J. Phys. Chem. 1983, 87,3267-3272
3267
Fate of Radiation-Inititated Charged Species in Liquid Alkane-CCI, Systems. Formation of Ion Pairs ( C C 1 3 ~ ~ ~ l ~by ) sGeminate olv Ion Recombination H . 4 . Gremllcht and R. E. Buhler’ Laboratory for phvsicel Chemistty, Swiss Federal Institute of Technoiogy, ETH-Zentrum, 8092 Zurich, Switzerland (Received: October 13, 1982; I n Final Form: February 1, 1983)
The well-known absorption centered at 470 nm in alkane solutions of CC14 is assigned to the CC13+cation within the solvent-separated ion pair (CCl,+~solvent~C1-),,1,. Its formation is explained by the recognition that a CC13 radical is produced by the dissociative electron attachment to CC14,within reach of the corresponding geminate alkane cation. Therefore, a positive charge transfer from the alkane cation to the CCl, radical is possible, at least for the least separated geminate ions. It is shown that very high mobility of the positive charge seems to be necessary to compete with the escape of the CCl, radical from the geminate region. In the subsequent ion recombination step the ion pair is produced. The stability of such solvent-separated ion pairs, due to weak interactions with the solvent,and the conditions for their formation are discussed. So far four ion pairs between CC13+and C(CH3),+as cations and Cl- and Freon-113- as anions have been identified. Their unimolecular decay is characterized by a low activation energy and, particularly, by a very low preexponential factor.
Introduction For some time hydrocarbon solutions of CC14have been studied in radiolysis. The primary aim of using CC14was to scavenge the negative charge. However, it was realized that CC1, somehow also takes part in reactions with cations. In almost all studies on alkane-CC4 systems (mostly glassy or polycrystalline systems)lP2a typical absorption band in the visible region (Amm = 470 nm) was detected, which was assigned through the many years to many different species: to the Ccl4-,, to a cation of cc14,4 to a charge-transfer complex C1CC14,5p6or to an ion pair (alkane(+)-Cl-).’ This visible band was obviously not understood, as there are arguments against any of these assignments (see Discussion). In order to elucidate the contribution of positive charge as well as negative charge to this visible band we concentrated on liquid systems. The well-known, rather strong caging effects8 on the transient species involved thereby were limited to the weak interaction with the nonpolar solvents. Due to spectral and kinetic results it was realized that the 470-nm band in hydrocarbon solutions of CC14displays characteristics very similar to those of the 500-nm band appearing in pure liquid CC14. The latter we assigned to the CC13+ cation within a solvent-separated ion pair (CC13+I(Cl-)sOlv.gJO From the results reported in this paper a reaction scheme is derived, which is able to explain the observed similarities between hydrocarbon solutions and pure CC14, and which fits the known experimental facts. The model is based on the recognition that a CC13radical is produced by the dissociative electron attachment to CCl,, within reach of the corresponding geminate alkane cation. Therefore, a positive charge transfer from the alkane cation to the CC13radical is possible, at least for the least separated geminate ions. In the subsequent ion recombination step again a solvent-separated ion pair (CCI3+IIC1-)solvis produced. In a recent paper’l we already reported from a classical electrostatic model calculation that such ion pairs might also be stable in methylcyclohexane due to solvent effects (polarizability and dielectric behavior). t Present address: Institute for Molecular Biology, ETH-Hoenggerberg, 8093 Zurich, Switzerland.
Due to this charge-transfer model (1) we gain a new understanding for processes in the very early stages of the interaction with ionizing radiation, and (2) we get a new insight in the ion neutralization processes, and the stability of short-lived solvent-separated ion pairs, and (3) earlier results in hydrocarbon glasses with CC14,hitherto unexplained, now fit the new model. Consequently it is possible to indicate the conditions for the formation of such ion pairs and to indicate the effect on the general neutralization processes.
Experimental Section The technique of pulse radiolysis with a Febetron 705 accelerator (Hewlett-Packard) for pulses of 2-MeV electrons has been described elsewhere.I2 A typical dose in the 2-cm optical cell was 300 Gy (30 krd). All times given for the spectra are from the start of a 50-ns electron pulse. The sample temperature was held constant to better than rt0.5 K by a temperature-controlled nitrogen gas stream from a liquid-nitrogen evaporator. The solvents methylcyclohexane (MCH: Fluka puriss.p.a.), isooctane (IO: Fluka puriss.p.a.), cyclohexane (CH: Fluka puriss.p.a.), cyclooctane (CO: Fluka purum), and CC14 (Fluka spectroscopy grade) were dried 3 times over molecular sieve (4A) for at least 6 h and then fractionated through a Fischer “Spaltr~hrkolonne”’~ with (1) W. H. H d in ‘Radical Anions”,E. T. Kaiser and L. Kevan, Eds., Interscience, New York, 1968, p 321. (2) J. E. Willard in “Fundamental Processes in Radiation Chemistry”, P. Ausloos. Ed.. Interscience. New York. 1968. D 599. (3) M. R. Ronayne, J. P. Guarino, and W. H: Hamill, J. Am. Chem.
