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Radiation-generated cations of 1,1-diphenylethylene and their kinetic behaviour at low temperatures
0146,-5724/~ $3.00+ 0.00 Cop~'~t ~ t990 i'enWaoe ~ ptc
Ra~knt. P/Ws. C&tm. Vol. 36, No. 6, pp. 715--719, 1990 Ira. J. I t a ~ t . ,4~1. Imteww., Part C Printed in Great Britain. All rights reserved
RADIATION-GENERATED CATIONS OF I,I-DIPHENYLETHYLENE AND THEIR KINETIC BEHAVIOUR AT LOW TEMPERATURES J. SCHMIDT,1 U. DECKEIt,t J. TEPLY,2 J. ig~s' and H. MAC *Central Institute of Isotope and Radiation Reaearch (CIIRR), Permoter~r. 15, 7050-Leipn$. G.D.R. and ZN~_~eJ__~Research Institute (NRI). l[el~ C.S.SR. (Received 27 N 0 q ~ , r 1989)
~---C~tions of I,I-diphenylethykne (DPE) we~ studied in electron-scavenger matrices in the temperattme rang 90-130 K. Nanmecond and micrmecond pulJe-radiolym, with optical absoq)t,on spectrmcopy in tim~ from 10-s to lO-~s was used. The bands of the aboorl~on spectra are m ~ on the b a h of • simplified reaction mechanism and kinetic cak'ulahons. Reaction rate constants for the dimeri~tion of the radical cations of DPE are given.
INTItODUCTION
IIgSL'LTS AND DgSC~SSION
in previous studies the absorption of DPE cations was investigated by pulse radioly$i$ at low (Schmidt el al., 1985) and room temperatures (Brede et al., 19824 H a y a ~ et al., 1978), and also by steady-i~tate experiments at 77 K (Hamill, 1968; H a y u h i e t a / . , 1978; ShJda and Hamill, 1966). The definite assignmerit of certain absorption bands is partly contradictory, and kinetic data are not available for low-temperature experiments. To extend our pulse radioly$is investigations on aromatic vinyl monomers ( ! ~ et al., 1985), we tried to get more information on DPE cations at low temperatures by kinetic analysis. By measurements using the pulse radiolysis apparatus in the NRI. [~et ('repl~ et aL, 1981), the detection times were increased from 0.05 to 20 ms, and so it became possible to observe temporal changes of the transient absorptions at temperatures lower than 110 K. Using these extended temperature and time regions and kinetic calculations for different absorption bands, further results concerning the cationic species of DPE are discussed.
Figure I presents the tran~ent absorption spectra of 0 . 1 5 m o i d m -3 DPE m BuC1 at l i 0 K at three different times after end of pulse. According to Brede et aL (1982), Hamill (1968) and Shida and Hamill (1966), tbe maxima at 390 and 520 ran are attributed to dimer radical cations "DPE2+ , the bands with maxima at 1000 and 550nm to monomer radical catiom DPE "+, and the maximum at 340 tun to the absorption of DPE radicals DPE" (Schmidt et aL, 1985). Transients of the BuCI matrix (Arai et al., 1976), according to the reaction
EXIPre.ilIMENTAL
The synthesis and purification procedures of DPE and of the electron-scavenger matrix /-butylchloride (BuCI) were described in Schmidt e t a / . , (1985). The same cryostat system (Schmidt et al., 1985) was used for me~___$utements at the ELITtype ~__~_terator (8 and 40 ns pulses with 30 and 70 Gy/pulse, respec~vely) in the CIIRR Leipzig. and at the 3.7MeV Testa ~___c~,;erator (2.5~s pulse, 125 Gy/pulse) in the NRI, l~d. Details o [ t h e optical detection system are given in Sclunidt et aL (1985) and Tepid, et al. (1981). t~: ~,.-<
BuCI ,,~ h" +, BuCI" +, B u ' , CI -,
(I)
where h" + are highly mobile holes and BuCi" + relaxed radical cations of the matrix, were not observed because of the relatively high DPE concentration. The product of the charge transfer reaction from the matrix to the vinyl monomer (M), h "+, BuCI'*, Bu + + M --, M'+ + BuCI, Bu"
(2)
was detected at ~ > 1000 and 550 nm. The t i ~ l v e d formation of these transients was not observed under our conditions and the __d~'~__y of these bands was only detected at temperatures ;~ IIOK. The curves in Fig. 2 describe the experimental decay of the 1000 nm band at 100, 110 and 130 K. Possible reactions for the monomer cation decay are dimerization (3) and neutralization (4):
M"+ + M M'* + C1715
kD
. "M~"
(3)
, products.
