyed first-order kinetics (Table IV), and appeared to be associated with the development of a band near 330 nm, although there was a further but slower growth in the region near 310nm (Table IV). These alterations were complicated by the somewhat faster decay observed at 360 nm (and 520nm). For times exceeding 5 ms, there was a slow, general decay which exhibited complex kinetics. On progressing to higher pHs (Fig. 2c), the initial intense absorption near 250 nm diminished, and at pH = 10.0 there occurred a discrete peak at 280 nm. While this latter feature decayed by apparent first-order kinetics (Table IV), the associated rate constant was larger than that occurring in this region at pH = 2.9. In addition, there was no discernible development of the 330 nm band at high pHs. Instead, the bands at 360 and 520 nm (with a slight shoulder at 420nm) now became the prominent features in the spectrum (Fig. 2c) with a ratio of e36o:~sz0of 2.95 : 1 ( - 8 % , pH = 2.9-10.1, Fig. 2b). At high pHs, there was a secondary growth of these peaks, the rate of which was somewhat faster than the decay rate at 280 nm (Fig. 2c). These bands decayed via second-order kinetics with the same rate-constant value (Table IV). Some long-lived absorption (5s scale) developed at shorter

wavelengths; however, it is uncertain if this feature was connected with decay at 360 and 520 rim. These latter peaks appear to be representative in the main of a single transient, although in the early stages there may be some contribution near 400 nm from another species as shown by the slight decrease in absorption occurring from 20 to 40/xs at pH = I0.0 (Fig. 2c). The extent of development for the bands at 360 and 520rim in acid media increased with pH, reaching an apparent maximum near pH 6 as shown in Fig. 2c. A plot of the ratio of E,,p(pH= 6):~,p,(pH < 6) vs proton concentration was linear with a slope of 9-+ 1 × I0~dm 3 mol -t. The initial rate of formation of these bands appeared to be slower at higher pHs; however, the situation was complicated by the concentration effect of cis-Pt(gly)2. At low concentrations, the formation rate exhibited some dependence on the amount of platinum complex whereas the apparent first-order rate constant appeared to be independent of starting material concentration above about 240 v.mol dm -3 (Table IV). The foregoing suggests the species associated with the bands at 360 and 520 nm is a secondary product which is formed in competition with another species with the latter's formation dependent upon proton concentration. In general, the reaction of OH and cis-Pt(gly): appears to yield five transients in addition to very long-lived product(s). The initial transient product, C,, absorbing in the region of 250-300 nm, appears to give rise to two products, C., (A=ax= 360 and 520nm) and C3 (Xm,~<250nm). The latter is favored at lower pHs and its decay may be associated in part with the slight development at pH = 2.9 of C, (A=,x= 330 nm). At high pHs the spectral changes near 400nm and perhaps at 280 nm (Fig. 3c) suggest an additional product, C~.

TABLE I V . KINE'nC DATA ON PRODUCTS FROM REACTION OF cis-Pt(gly).. WITH O H

Absorption change Decay and Initial Growth* Slower Growth Decay Growtht Decay:~

~, (nm) 250-260, 340-390 290-330, 40O-420 270-300 305-520 310-540

pH

k

[Pt(II)] (tzmol dm-3)

2.9

1.1 ---0.2× 10a s-t

253

2.9

4.2- 1.1 × 102 s-t

253

10.0

7.2 - 1.2 × I0" s-t 2.2- 1.l × 106 s-E 2.0±0.3 × 109 dma mol-~ s-~

ca. 5.7-10.0 4.8-10.0

81

188 240-2,35 80-936

*This growth was preceeded by a slight decrease in absorption at wavelengths greater than 350 nm (Fig. 2a). tThe growth appeared to be independent of the concentration of Cis-Pt(gly)~ above about 240 tzmol dm -3, whereas below this value there was some dependence on concentration. .~The second-order rate constant was calculated upon the basis that ~3,o= 440 mz tool-t and e~:o= 149 mz mol-~ (---15°~.Fig. 2b).