SOC.,84, 4230 (1962). (4) J. P. Guarino and W. H. Hamill, J . Am. Chem. Soc., 86,777 (1964). (5) P. F. W. Louvrier and W. H. Hamill, J. Phvs. Chem.. 73, 1702 (1969). (6) J. P. Suwalski, Radiat. Phys. Chem., 17, 393 (1981). (7) J. BOs, 0. Brede, R. Mehnert, G . Nilsson, P.-0. Samskog, and T. Reitberger, Radiochem. Radioanal. Lett., 39, 353 (1979). (8) R. E. Biihler and W. Funk, J. Phys. Chem., 79, 2098 (1975). (9) R. E. Biihler and B. Hurni, Helu. Chim. Acta, 61, 90 (1978). (10) R. E. Buhler, Radiat. Phys. Chem., 21, 139 (1983). (11) H.-U. Gremlich,T.-K. Ha, G. Zumofen, and R. E. Buhler, J. Phys. Chem., 85, 1336 (1981). (12) B. Hurni, U. Briihlmann, and R. E. Biihler, Radiat. Phys. Chem., 7, 499 (1975). (13) Fischer, Labor- und Verfahrenstechnik, Bonn-Bad Godesberg,
BR-Deutachland.
0022-3854/83/2087-3267$01 .50/0 0 1983 American Chemical Society
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The Journal of Physical Chemistty, Vol. 87,No. 17, 1983
TABLE I:
Gremlich and Buhler
Comparison of the Visible Band in Methylcyclohexane and CC1, Solution
MCH spectrum: band centered a t full width half-max kinetics: fast first-order decay
slow decay
+ lo-'
M CCl,
pure CCl,9
470 t 5 nm 130 nm
500 r 10 nm 160 n m
k,(-60 "C) = (4.59 f 0.25) X l o 5 s-' h , ( - i o o 0 c ) = (1.20 r 0.10) x 105 s-1 kl(- 120 "C) = (2.09 t 0.26) X lo4 s-' Eact = 13.9 t 1.3 kJ/mol log A = 9.04 n o contribution
h,(-22 "C) = ( 2 . 1 t 0.2) x 10's-1 k , ( + 20 "C) = (4.6 * 0.6) X lo' 5.' Eact = 10.9 t 2.1 kJ/mol log A = 9.57 small second-order contribution (corresponding t o free ions)
TABLE 11: Scavenger Studies o n Rate Analysis and Initial Transient Yieldsa scavenger
k , , 5.' G O €M-l , cm-'
none
(4.59 1435 0.24
G0,6000
ethanol
(lo-'
M)
k,, s-' G O €M-l , cm-I G,,,OO,
cyclopropane (satd)
k ,, s-' G O €M-' , cm-' G0,6000
cyclohexene
(lo-'
M)
k,, s-l G O €M-l , cm-I G036000
SF, (satd, 330 Gy)
k , , s-' G O €M-, , cm-' G036000
Freon-113 (10.' M )
- 100 "C
-60 "C
k , , s-I G O €W1-l , cm-' G0,600O
1
t
- 120 "C
0.25) x 105 (1.20 t 0.10) x 105 (2.09 2113 2594 0.35 0.43
t
0.26) x 104
1
thermodynamics = 13.9 f 1.3 kJ/mol, log A = 9.04
(1.02 2041 0.34
t
0.16) x
lo6
0.25) x l o 5 (1.27 1159 0.19
i.