(4)
716
J. SCIO~DT et aJ.
I II
0.2
,I
0.1
/
~1
~',l
1
400
,, .~_
_.~_
_ __0.__
600
=. __,,_?-
-
-
-'z,
-~--a-
800
I
1000
X(nm)
Fig. I. Absorption R)ectra of 0.15moldm ~ DPE in BuCI +0.lSmoldm ~ CCI4 irradiated with 8ns electron pull¢~ at I I O K , immediately (O), 10ps (O) and 45ps (/X) al'tcr the end of the pulse. More detailed investigations of the time behavior of the NIR band at 100Onto did not confirm the pseudo-first-order decay mechanism discussed in Schmidt et aL (1985). Good fits could be obtained by solving the differential equation (5) neglecting the recombination reaction (4):
~[M"] - -
= - k D [ M " ] ( [ M ] - [M"]).
~0~.,
<~
(5)
The term ( [ M ] - [ M " " ]) describes local redL)ction o f m o n o m e r concentration, [IV[], by the f o r m a t i o n o f m o n o m e r radical cations, M " , The solution o f equation (5) gives
[M" ']
[M0"
[M] ][M0" ] + ({M] - [M'-])c,*Dt~ '
(6)
with [Mo" ] as monomer cation concentration at pulse end. The calculated decay curves for 1000 nm are plotted in Fig. 2. The coincidence of calculated and experimental curves is very good a[ temperatures lOOK
O~
0
Fig. 2. Time bcbaviour of the 1000nm band of 0.15moidm "~ DPE in BuCI + 0 . 1 5 m o l d m ~ CCI~. (O) Estimated valu~ according to (6); (O) Estm~uxl vllues ~cording Io (7) ~ t h = - 0.828 and B -. 1.56 x 104 for I I O K . = = 0.560 and B - 868 × 10~ for 130 K.
Radiation-•eta'ratedcations of l,l-diphenylethylene • 110 K. A very good approximation can also be obtained by the use of time-dependent reaction rate constants given by
k(t)-Bt'-º,
0
superpmition of the a b a o q ~ o n bends of M'* and
"M~. means that it is not pouibte to degribe the dimeriaation reaction in d e c t r o n - p e ~ irradiated, highly viscous liquids by a clamcal iamglo-ltm-order reaction, in contrast to the results in liquid p h u e at room temperature (Bred¢ et al., 1982). An explanation of the differences in the kinetic behavior at room and low temperatures (ca 100 K) might be that the main ~ ob~,tved in the micro and millisecond time acak at low temperaturea
(7)
for a first or pseudo-first-order reaction (circles in Fig. 2). In our opinion there is some similarity between these two descriptions. Figure 2 illustrates that at temperatures • 100 K and concentrations of 0.15 tool dm -j DPE, no significant change in the 1000nm bcmd was observed at times • 2 5 ~ s . The reaction product of equation (3),"M/', has an absorption band at 390 ran. The experiments indicate, as demonstrated in Fi 8. 3, that immediately after the pulse at this wavelength there is already a relatively intense absorption at temperatures below 100 K. The spectrum in Fig. I shows this fact at !i0 K. The existence of this b i n d cannot be interpreted by fast dimer formation during the pulse because of reduced diffusion at the temperatures used here. Overlapping with the tail of the radical band may be excluded, too. The radical-formation rate constant is several orders of magnitude smaller than the dimer-cationformation rate constant (Brede et aL, 1982). The life-time of the radicals is also much longer than that of "Mr. An experimental explanation of the absorption immediately after the pulse was not possible. One can imagine that at 390 nm the monomer radical cations absorb, too. Kinetic treatment by superposition of dimer formarion, "M2" - A([M~' ] - [M'*]),
were local bimolecular reactions, where the concentration of M "+ may be similar to that of M.