W. L. WALTZ et al.

82

T h e r e a c t i o n of t r a n s - P t ( g l y ) : with O H gave rise initially to a n a b s o r p t i o n b a n d with a p e a k n e a r 260 n m (Figs. 3 a n d 4); h o w e v e r , the s u b s e q u e n t c h a n g e s were m a r k e d l y e f f e c t e d by pH. At p H = 2.9 (Fig. 3a), the a b s o r p t i o n i n c r e a s e d a n d shifted to l o w e r w a v e l e n g t h s , suggesting the s u b s e q u e n t f o r m a t i o n of a species w i t h a p e a k b e l o w 2 5 0 n m . In acid media, the d e c a y in a b s o r p t i o n at w a v e l e n g t h s b e l o w a b o u t 300 n m was c o n c o m i t a n t with g r o w t h of two b a n d s w i t h p e a k s at 3 4 0 n m (~ ----300 re: mot <) a n d c a . 5 4 0 n m (e = 65 m: mol-') and t h e s e c h a n g e s o b e y e d first-order kinetics (Table V). T h e v a l u e s of t h e s e rate c o n s t a n t s were the s a m e until p H = 4.3, w h e n the d e c a y b e c a m e s o m e w h a t f a s t e r t h a n the g r o w t h . T h e rates of t h e s e p r o c e s s e s i n c r e a s e d (Table V) a n d the d e v e l o p m e n t of a b s o r p t i o n at 340 n m was less (Figs. 3a a n d 4) as the p H was i n c r e a s e d . The d e c a y of the t r a n s i e n t at 340 a n d 540 n m a p p e a r e d to be a s s o c i a t e d in part with a s e c o n d g r o w t h in a b s o r p t i o n near 2 6 0 n m , w h i c h f o l l o w e d a n app a r e n t first-order rate law (Table V). T h e e x t e n t of this g r o w t h b e c a m e p r o g r e s s i v e l y less at higher p H s and was not o b s e r v e d at n a t u r a l pH. T h e d i s a p p e a r a n c e of this species by s e c o n d - o r d e r kinetics (Table V) gave rise to l e s s e r a b s o r b i n g p r o d u c t s , w h i c h a p p e a r e d to u n d e r g o longer term d e c a y (1-2 s scale). N e a r n e u t r a l pH, the m a j o r s p e c t r a l c h a n g e s o c c u r r e d at l o w e r w a v e l e n g t h s (Fig. 4b). The s p e c t r u m a f t e r 100 ~ s d e c r e a s e d b y s e c o n d - o r d e r kinetics (Table V) a l t h o u g h as n o t e d a b o v e t h e r e were l o n g - t e r m a l t e r a t i o n s to the r e s i d u a l absorption. In basic m e d i a (pH > 8.1), the d e c r e a s e in a b s o r p t i o n n e a r 2 6 0 n m b e c a m e f a s t e r a n d the