0.10) x
lo5
pact = 10 3 kJ/mol, log A = 8.1
(7.58 t 0.62) X l o 5 (8.21 i 0.70) x 1181 1665 0.20 0.28
lo5
1Eact = 1 7 t 5
n o t observable (4.64 658 0.11
i
(9.90 700 0.12
t
(1.14 r 0.20) x 2383 0.40
lo5
=24r5 1EactkJ/mol, log A = 13.20 2
kJ/mol, log A = 10.0
0.68) x 104
(4.93 t 0 . 7 7 ) x 105 (5.96 t 0.48) x 1173 1634 0.27 0.19
1 EactkJ/mol, = 16 4 log A = 9.64
lo4
t
a MCH with M CCI,, 300 Gy, , , A = 470 nm. G O € : pulse end yield, backextrapolated into pulse center (corrections $ 2 % ) . G,,6,,o: initial yield with assumed ~ ( 4 7 n0 m ) = 6000 M-'c m - ' ;
about 40 theoretical plates. Ethanol, cyclohexene, and aniline (all Fluka puriss.p.a.), N,N,","-tetramethyl-l,4phenylendiamine (TMPD: Merck-Schuchhardt for synthesis), 1,1,2-trichloro-1,2,2-trifluoroethane (Freon-113 TF quality of Hoechst), N20 (Sauerstoffwerke Luzern, 99%), cyclopropane (Merck-Schuchhardt, 99%), and SF, (Merck-Schuchhardt,99.9%) were all used without further purification. Results In order to understand the identity of the visible band in alkane solutions of CCl, the following effects were studied: effect of temperature (down to -120 "C) on transient absorbtion and kinetics, effect of various cationic scavengers, effect of electron scavengers as a replacement for CCl,, effect of various electron scavengers in competition to CCl,, and effect of choosing different hydrocarbon solvents. Figure 1shows the typical visible band centered at 470 nm in a MCH solution of M CC14 at -120 "C. For comparison the spectrum of the CC13+cation in pure CC14 is also shown.g In Table I the spectral and kinetic data of both systems are listed. It is obvious that both bands have a very similar width with a 30-nm shift in band position. Both first-order decay processes have low activation energies and both are characterized by a very low preexponential factor, indicative that similar processes must be involved. There is, however, one significant difference: in methylcyclohexane solution of CCl, the decay is entirely first order; there is no contribution from a slow
0 400
500
600
Figure 1. Comparison of the visible band in MCH and CCI, solution at the end of a 50-ns irradiation pulse. Spectrum 1: transient in pure CCI, (-22 OC, 200 Gy, ,A, = 500 nm) assigned to the CCI,+ cation within the solvent-separatedion pair (CCI3+((CI-),,, (e = 6000 f 1200 M- cm-', G = 1.3).g-'oSpectrum 2: transient in MCH with M CCI, (-120 OC, 300 Gy,h, = 470 nm) in this paper also assigned to the CCI,' within an ion pair (CCI,+//Ct)&. The scale for G values is based on eFF = e;:). For kinetic data see Table I .
The Journal of Physlcal Chemistry, Vol. 87,No. 17, 1983 3260
Charged Species in Liquid Alkane-CCI, Systems
lom3.G
2
L
(M-'cm")
1
0 300
600
400
A0 hm)
500
Flgure 2. Freon-1 13 (CFC12CF,Cl) as competing electron scavenger (all spectra In liquid MCH at -100 "C, 300 Gy,and 0.5 I.LS after pulse). M CC14, wkhout Freon-113, for comSpectrum 1: MCH with parison. Spectrum 2: MCH with M CC14 and M Freon-113. Spectrum 3: MCH with M Freon-113, without CCI,. Assignment: 480 nm, CCI,'; 440 nm, Freon-113(-); 380 nm, a radical from Freon-1 13.14 Note: In spite of the identical temperatures, the three systems have unequal viscosities (see text). The yields at a fixed time therefore are not necessarily comparable.
second-order process, as has been interpreted in pure CCl, to be due to the free-ion recombination. Table I1 gives further details for the rate constants It, and the initial yields (Got) for temperatures of -60, -100, and -120 "C. If the solute CCl, is replaced by Freon-113 (CF2C1CFClz),a completely different spectrum is attained, as shown in Figure 2. The new band corresponds to a radical from Freon-113 as indicated in an earlier paper.14 Studies of saturated methylcyclohexane solutions of N20 (-120 "C) or of SF, (-60 "C), without CCl,, again did not show the typical visible band. It is concluded therefore that CCl, must be involved in the reactions leading to the 470-nm band. Effect of Cation Scavengers. Aniline M) added to a MCH solution of M CCl, at -60 "C completely suppresses the 470-nm band. The typical aniline(+) cation band at 420 nm is produced instead. Aniline competes with the positive charge and the 470-nm band either is cationic itself or has a cation as its precursor. TMPD (5 X lo-, M) added to a MCH solution of M CCl, (-60 "C) also competes with the 470-nm band, producing the typical T W D + cation bands at 560 and 620 nm. The visible band is reduced by about 30%, whereas the yield of TMPD+ cation is calculated to be G(TMPD+) 3 0.12 (e636 < 20000'5). From this yield it is concluded that 5 X lo-, M TMPD primarily scavenges free ions. A detailed kinetic analysis for the decay of the remaining 470-nm absorbance is not possible due to spectral overlap. Cyclohexene M) was added to the MCH solution of M CCl, as a cation scavenger ( I (cyclohexene) = 8.9 eV, IJMCH) = 9.85 eVl6)). The effect on the decay rate and yield of the 470-nm band in experiments at -60 and -100 "C is given in Table 11. At both temperatures the initial yield of the 470-nm band is reduced by close to 20%. This indicates that cyclohexene is scavenging precursors. The effect on the decay rate is not clear: at -60 "C there is an increase, at -100 "C a decrease of the decay (14) B. Hurni and R. E. Biihler, Radiat. Phys. Chem., 15, 231 (1980). (15) T. Shida, J.Phys. Chem., 74, 3055 (1970). (16) (a) "CRC Handbook of Chemistry and Physics", 55th ed., CRC Press, Cleveland, OH, 1974; (b) J. C. Lorquet, Mol. Phys., 9, 101 (1965).
