J. Phys. Chem. 1994,98, 2877-2882
2877
Ion Pairs (CFC12+11Cl-)dvfrom Geminate Ion Recombination in Methylcyclohexane with CFC13: Formation, Reactivity, and Stability A. S. Domazou, M. A. Quadir, and R. E. Biihler' Laboratory for Physical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, 8092 ZEtrich, Switzerland Received: August 17, 1993; In Final Form: December 20, 1993'
Pulse irradiated samples of CFClJ in liquid methylcyclohexane (MCH) a t low temperatures exhibit an optical absorption with ,A, at 435 nm. From extensive scavenger studies with variation of dose, temperature, and solution composition, it is obvious that the 435-nm species has both a cation and an anion as precursor and behaves itself as a positive species. It is assigned to the cation CFC12+, produced by charge transfer from the solvent radical cation to the fragment radical CFC12' from CFCls- anion dissociation. The absorption is attributed to a charge-transfer band between CFC12+ and a solvent (or solute) molecule. Except for the very early time, the CFC12+ cation disappears a t lower doses by first-order kinetics up to four half-lives dependent on temperature, much slower than expected from geminate ion recombination, with kl(143 K) = (4.5 f 0.6) X lo3 s-l. The corresponding Arrhenius parameters are very low (Eact= 12.6 f 1.0 kl/mol, log A = 8.2 f 0.4). It is concluded that CFC12+and C1-on recombination are forming an ion pair with the CFClz+-absorber remaining unperturbed within the ion pair. The ion pair reacts with quadricyclane (Q), methyltetrahydrofuran (MTHF), and also with solvent radicals, the latter initiating a dose effect on the ion pair lifetime for higher doses. The rate constant with Q at 143 K is (8.7 f 0.4) X lo5 M-l s-l. The ion pair reactivity, as well as the very low preexponential factor, is difficult to explain in the context of contact ion pairs. Together with further arguments, it is concluded that the ion pair (CFC12+11C1-),Iv most likely must be solvent separated.
Introduction The fate of the radical cations and corresponding anions, as produced by various ionization processes, often is studied at low temperatures in solid matrices and by assuming that the negative entity has no consequence to the cationic behavior. However, in liquid systems fragmentations of the ions (positive or negative) may occur and strongly affect the fate of the ions. In systems with halocarbons, anionic fragmentation may play a vital role. The present study is extending the previously reported ion pair formation.1-3 Halocarbons have been widely used as electron scavengers in radiolysis. Especially CCI, has been studied extensively as a model halocarbon. In all CC4/alkane systemsin the solid (glassy or poly~rystalline)~ and in the liquid state,) a prominent absorption band in the visible region (A, 470 nm) was detected. The assignment of this band has been very controversial in the past; once it has been attributed to CC14-,5 then to a cation of CCgS to a charge-transfer complex CClpC1,7.* and to an ion pair (CCl4YCI-).9 However, we ascribed the 470-nm band to the CC13+ cation,) which later has been confirmed by others.1.10 Following our model for MCH solutionsof CC4, the CC13+cation is formed through positive charge transfer from the alkane cations to the CC13' radical, which derives from the dissociationof CC14--. On recombination with the anion (Cl-), it produces an ion pair (CCl,YCl-), contactlJ1 or solvent separated,12which eventually decays unimolecularly on a larger time scale. The absorption band due to CC13+ remains unperturbed, whether CCl3+ is a geminate ion, a free ion or an ion within the ion pair.13 Other halocarbons have also been studied in alkanes for comparison. Not all of them show similar behavior. Thus, for CF3C1, CF2C12, or CF3Br in 3MP glasses' and for CHC13 or CHZC12 in liquid MCH," the corresponding radical anions were observed with no formation of ion pairs. In contrast, CFCl3 or CBrCl3 in 3MP glasses produces absorption bands, which were assigned to the contact ion pairs CFC12+--CI- and CC13+--Br.1 Other ion pairs, which we already identified, are the ((CH3)3C+ICE-) in isooctane
-
*Abstract published in Advance ACS Absrracrs. February 15, 1994.
0022-3654/94/2098-2877$04.50/0
solutionsof CC,) and the (C2HC14+ICl-)in liquid MCH solutions of C2HC1~." Up to now, most of the radiolytic studies concerning CFCl) have been carried out in solid matrices. From ESR studies161* the CFCl3- radical anion as well as its fragment radical CFC12' could be identified. The formation of ion pairs CFC12+-CI- has been discussed by Truszkowski and Ichikawal but only in alkane glasses. In the CFCl3/3MP systems at 77 K they observed an absorption band at 430 nm, which they attributed to a contact ion pair CFC12+4!1-. This was based on the similarity of the characteristics and the behavior of the band to the 470-nm band for CC4. Neither kinetic analysis of its decay nor scavenger studies have been carried out. In the present pulse radiolysis work, it was examined whether ion pair formation also occurs in the liquid state. Therefore, MCH solutions of CFC13 at low temperature were studied, stressing kinetic analysis, effects of positive and negative scavengers, and dose effects, as well as temperature influences. From our results the existence of an ion pair between CFC12+ and C1also in liquid systems is confirmed. However, our experimental findings are in favor of a solvent-separated ion pair (CFC12+11Cl-) rather than a contact ion pair.
