1946
The Journal of Pbysicai Chemistry, Vol. 83, No. 15, 1979
15-207'0 of the toluene reacted on a molar basis),16 in agreement with previous studies,16-18while in irradiations of simulated polluted atmospheres with the xylene isomers added one at a time, a-dicarbonyls were observed only for o-xylene addition.43 This latter observation is probably due to a combination of the expected rapid reaction of the a-dicarbonyls glyoxal and methylglyoxal, which are presumably formed from m- and p-xylene, with OH radicals [analogous to the a l d e h y d e ~ ~ together l , ~ ~ ] with analytical difficulties for the gas chromatographic detection of these two a-dicarbonyls.
Acknowledgment. The authors gratefully acknowledge the assistance of F. R. Burleson and G. C. Vogelaar for carrying out the gas chromatographic analyses, and W. D. Long for valuable assistance in conducting the chamber experiments. This work was supported in part by the National Science Foundation, Applied Science and Research Applications (Grant No. NSF-ENV78-01004), References and Notes ( 1 ) A. C. Lloyd, K. R. Darnail, A. M. Winer, and J. N. Pitts, Jr., J . Phys. Cbem., 80, 789 (1976). (2) C. T. Pate, R. Atkinson, and J. N. Pitts, Jr., J . Environ. Sci. Health, All, l(1976). (3) B. J. Finlayson-Pittsand J. N. Pitts, Jr., Adv. Environ. Sci. Jecbnoi., 7, 75 (1977). (4) D. D. Davis, W. Bollinger, and S.Fischer, J . Phys. Chem., 79, 293 ( 1 975). (5) R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chem., 81, 296 (1977). (6)T. M. Sloane, Chem. Phys. Lett., 54, 269 (1978). (7) M. Hoshino, H. Akimoto, and M. Okuda, Bull. Chem. Soc. Jpn., 51, 718 (1978). (8) H. Aklmoto, M. Hoshino, G. Inoue, M. Okuda, and N. Washida, Bull. Chem. Soc. Jpn., 51, 2496 (1978). (9) R. A. Kenley, J. E. Davenport, and D. G. Hendry, J . Phys. Cbem., 82, 1095 (1978). (10) A. R. Ravishankara, S.Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Watson, G. Tesi, and D. D. Davis, Int. J. Chem. Kinet., 10, 783 (1978). ( 1 1 ) G. R. H. Jones and R. J. Cvetanovic, Can. J. Chem., 39,2444 (1961). (12) E. Grovenstein, Jr., and A. J. Mosher, J . Am. Chem. Soc., 92,3810 (1970). (13) J. S. Gaffney, R. Atkinson, and J. N. Pitts, Jr., J . Am. Cbem. Soc., 98, 1828 (1976). (14) D. Grosjean, K. Van Cauwenberghe, D. R. Fitz, and J. N. Pitts, Jr., presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 12-17, 1978.
Y. Wang, J. J. Tria, and L. M. Dorfman (15) K. R. Darnall, R. Atkinson, A. Glangetas, A. M. Winer, and J. N. Pitts, Jr., Envlron. Sci. Tecbnol., submitted for publication. (16) J. M. Heuss and W. A. Glasson, Envlron. Sci. Technoi., 2, 1109 (1968). (17) A. P. Altshuller, S.L. Kopczynskl, W. A. Lonneman, F. D. Sutterfield, and D. L. Wilson, Environ. Sci. Technoi., 4, 44 (1970). (18) C. W. Spicer and P. W. Jones, J . Air. Pollut. Control Assoc., 27, 1122 (1977). (19) J. N . Pltts, Jr., K. R. Darnall, A. M. Wlner, and J. M. McAfee, EPA-600 13-77-01 4b (1 977). (20) A. M. Winer, R. A. Graham, G. J. Doyle, P. J. Bekowies, J. M. McAfee, and J. N. Pitts, Jr., Adv. Environ. Sci. Tecbnol., In press. (21) J. H. Beauchene, P. J. Bekowies, J. M. McAfee, A. M. Winer, L. Zafonte, and J. N. Pitts, Jr., NASA Spec. Pubi., No. 336, 811-825 (1973). (22) G. J. Doyle, P. J. Bekowies, A. M. Winer, and J. N. Pitts, Jr., Environ. Sci. Technoi., 11, 45 (1977). (23) NO2and N0,vaIues were corrected for response to PAN [A. M. Winer, J. W. Peters, J. P. Smlth, and J. N. Pitts, Jr., Environ. Sci. Tecbnoi., 8, 1118 (1974)]. (24) E. R. Stephens, "Hydrocarbons in Polluted Air", Summary Report, Coordinating Research Council, Project CAPA-5-68, June 1973, NTIS PB 230 993/AS. (25) A. Glangetas, unpublished results. (26) R. Atkinson, D. A. Hansen, and J. N. Pitts, Jr., J . Cbem. fbys., 62, 3284 (1975); 63, 1703 (1975). (27) R. Atkinson and J. N. Pitts, Jr., J. Cbem. Phys., 68, 3581 (1978). (28) N. R. Greiner, J . Chem. Phys., 53, 1070 (1970). (29) R. P. Overend, G. Paraskevopoulos, and R. J. Cvetanovic, Can. J. Cbem., 53, 3374 (1975). (30) C. J. Howard and K. M. Evenson, J. Chem. Pbys., 64, 4303 (1976). (31) R. Atkinson, K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pltts, Jr., Adv. Pbotocbem., 11, 375 (1979). (32) J. A. Kerr, M. J. Parsonage, and A. F. Trotman-Dlckenson, "Strengths of Chemical Bonds" In "Handbook of Chemistry and Physics", 56th ed, The Chemical Rubber Co., Cleveland, Ohio, 1975-1976. (33) A. J. Haagen-Smit, C. E. Bradley, and M. M. Fox, Ind. Eng. Cbem., 45, 2086 (1953). (34) R. P. Taylor and F. E. Biacet, Ind. Eng. Cbem., 48, 1505 (1956). (35) G. B. Porter, J . Chem. Pbys., 32, 1587 (1960). (36) N. Padnos and W. A. Noyes, Jr., J. Phys. Cbem., 68, 464 (1964). (37) J. J. Bufalinl, S. L. Kopczynski, and M. C. Dodge, Environ. Left.,3, 101 (1972). (38) R. Atkinson, W. P. L. Carter, A. M. Winer, and J. N. Pitts, Jr., to be published. (39) E. Grovenstein, Jr., and A. J. Mosher, J. Am. Chem. Soc., 92, 3810 (1970). (40) C. T. Pate, R. Atkinson, and J. N. Pitts, Jr., J . Environ. Health Sci., All, 19 (1976). (41) D. G. Hendry and R. A. Kenley, J. Am. Cbem. Soc., 99,3198 (1977). (42) R. A. Cox and M. J. Roffey, Environ. Sci. Tecbnoi., 11, 900 (1977). (43) A. M. Winer, K. R. Darnall, R. Atkinson, and J. N. Pitts, Jr., Envlron. Sci. Tecbnoi., in press. (44) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, J . Phys. Chem., 82, 132 (1978).
Identity and Yield of Positive Charge Centers in Irradiated Chloro Hydrocarbon Liquids and the Rates of Their Interaction with Solute Molecules' Ylng Wang, John J. Trla, and Leon M. Dorfman" Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Recelved January 29, 1979) Publication costs assisted by the U.S. Department of Energy
Pulse radiolysis studies of the formation kinetics and the yields of various phenylcarbenium ions from several different solutes in 1,2-dichloroethane solution have been carried out. The results indicate that there are two kinetically distinguishable cationic species of the solvent which react selectively with the different solutes to form the phenylcarbenium ions. We suggest that one is a cation radical (yield 0.68 molecule/100 eV) and the other a carbocation (yield 0.20 molecule/100 eV). Rate constants for their separate reactions with selected aromatic compounds and with ammonia have been determined. Molar extinction coefficients have been estimated for benzyl cation, diphenyl cation radical, and anthracene cation radical. In our fast kinetic studies of carbocations in s o l u t i ~ n , ~ - ~ we have determined values for the rate constants of formation6 of these carbocations, we have indicated clearly our attention has been directed principally to the reactivity that the reactions observed, for which rate constants range of the various phenylcarbenium ions generated, Although 0022-3654/79/2083-1946$01.00/00 1979 American Chemical Society
The Journal of Physlcal Chemlshy, Vol. 83, No. 15, 1979
Reactive Charge Centers in Irradiated Chloro Hydrocarbons
from 4 X lo7 to 1.6 X 1O1O M-l s-l, may involve more than a single cationic species of the solvent as precursors. We have now investigated some of the details of the formation processes, and report here on the possible identity and the yield of the reactive positive charge centers in the irradiated chloro hydrocarbon solvent, and the manner in which charge is transferred to the solute molecules to produce the carbocations of interest. Three types of information about these reactive positive charge centers indicate clearly that two kinetically distinguishable cationic solvent species are involved in the charge transfer to solutes. These are their yields, the competition kinetics in some selected reactions, and the optical absorption spectra of the products formed with a specific scavenger.
