Inhibition, antiinhibition and enhancement of positronium formation in some different hydrocarbon—solute mixtures

Chemical Physics 66 (1982) 227-235 North-HoIkmd PubIisSngCom~vly

INDIBFTION, A.NTlNDIBITION AND ENHANCE.MENT OF POSITRONDJM FORMATION M SOkE DIFFERENT KYDROCARRON-sOLLJTE MIXTURES Caran WIKAWER Department

ofPhysical

Chemistry.

Universily of Ume;. S-901 87 Umeg, Sweden

Received 19 October 1981

Positron snnihiktion measurements have been performed in non-polar solvents with additions of ChHsCI. C&sCH2CI, _ _ CePra/C& and three different amines, DhfA, TPi and TMPD. Thehalogen derivatives of benzene behave Ii&etyypicalkhibitors. In the ‘two-solute” system it appears that CeFr2, which earlier has been shown to be a weak inhibitor, acts as an anti-inhibitor, when mixed with CC& in cyclohexane soltuions. The presence of ihe amines results in a powerful enhancement of the O-PSyield in CeHrr while this effect is strongly diminished in benzene solutions. The results ate discussed and interpreted in terms of the spur reaction model of Ps formation.

1. Introduction During the past five years, the positron annihilation technique, especially lifetime measurements, has been used in our laboratory to investigate and characterize the influence of halogen substituted organic molecules on the formation of 4% in different nonpolar hydrocarbons [ 11. The results of these investigations have been discussed and treated in terms of actual theories for Ps formation viz. the “hot Ps model” [2], the “Ore-gap model” [3] and the “spurmodel” [4]. Upon adapting experknental

data to equations derived from the

dif-

ferent theories it appeared that in terms of the “spur model” it was possible to give plausible qukntitative aa well as qualitative explanations for many obtained parameter values from a molecular as well as radiation chemistry point of view _ Furthermore, the parameter descriiing Ps inhibi-

tion in terms of the “spur .model”, was the only one which could be directly related to properties of the additives as well as the solvents. With regard to application of the “hot Ps model”, we found that for most of the systemsinvestig~ted the obtained parameter va!ues in many csses became unrealistic and also impossible to interpret on a molecular b&s_ Because many of our resuits hitherto 0301-0104/82/0000~0000/.5

presented, f&our .zn interpretation of positronium formation from a ‘kpur model” point of view, we consider that further investigations have to be performed in order to clarify the interaction between primary radiation induced species, electrons as well as positive holes, and solutes in different non-polar liquids. in one of our previous papers [ le] , where the influence of different bromobenzenes on the yield of o-Ps was investigated, we found a linear correlation between the inhibitor constant and the rate constant for electron scavenging. The inhibitor constant, Q, stems from the following expression

02.75 0 1982 North-Hohand

(1) which is normally used by us in order to quantify our inhibition curves. The above expression is based upon the assumption .that the Ps yield is proportional dative

to the relative fraction of geminate electrons which escape scavenging by electron scavengers introduced in the solvent [4b]. From the above equation, Q, might be expected to be equal to the reactivity parameter, 4, in the “scavening” function F(C) = (o&)1/2 / [I + ((u,Cj1!2 ] ) which has been introduced in radiation chemistry by Warman et al. [S]&e andI corresponds to the o-i’s intensity in the neat solvent snd at a certain solute