Experimental Section The technique of pulse radiolysis with a Febetron 705 accelerator (Hewlett Packard) for pulses of 2-MeV electrons has been described earlier.1g.20 All experiments were carried out at low temperatures (183-133 K) in the liquid state in a stainless steel cell of 2-cm optical path length. All times given are from the start of the 50-ns electron pulse. The dose per pulse was usually 30 Gy, with the exceptionof some experiments, where the dose was 60 Gy and some others, where the dose was varied from 12 to 600 Gy. Dosimetry was performed by calorimetry. The temperature was kept constant within *0.5 K by a temperaturecontrolled gas stream from a liquid nitrogen evaporator. The optical detection system consisted of a pulsed 450-W Osram Xe lamp, a Bausch und Lomb high-intensity monochromator, and Q 1994 American Chemical Society
2878 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 Absorbance
*
Domazou et al. Absorbance
1000
*
1000
10
14
40 10
6 20 2
-2
500 600 700 nm Figure 1. Transient absorption spectra of irradiated MCH solutionswith 0.2 M CFC13 at 143 K for 0.2, 10, and 200 ps after pulse start; dose = 60 Gy. 300
400
TABLE 1: X, (nm) Dependence on System Parameters
0
50
Absorbance
420
100 150 Microseconds
n
1000 I
1
I
200
I
l
S
N20 sat C-C&jsat l t 3M MTHF l t z M Q
T (K) 163 143 133
433f3
-
435+3 4 4 0 f S 440f4
-
-
-
4 4 0 f 10
430f 10
440*10
-
300
430f 5
-
a photomultiplier (Phillips XP 1003). The signals of the photomultiplier were detected by a fast transient digitizer Tektronix 7912 AD (bandwidth 100 MHz by the plug in) or Tektronix 2440 (bandwidth 350 MHz) and simultaneouslyby a slow digitizer (Datalab DL912,6-MHz bandwidth). Data were analyzed with a PDP 11/73 computer. Methylcyclohexane (MCH, Fluka purum, >98% GC) was passed through a column of aluminumoxide, dried over molecular sieve A4, and fractionated through a Fischer “Spaltrohrkolonne” (30 theoretical plates estimated). Methyltetrahydrofuran (MTHF, Merck, >98%) was distilled under a N2 atmosphere. CFClp (Fluka puriss, >99.5% GC), quadricyclane (Q, tetracyclo[3.2.0.0296.04J] heptane, Aldrich, 99%), NzO (Sauerstoffwerke Luzem, 99%), cyclopropane( c - C ~ HMerck-Schuchhardt, ~, 99%), and Ar (Sauerstoffwerke Luzern, 99.996%) were used without further purification.
ReSultS The typical spectrum of irradiated MCH solutions with 0.2 M CFC13 at 143 K is shown in Figure 1. The visible band, having its maximum at 435 nm and a FWHM = 120 f 5 nm, seems to correspond to the one observed in 1% CFC13/3MP glasses by Truszkowski and Ichikawa,’ although it appears to be 5 nm red shifted. The position of the maximum absorption is somewhat dependent on temperature and on solution composition (Table 1). The kinetic analysis of the band maximum reveals three characteristic time regions: The main part (for ca. t > 20 ps at 143 K) decays by first order. It is preceded by a rapidly disappearing additional spike (for ca. t < 15 ps at 143 K, Figure 2a). At temperatures lower than 153 K there is initially even a buildup detectable (Figure 2b). The kinetic behavior for the wavelengths from 370 to 5 10 nm is uniform. It is concluded that the band with ,A, = 435 nm corresponds to a single species. The dominant first-order decay was studied from 183 to 143 K at a dose 150 Gy. At higher temperatures the linearity test holds for 3-4 half-lives, at 143 K at least for 2 half-lives. A low activation energy of 12.6 f 1.0 kJ/mol and a very low preexponential factor, log A = 8.24 f 0.34, characterize the process (Table 2). During the initial spike the total spectrum remains the same. This is also true for the difference between
180
60
-60
A/
4 l u u + I
II
1 : : 0
1 ; 1.5
:
: 3
:
1 : :
4.5 M i c r0 3e c 0 nds
I 6
Figure 2. Rate curve of the 435-nm absorption in irradiated MCH solutions with 0.2 M CFCl3: (a, top) X = 450 nm, D = 20 Gy,T = 143 K and (b, bottom) X = 450 nm, D = 329 Gy,T = 133 K. Curve 1: Am = total absorption. Curve 2: AIQ = fintsrder contribution kckextrapolated to early times. Curve 3: A, = Am - A14 = spike absorption.