Experimental Section As previously d e s ~ r i b e d , the ~ t ~ source of the electron pulse was a Varian V-7715A electron linear accelerator, delivering 3-4-MeV electrons at a pulse current of about 300 mA for pulses of 100-1400-ns duration and 600 mA for pulses of 80-11s duration or less. 80-11s pulses were used in all the present experiments. The concentration of transients produced was on the order of lo4 M. Transient optical absorption signals were monitored with an RCA 1P28, RCA 7200, or RCA HTV 196 photomultiplier as detector. A Bausch and Lomb grating monochromator, Type 33-86-25, was used with various gratings which had dispersion factors of 6.4, 7.4, and 12.8 nm/mm. Exit slit widths were typically 0.25 mm. Appropriate Corning filters were used to minimize any photodecomposition and to exclude any second-order light. Bromodiphenylmethane, technical grade, was obtained from Chemical Samples Co.; triphenylcarbinol, 97%, from Aldrich; triphenylmethyl bromide from J. T. Baker, and dibenzylmercury from Alfa Inorganics. All the foregoing were purified as previously de~cribed.~ Benzhydrol, 99%, was obtained from Aldrich and was purified by crystallization from a solution containing equal volumes of triply distilled water and absolute ethanol (Commercial Solvents Co.). 1,l-Diphenylethylene, 97%, and 2,4-dimethyll,&pentadiene, 98%, were both obtained from Aldrich; the former was purified by partial freezing, the latter by vacuum distillation. Diphenyl, 999’0,was obtained from Chemical Samples Co. The solvent, 1,2-dichloroethane, Aldrich, 99% Gold Label, was purified by refluxing over reagent grade PzO5, and then distilling, with the first and last 15% fractions being discarded. After repeated cycles of freezing, evacuating and thawing, it was vacuum distilled into a storage bulb containing Linde type 4A molecular sieve which had been freshly prepared by heating at 200-300 OC, under vacuum (lo4 torr) for about 2 days. All the purified materials were stored under proper conditions. Pulse radiation dosimetry was performed in situ with the same 2 cm length reaction cells and the same optical detection system used in the reaction studies, with a double pass of the analyzing light beam. Two separate dosimeters were used, potassium thiocyanate (2 X M)9J0 and potassium ferrocyanide (1 X M).ll For Fe(CN)2- we took €420 = 1000 M-I cm-l, and since our observed ratio of E ~ ~ ~ ( C NtoS C ) ~~- ~ ~ ( F ~ ( C was N )7.6 ~ ~f- 0.15, ) we used e4,5(CNS)~= 7600 M-l c d after Baxendale.’O G(0H) was taken as 2.8 molecule/100 eV.12 Typically the absorbed dose in water with an 80-11s pulse was 1.0 X l O I 7 eV/g. Pulse reproducibility was h39’0 over periods of a few hours. The appropriate correction for stopping power was approximated by using the “electron density”, Z / A , to determine the dose absorbed in 1,2-dichloroethane (DCE) relative to that absorbed in water.13
Concentrotion of Ph2CHOH ( M x I O 2 ) 2 44 6 8 4 IO 12
2 0
-__
20 N
0
*
:
-
0
-
i t
.
-
%
2
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -,
16-
c
1947
-10 0
-8
z 0
.