kinetics were n o w c o m p l i c a t e d . T h e s e c h a n g e s were a s s o c i a t e d in part with the d e v e l o p m e n t of a b s o r p t i o n b a n d s a b o v e a b o u t 320 nm. A t p H = 10, the f o l l o w i n g s i t u a t i o n existed. A b o v e 320 rim, the a b s o r p t i o n i n c r e a s e d rapidly o v e r a b o u t 4 ~xs (Fig. 4b) a n d o b e y e d first-order k i n e t i c s (Table V). O v e r this s a m e time p e r i o d the 2 6 0 n m a b s o r p t i o n r e a c h e d m a x i m u m g r o w t h (ca. 2 p.s) a n d t h e n b e g a n to d e c a y . A f t e r 4~xs, t h e r e was a s u b s e q u e n t i n c r e a s e in a b s o r p t i o n in the regions f r o m a b o u t 320 to 400 n m a n d a b o v e a b o u t 460 nm, w h i c h f o l l o w e d first-order k i n e t i c s (Table V), a n d these c h a n g e s c o i n c i d e d with the d e c r e a s e s in a b s o r p t i o n obs e r v e d b e l o w 300 n m a n d f r o m 390 to 460 nm. The resulting s p e c t r u m c o n s i s t e d of primarily two b a n d s with p e a k s at 370 n m (E -~ 300 m: mol <) a n d ca. 610nm (E-72m'-mol '). T h e d e c a y of t h e s e b a n d s o c c u r r e d by s e c o n d - o r d e r kinetics w i t h the s a m e rate c o n s t a n t v a l u e (Table V), and the residual a b s o r p t i o n at s h o r t e r w a v e l e n g t h s a p p e a r e d to be stable. T h e r e a c t i o n of t r a n s - P t ( ~ y ) : with O H a p p e a r s to g e n e r a t e six t r a n s i e n t s in a d d i t i o n to very long-lived p r o d u c t s . T h e b e h a v i o r of the initial p r o d u c t , T~, a b s o r b i n g n e a r 2 6 0 n m is d e p e n d e n t u p o n pH. In acidic solutions, a rapid d e v e l o p m e n t of a species (T:) with p e a k s at 340 a n d 5 4 0 n m o c c u r r e d a l t h o u g h at p H = 2.9 the s p e c t r a l c h a n g e s (Fig. 3a) s u g g e s t e d a n additional t r a n s i e n t , T~, with a p e a k b e l o w 2 5 0 n m . T h e d e c a y of T: was a s s o c i a t e d with the d e v e l o p m e n t of a p e a k at 260 nm, T~. In basic media, two a d d i t i o n a l species were f o r m e d : T5 with p e a k s at 370 a n d 610 n m a n d T~, a b s o r b i n g at 405 n m a n d p e r h a p s also at 260 nm. To c h a r a c t e r i z e f u r t h e r the t r a n s i e n t s arising

TABLE V. KINETIC DATA ON PRODUCTS FROM REACTION OF

Acidity Acid

Near Neutral Basic

Absorption change

,k Inca)

Decay and Growth

250-280, 300-630

Slower Growth* DecayDecay; Initial Growth Slower Growth and Decay Slower Decay§

pH 2.9, 3.0 3.6 4.1 3.0-4. l 2.9-4.3 ca. 5.7, 6.3 10.0 10.0

trans-Pt(gly):

V~'ITHO H

k

[P t(II)] /grnol dm ~)

1.9_0.3 x l0 s s-'

88, 129

3.2=1.1×10; s ' 4. I = 1 . 2 x i 0 3 s ' 2.4= 1.1 x 10" s ~ 3.5-~0.4x 10~ dm ~ mol-' s ' 1.6=0.2 x 10~ dm ~ tool -' s -~ ca. 9 x 10" s ' ca. I × 10" s '

146 250 88-250 57-250 97-216 46, 102 46.102

260-310 250-350 250-360 320-480 320--400, 460--480 250-300, I0.0 410-430 300-520 9.0, ca. 10.0 6.5 = 1.2 × 10~ dm ~ caol ~ s '

46-197

*At pH = 2.9, the growth near 260 nm (and decay at 340 nm) was considerably slower (k = 48 = 21 s-'). +The second-order rate constant was calculated upon the basis that e,,,, = 520 ca" tool-'. -;The second-order rate constant was calculated upon the basis that E_,,,~~ 480 ca" tool-'. §The second-order rate constant was calculated upon the basis that e,-,, ~ 300 m: tool-' and e~,, = 72 m z tool '.