the total absorption, AW, and the fust-order contribution backextrapolated to early times, A14 (Figure 2a). The spike absorption is then A, = Am - ,414 and appears to be somewhat wider (FWHM = 155 f 10 nm at 143 K). The observable maximum absorption (Aw) increases with lower temperature T to become constant for T 5 153 K (Table 3). This is explainable by the slowing down of the decay kinetics at fixed time resdution, with Aw being constant as soon as thedecay within the irradiation pulse becomes negligible. The kinetics of the spike absorption is complex. Dose Effect. The initial maximum absorption, AM, of the 435-nm species was found to be proportionalto the dose from 12 to 500 Gy. Typical rate curves, normalized to 100Gy, are shown in Figure 3a for 143 K. Two aspects are striking: (a) The decay of the initial spike is dose independent. (b) The main decay rate enhanced with the dose D for D > ca.50 Gy, however, is constant for lower dose. The decay kinetics is still exponential, yet becoming less accurate with higher dose, indicating overlap with some other process. Figure 3b illustrates an almost linear dependenceof the observed first-order rate constant, &I&, with the dose. A similar behavior is observed at 163K, but the critical dose now is 40 Gy instead of 50 Gy. Effect of Ekctron Scavenger& NzO.In a saturated solution of N20 in MCH ([NzO],, = 0.1 18 M in cyclohexane21) with 0.2 M CFC13 the 435-nm band is decreased by 2 8 4 0 % dependent on temperature (Table 3). The decay rate is slightly larger (ca. 2 times for T = 183 + 143K, Table 2). Nevertheleas, its Arrhenius parameters are still very similar (Table 2). The initial spike is
Ion Pairs from Geminate Ion Recombination
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2879
TABLE 2: Obmved Rate Coaetrnts a d Arrbenius Parameters for the 435-nm Band Decay in the hesence and Absence of Scavengers, Dose = 30 CY S
T (K)
N2O s t
*
183 173 163 153 143
**
5.03 0.64 2.65 f 0.27 2.0 f 1.2 1.08 f 0.17 0.45 0.06
10-3 M MTHF
k1.h (104 s-1)
-
12.6 1.3 7.8 1.2 4.09 0.41 2.16 i 0.21 0.93 0.13
*
EW(kJ/mol) log A
C-C& Sat
*
* *
-
11.7 1.0 8.26 h 0.91 4.50 0.27 2.96 f 0.48
-
10.2 1.4 6.2 f 1.2 2.47 0.28
*
68 f 18 28.6 6.1 9.4 f 1.5
-
17.8 f 3.1 11.5 & 1.1
10.7 1.5 8.1 1 f 0.36
*
* Arrhenius Parameters 14.2 * 1.0 12.5 * 1.9 9.18 f 0.32 8.88 * 0.61
12.6 f 1.0 8.24 0.36
MQ
*
TABLE 3: Scavengers' Effect. on 435-nm Absorption at Different Temperatures, Dose = 30 CY S
T(K)
Atot
Asp
A~pIAtot
183 173 163 153 143
18 19
2 4 6 7 7
0.12 0.21 0.23 0.25 0.24
25 28 29
&tot
-38
40 4 0 -32 -28
AAnp
4 5 4 5 -27 -30 -23
1.1 x 1 w M MTHF
&& sat
NzO s t
utot
-
-28 -24 -2 1
-
U
-
p
+45 +51 +43
-
M U n
-
-
4
no spike
-3 1 -24 -24
-57 -30
MQ M U n
U p I
-33 -26 -31 -37
nospike nospike
-
-67 -59
-
a A = absorbance X 1OOO. Am = total initial absorbance. AIQ= first-order contribution backextrapolated to pulse position. A, = Ala - Ala = absorbance due to spike only. PAUn= W reduction of total absorbance due to scavenger. AA,, = 46 reduction of spike absorbance due to scavenger.