228

G. IVikatrd~./Pcx&onim

formation & hydrocarbon-solute

concentration, while fl is an adjustable parameter. Just recently Casanovas et al. [6a] have published electron scavenging rate constans of C,H5 Cl in different solvents. In order to see if the same relationship also holds for C6HjCI we decided to extend our earl& inhibition-data [lb] by investigating the in&ibitor efficiency of C6H5CI in neo-C6H1? and nC$=+4. in another paper Grob and Casanovas [6b] concluded on the basis of steady-state scavenging measurements that benzylchloride, which is a very efficient e!ectron scavenger, should trap electrons nondissociativeiy. According to thGs statement we expect that C,H, CH, Cl, similarly to C,F, , CS, and biphenyl, which are known to scavenge electrons nondissociatively . either will enhance or gve a minimum in the o-?s yield. However, earlier Hagemann 2nd Schwan [7] have shown in pulse radiolysis experiments that benzylchloride undergoes dissociation upon electron attachmenr, creating benzylradical and chloride ions. According to these results we expect that C,HSCHZCl, similarly to C6HsCl and C,H,Br should act as a powerful inhibitor. In order to see if it is possible to use the Positron-annihilationmethod as a probe to judge which of the above statements is the right one, we considered to start a detailed invesrigation of the influence of Cs H5 CH2 Cl on the o-Ps yield in CgH12. Our earlier investigation concerning mixtures of C,F12/C6H,, [lfl has been extended_ In the present paper we report results from experiments when a second solute, viz. CCI,, is added to the system CSF12/CSH12. Recently LCvay et al. [S] investigated the enhancement of o-Ps formation in C6H12 due to scavenging of fast positive holes by pyridine. In previous investigations, Warman and co-workers [9] with the’aid of pulse radiolysis as well 2s steady-state radiolysis have determined rate constants of positive hole/electron scaven_tig of Nddimethylaniline (N,N-DM_4), tetramethylpara-phenylenediamlne (TMPD) and tripheny!amine (TPA). These compounds scavenge positive holes at almost the svnc rate. However, N,N-DMA and TMPD have low reactivity towards electrons, while that of TP.4 is comparatively high [9b]. According to the interpretation by Ldvay et al. it is expected that these substrates will increase the o-Ps yield in C6Hl2, and act as weak inhibitors in CsHs. In order to see whether or not tNs i_nterpret;ilion

mirtures

holds, we decided to investigate the influence of these compounds on the yield of o-P:, in C6H,, , where fast ho!es are created and CgHg where no fast holes have been detected.

2. Experimental The expertiental procedure for obtaining lifetime spectra has been described before in earlier reports from this laboratory [I].

3. Results All Iifetime specrra of the systems investigated could be resolved into three components, with lifetimes rO, rl and ‘i-, and the corresponding intensities Zo,Zl andZ2, where TV andZz have been attributed to o-Ps. We found that rz in one 2nd the same solvent was almost unaffected by the presence of different solutes, which implies that thermalized Ps atoms do not react with the additives under investigation. Otherwise considerable quenching effects caused by the solute would have been observed in the lifetime spectra. On the other hand we found that the intensity of the long-lived coimponent,12, was markedly influenced upon increasing addition of substrate molecules. The presence of a solute in many cases gave a decreased yield of oPs as compared to the neat solvent. However, for some systems we also obtained an increase of o-Ps yield. This enhancement was observed at small solute concentrations (lo-’ M). In fig. 1 the vzriation of the o-Ps intensity as a function of the solute concentration is displayed for the solvents n-hexane, ‘I,2dimethyibutane (neohexane) 2nd cyclohexane with admixtures of C6H,CI 2nd C,H,CH,Cl. The two solutes behave like typical inhibitors ca&ng no mjnima ins the o-Ps yield. Fig. 2 shows the o-Ps yield for the system CgH12/ CsFr2, where three different, but constant, initial concentrations ofC6F12 have been used, and the effeet of in&easing concentrations of a second solure, viz. Ccl4 +can be discerned. Upon comparing the resuits from +&ese “two-solute” systems with those from the “one-solute” system (CC1,/C,H12) it appears. that a distinct zlrhough rather small anti-inhibition

effect is

G. Wikarder/Posi&vnium formation in hydrocmbon-soiute

mlrrures

229

0 0

O=C&Cl

in

o=C~H,CH,Cl

n-&H, I” C,H,,

A

I

0

3

1

0.001

0.005

001 C/n!d

0.05

0.1

05

1.0

en-=

o-Ps yield as a function of inhibitor concentration in nC6H14 , nenCgH~~ and C6H12. The curves are drawn according best fits of experimental data. The uncertainties are roughtly -+O.SA. Fii.