A pulse radiolysis study of the reactions of platinum(lI) glycine complexes in aqueous media from the reactions of cis- and trans-Pt(gly):, the effects of the addition of Cu(CIO3: (oxidizing agent) and K, Fe(CN)~ (reducing agent) on the behavior of the transients were investigated, and the quantitative results are presented in Table VI. As a general observation, the presence of Fe(CN)6'(or its mono- and di-protonated forms at pH ~ 3)'tr~ resulted in the development of stable absorption at about 420 nm, which is most instances represented a peak that is indicative of Fe(CN)6 ~-"~. For cis-Pt(gly),., the effects of Cu(II) were studied at pH = 3.0 and ca. 5.7, and no detectable alteration in transient behavior was found at 270 nm. For the former pH, the decay rate for the absorption at 360 and 520 nm was increased by the presence of Cu(II) (Table VI) whereas at pH = 5.7 the effect was less.* At this latter pH, the presence of Fe(CN)6'- resulted in complex kinetic changes (Table VI) and in the development of a stable absorption peak at 420 rim. For pH = 2.9, the rate of decay for the absorption at ca. 260 nm (Fig. 2a) was increased by Fe(CN)6'- and was concomitant with growth of a stable peak at 420nm. Additional observation was that the presence of 102 ~mol din-' K3Fe(CN)6 at p H = 5 . 7 had no effect on the transient behavior at 360 and 520 nm. Studies of the behavior of the trans-Pt(glyh system in the presence of Cu(II) were performed at pH = 2.9 and 10.0. At the lower pH, only the decay of the second 260nm-transient (Fig. 3a, 150ms curve) appeared to be reactive towards Cu(II) as *At 595 ~mol dm -3 Cu(II), the results suggest that kcu is about 3 × 106 dm3 tool-' s-'.

evidenced by an increase in its rate of decay (Table VI). While the decrease in absorption at 260 nm (Fig. 3b) at pH = I0.0 exhibited some dependence on copper concentration, the changes were complex. Under the same conditions, the presence of Cu(II) enhanced the rate of decay of the species absorbing at 370 nm (Table VI). At pH = 3.3, the decrease in the peak at 260 nm (Figs 3a and 4a) was accelerated by Fe(CN)6'-, and the rate of this change coincided with that for the formation of the ferricyanide peak at 420 nm (Table VI). No development of the transient absorbing at 340 nm (Fig. 3a) took place under these circumstances. For pH = 8.4, the rate of decay in absorption at 260 nm was enhanced (Table VI) and the absorption of the transient at 370 (and 610 nm) was somewhat suppressed. The subsequent decay of this transient did not however appear to be effected by Fe(CN)6'- although stable residual absorption was observed at 420 nm. As a qualitative observation, saturation with O., of the solutions for trans-Pt(gly),. at pH = 10.0 and for cis-Pt(gly)z at pH - 5.7 resulted in about a two-fold enhancement in the rates of decay for the species absorbing at 370 nm (trans-) and at 360 nm (cis-). To determine if the reactions of hydroxyl radical with cis- and trans-Pt(gly)~_ led to the degradation of the glycinato ligand or to the release of free glycine, tests for NH3 and glycine were performed on pulsed-irradiated solutions at various pUs (see Experimental). The solutions were pulsed two to five times so that the cumulative amount of OH formed was about 20-60% of the initial platinum complex concentrations. Under these conditions neither NH3 nor glycine could be detected. If these materials were

TABLE VI. KINETICEFFECTSOF CH(CIO,)z AND K, Fe(CN), ON THE T1La,NSIENTS* Complex

Additive

[Additive] (/zmoldm -3)

83

pH

cis-Pt(gly):

Cu(C10,h K,Fe(CN)6 K, Fe(CN)~

115, 343 12, 17 32, 91

3.0 2.9 ca. 5.7

trans-Pt(gly):

Cu(CIO,): Cu(C10,): K,Fe(CN)6 tGFe(CN),

82, 147 58, 242 10-33 22

2.9 10.0 3.3 8.4

k (din ~ tool -~ s-') 1.7 _+0.8 x 3.7-- 1.1 x 3.0---0.7x 3.7 ~- 1.4 x 4.2 -~0.3 x 9.9 --+0.3 x 2.5 ---0.8 x 3.8 -- 0.2 x