From the fits, the rate constants, kD, for the dimerization reaction of DPE "÷ are e~a~aated in Table I. The rate constants estimated via the matrix viscosity at temperatures • i 20 K are much smaller than the experimental values (Table I). The dimerization of DPE" ÷ in BuCI matrices, like that of styrene radical cations (B,~ et aL, 1985), cannot be explained by molecular diffusion alone because of the very high viscosities at 90-130 K. From the good coincidence of the experimental curves with their estimates (curves and points in Figs 2 and 3, respectively) it may be deduced that the dimerization reaction (3) must prevail over the neutralization of the monomer radical cations (4). From the temperature dependence of the rate ¢onstallts, k D (Table !), in the temperature range investigated an activation energy of 17.5kJmol-' was obtained (Fig. 4, curve A). Possible __dec~__yreactions of "M/' are tnmerization "(9) and neutralization (10):
(8)
where A is a constant factor, and of simultaneous monomer decay (6) gives good fits (points) for the experimentally observed growth of this band (curves in Fig. 3). For this reason the 390 n m band deviating from Schrnidt et aL (1985), must be interpreted as a
13OK
717
"M2' + M kN2
"M; + a
110K
, "M3'
(9)
. products.
IOOK
(10)
9OK
1.O
"12
oo
-10 -~
1 10-e
1 t0-~
l 10-4
l iO-~
Time(s) Fig
3. TemlX'~tur~ &.pem~.uce of the 390~n sl~x)~t)on of 0.15moldm -) BuCI + O.15 mol dm- ~ CCI, (curves--experimental values, point.v--estimated).
DPE
m
718
J. S C I O ~ T
Table I. ~ u d
al.
et
and ¢ ~ m l u K l ' talc co.,-~,.,,~ for the ~ ¢ o n
TIK]:
90
93
3 x I0"
I 0 ~'
%
msctmz ol D P E ' " at diffenmt Icmperatutes
100
II0
120
130
298'
2.7 x 10'~
3.3 x I0'*
10'
4 ~ 10~
1.2 x 1 0 "
10'
I0"
5 x 10'
7 x 10*
Experimental values of d~L,,.on nile com~nts [dm' tool ' s '] Estimated values of dimcri~tion rate co,m~nl~
2 x 10 ~
[din' tool ' s ']
3 x 10 "~
13 x 10~
4 x 10 '
'Us,,,l the Em.Mcin-~okcz formula and vimo~ty val,,,-liv¢~ in I ~ bFmm Brede eta/. {1982)
(1959).
The static hinderam:e of dimcr DPE radical cation reactions is relatively high; thus reaction (I0) is more probable The ~ y of the dimer band was fitted by expression (I I), taking into consideration neutralization and recombination reactions (Kao and Hwan8. 1984):
The following reasons exclude the observation of D P E trimerization reactions under our conditions at low temperatures. (I) Trimerization of "DPE2+ was not detected in pulse radiolysis experiments in liquid alkyl chloride solutions at room temperature (Brede et al., 1982). (2) The comparable tnmenzation rate constant of styrene radical cations at room temperature is 4 orders of magnitude smaller than the dimenzation rate constant (Brede et ai., 1982). For this reason, this reaction is too slow for observation in our time scale at low temperatures.