diminished in proportion to the total absorption; therefore, Alp/ Am remains unaffected. It is concluded that the 435-nm species must have a negatively charged precursor. Effect of Cation Scavengers. Cyclopropane. c - C ~ H as ~,a positive scavenger, is expected to react by H or H2 transfer rather than by charge transfer with MCH+ cati0ns2~1*3(Zp(c-CgH6) = 10.1 eV," Z,(MCH) = 9.85 eV29. Yet, at the concentrations used (1.19 M as a saturated solution21) direct effect on c-CoH6 might produce some c-C,Hs+. Cyclopropane (saturated solution of c - C ~ Hin~MCH with 0.2 M CFCl3) reduces the initial yield of the visible band by 21-28% depending on temperature (Table 3). Thedecayrateislarger byafactorof2.5-3.5. The Arrhenius parameters, however, are still the same within experimental error (Table 2). A new, very short lived, unspecific absorption (ti12 = ca. 1.5 @ at t = 143 K),covering a wide X range, is obscuring any judgement of the effect on the initial spike. The decay of this new band appears to follow the kinetics of geminate ion recombination,aschecked by thelinearitywith ther0.6ratelaw.26 Since such a broad band can also be seen in the C-CBH~/MCH system without halocarbons,27 it must be related to the c-C3H6+ cation, as observed by Shida et aL2* In summary, c - C ~ Hreacts ~ with the precursor and has very little effect on the decay rate of the 435-nm species. Hence, the precursor has a positive charge. Quadricyclane. Quadricyclane, Q, was added to a solution of 0.2 M CFC13 in MCH as a positive scavenger (Z,(Q) = 7.86 eV29). At temperatures from 183 to 153 K, 1 P 2 M Q reduces the visible band by about 302, the initial spike by at least 60% (Table 3). A new, rather weak, absorption at 650 nm is detected, most likely due to the norbornadiene (bicyclo[2.2.l]hepta-2,5diene) cation, NBD+,the isomerizationproduct of Q+.29-31With higher Q concentration, the 435-nm band is drastically lowered to a minimum, which corresponds to the tail of the 650-nm absorption of NBD+. The decay rate of the 43541x1 band also strongly increases. For Q concentrations up to 0.16 M (typical dose of 30 or 60 Gy) and temperatures from 143 to 163 K,there is always a very good first-order fit over more than 3 half-lives. The observed rate constant, k'l,,,b, changes linearly with [Q] (Figure 4). This indicates that Q also reacts with the 435-nm transient in a pseudefirst-order process. Thus, the 435-nmspecies not only has a cation as a precursor but must be cationic itself. Methyltetrahydrofuran. Methyltetrahydrofuran, MTHF (Ip = 9.34 eV39, is expected to react with the primary solvent cation by proton transfer.7.33 Even if a charge transfer should occur,
A b s o r b a n c e K 1000 110
I 80
50
20
4
2
0 0
200
400 D o s e / Gy
600
m e 3. (a, top) Decay rate curves, normalid to 100 Gy, of the 435nm absorption in MCH solutions with 0.2 M CFCI, irradiated with different dosts; T = 143 K, X = 450 nm. (b, bottom) Corresponding observed first-orderrate constantsof the 435-nmabsorption decay, &I&, as a function of dose.
since Z,(MTHF) < Z,(MCH), the resulting MTHF+ would again react with MTHF by proton transfer." In a 10-3 M solution of
2880 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
TABLE 4 Rate Data for MCH + 0.2 M CWl3 in the Presence of Q
I O 4 5.'
k{,obs/
Domazou et al.
20
163 15
153 143
35-40 60 60 60
3.73 f 0.17 4.24f0.29 2.12 j: 0.38 1.42f 0.27
6.4i 2.6 9.21a0.64 8.54f 0.51 8.69f 0.38
2.0 f 1.2 4.59f0.15 2.85 f 0.69 0.67 f 0.10
70.7 70.7 8.7
From [Q]-depcndence. b For 60 Gy there is a slight dose effect; for best values see Table 4.
10
5
0 0
40
120
80
160
[Ql
/ M Figure4. Observed first-orderrate constantsfor the decay of the 435-nm absorption, kl,ob,in irradiated MCH solutions with 0.2 M CFCI3 as a function of Q concentration; X = 450 nm, 7' = 163 K,D = 60 Gy.
MTHF in MCH with 0.2 M CFCl3, the initial yield of the 435nm transient is decreased by 24-3 1%, depending on temperature (Table 3). The apparent first-order decay rate is faster by a factor of 2 1-34 (T-dependent), and the correspondingArrhenius parameters are much larger (Table 2). The initial spike is much less pronounced and is detectable only for temperatures lower than 153 K. 0.1 M MTHF completely removes the band. In summary, MTHF reacts with the precursor, as well as with the 435-nm species. Both, precursor and the 435-nm species, appear to carry a positive charge.
Discussion Assignment of the 435-om Band: hoposed Mechanism. The transient absorption with ,A, = 435 nm is comparable to the absorption observed by Truszkowski and Ichikawa for CFC13 in 3MP glasses (77 K, A,, = 438 nm).' From scavenging studies with N20, c-C&, methyltetrahydrofuran (MTHF), and quadricyclane (Q), we conclude that the 435-nm species has both a cation and an anion as precursors. The decay kinetics after the initial spike is of first order (dose I 50 Gy), and the Arrhenius parameters are characterized by low values, particularly by a very low log A. Since Q and MTHF, as positive scavengers, substantially increase the decay rate, the 435-nm species itself must also have a positive charge. At low temperatures ( T I 153 K), there is even a buildup detectable (Figure 2b). This behavior makes clear that the 435-nm band may be compared with the one at 470nm observed in liquid MCH with CC4, which we attributed to the CC13+within a solvent-separated ion pair (CC~~+IICI-),I,.~ In analogy, we now assign the 435-nm band to the CFC12+cation, which is formed by a positive charge transfer from the solvent radical cation MCH+ to the CFC12' radical, originated from the fast anion fragmentation:
r MCH'.
