765
COMMUNICATIO:NS TO THE EDITOR large, ie., the creation of the aew bond with extremely high vibrational energy of the T-C-X bonds. (18) This research was supported by A.E.C. Contract No. AT-(111)-34,Agreement No. 126.
CHENG T. TING F. S.ROWLAND
DEPARTMENT OF CHEMISTRY~~ OF CALIFORNIA UNIVERSITY IRVINE, CALIFORNIA 92664
R,ECEIVEDDECEMBER 1, 1967
Electron Capture Processes in Irradiated Gaseous HCI
Sir: It has been previously shown' that the yield of H2in the radiolysis of HCI gas is depressed by low concentrations of SF6 and that this effect is consistent with an assumption that thermal electrons react in HCl by a process which gives a net yield of one molecule of H2 per electron, and which competes with electron scavenging by SF6 e-
+ SF6
SFe-
(1) At a dose rate of' 3 X 10l2eV ~ m sec-l - ~ (HCI pressure 650 torr) the coinpetition was observed in the SFs concentration range 0-2 X mole %. This finding shows that the results cannot be attributed to a competition between reaction 1 and the recombination reaction e-
+ H&1+
----t
--f
H
+ HC1
(2)
if kl = 3 X lo-' and le2 5 cm3 molecule-' sec-'. A competition between reaction 1 and the dissociative capture process e--
+ HCl +H + C1-
+ nHCl +H + Cl-(n
- 1)HCI
(4) The threshold energy of this process could be less than that for simple dissociative capture (reaction 3) because of solvation of the anion. A process of this type has been suggested previously to explain results obtained in the radiolysis of both gaseous4 and liquid HC1.616 We report here further evidence in support of a process such as reaction 4. in this system. Stationary-statle kinetic treatment of the competition between reactions 1 and 4 gives 1/Gf(H2) = l/G(e-) kl [SIF6]/G(e-)k4[HCI]" where Gf(H2) =
+
4
[SFe]
/ [H CI]
8
6
IO
x IO'
Figure 1. Kinetic plot for effect of SFBon G(H2) from HCl at different HCl pressures (7 radiolysis, with dose rate ca. 1013 eV cm-a sec-1, 20'); HC1 pressure (atm): 0,1; 0, 2; 0,3; W, 4.
G(H2) - 4.0 is the contribution of reaction 4 to the H2yield. A study of the dependence of G(H2) on the concentration ratio [SF6]/ [HCI] at different HC1 pressures gave results which are consistent with this expression (Figure 1). A value of n = 3.3 is obtained from the results over the pressure range 1 to 4 atm. In the absence of SF6, G(H2) = 8.0 f 0.2 was found to be independent of the HC1 pressure over this range. The results are most simply interpreted in terms of a process represented by eq 4, although the nature of the actual reaction involved is speculative at present. One possibility is that associated HC1 molecules, present at an equilibrium concentration nHCl=
(3)
can also be excluded, since the threshold energy of this process is 0.6 ==I 0.2 eV13although a small fraction (ca. 5%) of the {electronsmay react with HCl by this process during thermalization.' An alternative explanation is that, at the pressures of HC1 used in the radiolysis, a hydrogen-bonded agglomerate of HCI may be involved in the reaction with thermal electrons, which may be represented by e-
2
"0
(HCI),
capture electrons e-
+ (HCI),
--j
H
+ Cl-(n
- 1)HCI
Alternatively, an electron-HC1 complex may be formed which reacts with HCl.4-6 The results could also be interpreted in terms of the formation of an electronHCI complex which persists until ion neutralization occurs, the latter process involving the reaction HzClf
+ (HCI),-
+H
+ (n + 1)HCl
However, this seems improbable, particularly in view (1) R.9.Davidow, R. A. Lee, and D. A. Armstrong, J . Chem. Phys., 45, 3364 (1966). (2) B. H.Mahan and C. E. Young, ibid., 44,2192 (1966). (3) D. C.Frost and C. A. McDowell, ibid., 29, 503 (1958). (4) R.A. Lee, R. 9. Davidow, and D. A. Armstrong, Can. J . Chem., 42, 1906 (1964). ( 5 ) D. A. Armstrong, ibid., 40, 1385 (1962). (6) R. C. Rumfeldt and D. A. Armstrong, J. Phys. Chem., 68, 761 (1964). Volume 78, Number 9 February 1068
ADDITIONS AND CORRECTIONS
766 of a recent finding,' which indicates that in pure HC1 ion neutralization does not contribute to Hz formation. (7) R. A. Lee, Nature, 216, 58 (1967), (8) Chemistry Division, Argonne National Laboratory, Argonne, Ill. 60439.