rate. However, the decay is still characterized by a similar low activation energy and a very low preexponential factor. Cyclopropane with an ionization potential of 10.1 eV is not expected to scavenge by charge transfer from MCH+ cations. However, it is known to react with the alkane cations by H or H2tram3fer.l' Correspondingly a saturated M CCl, reduces MCH solution of cyclopropane with the 470-nm band by about half without affecting the decay rate (Table 11). This indicates that the alkane cation is precursor to the 470-nm species. Ethanol is again cation scavenger by a different mechanism than charge transfer from the alkane cation (Ip(EtOH) = 10.5 eV"). Ethanol is known to scavenge alkane cations by proton transfer.18 The result as shown in Table I1 however clearly shows that M ethanol is unable to scavenge the precursor of the 470-nm species. Instead the decay rate is drastically increased (about 5-8 times). Also, the large preexponential factor derived for the first-order decay makes clear that the 470-nm decay now corresponds to quite a different process. Effect of Electron Scavengers. A saturated solution of SF6in MCH (saturated at room temperature) with M CC14 at -100 "C reduces the initial yield of the 470-nm band to whereas the decay rate is smaller only within experimental error (Table 11). It is concluded that the precursor of the 470-nm species not only is a cationic species but also is dependent on the negative entity. If M Freon-113 (CF2C1CFC12)is added to a MCH solution of M CCl, at -60 and -100 "C, there is again a reduction by about 22% in the 470-nm absorbance (Table 11). As shown in Figure 2 there is a new band at 440 nm appearing, which corresponds to the Freon(-) anion.14 For the system at -60 "C the decay rates of the 470-nm band with and without Freon-113 remain within experimental error, whereas at -100 "C the rate with Freon drops to about half. This drop can be related to the experimental finding that the viscosity of the system with Freon-113 must be higher than without Freon-113 (the melting point of the system with Freon-113 was measured as -104 "C, without Freon-113 as -126 "C). The low activation energy and very low preexponential factor for the system with Freon suggest that the same type of decay process is involved as without Freon. At 440 nm the decay rates are also of first order with k(-60 "C) = (4.8 f 0.6) X lo5 s-l and 12(-100 "C) = (6.6 f 0.8) X lo4 s-l. Both values are slightly higher than the corresponding values at 470 nm; however, they still remain within experimental error. Due to the spectral overlap clear-cut interpretation of the decay rates is difficult. However, Freon-113 again supports the conclusion that a precursor of the 470-nm species must be a negative ion as well. Variation of Positive Charge Mobility. As it is proposed that positive charge transfer to CC13radicals from electron attachment to CCl, is occurring, it is of interest to study the influence of the positive charge mobility. For this purpose the solvents cyclohexane, methylcyclohexane, cyclooctane, and isooctane were compared (each experiment with M CC14). Cyclohexane and MCH are known to have a higher mobility for the positive charge than expected from diffusion-controlled processes: there is a resonance charge transfer i n v o l ~ e d ' ~(Table * ~ ~ 111). (17) J. M. Warman, P. P. Infelta, M. P. de Haas, and A. Hummel, Chem. Phys. Lett., 43, 321 (1976). (18) J. W. Buchanan and F. Williams,J. Chem. Phys., 44,4377 (1966). (19) (a) M. P. de Haas, J. M. Warman, and P. P. Infelta, Chem. Phys. Lett., 31,382 (1975); (b) A. 0. Allen, Natl. Stand. Ref. Data Ser. (US., NatE. Bur. Stand), No. 57 (1976). (20) E. Zador, J. M. Warman, and A. Hummel, Chem. Phys. Lett., 23, 363 (1973).