.. .. .. .. ..
c E 2 L (CFCI~-)+
I
+
..
1
Therefore, the probability is high that CFC12' is still adjacent to C1- when the fast MCH+ is approaching C1-. Free ions and geminate ions of large separation recombine too late to meet a CFC12' radical. Only close distant geminate ions will have a chance to undergo charge transfer to CFCl2' in competition to geminate ion recombination.' At very early times, before eiIv reacts with CFCl3, geminate recombination will be between MCH+ and e;,,. At very late times (after diffusional escape of CFCh'), it will be between MCH+ and C1-. The radical CFC12' absorbs in the range 365415 nm with a maximum at about 385 nm.35 It is expected that it is not interfering with the 435-nm band, except for a small additional absorbance in the UV slope, observed as a slightly faster initial decay. In analogy to the results for the CCld/MCH system: where quantum chemical calculations made clear that an isolated CC13+ has no absorption in the visible?*3*we assign the transition at 435 nm to a charge-transfer band between CFC12+ and a solvent (or solute) molecule. This explains why A- is slightly dependent on system parameters like temperature and composition (Table l), even though the dependence is not so pronounced as in the system with CC14.2 Charge-transfer bands are known to be very sensitive to environmental changes. As our cationic absorption at 435 nm is not disappearing on ion recombination, the cation must survive as a contact or solventseparated ion pair (CFCI~+ICI-),I,, whose unimolecular decay is characterized by the very low log A (Table 2). For a structural discussion, see below. Reactivity of the Ion Pair. With N20 and Cyclopropane. Although thesescavengerswereusedin highconcentrations(0.1 18 M for N2O saturated, 1.19 M for c-C3H6 saturated), the decay rate of the ion pair increased only by a factor of about 2 4 (Table 2). If this would be due to a direct reaction of the ion pair with the solute, the corresponding rate constant, e.g., at 163 K, would be about 400 times (N2O) or 2000 times (c-CsH6) smaller than kdiff. This contrasts to the reactivity of typical charge scavengers. For a discussion, see the Structure of the Ion Pair subsection. With Quadricyclane. Q is not only scavenging the precursor of CFC12+ but is also accelerating its decay. This must be explained by a charge-transfer process due to their ionization potentials (Zp(CFC12') = 8.17 eV, ZJQ) = 7.86 eV). Reaction 2, therefore, competes with the unimolecular decay (1) of the ion pair.
ki
MCH
(CFCIs+JCI-) + (CFC12' 1 Cl-)
cr. . . . .
The charge-transfer process follows the ionization potentials involved (Z,(MCH) = 9.85 eVZ5and Zp(CFC12') = 8.17 eV39. It occurs whenever the positive charge (MCH+)reaches C1-, before the CFCl2' radical is able to escape from the Coulombic field by diffusion. This is favored in MCH as the solvent radical cation is known to move by a resonance charge-transfer process. At 143 K it moves about 200 times faster than diffusion.36.37 The diffusional escape of the CFC12' radical may be calculated by Brows's law. The root mean square displacement in MCH at 143 K is estimated to be about 0.5 A in 1 ns or 5 A in 100 ns.
(CFC12+1CI-)
+0
CFCI,
-
(1)
kz
(0'
+ CFQ' + CI-)
(2)
I
strudure unknown
It is rather surprising that Q is able to pull the positive charge out of the ion pair (see discussion on structure below). The observed pseudo-first-order decay rate constant k'l,ob is dependenton theconcentrationofQ: k ' ~ . = ~ bk~+ k2[Q] (Figure 4). The results are given in Table 4. The constant kl, so calculated, is comparable with the values derived without Q. At 143 K, k2 is about an order of magnitude smaller than the estimated diffusion-controlled rate constant (kdd143 K) = 8.7 X 106 M-I s-l); k2 also appears to be surprisingly little affected by temperature.
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2881
Ion Pairs from Geminate Ion Recombination
TABLE 5: Ion Pairs Identified So Far chemical system' CC4 neat MCH + CC4 isooctane + CC4 frcon-1 13 (CFCl2CF2Cl) + Cc4 MCH + pentachloroethane (C2HC15) MCH + CFCl3 MCH = methylcyclohexane.
ref
ion pairproduced (cc13+lci-)MIv
T (K) 25 1 153 (cc13+1c1-)MIV (C(CH~h+lCl-)lo~v 173 (CC13+lfreon-),.,lv 239 (C~HCI~+~CI-),I, 153 153 (CFC~~+~CI-),I,
33 13 3 3 43 13 44
With Methyltetrahydrofuran. The low concentration of 10-3 M MTHF is speeding up the ion pair decay by up to 35 times (Table 2). A charge-transfer reaction of MTHF with CFC12+ is impossible due to its higher ionization potential. The reaction most likely is with the CFC12+ion by addition to give the oxonium cation.39 This explains the very different Arrhenius parameters (Table 2).