[C0] • " ['M~'] =['M~'][Co] + [ ' M ~ ] ( I - e
")'
where I/~ =kN, x [Col, 10 4
sOe
-
10 T
8
10 3
o 7
7
102 ( o z_J
\
10 6 I0
,0
"-t 10 ~
104
1 0,8
; 09
l 10 1/Tx
Fig. 4. T e m p e r a t u r e
dependence
1 11 IOI(K
o f the d i m c t i z a t i o n
l I 2
100
-1 )
rzt¢ constant
k~,[Co] (B).
k o (A)
and
ncutrsli=,=tion
rate
(11)
R.sdiation-llencrated
cations
['M~.] is the average dimer radical cation concentration, and [Co] it the stationary value of charlled m the t a m p k . The points in the decayin$ part in Fig. 3 denote the calculations according to equation (I I). In the long-time tail of the __d~__ycurves there arc some deviations from the predicted time behaviour (I I), which m a y be due to overlal~ng of the increasing radical band at 340 rim. The estimated recombination rates, kN2 × [Co], are also plotted in Fig. 4, curve B. An activation energy of 17.2kJ m o l - ' can be derived. One of the products of reaction (10) may be excited molecules, from which, in a following step, result radicals with an absorption maximum at 340 um (Hayashi et al., 1978; Schmidt et at., 1985). To investigate radical kinetics, further experiments have to be done, because there are different routes for radical formation---one directly from radiationexcited molecules, another indirectly by the neutralization of ionic species. Kinetic calculations for the 550 run decay in the temperature range 90-1 l0 K tend more to a neutralization than to a dimerization reaction, so that the nature of this band is not quite clear. Possibly there is also overlapping with bands of other species. SUMMAII¥
These results on the pulu~-radiolytically generated DPE cations show that the kinetics did not change in the temperature range 90-130 K, where the viscosities differ widely from 2.5 × 10 '° Pa s (glassy state) to about 10°Pas (viscous liquid). In extension to previous results it was found that it is not possible
of l,l-.diphenylethyk.n,¢
719
to ~ b e monomer decay and dimer formation m the expanded temIx'ramre , a d time rtn$et by maple l ~ u d o - f i n t - o r d e r k i u e t ~ . Good fits are ~ven by cak'ulations taking into account hisher local conzentratiom of ionic species than m a homoigneous distribution, it is demonstrated that the I~alte radiolDis technique, in connection with abeorption spectroscopy, in a wide temperature and time range at low temperatures, is very useful for the treatment of the first steps of radiation-induced reactions of aromatic vinyl monomers. On the b a ~ of a simplified reaction mechanism and kinetic calculations, the reaction rate constants for dimerization at different temperatur~ are estimated, they are higher than the values expected for diffusion-controlled reactions. An activation energy for the dimerization reaction of DPE radical cations of 17.5 kJ tool-' was obtained.
ItEt~-m~
Anti S., Kira A. and lmamura M. (1976) J. Phys. Chem. g$, 1968. B6s J., Schmidt J., Mai H., Ha]big M. and Decke~ U. (1985) Radar. Phys. Chem. 26, 531. Bred¢ O., B6s J., Heln~treit W. and Mehneft R. (1982) Radiat. Phys. Chtnn. 19, !. Denney D. J. (1959) J. Chem. Phys. 30, 159. Hamill W. H. (1968) Rad/ca/Iocu. I n ~ , New York. Hayuhi K., lrie M., IAndenau D. and Schnabel W. (1978) Radiat. Phys. Chem. !1, 139. Kao K. C. and Hwan 8 W. (1984), Electrical Tramport m SoIMa. p. 297. Mir, Moskow. Schmidt J., ~ J., Mai H., Halbiil M. and Decker U. (1985) Rm~t. Phys. C&,m 26, 543. Shida T. and Hamill W. H. (1966) J. Chem. Phys. 44, 4372. Tepl~ J., Jtnov~y I., Sojka B. Vocilka J. and Fojtik A. (1981) Chem. /.~ty 7~, 758.