1.

O=0.2 K

to

&F,

A=O.l M &Fu

O=OOS K &F,,

e=ca,

I

( P-J= W,,

h

0

a005

001

-005

0.1

OS

1.0

[CU.]/mclhi’

Fig. 2.02s yield as a function ofCC&-concentmion in CgK12, witi-~three tifeerent initial concentrations of CeF,s- The CUI-WS are drawn according to best fits of experim&al data. Thd uncertainties are roughly +0.5%.

230

Fig. 5.04’~ yield as a function of C&,,-concentration in C6H12, with two different initial concentrations are ctmvn according to best fits ofexperimentaI data. The uncertain&s are roughly 4.5%.

cbtained when the other solute, C,F,, t has been added to the solvent. in fig. 3 we display the o-E’syieId for two different, although constant, initid concentrations of CC14 in C6H12 and the effect ofincrensing concentrations of C6 F12 in these two systems. From the figure it appears that a distinc: enhancement of the G-Ps yieId is

ofCC4.

The curves

the o-Ps yield. However, the effect is smaller than that obtained for C,H,, _We also fmd that for solute concentrations tess than C = 1 M, there is almost no influence of NJJJXUA on the o-Ps yield. Eiowever,

r

obtained when the concentration of the second additive is increased. Fin&y in fig. 4 we display the influence of N$DMA, TPA and TMPD on the yieId of 02% in C,H,,

and C,H,. TPA and TMPD are solids at room temperature and due to their limited solubility in CGH,, and C,H, the concentration range of these two solutes is r~rher small. in order to increase the solubility of TffPD in C,H,, runs were made at 43OC. At &is kmpemture the largest concentration of TMPD appeared to be approximateiy 10 times higher than that obtained at room temperature. However: from fig. 4 it appears that the three additives have a positive inEusncc on the o-Ps yield as compared to neat C6H12. For !ow solute concentrations (C < 0.1 M) we find :~IJ: the capability of en!!ancing the o-Ps yield is comparable for N,N-DMA and TMPD, while that of TPA ;tp~~Is to be somewhat higher. For NJ%DMA we obtai!: a maximum in *he OPS yield at kO.4 M. A further increase in the Nfi-DM.4 concentration results in 3 t&t decrease in the o-Ps yield. In C6H6, on the orl;er hmd, we find that only TMPD and TPA enhance

hi,

?S&.4,oPs yield as a function of solute concentr2tion in C6KU and C$&. The curves are tidal fits. The uncertainties are roughly t 0.5%.

when this cdncentration of N,N-DMA is exceeded, it appears t&t the solute behaves I.&e a we& inhibitor_

4. Discussion For most of the systems investigated, small solute concentrations have been used. The reason for this limitation of the substrate concentration is as follows. From a physicochemical point of view; it is often more realistic to use non-polar solutions with small admixtures of solutes upon investigating the infmence of additives on the formation of o-Ps. Too concentrated solutions will most probably influence the structure of the solvent and in this situation effects caused by the solute are not investigated but rather the behaviour or Ps in the solute, diluted to solme extent with a non-polar hydrocarbon. From radiation chemistry it is well known that the ener,9 deposition of bw energy particles in diluted solutions almost exclusively occurs in the solvent molecules [lo]. This means that the primary species which are generated when injected positrons are thermalized in the solutions, originate from the solvent and not from the solute. In addition a high concentration of the additive means that the solute molecules themselves will contribute considerably to the primary processes of radiolysis and will also most certainly affect rate constants between electrons and solute molecules, as well as between electrons and positrons. The connection between early processes in radiation chemistry and the nature of the