-
H2C-FH2 I
+ CFC12*
H2C.0,CH-CH3
H2CI
FH2
H2C.0,CH-CH3
(3)
I+
CFCI2
+
The apparent decay rate constant with MTHF is kapp= kl k3[MTHF]. At 163 K (e.g.), this yields k3 = 6.6X 108 M-l s-l, which is about 10times kdia. This is explainable by the fact that MTHF (n donor) easily complexes with halocarbons (electron acceptors), particularly at low temperature.& This means that the local, effective concentration of MTHF near CFC13 is higher than the bulk concentration. From scavenging of the precursor cation MCH+ by proton transfer, methyleyclohexyl radicals R' are produced (MTHF + MCH+ MTHFH+ R'). These radicals R' may also react with the ion pairs by reaction 4 and thereby enhance the ion pair decay, as will be discussed for the dose effect (see below). Dose Effect. From the normalized curves in Figure 3a it is clear that the mechanism of ion pair formation is independent of dose. However, for a dose above ca. 50 Gy the ion pair decay is perturbed by some other process. The observed rate constant, kl,,,k, is constant up to about 50 Gy, then it changes almost linearly with dose. There must be an additional reaction of the ion pair with some transient, of which the concentration is increasing with dose. Of the many transients possible, only a radical R* appeantobeabletoreact with theion pair. Themethylcyclohexyl radical, produced from primary solvent excitation, seems to be the most likely one. Its concentration is dose dependent, and its ionization potential (compared to Z,(cyclohexyl) = 7.66 eVZ5)is expected to be lower than Zp(CFC12*)= 8.17 eV. Consequently, reaction 4 is competing with reaction 1:
-
(CFCl2+1CI-)
+
+ R'
-
, (R'
+ CFCI; + Cl-) ,
(4)
I
structure unknown
This competition is very similar to the scavengingwith Q,except that R' is not homogeneously distributed. At the early time of the initial spike (t < 5 MS at 143 K) the primary spurs are not yet overlapping. The CFC12+cation (geminate ion or ion within the ion pair) and the radical R*in reaction 4 are from the same spur. Because the spur composition is unaffected by dose, no dose effect is seen for the spike kinetic. For the later time of nonhomogeneous kinetics the expanded spurs increasinglyoverlap. Therefore, the radicals R' from different spurs sum up, and a dose effect becomes possible. This is observed at 143 K for t > 5 MS. For a dose I50 Gy, only radicals R' within the same spur are able to react with CFC12+. The corresponding kl,obare the real kl for unimolecular decay of the ion pair. In this case, the spike contribution (Asp= Atot- ,414) represents the amount of ion pairs reacting by process 4 within the spur.
lifetime 7 33 ns 33 ps 810 ns 52 ns 2.6 ps 64 PS
Arrhenius parameters for first-order decay log A (kl/mol) 9.6 9.0 7.4 9.0 8.6 8.2
10.9 13.9 4.8 8.8 9.5 12.6
Structure of the Ion Pair. ( 1 ) The Available Models. From various attempts to explain the stability of the ion pairs, originally considered for (CCl3+(Cl-),the following two models are available for discussion: (a) The solvent-separated ion pair was initially proposed by Biihler et a1.3J3,41,42After having shown by quantum chemical calculation that a contact ion pair of C3, symmetry (Cl- above the plane of CC13+)does not have any potential barrier against neutralization to CC14,we suggested a solvent-separated ion pair as the more likely model. A classical electrostatic model calculation, with the solvent simulated as a face-centered-cubic lattice and the ions varied in relative position, resulted in a small energy minimum for the structure of the two oppositely charged ions separated by just one solvent molecule:41 e.g., (CCl3+lCC141Cl-),1, or (CC~~+IMCHICI-),I,. This was taken as a partial argument for the model of solvent-separated ion pair. (b) The contact ion pair was again picked up by Reed and Weinhold et al.," however now with a C , symmetry: C1- being in-plane with the planar CC13+,along one of the three C-Cl axes (C12C-Cl+.-Cl-). They showed by quantum chemical calculation that such a contact ion pair may be stable. They also concluded that both sides of the planar CC13+must be occupied by solvent molecules (binding energy 114.5 kJ/mol per side). This rather strong complex of the cation with solvent molecules agrees very well with the assignment of the visible band to a charge-transfer transition between CC13+(in this paper CFC12+) and a solvent molecule.2J2J7 These two structural models have to be compared with the experimental findings in this and earlier papers. (2) The Arrhenius Parametersfor the Zon Pair Decay. All ion pairs detected so far decay unimolecularly and show, besides a low activation energy, a very low preexponential factor in the range of 7.4 Ilog A I9.6 (Table 5). Such a low factor usually is associated with the formation of a tight ("rigid") activation complex relative to the reactant molecules. It corresponds to a large negative entropy change, related to a loss in the degree of freedom in the transition A solvent-separated ion pair losing the spacing molecule in the activated complex would be compatible with such a low preexponential factor. Contact ion pairs are much less likely to explain the loss of the degrees of freedom. ( 3 ) Scavenging the Positive Charge with Quadricyclane or Solvent Radicals. In the present study on CFCl3 in liquid MCH it has been shown that quadricyclane (Q) and solvent radicals R* (most likely methylcyclohexyl), having the characteristics of positive scavengers, actually react with the ion pair. The decay of the cationic absorber (CFC12+ solvent) becomes faster. Q (or R') will only be able to initiate charge transfer if the resulting Q+ (or R+) does not increase the ion separation, otherwise the process would have to overcome the Coulombic energy. Three possibilities may be discussed: (a) Charge transfer would be possible in both structural models if the complexed solvent molecule S at CC13+(or CFC12+) could be exchanged with Q. Yet, from the calculation by Reed and Weinhold et al.11 the binding energy of S to CC13+is so large (ca. 100 kJ/mol) that such an exchange appears to be very unlikely or at least very slow.