10" 109t 10a, 10':[: 10s§ 106 109.* 109

h (nm) 360, 520 260-430 340-520 340-520 260 370 270--440 260-280

*The concentrations of the platinum complexes were in the range of 109-308 #tool dm -3, and the relative concentrations of complex to additive were such that greater than 82% of the hydroxyl radical formed was expected to react with the platinum materials. 1"Decay in absorption at h = 260 and 270 nm, and growth at 340---430nm. This reaction may be complex as the rate constants for the decay tended to be less than that for the growth and were somewhat lower at 260 nm than at 270 nm. ~tThe kinetics of these reactions may be complex; however, they could be fitted to two consecutive, pseudo first-order rate laws. §Effect on second 260 nm--transient. **Decay in absorption at h < 300 nm and growth at h > 320 nm.

W.L. W~,LTZ et al.

84

present, they amounted to less than 10% of the total amount of OH generated.

reaction: ¢2)

-*. DISCUSSION 4.1 Reactions of hydrated electron The reactions of cis- and trans-Pt(glyh with e~q gave rise in each case to the formation of a transient product, which absorbs at short wavelengths (Fig. 1) and which decays by secondorder kinetics (Table I). The rate constants for the formation of the transients are near those expected of diffusion-controlled processes, and while they are of the same order of magnitude as found for the reactions of eL with other platinum(II) complexes "':'2~, they are considerably larger than those for the reactions of the various forms of uncoordinated glycine':'. The two transients exhibit essentially the same kinetic behavior; however, the spectral data indicates that they are different species. Conceivably these transients may arise either from electron attachment to the carboxyl group followed by intra-electron transfer to the metal center and/or from direct electron addition to the platinum center. In the case of copper triglycine, both mechanisms are operable on about a 50-50% basis, and for the former mechanism, the rateconstant of intra-electron transfer is 1.7-0.3 x 10~s -''~'~. In contrast, for cadmium glycinate''~' direct electron addition to the cadmium center appears to occur. For free glycine, electron attachment to the carboxyl group is the initial step in a complicated sequence leading in part to reductive deamination and to the formation of the c~-carboxyl alkyl radical, -CHzCO: -'I9-23'. In the present work, no direct evidence for electron attachment to the carboxylate group has been found. If this mechanism is operable, then the rate constant for the subsequent intra-electron transfer step would be in excess of 10' s ', which would be considerably greater than that observed for copper triglycine. In addition, Willix and Garrison~2~'have proposed that the reaction of e2q and Cu(gly): may generate .CH:CO:-, which absorbs at 350 nm'::', but was not observed in our study. Our results are compatible with the simpler mechanism of direct electron addition to the platinum center, equation (1): (1)

e2q + P t(II)---, Pt(I).

The decay of the transients by second-order kinetics and eventual formation of metallic suspensions is suggestive of a disproportionation

Ptll) ~- Pu I)-- Pt(O)- PtftI).

4.2 Reactions of H atom The reactions of both cis- and trans-Pt(gly): with H atom lead to the development of a series of intensely absorbing products (Fig. l/. The rate constants for the formation of the initial products are slightly greater than those for the corresponding reactions of e~ (Tables I and IlL and they are in general very large for H atom reactions such as for free glycine '~:. Both the spectral and kinetic data indicate that the products are different for the two geometrical isomers and also differ from those arising from the electron reactions. For both systems, the initial products appear to give rise to two subsequent transients, and metallic suspensions can be formed on multiple-pulsing. For the case of trans-Pt(gly):, the data support the following mechanism with steps !4) and (5) being apparent first-order (Table II) and step (6) being complex: (3)

H + Pt(II) -'+ H,

(4)

H, ~ H:

(5)

H:--' H~

(6)

H~PtI0).

