26
SHAMIM A. CHAUDHRI AND E(.-D. ASMUS
extinction data or, alternatively, to some systematic complexity concerning its use as a positive-ion scavenger in these particular systems. Acknowledgments. We thank Miss Ruth Gibbons who devised the method for determining extinction COefficients for aromatic amine radical cations and for making some of the measurements as part of her under-
graduate research project in the School of Molecular Sciences. We acknowledge the help of our colleagues at the Cookridge High Energy Hadiation Research Centre, Particularly Dr. G. A. Salmon, and the access to the pulse radiolysis facility given by its Director, Professor Sir Frederick Dainton, F.R.S. H. D. B. and D. G. thank the S.R.C. for financial support.
Ion Yields and Ion Neutralization Processes in Pulse-Irradiated Acetone by Shamim A. ChaudhrilEand K.-D. Asmus* Hahn-Meitner-Institut fur Kernforschung Berlin GmbH, Sektor Strahlenchemie, Berlin, Germany (Received J u n e 7 , 1971) Publication costs assisted by Hahn-Meitner-Institut far Kernforschung
Ion yields and ion neutralization processes have been studied in pulse-irradiated acetone solutions using optical and conductivity methods. The yield of free (CH3)zCO- ions has been determined to be Gfi = 1.20 i 0.20 by conductivity measurements. Tetranitromethane is reduced by (CN,)zCO- ions to form C(NOZ)~-ions with k = 1.2 X 10'O sec-l. Nitroform ions are neutralized by solvated protons with k = 3.6 X IOs M-l sec-l, and C(NO2)gH dissociates with 1.6 X l o 3sec-'. The equilibrium constant of C(NO2),H e C(NOZ)~Hsol+in acetone has been determined to be 4.5 X M . The yields of homogeneously distributed C(NO&anions, anthracene anions, and chloride ions from the reduction of tetranitromethane, anthracene, and carbon tetrachloride, respectively, exceed Gfi((CH,)zCO-). The influence of equilibrium conditions and possible activation energies on the neutralization process are discussed. Equivalent conductances have been deterions (A = 130 Q-' cmz equiv-l) and anthracene anions (A = 135 em2 equiv-') in mined for c(N0~)~acetone.
+
Introduction Acetone is knowrl to be a good electron scavenger, The reactionlb to
(CH&CO
+ e- +(CH,)&O-
(l)
and the associated acid-base eauilibrium2 (CH8)zCOH
(CH3).&O-
+ H+
have been studied in detail in irradiated aqueous solu, tions (kl = 5.9 x 109 M-1 set-1 and p ~ ( =~ 12.2). Both the (CH3)&0- anion and the (CH,),&OH radical were found to act as reducing species by transferring their radical electron to other solutes.3 The formation of a reducing species has been observed when acetone mas used as a solvent. Aromatic compounds such as naphthalene, anthracene, etc., for example, were shown t o produce short-lived anions and triplets in irradiated acetone.4 The observed yield of aromatic anions in the electron-transfer reaction
Ar-
+ (c&)zCo + [H"]
T h e Journal of Physical Chemistry, Vol. 76, N o . 1, 1972
(3)
has been associated with the yield of free ions produced upon irradiation of the solvent. Literature values of G(free ions), however, vary from 1.2 to 1.7 for the same ~ystem.~-6This, presumably, is mainly due to uncertainties in the extinction coefficients and the very short lifetimes of the anions. The aim of the present work has been to obtain information on ion formation and ion neutralization processes by combined optical and conductivity pulse radiolysis experiments. The conductivity method has been successfully applied in pulse radiolysis studies of aque(1) (Lt) Postdoctoral Fellow from the Pakistan Atomic Energy Commisslon, Karachi, with a grant from the Alexander von HumboldtStiftung, Bad Godesberg, Germany; (b) 8. Gordon, E. J. Hart, M. 8 , Matheson, J. Rabani, and J. K. Thomas, Discuss. Faraday Thomas, Soc.9 36, 193 (1963) ; (c) E. J. Hart, S. Gordon, and J. J . P h y s . Chem., 68, 1271 (1964). (2) K.-D, A ~ A. Henglein, ~ ~ A. Wigger, ~ , and G. Beck, Ber. B u n senges. Phys. Chem., 70, 756 (1966). (3) See, for example, (a) K.-D. Asmus, A. Henglein, A t . Ebert' and J. P. Keene, ibid., 68, 657 (1964); (b) K.-D. Asmus, A. IVigger' and A. Henglein, {bid., 70, 862 (1966). (4) 8 . Arai and I. M. Dorfman, J . Phys. Chem., 69, 2239 (1965). (5) E. Hayon, J . Chem. Phys., 53, 2353 (1970). (6) M. A. J. Rodgers, Trans. Faraday SOC., 67, 1029 (1971).
ION
YIELDS AND
ION
NEUTRALIZATION PROCESSES
27
T I " MI
ous solutions' and recently also of alcoholic solutions.s.O Acetone is expected to be quite a suitable solvent as well since equivalent conductances of ions are fairly high with ca. 180-200 Q-' emz lo per equivalent of an ion pair.