3270
Gremiich and Buhler
The Journal of Physical Chemistry, Vol. 87, No. 17, 1983
TABLE 111: Characteristics for Hole Mobilities in Hydrocarbon Solvents
solvent
Gfi(20 'C)IQb
cyclohexane me thy lcyclohexane cyclooctane isooctane
0.15 0.12 0.17 0.33
M+( 22 OC),19a c m z V-I s'l
k(solv+ t pyrene),20 M-I s-l
15 x 10-3 5 . 8 x 10-3
7 x 10" 2 x 10"
1 x 10-3
2 x 10'0
comments resonance charge transfer resonance charge transfer diffusion controlled cation fragmentation
a x 109
TABLE IV: Effect o f Solvent Variation o n Cationic Absorptions hl,,
solvent
nm
decay kinetics
thermodynamics of decay
methylcyclohexane
470
Eact = 1 3 . 9 i 1 . 3 kJ/mol log A = 9 . 0 4
cyclooctane
680
isooctane
450
first order for k , see Table I1 second order k z / e ( 2 0 "C) = 7.4 x lo7 c m s-l free-ion recombination first order k , ( - 6 0 "C) = (1.6i 0.2) X l o 6 s-' kl(- 100 "C) = (8.6 * 1.0)X l o 5 s-'
Cyclooctane (Ip= 10.06 eVleb) and isooctane (Ip= 9.86 eVlBa)are expected to follow diffusion limitsz0(at least a factor of 10 slower). The experiments with MCH are reported above (Tables I and 11); they serve as a reference system. Cyclohexane with a melting point of 6.5 "C was studied at room temperature (20 "C). The Viscosity of the system (vcH(20 "C) = 0.979 cp, 7cH(8"c)= 1.234 cPZ1)turned out to be too low to allow detection of the visible band. In the system with MCH at 25 "C (vMCH(25 "C) = 0.692 cPZ1)there was also no chance to detect the visible band. The temperature had to be lowered to -60 "C ( ~ ~ ~ ~"C) ( -=650cPZ2)to allow the observation of the 470-nm decay. The results with cyclooctane and isooctane are given in Table IV and Figure 3 together with the data for MCH. The cyclooctane solution is clearly unable to produce the 470-nm species. Instead the cyclooctane(+) cation with ,A, = 680 nm is seen. Its decay is second order with k / c (20 "C) = 7.4 X lo7 cm s-l. It corresponds to the free-ion recombination. The viscosity of cyclooctane at 20 "C with 2.6 cP is high enough to allow comparison with the MCH system. It is concluded that different charge mobilities might be the reason for the drastic difference between the systems (see Discussion). The results for the isooctane solution at -60 and -100 "C are surprising, if compared with cyclooctane, since from the point of view of charge mobility there should be no substantial difference. There is however a dominant visible band centered at 450 nm with its first-order decay being very similar to the 470-nm characteristics: low activation energy and very low preexponential factor.
Discussion New Reaction Model. From the results reported in this paper it is obvious that the 470-nm band simultaneously has a cation and an anion as precursors. This appears to be the reason for the early confusion in assignments CC14(ref 3, later revoked) or a cation of Ccl4.I The reported assignment to the charge-transfer complex C1.CC145r6produced from Cl- + CC4+recombination assumes that there is a simultaneous charge and energy transfer to allow for the ionization energy of CC14 (Ip(CC14)= 11.45 eV, Ip(MCH) = 9.85 eV16). There is so far no proof for such a transfer of an excited hole in the liquid hydrocarbon. A stronger argument against the CI.CC1, charge-transfer(21) Landolt-BBmstein, Zahlenwerte und Funktionen, Bd. 11,5. Teil, Springer-Verlag, West Berlin, 1969. (22) B. Hurni, unpublished, 1975.
C(CH,),+ within (C(CH,),C ~~c1-)sok
E,,
= 4.8 f 1.0 kJ/mol log A = 7 . 4
I
0 .
400
I
500
assignment CCl,' within (CC13+I Icl-)& cyclooctane ( t ) free cation
I
600
A(nm)
I 700
I 800
Effect of Solvent Variation (all spectra: 280-300 Gy,0.5 after pulse). Spectrum 1: MCH with lo-' M CCI, at -100 "C (7= 5 cP). Spectrum 2: cyclooctane with lo-' M CCI, at 20 ' C (qm = 2.6 cP). Spectrum 3: isooctane with lo-' M CCI, at -100 "C.