While for the cis- system step {4) could not be kinetically resolved from step (3) owing to the smaller spectral changes, the mechanism appears to be of the same type. The foregoing observations may potentially be representative of either H-abstraction or H-addition reactions. Electron transfer appears to be eliminated as the products of the reactions of e,q and H atom differ. Free glycine reacts with H atom (and OH) via hydrogen abstraction at the methylene group to yield an aminocarboxyalkyl radical with an intense peak near 250nm '~*-:~. While the transients observed here may be metal complexes with ligand-radicals and/or their fragmentation products, it is notable that the platinum complexes react much faster than free glycine. Since complexation is anticipated to lower the electrondensity on the ligand and as the rates of H-addition reactions are frequently higher than those of abstraction "~', we would suggest that the initial products are ones involving H-adduct formation. Steps (4) and (5) may represent the loss of coordinated glycine, which would be promoted b~

A pulse radiolysis study of the reactions of platinum(II) glycine complexes in aqueous media

the acidic media. The complex kinetics associated with equation (6) would then reflect disproportionation reactions involving species with differing content of glycine. 4.3 Reactions of hydroxyl radical Both cis- and trans-Pt(gly),_ react rapidly with OH (Table III), giving rise to a series of intensely absorbing transients (Figs. 2-4). Of direct interest are the related questions of whether these species can be characterized as Pt(III) systems and whether or not degradation and/or loss of coordinated glycine occurs either initially or after the reaction of OH with the platinum glycinates. Considering the latter aspect first, hydroxyl radical reacts with the various protonic forms of free glycine to yield transients with peaks near 250nm {''''~). The mechanism appears to involve abstraction of H atom predominantly from the methylene group although at high pHs the evidence suggests that the unprotonated amine group may now be the preferred site ''9":}. Ammonia is one of the major final products "9~. Several features mitigate against the occurrence of degradation and/or loss of co-ordinated glycine in the reaction of OH with Pt(gly),., the principal one being that neither NH3 nor free giycine could be detected as a final product. The absence of free glycine is in accord with the observation that the transients for the two systems have similar yet differing spectral properties whereas they would be expected to be the same if the reactions of these species involved release of free glycine.* The close spectral similarities between the transients observed in this study and that from the reaction of OH and Pt(NH3), z'{z}imply that ligand attack at the methylene group of glycine probably does not occur. It is expected that complexation like protonation would lower the reactivity of the amine group towards H abstraction by OH. Based in part on the apparent lack of viability of a ligand attack mechanism, we would propose that the reaction of OH involves metal-centered attack with the resulting transients being platinum(III) complex ions in which the glycine remains intact and co-ordinated to the platinum center. Support for these species being platinum(III) entities is found in the fact that under certain conditions (Table VI) they can react with Cu(II) and Fe(CN)# ~*While the partial release of glycine from one co-ordination site might account for such spectral differences, it would be anticipated that such action would lead to the formation of free glycine at some of the pH conditions used here.

85

with the latter yielding Fe(CN)~ 3-, although the complicated kinetics observed in a number of instances imply that more than one platinum species is present and reacting with these substances. Such behavior is in agreement with previous reports on reactivity of Pt{III) materials towards oxidizing and reducing agents [see Refs (1) and (2) and Refs therein]. The observation that the longer-lived transients generally decay via secondorder rate laws is also suggestive of Pt(III) disproportionation to yield Pt(II) and Pt(IV) products. The mechanism of metal-centered attack by OH also appears to be operable in the reactions of Pt(II) amines "':}, Cu(II) and Ni(II) amines and glycine {2~-:7'. and Ni(II) ethylenediaminetetra-acetate (NiEDTA"-) ~:8~.While the subsequent reactions of the resulting trivalent metal complexes frequently lead to ligand degradation, it does not occur with the platinum amines and glycine systems or with NiEDTA:-. In these instances, complexation appears to afford a certain degree of radiolytic protection for the co-ordinated organic entities. The kinetic and spectral behavior of Pt(III) glycine species observed in this investigation bear a strong resemblance to that of the Pt(III) ethylenediamine and related amine systems :'}. In both instances the initial transients generally exhibit a single peak below 300 nm whereas the longer-lived and secondary species are characterized by two peaks absorbing above 300nm, and the interrelation between the various transients is strongly dependent upon pH. Owing to this dependence and to the greater number of species encountered in the glycine systems, detailed kinetic analyzes are not feasible; however, the following possible mechanisms provide consistent interpretations of the major features and parallel that of the platinum(III) ethylenediamine case. For trans-Pt(gly),., six species (T~-T6) appear to be formed whereas five (C~-C~) appear to be present for cis-Pt(gly),, if it is presumed that the changes observed at very long times are representative of relative stable final products. The initial product, T,, of trans-Pt(gly),, absorbs at 260nm, and at pH's below about 4.5 the following scheme is suggested (Table V): (7) (8) (9)