@
Experimental Section Solutions. The experiments were carried out wit,h acetone of "pro analysis" grade (Merck). Traces of water were removed by refluxing t,he solvent wit,h anhydrous CaCI2and suhsequent,ly distilling under a const,ant flow of pure argon. A part of the column was heated to about 120" to avoid migrat.ion of ions. Tetranit,romethane (Fluka purissimum p.a.) was purified by several washing procedures with triply distilled water. HCIOl and NaOH were reagent grade and sodium isopropylate was prepared from sodium metal and isopropyl alcohol. The pK of HCI04 in acetone was determined to be 2.0 0.2 by a conductivity method. I n the concentration range used for our experiments, HClO, was therefore completely dissociated. Prior to t,he addition of solutes all samples (0.5-1 I.) were bubbled with specially pure argon ( [C(NO2)3-] is given by8
LO = kn[Hm~+]
(11)
jcd
Accordingly a plot of ICo vs. [Hs,l+] gives a straight line (Figure 2). From the slope the neutralization rate constant ICn = 3.6 X 108 M-l sec-l is derived. The rate constant for the dissociation, kd, could be obtained from the intercept. Since it is very small, the error, however, is very large. It is calculated, therefore, from the known relationship kd
= =
may be obtained from a plot of the remaining optical absorption or conductivity after the partial neutralization of the C(N02)3-ions as a function of Hsol+concentration.8 Since it is difficult to adjust the proton concentration in the interesting range below 10-j M with the desired accuracy a second independent method using y radiolysis was also applied to determine the dissociation constant K5.8 In irradiated solutions of tetranitromethane ( lom3 &I) the C(N02)3- ion Concentration equals the Hsol+ concentration, so that the equilibrium (reaction 5 ) can be written in the form [C(N02)3-I2 = Kj[C(NO,),H]. The C(N02)3- ion concentration can be measured directly, the C(N02)3Hconcentration after dilution with HzO, i.e., complete dissociation.
160
120
IHCIO,] .IO6, M
with the (CH3)260- ion having been formed according to reaction 1. The reaction kinetics are of pseudo-first order since the (CH3)&O- concentration (ea. 5 X lo-’ M/l-psec pulse) is small compared with the tetranitroto 5 X ill. methane concentration used of 5 X Consequently the half-life, tl,2, of the electron-transfer process (reaction 4) is inversely proportional to the tetranitromethane concentration. The bimolecular rate constant for the C(S02)3- ion formation is calculated to be =
LO
+
C(Noz)3-
kq
0
x
K x k , = 4.5 x 1.62 X lo3 sec-I
3.6
x
lo8
In pulse-irradiated solutions of M tetranitromethane in acetone, nitroform ions are produced with an initial yield of G(C(n’02)3-) = 2.15 f 0.1. This is calculated from the change in optical absorption immediately after the pulse (Figures l b and d), the absorbed dose, and the known extinction coefficient of c(xo2) 3-. From the initial change in conductivity and the C(NO2)3- concentration, and using eq I, the equivalent conductance of the C(N02)3- Hsol+ion pair in acetone is calculated to be A C ( N O ~ ) ~ - + = H ~220 ~ ~ +* 10 Q-1 em2 equiv-1. Since AHSO,+ in acetone is known to be 90 Q-1 cm2 equiv-’,’O the equivalent conductance
+
(13) A. A. Frost and R. G. Pearson in “Kinetics and Mechanism,” 2nd ed, Wiley, New York, N, Y., 1963, Chapter 8, p 160.
IONYIELDSAND IOKNEGTRALIZATIOK PROCESSES
29
of a nitroform ion is calculated t o be AC(NO%)~= 130 f 10 f2-l cm2 equiv-I. This value is very similar to that of other large ions such as hio3- (A = 120 f2-I em2 equiv-‘) or C104- (A = 115 f2-1 em2 equiv-l) in acetone. 10 6. Solutions of Anthracene and Carbon Tetrachloride. Both anthracene (An) and carbon tetrachloride are also reduced in irradiated acetone solutions. The electrontransfer processes (CH3)2CO-
+ An +(CH3)&0 + An-
(6)
and (CH3)zCo-
+ cc1,
I
060 --f
(CH3)ZCO
+ CCI3 + C1-
(7) are followed by the neutralization of the An- and C1ions. The formation and disappearance of the anthracene anion could be observed simultaneously at 720 nm and by conductivity measurements. Using EAn-720nm = 1.0 X lo4 M-I cm-14 an initial yield of G(An-) = 1.7 f 0.2 is calculated for a solution of 5 X M anthracene in acetone. The true yield of Anions, however, might be considerably smaller than 1.7. As indicated in some recent work6 the absorption at 720 nm is due not only to negative but also t o positive anthracene ions. From this and the observed conductivity signal the equivalent conductance i l ~ s+o A nl- + = 225 f 20 f2-1 cmz equiv-I is derived. The equivalent conductance of the anthracene anion of A A ~ -= 135 f 20 W 1em2equiv-l is, therefore, quite similar to that of the nitroform anion. The presence of some positive ions mould have only little effect on the A values because all ions in acetone have quite similar equivalent conductivities. l o In solutions of CC14 in acetone only changes in conductivity could be observed. Similar to the tetranitromethane and anthracene solutions the conductivity is increased upon pulse irradiation due t o the formation of Cl- and Hsol+. The subsequent neutralization of C1- then causes the decay of the signal. The chloride ion yield can be calculated from the known equivalent - 90 105 = 195 f2-l conductances A H , , ~ + + c ~ = emz equiv-’ lo and the change in conductivity extrapolated to the middle of the electron pulse. For soluM CC14,G(C1-) = 1.40 f 0.10 was tions of 2.5 X obtained. The neutralization processes of anthracene anions and chloride ions are much faster than that of the nitroform ions. The half-life of these ions is ca. 10-20 psec in LL neutral” solutions. The observed conductivity signals disappear upon addition of