‘2-
-6 E
FQ.#+?ii ----
- - _ -lk
-4
0
Ei U X
-2
0
I
I
~
I
I
I
I
I
I
l
~
I
0~
EN I
Results and Discussion Yields of Positive Charge Centers. It was found early on in this work that the relative yield of a particular phenylcarbenium ion was markedly dependent upon the nature of the compound used as its precursor, Le., upon the compound scavenging the solvent cationic species. This suggested the presence of more than one cationic species of the solvent with selective reactivity toward different solutes. Accordingly, we determined the radiation chemical yields of phenylcarbenium ion in the following different precursor systems: benzyl cation from dibenzylmercury, benzhydryl cation from both bromodiphenylmethane and diphenylmethanol, and trityl cation from both bromotriphenylmethane and triphenylmethanol, in 1,2-dichloroethane solution. It was necessary, first, to establish that the yield of each cation reached a well-defined maximum as a function of the precursor concentration, an indication that the cationic species of the solvent were being completely scavenged. Such yield-composition curves are shown in Figure 1for PhCH2+and Ph2CH+,and in Figure 2 for P h Q , in which the relative optical density at the absorption maximum of the carbenium ion is plotted against precursor concentration. The curves all reach a well-defined plateau. These
I
1948
The Journal of Physical Chemistry, Vol. 83, No. 15, 1979
TABLE I: Yields of Phenylcarbenium Ions for Different Precursor Compounds in DCE Solution molar extinction G value cation precursor coeff of cation (molecule/ obsd comwd ( M - I cm") 100 e V )
PhCH,' Ph,CH+ Ph,CH+ Ph,C* Ph,C*
(PhCH,),Hg Ph,CHBr
0
["3l/[Ph3COHI .04 .06
.02 I
I
1
'
1
,
0.67 0.87 0.18 0.86 0.21
16 000 38 000 38 000 40 700 40 700
Ph, CHOH Ph,CBr Ph,COH
Y. Wang, J. J. Tria, and
.08 I
L. M. Dorfman
,
.IO
0
0
91"
D
7
maximum optical densities, after the small, but necessary corrections for any spectral overlap of other species generated, were used together with the pulse dosimetry and the molar extinction coefficient of each phenylcarbenium ion observed, to determine the yield of phenylcarbenium ion and hence the yield of the solvent cationic species from which it is formed. The results obtained are shown in Table I. The extinction coefficients for benzhydryl cation14and for trityl cation15 are taken from the literature, the latter value being the mean of two very similar values. The extinction coefficient for benzyl cation, 16 000 M-'cm-l, was estimated in the following way from our own kinetic constants for the natural decay (second order) of the phenylcarbenium ions. We had previously determined the rate constants6 for the reactions Ph3C+ C1- and Ph2CH+ C1- from second-order decay curves by using the extinction coefficients in Table I, the values being respectively 8.0 X 1O1O and 9.1 X 1O1O M-l s-l. From this it is reasonable to assume that the rate constant for the analogous reaction of benzyl cation, namely, PhCH2+ C1-, will have a rate constant of 10.0 X 1O1O M-l s-l. Using this value and the linear second-order plots of several decay curves for benzyl cation (slope = k / 4 , we obtain E = 16000 at 364 nm. The ratios of the yields from the bromide to that of the carbinol are not dependent on the extinction coefficients, but the absolute magnitudes are. The agreement for benzhydryl and trityl is excellent. The experimental uncertainty in these yields, apart from the uncertainty in the extinction coefficients, is about f 5 % . The yields in Table I lead to the obvious suggestion that there are two separate solvent cationic species reacting with the solute, with yields of 0.67 and 0.20 molecule/100 eV. Only one of them reacts with the carbinols, the other reacts with dibenzylmercury, and both react with the bromides. We suggest that one is a radical cation of the solvent and the other a solvent cation formed after C-C1 cleavage. Competition Kinetics. It seemed worthwhile to determine whether the two solvent cationic species, which were characterized by different yields, were also kinetically distinguishable, In that event, their existence and their separate roles in reacting with solute molecules would be further confirmed. Accordingly we carried out a series of competition kinetic experiments in which we monitored (by determining the relative yields of phenylcarbenium ion) the competition for the solvent cationic species between the precursor compound of the phenylcarbenium ion on the one hand and ammonia, which is known4 to scavenge cationic species, on the other:
+
+
+
s++ P S+ + NH,
P+ + s
-
---*
products
(1)
(2)
This competition was carried out with both [PI and [NH,] high enough to compete overwhelmingly with the natural decay of the solvent cation, namely, S+ + C1-. The concentration of P was chosen at, or near, the plateau of its yield curve. The procedure followed was to determine the
n
I
N+
Figure 3. Competition kinetics plots in DCE solution following 80-ns pulses: X , benzyl cation at 364 nm (dibenzylmercury competing with ammonia); 0 , trityl cation at 434 nm and, 0, trityl cation at 409 nm (both for triphenylmethanol competing with ammonia).