creation of Ps ato.ms in liquid media -was fast proposed by Mogensen in the “spur model” [4a]. At that time most of the results obtained from lifetime measurements, e.g. inhibition curves, were explained in 2 qualitative w&y and no equation was deduced for the quantification of experimental data. However, some years ago Uvay and Mogensen [4b] attempted to quantify inhibition curves by using eq. (l), which is based upon the assumption that the relative fraction of geminate [l 1J electrons, which escape scavenging by eIectron acceptors introduced in the solvent, should be accessible for Ps formation. Eq. (1) has been fitted to experimental data (Cd 0_2 M) for the systems displayed in figs. 1 and 2. The results of the fittings, as well as eaflier results from investigations concerning monohalogenated benzenes, are displayed in table 1. Using the simple ion-pair mode1 for electron scavenging, the constant crs is found to be equal to k/A, where k is the bimolecular rate constant for electron scavenging and h is the frequency for geminate recombination [12]. An attempt to elucidate the degree as well as the magnitude of the variation of A in different solvents was recently performed in a theoretical paper by Crumb and Baird [13a]. These authors found a slight variation in h for different non-polar organic solvents, all of the vaIues being of the same order, viz. lOI s-l _ The only exception was benzene, where the recombination frequency appeared to be somewhat higher, 2.5 X 1Ol3 s-1. Since eq. (1) is based upon the same phenomenological model as that out-

Table 1

Positronium inhibition parameters zccording to eq. (1) of tivestigated systems and rate constants for electron scave&

of ChH$1

and C6H5CH2Cl

LY(K’)

System

10.3 z 1.4b)

0.64 f 0.62 f 0.41+ 0.30 i

15.7 _+0.4

0.65 2 0.02

30.0 r 0.2b) 22.5 i; 0.6 20.6 I 0.9

0.82 f;o.olb)

16.3 2 0.6

0.63 * 0.04

9.1 f 0.4 8.2 + 0.2 4.0 ko.gb)

C&CHzCi/C6Hn CC4lCsH12

0.05 M

CsFIZ f CCL&H15 0.1 bl CsF,2 + CCWC~HIZ 0.2 hf C6F,2 + CCI+/CsH12 a) See ref. [6a].

b) See ref. [lb].

&i,X

B

@ See ref. [6b].

0.03 0.02 0.07 b) 0.04 b)

0.69 c 0.03 0.63 + 0.05

10

lo-‘2 IW-’ s-1) *Ia)

2.9 + 0.3 a) 4.9 + 0.5 a) 9.0 k 0.9 a) 3.85C)

G. Wikzrzder/Pojitronium

232

formorion in hydrocarbon-solute

mirhues

weakest bond of the substrate molecule must also be of importance for the additive when acting as an inhibitor. In a paper published by Grob and Casanovas [6b] it is concluded that CsH5CH2C1 is a powerful electron scavenger with ke-cg~2~ - 3.85 X 1012 M-1 s-1 in

Fig. 5. Correlation between Ps inhibition parameter (a) and rate cocstant (k) for electron scrtveng@ of monohdogeilated benzenes in different non-polar solvsnts.

lined by Schuier et al., one should expect that a would reflect the electron scavening ability of the solute, protided thar X does not differ too much between different solvents. In fig. 5 we show rhe correlation between k and a-values of different solutes as obrained from eq. (1). For comparison earlier data for @.Y as well as dBr are included [lb,e]. From this figure it appears that the correlation between rate constant data of monohalogenated benzenes and the inhibitor parameter. a, is raiher

poor.

Most striking is the fact that althoguh kc_+oct values in C8H,, and neo-C6Hi, are considerably greater than those for @Cl in C6KlZ and ~z-C~H~~ as well as for @Br in CBH12 and ,r-CgH14: we find that gC1 is a rather moderate inhibitor. From gas-phase data [ !3b] as well as pulse-radiolysis investigations it is verified that @Cl and OBr react dissociatively upon encounier with electrons. The bond strengths of the actua! substrates displayed in fig. 5 are 3.7 eV (@Cl) [I&] snd3.;17eV@Br) [15].Holroyd [16] hasassumed that a dissociative attachment reaction most generally proceed through an intermediate excited mo!ecuIar ion-complex: .AB+e-+AB”--tA+B-.