T~ + I"I"~ T3

TI "->T: T,. ~ T,

ks <~ 10~s-I k9 =2.4 x 10: s-'

(10) T4 + T:-~products k,o = 3.5 x 10~dm} mol-' s-'.

W. L. "VVAL'I-Zet al.

86

The rapid spectral changes at pH = 2.9 (Fig. 3a) imply not only the existence of T~ ( h ~ < 250 nm) but also that reaction (7) may approximate an equilibrium situation. Such a situation in conjunction with reaction (8) being slow can account in part of the progressively slower development of T : ( A ~ = 3 4 0 and 540nm) with decreasing pH (Table V). The species, T , which is representative of the second growth at 260 nm (Figs. 3a and 4). is considered to be different from T, on the basis that it is derived from T, and that its level of maximum absorption is greater than that attained by T,. The observations that the extent of growth of T: (and T~) decreases but its rate increases on proceeding towards neutral pH indicates the onset of reaction(s) competitive with step (8). Such competition may be associated with reaction (11):

further increase in absorption in this region, suggesting the formation of C: I~,~,< 250nm) in analogy to that for the formation of T:. The kinetics of the decay in this absorption (Table IV) appear to be related in part to the development of C~ (,~,~, = 330 nm). While this behavior is similar to that for the formation of T: with a peak at 340 nm, the subsequent decay of C~ does not Nve rise to a secondary-growth at 260nm as in the case of trans-Pt(gly):. The situation is complicated at p H = 2 . 9 by the presence of C:(A-~=360 and 520 nm) and its subsequent disappearance. The increasing development of C: with pH (Fig. 2b) indicates that it is a secondary product formed in competition with another species with the latter being formed at lower pHs, and the following reactions are proposed to account for this:

(11) T~+T,--~products k , ~ = l . 6 x l 0 9 d m 3 m o t - ' s t,

114)

which would also account for the second-order decay kinetics observed at 260 nm (Table V) near neutral pH, and by the onset of reaction (12):

(15)

(12)

Tt~T~.

On proceeding to basic media, the rates of formation and decay of T, tend to increase with pH. These changes, related in part to the formation of T_~ (h=,~=370 and 610nm), suggest that reaction (12) may be base catalyzed, and this type of feature has been observed in the case of platinum(liD ethylenediamine':'. The situation is made complicated by the presence of T6 above pH - 9. While this latter species seems to be formed in competition with T~, the longer term kinetic changes (Table V) suggest that its decay is concomitant with the further development of T~ (Fig. 3b). The decay of T~ obeys second-order kinetics: (13)

Ts + T s + p r o d u c t s

k13 =

6.5 x 10~dm 3 mol < s -~.

The general features encountered in the reaction of cis-Pt(gly): with OH tend to parallel those of the trans-system; however, the absorption changes of the former are less clearly resolved, particularly at low pHs (Fig. 2). The initial product (C,) in the cis-case absorbs in the region of 250-300nm although its peak absorption may be at lower wavelengths. At the lowest pH (pH = 2.9), there is a *For the platinum glycine systems, a possible tetrahedral structure can be ruled out as the transients for the cis- and trans-forms would now be expected to be the same.

C.-H-+C, C,--'C:.