Ra~knt. P/Ws. C&tm. Vol. 36, No. 6, pp. 715--719, 1990 Ira. J. I t a ~ t . ,4~1. Imteww., Part C Printed in Great Britain. All rights reserved
RADIATION-GENERATED CATIONS OF I,I-DIPHENYLETHYLENE AND THEIR KINETIC BEHAVIOUR AT LOW TEMPERATURES J. SCHMIDT,1 U. DECKEIt,t J. TEPLY,2 J. ig~s' and H. MAC *Central Institute of Isotope and Radiation Reaearch (CIIRR), Permoter~r. 15, 7050-Leipn$. G.D.R. and ZN~_~eJ__~Research Institute (NRI). l[el~ C.S.SR. (Received 27 N 0 q ~ , r 1989)
~---C~tions of I,I-diphenylethykne (DPE) we~ studied in electron-scavenger matrices in the temperattme rang 90-130 K. Nanmecond and micrmecond pulJe-radiolym, with optical absoq)t,on spectrmcopy in tim~ from 10-s to lO-~s was used. The bands of the aboorl~on spectra are m ~ on the b a h of • simplified reaction mechanism and kinetic cak'ulahons. Reaction rate constants for the dimeri~tion of the radical cations of DPE are given.
INTItODUCTION
IIgSL'LTS AND DgSC~SSION
in previous studies the absorption of DPE cations was investigated by pulse radioly$i$ at low (Schmidt el al., 1985) and room temperatures (Brede et al., 19824 H a y a ~ et al., 1978), and also by steady-i~tate experiments at 77 K (Hamill, 1968; H a y u h i e t a / . , 1978; ShJda and Hamill, 1966). The definite assignmerit of certain absorption bands is partly contradictory, and kinetic data are not available for low-temperature experiments. To extend our pulse radioly$is investigations on aromatic vinyl monomers ( ! ~ et al., 1985), we tried to get more information on DPE cations at low temperatures by kinetic analysis. By measurements using the pulse radiolysis apparatus in the NRI. [~et ('repl~ et aL, 1981), the detection times were increased from 0.05 to 20 ms, and so it became possible to observe temporal changes of the transient absorptions at temperatures lower than 110 K. Using these extended temperature and time regions and kinetic calculations for different absorption bands, further results concerning the cationic species of DPE are discussed.
Figure I presents the tran~ent absorption spectra of 0 . 1 5 m o i d m -3 DPE m BuC1 at l i 0 K at three different times after end of pulse. According to Brede et aL (1982), Hamill (1968) and Shida and Hamill (1966), tbe maxima at 390 and 520 ran are attributed to dimer radical cations "DPE2+ , the bands with maxima at 1000 and 550nm to monomer radical catiom DPE "+, and the maximum at 340 tun to the absorption of DPE radicals DPE" (Schmidt et aL, 1985). Transients of the BuCI matrix (Arai et al., 1976), according to the reaction
EXIPre.ilIMENTAL
The synthesis and purification procedures of DPE and of the electron-scavenger matrix /-butylchloride (BuCI) were described in Schmidt e t a / . , (1985). The same cryostat system (Schmidt et al., 1985) was used for me~___$utements at the ELITtype ~__~_terator (8 and 40 ns pulses with 30 and 70 Gy/pulse, respec~vely) in the CIIRR Leipzig. and at the 3.7MeV Testa ~___c~,;erator (2.5~s pulse, 125 Gy/pulse) in the NRI, l~d. Details o [ t h e optical detection system are given in Sclunidt et aL (1985) and Tepid, et al. (1981). t~: ~,.-<
BuCI ,,~ h" +, BuCI" +, B u ' , CI -,
(I)
where h" + are highly mobile holes and BuCi" + relaxed radical cations of the matrix, were not observed because of the relatively high DPE concentration. The product of the charge transfer reaction from the matrix to the vinyl monomer (M), h "+, BuCI'*, Bu + + M --, M'+ + BuCI, Bu"
(2)
was detected at ~ > 1000 and 550 nm. The t i ~ l v e d formation of these transients was not observed under our conditions and the __d~'~__y of these bands was only detected at temperatures ;~ IIOK. The curves in Fig. 2 describe the experimental decay of the 1000 nm band at 100, 110 and 130 K. Possible reactions for the monomer cation decay are dimerization (3) and neutralization (4):
M"+ + M M'* + C1715
kD
. "M~"
(3)
, products.