LABORATORY OF RADIATION CHEMISTRY SCHOOL OF CHEMISTRY THEUNIVERSITY UPON TYNE, 1, ENGLAND NEWCASTLE
G. R. A.
JOHNSONS
J. L. REDPATH
RECEIVED DECEMBER 4, 1967
Activation Energies for Reactions of
the Hydrated Electron Sir: It has been stated recently that the lowest activation energy for reactions of the hydrated electrons, eaq-, is that for diffusion in water, Le., 3.5 kcal mole-'. An unfortunate misprint quoted our value for the eaqNO2- reaction wrong1y.l We have measured the temperature dependence for the reactions of hydrogen ions, H+, and nitrite ions, NO%-,with eaq-. The reagents used were HC104 and NaNOz Analar grade (Hopkin and Williams Ltd.). The pulse radiolysis technique2i3was used and the rate of disappearance of the eaq- at 650 nm after a pulse of about 10 rads was measured. In repeat observations, the rate constants at 20" were found to be k(eaq- H+) = 2.2 X 1Olo M-' sec-l and k(eaqNOz-) = 3.2 X lo9 M-1 sec-1, in agreement with previously reported
+
+
+
values.' Under our experimental conditions we were able to control the temperature over the range from 20 to 80" within f1". The activation energies calculated from Arrhenius plots were found to be for the reactions eaq- with H+ and NOz- 2.6 and 1.7 kcal mole-', respectively, the uncertainty being smaller than f10%. These values are substantially lower than the activation energy for diffusion in water.' The mechanism by which eaq- are transferred through water is therefore likely to be different from normal diffusion. It also excludes any other process with an energy requirement significantly in excess of 1.7 kcal mole-', such as dipole reorientation of water molecules which requires about 4 kea1 m01e-l.~ Acknowledgment. The data reported belong to a series of experiments carried out in collaboration with Dr. A. 0. Allen and Dipl. Ing. K. U. Willamowski at the Paterson Laboratories. (1) M. Anbar, E". B. Alfassi, and H. Bregman-Riesler, J. Am. Chem. SOC.,89, 1263 (1967). (2) J. P.Keene, J. Sci. Instr., 41,493 (1964). (3) J. P. Keene, "Pulse Radiolysis," M. Ebsrt, J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press, London, 1965, P 8. (4)M.Anbar and P. Neta, Intern. J . Appl. Radiation Isotopes, 16,227 (1965). (5) C. H. Collie, J . B Hasted, and D. M. Ritson, Proc. Phya. Soc., 60, 147 (1948).
PATERSON LABORATORIES B. CERCEK CHRISTIE HOSPITAL AND HOLTRADIUM INSTITUTEM. EBERT MANCHESTER 20, ENGLAND ACCEPTED AND TRANSMITTED BY THEFARADAY SOCIETY (SEPTEMBER 26, 1967)
A D D I T I O N S AND C O R R E C T I O N S 1967, Volume 71 L. R. Romsted, R. Bruce Dunlap, and E. H. Cordes: Secondary Valence Force Catalysis. V. Salt Effects on Certain Detergent-Catalyzed Organic Reactions. Page 4581. Through no fault of the authors, the communication contains the incorrect designation of methyl orthobenzoate as methyl o-benzoate.-THm EDITORIAL STAFF.
The Journal of Physical Chemietry