The linearity in the plot of e~pp{pH--- 61: e,op(pH < 6) for C: vs proton concentration is in agreement with the above mechanism, and the resulting slope of 9 x 103dm3mol ' yields the ratio of k~:/kEs. Assuming that the value of 2.2 × 10t s-' for the growth of C: in neutral and basic solutions and at high concentrations of cis-Pt(Ny): (Table IV) is indicative of k,~, then k,~ is estimated to be of the order of 2 x 10"~dm-~tool -~ s -~. At pH = 10, the spectral and kinetic changes (Fig. 2c and Table IV) indicate the presence in the early stages of an additional transient, C~. This situation resembles that encountered with trans-Pt(gly): and as such the decay of C~ could contribute to the development of C: although the decrease in the apparent absorption coefficient of C: at high pHs may indicate additional reactions of C,. The long-term behavior of C: involves its disappearance via second-order kinetics (Table IV): (16) C: + C:-+ products k ~ = 2.0x 10~dm~mol "s". Considerable attention has recently been focused on the question of the compositions and thus structures of Pt(III) transients containing halide and/or organic amine ligands"-"-'9->. Various structures including distorted octahedral, square planar, square pyramidal and trigonaI bipyramidal have been proposed, and it has been suggested that Pt(III) entities may exhibit several of these structures.* Several features of this study are germane to these issues. As no free gl,vcine was detectable as a final product and thus presuming

A pulse radiolysis study of the reactions of platinum(II) glycine complexes in aqueous media that the reactions of the transients are not ones involving opening and closing of the ligand ring, it is c o n c l u d e d that no one structural type can adequately account for the number of o b s e r v e d transients. This lends credence to the earlier proposals that Pt(III) c o m p l e x e s can manifest different structures. The general spectral and kinetic features of the transients derived f r o m the reactions of cis- and trans-Pt(gly), with OH are not only similar to each other but also to the platinum(III) transients containing saturated organic amine ligands such as ethylenediamine"-" although the similarity is less with the platinum(III) halide c o m p l e x e s ":9-3". Such c o r r e s p o n d e n c e b e t w e e n the glycine and amine systems suggests basic similarities in compositions and structures. In the case of the platinum(III) organic amines and in particular the ethylenediamine system, where a more c o m p r e h e n s i v e kinetic analysis was feasible than that here, arguments were presented to support the proposal that the initial products, which exhibited single absorption peaks below 300nm, were six coordinate c o m p l e x ions of distorted octahedral geometry"-". The effect of p H was attributed in part to the presence of water and/or hydroxide ion as ligands in the labile fifth and sixth positions. For the longer-lived intermediates having two absorption peaks above 3 0 0 n m , it was p r o p o s e d that they were five coordinate species with the possible limiting forms of square pyramidal or trigonal bipyramidal. In the main, these contentions appear to be applicable to the glycinato complexes. Both limiting structures of square pyramidal and trigonal bipyramidal can account for the similar yet differing characteristics b e t w e e n the cis- and trans-intermediates exhibiting two peaks a b o v e 300nm. Because of the uncertainty as to whether C, is similar to T2, T~ and C,., it is not possible to distinguish b e t w e e n these possible structures; however, it is n o t e w o r t h y that the trigonal bipyramidal type of structure would be e x p e c t e d to generate a two to one ratio of such species in f a v o r of the trans-system whereas a square pyramidal structural type would yield a one to one correspondence. Acknowledgements--We wish to express our appreciation to the National Research Council of Canada for financial aid, and to the Atomic Energy Control Board of Canada for use of the accelerator facilities. It is also a pleasure to acknowledge the contributions of Drs. R. L. Eager and D. K. Storer at various stages in this study. REFERENCES 1. D. K. STORER,W. L. WALTZ,J. C. BRODOVrrCHand R. L. EAGER, Int. J. Radiat. Phys. Chem. 1975, 7, 693. 2. J. C. BRODOVlTCH,D. K. STORER, W. L. WALTZ and

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