(4)
716
J. SCIO~DT et aJ.
I II
0.2
,I
0.1
/
~1
~',l
1
400
,, .~_
_.~_
_ __0.__
600
=. __,,_?-
-
-
-'z,
-~--a-
800
I
1000
X(nm)
Fig. I. Absorption R)ectra of 0.15moldm ~ DPE in BuCI +0.lSmoldm ~ CCI4 irradiated with 8ns electron pull¢~ at I I O K , immediately (O), 10ps (O) and 45ps (/X) al'tcr the end of the pulse. More detailed investigations of the time behavior of the NIR band at 100Onto did not confirm the pseudo-first-order decay mechanism discussed in Schmidt et aL (1985). Good fits could be obtained by solving the differential equation (5) neglecting the recombination reaction (4):
~[M"] - -
= - k D [ M " ] ( [ M ] - [M"]).
~0~.,
<~
(5)
The term ( [ M ] - [ M " " ]) describes local redL)ction o f m o n o m e r concentration, [IV[], by the f o r m a t i o n o f m o n o m e r radical cations, M " , The solution o f equation (5) gives
[M" ']
[M0"
[M] ][M0" ] + ({M] - [M'-])c,*Dt~ '
(6)
with [Mo" ] as monomer cation concentration at pulse end. The calculated decay curves for 1000 nm are plotted in Fig. 2. The coincidence of calculated and experimental curves is very good a[ temperatures lOOK
O~
0
Fig. 2. Time bcbaviour of the 1000nm band of 0.15moidm "~ DPE in BuCI + 0 . 1 5 m o l d m ~ CCI~. (O) Estimated valu~ according to (6); (O) Estm~uxl vllues ~cording Io (7) ~ t h = - 0.828 and B -. 1.56 x 104 for I I O K . = = 0.560 and B - 868 × 10~ for 130 K.
Radiation-•eta'ratedcations of l,l-diphenylethylene • 110 K. A very good approximation can also be obtained by the use of time-dependent reaction rate constants given by
k(t)-Bt'-º,
0
superpmition of the a b a o q ~ o n bends of M'* and
"M~. means that it is not pouibte to degribe the dimeriaation reaction in d e c t r o n - p e ~ irradiated, highly viscous liquids by a clamcal iamglo-ltm-order reaction, in contrast to the results in liquid p h u e at room temperature (Bred¢ et al., 1982). An explanation of the differences in the kinetic behavior at room and low temperatures (ca 100 K) might be that the main ~ ob~,tved in the micro and millisecond time acak at low temperaturea
(7)
for a first or pseudo-first-order reaction (circles in Fig. 2). In our opinion there is some similarity between these two descriptions. Figure 2 illustrates that at temperatures • 100 K and concentrations of 0.15 tool dm -j DPE, no significant change in the 1000nm bcmd was observed at times • 2 5 ~ s . The reaction product of equation (3),"M/', has an absorption band at 390 ran. The experiments indicate, as demonstrated in Fi 8. 3, that immediately after the pulse at this wavelength there is already a relatively intense absorption at temperatures below 100 K. The spectrum in Fig. I shows this fact at !i0 K. The existence of this b i n d cannot be interpreted by fast dimer formation during the pulse because of reduced diffusion at the temperatures used here. Overlapping with the tail of the radical band may be excluded, too. The radical-formation rate constant is several orders of magnitude smaller than the dimer-cationformation rate constant (Brede et aL, 1982). The life-time of the radicals is also much longer than that of "Mr. An experimental explanation of the absorption immediately after the pulse was not possible. One can imagine that at 390 nm the monomer radical cations absorb, too. Kinetic treatment by superposition of dimer formarion, "M2" - A([M~' ] - [M'*]),
were local bimolecular reactions, where the concentration of M "+ may be similar to that of M.