initial optical density of P+without added ammonia, ODp+, and with added ammonia, ODlp+, over a range of concentrations. For the competition consisting of reactions 1 and 2, these optical densities are related by -ODp+ -
kz["31 I=.-----(3) OD'p+ klP1 A plot of (ODp+/OD'p+)- 1against [NH3]/[P] will give a straight line with slope = k z / k l . Since kl is known6 for various P molecules, the value of k z may be determined for different P. The purpose of the experiment then is to determine whether values of k2 depend on the choice of P used in the competition. If this were so, as turns out to be the case, then clearly S+ cannot represent a single solvent cationic species, but must represent kinetically distinguishable species, S1+ and Sz+ (as indicated by the yield data), each having a different reactivity toward ammonia. Such experiments were carried out with dibenzylmercury, trityl alcohol, and benzhydryl bromide as precursor compounds. In each case the initial optical density of P+ a t the middle of the 80-11s electron pulse was determined by extrapolation back of the rate curve (for the decay of P+ in the reaction P+ 4- NH3 as well as its combination with the counterion). The competition for S+ between dibenzylmercury and ammonia was carried out over roughly a 50-fold range of [NH3]/[(PhCHz)2Hg]. A plot of eq 3 for these data is shown in Figure 3. The slope of the straight line obtained is 0.52. Since the value of kl, the formation contant for benzyl cation from dibenzylmercury,6 is 1.3 X 1O1O M-' s-l, we obtain for the ammonia reaction rate constant, let us call it kza, the value (6.8 f 1.5) X lo9 M-l s-l. A confirmation of the value of kZa,as well as some indication of the nature of the solvent cation, S1+, can be obtained from the reaction with diphenyl. If S1+is a radical cation, it should reactlBwith diphenyl to produce the diphenyl radical cation as product, and should yield the same value of k2a in the competition: SI+- Phz = Phz+*9. Si (la>
+ SI++ NH3 = product
(24
The value of kla is known from earlier work1' to be 1.3 X 1O'O M-' s-l, From competition experiments carried out over a 20-fold range of [NH,]/[Phz], in which the yield of Phz+., which has an absorption band a t 690 nm, was
The Journal of Physical Chemistry, Vol. 83, No. 15, 1979 1949
Reactive Charge Centers in Irradiated Chloro Hydrocarbons
monitored, we obtain k2, = (5.6 f 1.1)X lo9 M-l s-l. This is in good agreement with the value 6.8 X lo9 obtained in the dibenzylmercury experiments. The mean is 6.2 X lo9 M-1 s-l Simiiar data for the competition for S+ between trityl alcohol and ammonia were obtained over roughly a 40-fold range of [NH,]/[Ph,COH]. The plot of eq 3 for these data is shown in Figure 3. The straight line obtained has slope = 23.1. Since the value of kl,the formation constant for trityl cation from trityl alcohol6 is 5.7 X lo8 M-’ s-l , we obtain for the ammonia competition constant in this case, k2b = (1.3 f 0.4) X 1O1O M-l s-l. Clearly the values of kza and kzb are different, from which we must conclude that in the two cases the different P molecules and ammonia are reacting with distinguishable solvent cationic species:
-
S1+ + NH3 product kza = 6.2 x 109 Sz+
+ NH3
k2b
-
product = 1.3 X lolo
(24
Ob)
as indicated by the yield data. With benzhydryl bromide as the precursor molecule, in similar experiments, a more complex situation obtains, if the conclusion drawn from the yield data is correct, for this compound reacts with both S1+ and Sz+. The reaction scheme will then be represented by
S1++ PhzCHBr Sz+ + PhzCHBr
-
Ph2CH+ + Br
+S PhzCH+ + Br + S
(4)
(5)
along with reactions 2a and 2b. Solution of the kinetic equations for this reaction scheme yields h[Si+]o k2,X
-- - + OD’p+
ODpt
kq
+
k5[Sz+]o k2bX
k5
+
[Sl+IO+ [SZ+lO
(6)