Flgure 3. ps
complex assignment is derived from our theoretical correlation of the charge-transfer energy with the ionization potential of electron donor involved (CC14) for a fixed electron acceptor (C1 atom).23 From this correlation the charge-transfer band for Cl.CC14 is expected at about 280 nm (4.5 eV), not at 470 nm (2.6 eV). If the latest assignment to an ion pair (alkane(+)-Cl-) holds, then one should be able to detect the absorption band of the alkane(+) (typically in the 600-900-nm range) simultaneously with the ion complex band at 470 nm. This is not the case. Therefore, we are forced to propose a new assignment in the context of a new reaction model, which takes care of all the known experimental findings. Our model is based on the similarities in spectral characteristics and decay kinetics to the 500-nm band in pure CC14 (Table I). It is proposed therefore that the 470-nm band is also due to the CCI3+within a solventseparated ion pair ( C C l ~ + ~ ~ C l ~The ) 8 0 spectral ~v. shift by 30 nm relative to the CC14solution is related to a solvent effect on CC13+. We have already reported on a classical electrostatic model calculation of the solvent effect1' (dielectric characteristics and polarizability of solvent and ions) which indicated for both solvents, CC14and MCH, a potential minimum for the solvent-separated ion pair. The (CCl3+IIC1-),,, ion pair is produced as shown in Figure (23) R. E.Biihler, Radiat. Res. Reu., 4, 233 (1972); for the chargetransfer complex Cl.CC1, see Figure 2 and Table 10.
Charged Species in Liquid Alkane-CCI, Systems
MCH
+
MCH+
e'
I I
MCH+
+
cci;
+
CI-
-
geminate d i s t r i b u t i o n
MCH
+
CCI;
+
Ci-
I
t
(cci;
11 ci-)
so1 v
Flgure 4. Reaction model for the charged species In llquid methylcyclohexane.
4. The reaction model is based on the recognition that a CC13 radical, as produced from the dissociative electron attachment to CC14, is located within the range of the geminate ion separation, and therefore is ready to accept the positive charge from the geminate alkane cation (Ip(MCH) = 9.85 eV, Ip(CC13)= 8.78 eV21). The diffusion of the CC13 radical away from the C1anion is competing with the alkane(+) cation approaching the anion. For large geminate ion separations the CCl, radical might have escaped before the cation comes close to the anion. A simple diffusion calculation based on Brown's law indicates that the CCl, escape from C1- is rather slow (for MCH at -60 OC: 0.6 A in 100 ps, 2 A in 1 ns, 15 A in 50 ns). Therefore, there is rather little chance for CC13 to escape from C1- within the time scale of geminate recombination. In other words, when the geminate cation approaches the anion C1-, the CC13 radical, in most cases, is still there. Furthermore, a simple electrostatic calculation for the interaction between the two geminate ions and the dipole moment of CCIB( p = 0.35 D%),where the latter is located between the ions, has been carried out. It was found that the interaction energy is greater than thermal for ion separations smaller than about 10 A. This means that CC13 may orient itself within the field of the geminate ions. (24) p(CC1,) = 0.35 D as calculated from data in ref 11. (25) J. B. Hendrickson, D. J. Cram, and G. S. Hammond, "Organic Chemistry", 3rd ed., McGraw-Hill, New York, 1970. (26) C. Capellos and A. 0.Allen, J. Phys. Chem., 72, 4265 (1968). (27) Another alkane cation absorbing at 450 nm was reported by P. W. F. Louvrier and W. H. Hamill for 2,2-dimethylheptane (J.Phys. Chem., 72, 3878 (1968)). Since all other alkane cations so far known absorb at longer wavelength, this 450-nm band moat likely also corresponds to the tert-butyl cation. (28) G. A. Olah, J. R. DeMember, A. Commeyras, and J. L. Bribes, J. Am. Chem. SOC.,93,459 (1971). (29) J. M. Warman, "Proceedings of the NATO Advanced Study Institute on Fast Processes and Labile Species in Chemistry and Molecular Biology", Capri, Italy, Sept 1981" (see data for cyclohexane and transDecalin). (30) B. Hurni and R. E.Btihler, Chimia, 29, 526 (1975). (31) H.-U. Gremlich, Thesis, ETH-Ziirich, Zurich, Switzerland, No. 6458, 1979 H. U. Gremlich and R. E.Biihler, presented at the IUPAC Conference, Vancouver, British Columbia, Canada, 1981.