Onz reasonable explanation

for the low correlation between inbibiror constants and rate constants displayed in fig. 5 might be that in addition to the rate constant of electron scavenging the strength of the

WI,,From steady-state scavenging experiments these authors claimed &at the electron scavenging process of this substrate should be non-dissociative, due to the fact that no characteristic products, neither benzene nor toluene, could be found from irradiated cycIchexane-benzylchloride mixtures. On the other hand, Hagemann and Schwarz [7] found from pulse-radiolysis experiments that benzylchloride dissociates upon eiectron attachment with quantitative formation of benzyl radicals. From fig. 1 it appears that C,H,CH,Ci is a rather strong inhibitor in C6H12, even more powerful than e.g. $21 in neo-CgHZ4. The influence of C,H,CH,Cl on the formation ofo-Ps in CgH12 is also very similar to that obtained for C,H,Cl, and as mentioned earlier this latter substance is known to scavenge electrons in a dissociative manner. Taking the above facts into consideration it is apparent that benzylcbloride reacts dissociatively with spur electrons in accordance with results from the experiment performed by Hagemann and Schwarz [7], and apparently the conclusion drawn by Grob and Casanovas [6b] must be wrong. Recently Robinson and Salmon [ 171 with the aid of pulse radiolysis measurements determined the rate constant for electron scavenging of C, F,, to be 7 X 10” M-l s-1 in C,Ht2. From steady-state scavenging ,technique measurements it is well known that C6 F12 is a powerful electron scavenger, with us = 30 LM-i in C6H,,, producing C,F,,H when used as electron scavenger in C6H12 [18]. Earlier Holroyd et al. estirZed the rate constant for electron scavenging of CCI, to be 2.7 X 1012 M-1 s-l in C,H12 f16]. From earlier experiments [lb], we know that Ccl, is a powerfill positronium inhibitor. Assuming that C6FL2 scavenges electrons dissociatively, we expect to find that an addition of both of these electron scavengers would increase the inhibition of o-Ps, compared to the result obtained when only CC!, is used as inhibitor. On the other hand, if Cg FIZ reacts nondissociatively with electrons we expect to find an anti-inhibition effect.

G. IVikanderjPosifronium formanin in hydrocarbon-solute

From fig. 2 it appears that a distinct anti-inhibition effect can be discerned when CC14 is mixed with C6F12 in C,H,,. This is even more pronounced in fig. 3 when the concentration of CC14is fixed and the concentration of C6F12 is varied. Upon adapting experimental data from fig. 2 to eq. (l), we find that %ccl, decreases from 30 M-1 in neat C6H,, to 16.3 when 0.2 M C6 F12 has been added. Even such a small concentration as 0.05 M C6F12 reduces *cl4 to 22.5 (cf. table 1). In a previous investigation, Kennedy and Hanrahan [I91 assumed that the thermal C-C6Fil radical could hardly be precursor of C6FllH, but rather thermal electrons or negative ions are included in the creation of the end-product C6F,,H when the system C6F12/ C6H,, is exposed to yradiation. In our earlier investigation of the influence of C6Fl.3 on the yield of o-l’s we made the assumption that C6F12 scavenged spur electrons undissociatively [ 1fj . With the results from figs. 2 and 3 we consider that the assumption of molecular anions is confumed. With regard to the fate of released spur electrons it is most probable that the following competitive prccesses take place in the spur after its thermalization e- + C6H& + C6Hjj2 (recombination) e-i-C6F12+CgFi2,

,

(0 01)

e- t CC14--f CCI; t Cl- ,

(III)

e-•e++Ps

@V)

eitC6Fiz+Ps+C6F12, e’+Ci--+

[Ccl-,e+] _

03 (vr)

Within this scheme, the efficiency of C6 F12 as an anti-inhibitor is primarily determined by the competition between processes (II) and (III). The o-Ps formed from reactions (IV) and (V) gives a contribution to th long lifetime (3 ns). At present it is known that as soon as the halide ion, Cl-, has been formed, the electron is so tightly bound that it wi!l no longer be accessible for the positron to fom Ps [ZO] . This means that the complex [Cl-, e+] which can be considered to be 2 type of coulombic ion-pair, does not contribute to the long lifetim::. Instead it has a lifetime comparable to that of the free positron (400 ps). From our experimental