From the fits, the rate constants, kD, for the dimerization reaction of DPE "÷ are e~a~aated in Table I. The rate constants estimated via the matrix viscosity at temperatures • i 20 K are much smaller than the experimental values (Table I). The dimerization of DPE" ÷ in BuCI matrices, like that of styrene radical cations (B,~ et aL, 1985), cannot be explained by molecular diffusion alone because of the very high viscosities at 90-130 K. From the good coincidence of the experimental curves with their estimates (curves and points in Figs 2 and 3, respectively) it may be deduced that the dimerization reaction (3) must prevail over the neutralization of the monomer radical cations (4). From the temperature dependence of the rate ¢onstallts, k D (Table !), in the temperature range investigated an activation energy of 17.5kJmol-' was obtained (Fig. 4, curve A). Possible __dec~__yreactions of "M/' are tnmerization "(9) and neutralization (10):
(8)
where A is a constant factor, and of simultaneous monomer decay (6) gives good fits (points) for the experimentally observed growth of this band (curves in Fig. 3). For this reason the 390 n m band deviating from Schrnidt et aL (1985), must be interpreted as a
13OK
717
"M2' + M kN2
"M; + a
110K
, "M3'
(9)
. products.
IOOK
(10)
9OK
1.O
"12
oo
-10 -~
1 10-e
1 t0-~
l 10-4
l iO-~
Time(s) Fig
3. TemlX'~tur~ &.pem~.uce of the 390~n sl~x)~t)on of 0.15moldm -) BuCI + O.15 mol dm- ~ CCI, (curves--experimental values, point.v--estimated).
DPE
m
718
J. S C I O ~ T
Table I. ~ u d
al.
et
and ¢ ~ m l u K l ' talc co.,-~,.,,~ for the ~ ¢ o n
TIK]:
90
93
3 x I0"
I 0 ~'
%
msctmz ol D P E ' " at diffenmt Icmperatutes
100
II0
120
130
298'
2.7 x 10'~
3.3 x I0'*
10'
4 ~ 10~
1.2 x 1 0 "
10'
I0"
5 x 10'
7 x 10*
Experimental values of d~L,,.on nile com~nts [dm' tool ' s '] Estimated values of dimcri~tion rate co,m~nl~
2 x 10 ~
[din' tool ' s ']
3 x 10 "~
13 x 10~
4 x 10 '
'Us,,,l the Em.Mcin-~okcz formula and vimo~ty val,,,-liv¢~ in I ~ bFmm Brede eta/. {1982)
(1959).
The static hinderam:e of dimcr DPE radical cation reactions is relatively high; thus reaction (I0) is more probable The ~ y of the dimer band was fitted by expression (I I), taking into consideration neutralization and recombination reactions (Kao and Hwan8. 1984):
The following reasons exclude the observation of D P E trimerization reactions under our conditions at low temperatures. (I) Trimerization of "DPE2+ was not detected in pulse radiolysis experiments in liquid alkyl chloride solutions at room temperature (Brede et al., 1982). (2) The comparable tnmenzation rate constant of styrene radical cations at room temperature is 4 orders of magnitude smaller than the dimenzation rate constant (Brede et ai., 1982). For this reason, this reaction is too slow for observation in our time scale at low temperatures.