The Journal of Physical Chemistry, Vol. 87, No. 17, 1983 3271
For the free ions, due to their large separations, the diffusional escape of CC13 dominates. The chance for the alkane cation to find a CC13 radical before these radicals have reacted otherwise is very minor. These conclusions fit the experimental finding that no free-ion contribution can be detected. The yield values for the CC13+cations, as calculated from C ~ , ~ ( C C=~6000 ~ + )f 1200 M-' cm-' (taken as identical with cm(CC13+)in CCl4lo)are given in Table 11. These Govalues are much smaller than any expected total ion yields in alkane solutions and also smaller than the corresponding CC13+yield in pure CC14 (Go(CC13+)= 2.5 in CC1410).This is due to the competitive situation as described above. There are two ways by which CC13 radicals may miss the positive charge (a) by the CC13 escape from the region of the geminate ions and (b) by the very short distant geminate ions, where neutralization happens with thermalized electrons, before electron attachment to the C C 4 occurs (average separation between CC14 molecules in a M solution is about 60 A). The experimental G(CCl,+IICl-) of about 0.43 at -120 "C actually corresponds roughly to half of the geminate ions scavenged by CC1t2as estimated from the square root law of ~cavenging,~ assuming a = 10 M-' for CC14 in MCH. Scavenger Effects. The model makes obvious that both the electron scavengers as well as cation scavengers affect the yield of CC13+. The details of their effect can be discussed as follows: For M aniline, the yield of aniline(+) is substantially higher than the suppressed yield of CC13+. This means that M aniline competes not only with CC13 but also directly with alkane cations; there is also a contribution from the free MCH+ cations and from the geminate MCH+ cations, which missed the CCl, radical. The fact that we measure such a large aniline(+) yield in the microsecond range indicates that aniline(+) might also be stabilized as an ion pair (aniline(+)lICl-);otherwise, the aniline(+) yield should have already dropped to Gfi. For 5 X lo4 M TMPD, with G(TMPD+) 2 0.12 it must be concluded that this low concentration of TMPD is primarily scavengingthe free alkane cations. However due to the 20-30% reduction of the 470-nm band TMPD also competes for the positive charge: by scavenging either the precursor of CC13+(e.g., the geminate alkane cation MCH+) or the CC13+within the long-lived ion pair. As the concentration of TMPD is rather low, direct scavenging of geminate MCH+ is expected to be small, so that the dominant part of the 30% reduction in 470-nm absorbance seems to relate to the reaction of TMPD with the ion pair: (CC13+IICl-) + TMPD
-+
TMPD+
(Ip(TMPD)= 6.7 eV, I (CCl,) = 8.78 eV). As we cannot observe a TMPD+ yiedhigher than the free-ion yield, we must conclude either that TMPD+ cations immediately disappear by neutralization with the adjacent C1- or that a possible ion pair (TMPD+IICl-) has a shorter lifetime than our time resolution. Cyclohexene (in a M solution) is scavenging by charge transfer like aniline and TMPD. The ionization potential I (cyclohexene) = 8.9 eV is higher in this case than I,(C&) = 8.78 eV. A direct reaction with the ion pair is therefore not possible. The precursor MCH+ is reduced by cyclohexane. Cyclopropane (saturated solution) typically reacts with alkane cations by H or H2transfer" but is not expected (32) We thank one of the referees for pointing out this fact. (33) J. M. Warman, K.-D. Asmus, and R. H. Schuler, J.Phys. Chem., 73, 931 (1969).
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to react with CC13+. The experimental facts agree with this expectation. Ethanol M solution) is unable to reduce the CC13+ yield in spite of an expected H+transfer from the alkane(+) cation. It means that the scavenging process is too slow in comparison to the charge transfer. Ethanol is still expected to scavenge the free alkane cation; however, this is without effect on the 470-nm band. Due to the high polarity ethanol preferentially takes part in the solvation of the ion pair. Its high nucleophilic character relative to C1- then promotes fast reaction with CC&+to yield trichloromethyl ethyl etheraZ5 SF6(saturated solution) and Freon-113 M solution) are direct competitors for the electron. Therefore, fewer CCl, radicals are available for the charge-transfer process. There is no effect on the ion-pair decay process, as expected. Positive Charge Mobility. The results for MCH and cyclooctane make clear that the rate of charge transfer is very important for the competition between the CCl, escape from the C1- and the mobility of the positive charge toward its geminate C1-. In MCH the charge approach toward C1- (i.e., toward the CC13radical) is about 20 times faster than in cyclooctane (see rate constants in Table 111). As a consequence, in MCH the CC13+is produced and due to the ion-pair formation displays a much longer life than expected for the geminate ions ( T ~ (ion / ~ pair) = 1.5, 5.8 and 33 p s for -60, -100, and -120 "C, respectively). In cyclooctane only the cyclooctane cation is detected. Its yield corresponds to the free-ion part. For isooctane, results analogous to cyclooctane were originally expected. Instead an absorption centered at 450 nm appeared with decay characteristics again similar to ion-pair decay and not typical of free ions. It has been shown earlier26that the initially produced parent cation of isooctane quickly fragments into a tert-butyl carbonium ion. This fragment ion (I,(tert-butyl radical) = 7.4 eVz6) is unable to transfer the charge onto CCl, !Ip= 8.78 eV). Therefore, on ion recombination, an ion pair (C(CH,),+lICl-)mlvis formed fully analogously to the (CCl3+IICl-),l, ion pair. The fragment ion +C(CH3),absorbs at 450 nmn and the decay of this new ion pair is again characterized by a low activation energy of 4.8 kJ/mol and a very low preexponential factor of log A = 7.36 (Table Iv). Following infrared and Raman studies by Olah et aLZ8the tert-butyl cation has CBUsymmetry in solution and therefore the positive charge on the carbon center is more apt to interact with a solvent molecule (shared by the two ions of the ion pair). Temperature Effect. From the temperature dependence of the rate constants, the thermodynamics of the ion-pair decay processes are derived, as discussed above. However, the strong increase of the initial yield G,(ion pair) (Table 11) with lower temperature is not explainable by the slowdown of the decay process. Within our reaction model this effect must be correlated to differences in the initial, very fast, competing processes: the resonance chargetransfer process of the solvent cation (MCH' mobility) toward the negative species is expected to be much less temperature dependentz9 than the diffusional escape process of the CC13radical from the C1- anion. As a consequence, fewer CC13 radicals are able to escape from the geminate ion range at lower temperature and therefore
Gremlich and Buhler
more CC13+cations are produced.
Conclusions (1)The model of charge transfer from the solvent cation to a CCl, radical within the range of geminate ions is able to clarify substantially the mystery of the visible band being formed in many alkane solutions with CC4. (2) The reactions in this very early time scale (geminate ion recombination) appear to be very sensitive to weak interactions with solvent molecules and to the microscopic movement of neutral and charged species within the field of the ions. (3) Due to the stability of the ion pairs being formed in the very early stage of the ion recombination, they serve as a direct probe in these extremely fast ionic processes. (4) In systems where such solvent-separated ion pairs are formed, the geminate ion neutralization is a two-step process: recombination of ions, followed by neutralization within the ion pair. We called this "a delayed geminate ion neutrali~ation".~ (5) So far we have identified the following solvent-separated ion pairs: ion pair
ref
first reported
(CCl,' I c c l , l C l - ) c ~ ~ ( c c i , +I M C H I C ~ - ) & (CC1,' \FreonIC1-)Freon-,, (( CH,),C+ 110ICl-),o
9, 10 this paper 14 this paper
1975,O 1979,l 1975,O 197g31
(6) From these examples we suggest the following generalized conditions for ion-pair formation: (i) The stability of solvent-separated ion pairs seems to be based on the weak interaction with the solvent. It is critically dependent on the polarizability and dielectric characteristics of the solvent. Some solvent might be unable to stabilize such ion pairs. No quantitative statement is possible at the present time. (ii) The cation of the pair should correspond to a lower ionization potential than the one of the solvent, to avoid immediate charge transfer to the separating solvent molecule. Contact ion pairs are expected to collapse immediately.'l This means that there is little chance for the (MCH+IMCHIC1-),,lv ion pair as proposed by Bos et ale7 (iii) If CC13+is produced from the charge transfer to CCl, from the anion dissociation, then the following conditions seem to be necessary: (a) a very fast positive charge transfer is required (resonance charge transfer), (b) the geminate ions involved must be short distant, and (e) the ionization potential of the solvent must be greater than IP(CCl3). Further experiments in support of the charge-transfer model and of the ion-pair formation, as presented in this paper, are in progress.
Acknowledgment. We thank G. Natterer for his valuable technical assistance and the Swiss National Science Foundation for the support of this project. Registry No. CC14,56-23-5; CC13+,27130-343; Cl-, 16887-00-6; C(CH3)3+, 14804-25-2; CF2ClCFC12, 76-13-1; SF,, 2551-62-4; freon-113-, 75358-67-7; aniline, 62-53-3; Nflfl'fl'-tetramethyl1,4-phenylenediamine, 100-22-1; cyclohexene, 110-83-8; cyclopropane, 103-65-1; ethanol, 64-17-5; cyclooctane+, 62667-71-4; cyclohexane, 110-82-7; methylcyclohexane, 108-87-2;cyclooctane, 292-64-8; isooctane, 540-84-1.