where [Sl+l0and [S2+l0are the initial concentrations of these solvent cationic species, and X = [“,I/ [Ph2CHBr]. Unlike eq 3, this formula will not generally yield a straight line. Taking [Sl+]o/[Sz+]o= 3.3 from the yield data, and k4 k5 = 1.6 X 1O1O M-’ s-l as determined previously: since we have determined kza = 6.2 X lo9 M-l s-l, and k2,, = 1.3 X 1O1O M-l s- l, the only unknown quantity is k5,which was then determined by varying its value to obtain the best fit of the calculated curve for eq 6 to the experimental data points. The best fit was obtained with k5 = (4 f 1) X lo9, as shown in Figure 4, where two other calculated curves have been included to show the effect on the curve of varying kb A small improvement in the fit can be obtained if both kza and k5 are allowed to vary. Optimum fit is then obtained with k2, = 5.6 X lo9 (well within error of the 6.2 X lo9experimental value) and k5 = (4 f 1) X lo9. Because the curvature of the calculated line is small with respect to the substantial scatter of the data points, this simultaneous variation of both kzaand k5 in the calculated curve is not a very sensitive test for k2,. However, it is noteworthy that for all choices of hz, between 6.2 X lo9 and 5.3 X lo9, best fit is always obtained with h5 = (4 f 1) X lo9. The value k4 = 1.2 X 1O1O M-l s-l, which follows from this value of 1125, is typical for a diffusion-controlledelectron transfer reaction in this solvent.16 Identity of S1+and S2+.It seems firmly established by the foregoing data for the yields and by the competition kinetic studies that the radiolysis of DCE forms two kinetically distinguishable solvent cationic species which may react selectively with different precursor compounds to
+
i1
Q 1 1 00
10
05
[N H3
/[
15
P W Br
Figure 4. Competition kinetics plot for dibenzyl bromide competing with ammonia in DCE solution. Optical density of benzhydryl cation taken at 449 nm. Curves computed from eq 6 with k , = 1 X los,---; 4 X log, -; and 7 X IO9,
-----..
form the respective phenylcarbenium ions. The yields of the two species are 0.67 and 0.20 molecule/100 eV, with the larger yield species identified as the one reacting with ammonia with a rate constant of 6.2 X lo9 M-l s-l, To attempt to identify these species we turn to some observations in the gas phase by photoionization mass spectrometry and ion cyclotron resonance spectroscopyls along with previous radiolytic data in low temperature g l a s ~ e s l ~as- well ~ ~ as in liquid DCE.l6lz3 From the work in glasses and in liquid DCE, it is well-known that aromatic solutes such as diphenyl, anthracene, and many others form the respective radical cations upon irradiation of their solutions, very likely by charge transfer with radical cations of the solvent. In the gas phase studies,18both the parent radical cation, C1CH2CH2Cl+.,and the radical cation, C1CHCH2+. (formed by detachment of HCl), were observed. We suggest that one or both of these radical cations are responsible for the yield of 0.67 molecule/100 eV, identified with S1+,a proposal which is substantiated by further experimental evidence. In our work, these two radical cations would be indistinguishable if they have similar rate constants for the charge transfer reactions. The other species, S2+,may be a chloroethyl cation, CICHzCHz+,formed by cleavage of a C-C1 bond in DCE. This carbenium ion may be present in the form of a cyclic chloronium ionla CICH,CH,+ e CH,-CH,+ \ I
c1
(7 1
which is estimatedz4 to be more stable by about 15 kcal/mol than the acyclic form. Titration with Diphenylethylene. Further supporting evidence for the identification of S1+as a radical cation, and particularly of Sz+as a solvent carbenium ion (these species do not have optical absorption bands in an accessible region of the spectrum) was obtained by “titrating” with a reactant which yields a unique and observable product. We chose 1,l-diphenylethylene, which reacts differently with each species: Si+ + PhzC=CHz S + PhzC=CH2+. (8)
-
--+
Sz+ + PhzC=CHz
PhzbCHz(S2) (9) The product of reaction 8 is a radical cation (the ionization potentials of diphenylethylene and DCE being, respectively,