[C0] • " ['M~'] =['M~'][Co] + [ ' M ~ ] ( I - e
")'
where I/~ =kN, x [Col, 10 4
sOe
-
10 T
8
10 3
o 7
7
102 ( o z_J
\
10 6 I0
,0
"-t 10 ~
104
1 0,8
; 09
l 10 1/Tx
Fig. 4. T e m p e r a t u r e
dependence
1 11 IOI(K
o f the d i m c t i z a t i o n
l I 2
100
-1 )
rzt¢ constant
k~,[Co] (B).
k o (A)
and
ncutrsli=,=tion
rate
(11)
R.sdiation-llencrated
cations
['M~.] is the average dimer radical cation concentration, and [Co] it the stationary value of charlled m the t a m p k . The points in the decayin$ part in Fig. 3 denote the calculations according to equation (I I). In the long-time tail of the __d~__ycurves there arc some deviations from the predicted time behaviour (I I), which m a y be due to overlal~ng of the increasing radical band at 340 rim. The estimated recombination rates, kN2 × [Co], are also plotted in Fig. 4, curve B. An activation energy of 17.2kJ m o l - ' can be derived. One of the products of reaction (10) may be excited molecules, from which, in a following step, result radicals with an absorption maximum at 340 um (Hayashi et al., 1978; Schmidt et at., 1985). To investigate radical kinetics, further experiments have to be done, because there are different routes for radical formation---one directly from radiationexcited molecules, another indirectly by the neutralization of ionic species. Kinetic calculations for the 550 run decay in the temperature range 90-1 l0 K tend more to a neutralization than to a dimerization reaction, so that the nature of this band is not quite clear. Possibly there is also overlapping with bands of other species. SUMMAII¥
These results on the pulu~-radiolytically generated DPE cations show that the kinetics did not change in the temperature range 90-130 K, where the viscosities differ widely from 2.5 × 10 '° Pa s (glassy state) to about 10°Pas (viscous liquid). In extension to previous results it was found that it is not possible
of l,l-.diphenylethyk.n,¢
719
to ~ b e monomer decay and dimer formation m the expanded temIx'ramre , a d time rtn$et by maple l ~ u d o - f i n t - o r d e r k i u e t ~ . Good fits are ~ven by cak'ulations taking into account hisher local conzentratiom of ionic species than m a homoigneous distribution, it is demonstrated that the I~alte radiolDis technique, in connection with abeorption spectroscopy, in a wide temperature and time range at low temperatures, is very useful for the treatment of the first steps of radiation-induced reactions of aromatic vinyl monomers. On the b a ~ of a simplified reaction mechanism and kinetic calculations, the reaction rate constants for dimerization at different temperatur~ are estimated, they are higher than the values expected for diffusion-controlled reactions. An activation energy for the dimerization reaction of DPE radical cations of 17.5 kJ tool-' was obtained.
ItEt~-m~
Anti S., Kira A. and lmamura M. (1976) J. Phys. Chem. g$, 1968. B6s J., Schmidt J., Mai H., Ha]big M. and Decke~ U. (1985) Radar. Phys. Chem. 26, 531. Bred¢ O., B6s J., Heln~treit W. and Mehneft R. (1982) Radiat. Phys. Chtnn. 19, !. Denney D. J. (1959) J. Chem. Phys. 30, 159. Hamill W. H. (1968) Rad/ca/Iocu. I n ~ , New York. Hayuhi K., lrie M., IAndenau D. and Schnabel W. (1978) Radiat. Phys. Chem. !1, 139. Kao K. C. and Hwan 8 W. (1984), Electrical Tramport m SoIMa. p. 297. Mir, Moskow. Schmidt J., ~ J., Mai H., Halbiil M. and Decker U. (1985) Rm~t. Phys. C&,m 26, 543. Shida T. and Hamill W. H. (1966) J. Chem. Phys. 44, 4372. Tepl~ J., Jtnov~y I., Sojka B. Vocilka J. and Fojtik A. (1981) Chem. /.~ty 7~, 758.