233

mixtures

data it appears that the anti-inhibition effect is observed at comparatively low relative concentrations of C6F12* This indicates the following: (a) The rate of reaction (II) must be comparable with that of (III), which is also experimentally proved in the investigation performed by Robinson and salmon. (b) The lifetime of the molecular anion, C6 FiI: must be greater than the time for creating Ps in the spur,i.e.> 10 ps. This might be verified with the aid of picosecond spectroscopy [2 11. The last part of this discussion will be devoted to results obtained when N,N-DMA, TPA and TMPD are used as additives. From pulse-radiolysis investigations performed by Warman et al. [9a,b], it is confirmed that this type of compound has a high reactivity towards positive holes. Rate constants for positive hole scavenging as well as electron scavenging of these substances are summarized in table 2. The ionization potentials of these additives are rather low (see table 2) compared to those of C6H,, and C,H,, 99 and 9.25 eV respectively [15]. Warman et 21. [22] have also established that fast positive holes are generated in C,H,, upon irradiation, while the existence of such mobile species has never been detected in irradiated C6H6. The reaction mechanism between these amines and corresponding positive holes is charge transfer. The mobility of the positive hole is considerably greater than that for a molecular ion 1221, which means that the product of the charge-transfer reaction e.g. DMAf, has a reduced rate of scavenging spur electrons_ Besides, it is also verified that N,NDMA and TMPD are poor electron scavengers, while I’PA appears to be rather effective (see table 2).

Tabie 2 Ionization potentials and rate constants of positive hole/ electron scavenging of used amines SOlUte

k,,x?o-?'

&I-’

N,N-DMA TPA TMPD

s-1)

2.9 + 0.4a) 2.8 b) a.5 b)

k,-+,x

10-r* (Xi-1s-1)

I* (ev)

cQ.38 a) co.1 c) 18”)
7.2 a) 6.9 d) 6.5 e,

a) See ref. [9a]. b) See ref. [9b]. c) Seeref. [6a]. d)See ref. [23]. e, See ref. [24].

234

G. IVikanderjPosirmnium formatibn in hydrocarbon-solure mixtures

‘!&king the above fac?ors into consideration, assuming that ‘Ihe positron, which is a light particle, has a comparatively high mobility (the mobility of this antiparticle hasnot yet been determined),we consider that it is probable that the following processes are irnportant in the terminal spur of C6H12, and consequently have a direct influence on the yield of o-Ps {cf. the discussion in ref. ES]).

CgH& + e- -+ C,H,*, (recombination) C6Hf2 + A + A+ + C,H,,

,

(charge-transfer) ,

(I’) cu’,

e-+A++A*.

(III’)

e-+e++Ps,

W’)

Addition of arnines to C6H12 diminishes the importacce of (I’). Since A+ has 2 lower mobility than CsG2, the relative rate for the light positron to scawnge electrons will increase and consequently the oPs yield must be enhanced compared to neat C6H12. From fig. 4 it appears that this effect is clearly discerned at small concentrations of the additives (C< O.! M). From this figure it can be seen that TPA is masr effective as enhancer and this is possibly due to the fact that this molecule also has effective electron scavenger properties, which means that the following competitive reaction takes place in the SpUi TPAt-e-+TPA-.

WV’)

However, this captured electron is certainly delocahzed and loosely bound to the TPA molecule. Further, the lifetime of this moIecular anion must be of the order of or greater than that for the creation of Ps, so that the positron can remove the electron and form Ps, i.e. ef+TPA--+Ps+TPA.