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2882 The Journal of Physical Chemistry, Vol. 98, No. 1 1 , 1994
(b) With the solvent molecule S blocking both sides of the planar CC13+(or CFC12+),the only approach to CC13+is in-plane or near the plane (estimated accessibility about 1/3 of 4 ~ ) This . is true for the solvent-separated ion pair. In this case charge transfer to Q is possible, as the ion separation (initially CC13+.-Clor CFC12+4!1-, finally Q+-Cl-) remains about the same, corresponding to no change in Coulombic energy. Also, the Coulombic field is rather small due to the neutral molecule separating the ions. It is expected that the resulting pair Q+-Climmediately neutralizes, since no ion pair so far could be observed with a parent radical cation. (c) For the model of contact ion pair, the in-plane approach is partly blocked by the anion. From the opposite side, Q has easy access; however, for a charge transfer to happen, it would have to pull the positive charge to a much larger distance from the anion. The only two possibilities left for a reactive approach are in the plane of the CC13+(or CFC12+) and as direct neighbor of the anion. Thus, the collisional efficiency for reactions of Q with thecontact ion pair must bevery small (estimated accessibility less than 1% of 4 ~ ) .Contrary to this model, the experimental rate constant k2(Q ion pair) is near the diffusion limit (k2 at 143 K is about 1/10 of kdiff). It is concluded that the solvent-separated ion pair is the more likely model to explain the reactivity of the ion pair with quadricyclane or a solvent radical R'. ( 4 ) Reactions with N20and Cyclopropane. The effect of high concentrations of N2O or cyclopropane is so small (see above) that it must be correlated with the scavenger molecules taking part in the solvation of the ion pair. An exchange in the solvation shell only is expected to be of negligible influence. Yet, the scavenging molecules of such high concentrations may compete with the solvent for the position between the ions in a solventseparated ion pair. For cyclopropane the statistical ratio of solvent to solute is about 1:8. For N20 it is smaller (1:lOO); however, N20 must be in preference due to its polarity. It is conceivable that the lifetime of a solvent/solute-separatedion pair is dependent on the separating molecule. The slightly shorter lifetime appears to be in favor of the model of a solvent-separated ion pair. ( 5 ) Spectra of the Geminate and Free CClj+ Ions. In the system of neat degassed, liquid CCl4 (251 K)l3 the transient absorption at 500 nm, due to CC13+,did not show any A, shift, whether CC13+ was within the ion pair or an unpaired ion (geminate or free ion).42 For a contact ion pair one would expect a disturbance of the electronic transition of CC13+by the adjacent C1-. As there is no shift of ,A, detectable, the model of contact ion pair is rather unlikely. Conclusion. From the whole of arguments, particularly from the special Arrhenius parameters and the reactivity toward radicals and quadricyclane, it is concluded that the model of solvent-separated ion pairs is more likely than the one of contact ion pairs.
+
Acknowledgment. We thank the Swiss National Science Foundation for support.
Domazou et al.
References and Notes (1) Truszkowski, S.; Ichikawa, T. J . Phys. Chem. 1989,93,4522. (2) Biihler, R. E.; Ha, T.-K. Radiat. Phys. Chem. 1989,34, 539. (3) Gremlich, H.-U.; Biihler, R. E. J. Phys. Chem. 1983, 87, 3267. (4). (a) Hamill, W. H. in Radical Anions; Kaiser E. T., Kevan, L., Eds., Interscience: New York, 1968; p 321. (b) Willard, J. E. In Fundamental Processes in Radiation Chemistry; Ausloos, P., Ed., Interscience: New York, 1968, p 599. (5) Ronayne, M. R.; Guarino, J. P.; Hamill, W. H. J . Am. Chem. Soc. 1962,84,4230. (6) Guarino, J. P.; Hamill, W. H. J. Am. Chem. Soc. 1964,86,777. (7) Louwrier, P. W. F.; Hamill, W. H. J. Phys. Chem. 1969, 73, 1702. (8) Suwalski, J. P. Radiat. Phys. Chem. 1981, 17, 393. (9) B b , J.; Brede, 0.; Mehnert, R.; Nilsson, G.;Samskog, P.-0.; Reitberger, T. Radiochem. Radioanal. Lett. 1979, 39, 353. (10) Klassen, N. V.; Ross, C. K. J . Phys. Chem. 1987, 91, 3668. (11) R e d , A. E.; Weinhold, F.; Weiss, R.; Macheleid, J. J. Phys. Chem. 1985,89, 2688. (12) Ha, T.-K.; Biihler, R. E. Radiat. Phys. Chem. 1988, 32, 117. (13) Biihler, R. E., Hunri, B. Helu. Chim. Acta 1978, 61, 90. (14) Katsumura, Y.; Biihler, R. E. Radiat. Phys. Chem. 1989, 34, 543. (15) Quadir, A. M.; Domazou, A. S.; Biihler, R. E., to be published. (16) Bonazzola, L.; Michaut, J.-P.; Roncin, J. Chem. Phys. Lett. 1988, 149, 316. (17) Symons, M. C. R.; Wyatt, J. L. J. Chem. Res. ( S ) 1989, 362. (18) Hasegawa, A.; Shiotani, M.; Williams, F. Faraday Discuss. Chem. SOC.1978, 63, 157. (19) Gersbach, P.; Biihler, R. E. Proceedings of the Fourth Working Meeting on Radiation Interaction; ZfI, Akad. Wiss. DDR: Leipzig, 1988; p 51. (20) Hurni, B.; Briihlmann, U.; Biihler, R. E. Int. J . Radiat. Phys. Chem. 1975, 7, 499. (21) Horsman-van den Dool,L. E. W.; Warman, J. M. Interuniuersitair Reactor Instituut Rapport 134-81-01; Mekelweg 15: Delft, The Netherlands. (22) Ausloos, P.; Scala, A. A.; Lias, S. G. J. Am. Chem. SOC.1966,88, 1583. (23) Scala, A. A.; Lias, S. G.;Ausloos, P. J . Am. Chem. SOC.1966,88, 5701. (24) Dewar, M. J. S.; Worley, S. D. J. Chem. Phys. 1969, 50, 654. (25) Pottie, R. F.; Harrison, A. G.;Lossing, F. P. J. Am. Chem.SOC.1961, 83. 3204. (26) van den Ende, C. A. M.; Warman, J. M.; Hummel, A. Radiat. Phys. Chem. 1984, 23, 55. (27) Gebicki, J. L.; Biihler, R. E., to be published. (28) Shida. T.: Takemura. Y. Radiat. Phvs. Chem. 1983. 21. 157. (29) Haselbach, E.; Bally, T.; Lanyiova, Z.; Baertschi, P. He$. Chim. Acta 1979, 62, 583. (30) Gebicki, J. L.; Gebicki, J.; Mayer, J. Radiat. Phys. Chem. 1987,30, 165. (31) Quadir, A. M.; Domazou, A. S.; Biihler, R. E., to be published. (32) Achiba, Y.; Kimura, K. Chem. Phys. Lett. 1976, 39, 515. (33) Gallivan, J. B.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2378. (34) Dainton, F. S.; Salmon, G. A. Proc. R . SOC.London, Ser. S 1965, 285, 319. (35) Tsai, B. P.; Johnson, R. D., 111; Hudgens, J. W. J . Phys. Chem. 1989, 93, 5334. (36) Biihler, R. E. Can. J . Phys. 1990, 68, 918. (37) Katsumura, Y.; Azuma, T.; Quadir, A. M.; Biihler, R. E., to be published. (38) Gebicki, J. L. et al., to be published. (39) Perst, H. In Carbonium Ions; Olah, G.A., Schleyer, P. v. R., Eds., John Wiley & Sons: New York, 1976; Vol. 5, 1961. (40) See, e.&: Foster, R. Organic Charge-Transfer Complexes;Academic Press: London and New York, 1969; p 408. (41) Gremlich, H.-U.; Ha, T.-K.; Zumofen, G.;Biihler, R. E. J . Phys. Chem. 1981,85, 1336. (42) Biihler, R. E. Radiat. Phys. Chem. 1983, 21, 139. (43) Hurni, B.; Biihler, R. E. Radiat. Phys. Chem. 1980, 15, 231. (44) This paper. (45) See, e.&: Robinson, P.J.; Holbrook, K. A. Unimolecular Reactions; Wiley-Interscience: New York, 1972; Chapter 6.2.