W)

Otherwise a smalier enhancement exerted by TPA compared with the other amines should have been observed_ This iast argument also holds for C&,-soiutions where we notice that the presence ofN,WDiMA and TMPD has very Iittle influence on the o-Ps yield, while TPA in this solvent stih acn in an enhancing manner in accordance with reaction (VI’). From fig. 4 it also appears that even if the temperature is raised, which should mean that the ability of TMPD to scavenge electrons increases, TPA is most effective in enhancing the 02s yield in C6H6. If we instead consider the electron as the fastest

particle and the positron as w&Uas the molecular cations are slowly _moving particles, the above mentioned reaction scheme does not hold for l$H,, so!utions. One possible secondary effect which might contribute to the early processes proceeding in the spur is the fo?.lowing C6H& + A + C,H,,

f Ai f e- ,

i.e. energy transfer from excited CsH12 molecules to the used amines. The existence of such excited shortlived fast moving states has recently been detected by Tabata et al. [36] _This secondary reaction should be reasonable due to the fact that the amines used in the present investigation have low ionization potentials (see table 2).

Earlier Warmanet al. [9b] hwe found that addition of TPA, _NJLDMA and TMPD to CGH,, solutions containing CH3CI, increased the yield of methane, which might be due to the interaction between excited states and the amines. An experiment in order to verify if the enhancement in C,H,, is due to secondary effects would be to use NH3 as a positive hole scavenger. This molecuie has a high ionization potential and does not react with electrons [Sal. Experiments of this kind will be performed in the near future. As concerns the N,N-DMA curves, we notice that a maximum in the b-Ps yield of CSH,, is obtaked for C = 0.4 M and with a further increase in the DMA concentration we fmd that DMA acts as a weak inhibitor. This pattern appears also for C6H6-solutions when the concentration of DMA exceeds 1 M. It is tempting to suggest that this should be an effect of cluster formation of DMA-molecules, which like akohol clusters should trap electrons. This is one of the explanations used by JXvay et al. [S] to explain the fact that pyridine acts as a weak inhibitor above a certain concentration limit. In fact the shape of the oPs yield curve of DMA-C6H12-solutions is very similar to that obtained for pyridine-C6H12soliitions . However, since no investigation of the radiation chemistry of N,N-DIMA hitherto has been presented we wili not speculate further in what happens in the high concentration range.

C. Wikander/“osirronium form&ion

Acknowledgement The author than!! Ole IMogensen and John Warrnan for fruitful and valuable discussions during the preparation of the manuscript. It is a pleasure to acknowledge the help of Miss Lilly SjMander for technicaI assistance as well as the origina! drawings. This work was partly spor?sored by grants from Swedish Natural Science Research Council.

References [l]

(a) G. Wikander, Chem. Phys. Letters 43 (1976) 344; (b)G. Wikander, Chem. Phys. 38 (1979) 181; (c) G. Wikander, Chem. Phys. 39 (1979) 21; (d) G. W&ander, Chem. Phys. 39 (1973) 309; (e) G. Wikander, Chem. Phys. 49 (1980) 153; (13 G. IVikander, Chem. Phys. Letters 77 (1981) 120; tg) G. Wikander,Chem. Phys. Letters 80 (1981) 361. [2] (a&S.J. Tao and 1.X Green, J. Chem. Sot. (A) (1968) (b) I&.

Wild and H.J. Ache, J. Chem. Phys. 65 (1976)

[3 j Fl7V.L Goldanskii, At. Energy Rev. 6 (1968) 3; (LQV.I. Goldanskii, O.V. Koldaeva and V.P. Shantarovich, Khim. Vys. Energ. 8 (1974) 124. [4] (a) OE. Ilogensen,Chem. Phys. 60 (1974) 998; &I) 3. L&y and OE. Mogensen, J. Phys. Chem. 81 (1977) 373. [5] J_M. Warmen, K.-D. Asmus and R.N. SchuLer, J. Phys. Chem. 73 (1969) 931. [6] (a) I. Casanovas, R. Grab, D. Gretot, 3. Blanc and Z. &fatthieu, J. Electrost. ? (1979) 227; (b) R. Gmb and J. Casanovas, Radiat. Phys. Chem. 15 (1980) 325. [7] RJ. Hagemann, and H.A. Schwarz, J. Phys. Chem. 71 (1967) 2694.

in hydrocarbn-solute

mirstures

23.5

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