P. F. de Violet, R.
1698
“complete” experiment described in Figure 2 one can say that mechanism I prevails only at the beginning of the reaction (close to point A). When appreciable superoxide concentrations are formed, mechanism I1 takes on more and mbre importance and largely prevails in the interval BC. Mechanisms such as I11 and IV become competitive when, again, a net production with I1 only in interval of superoxide can be observed. The prevalence of mechanism V occurs when practically all the peroxide initially present is converted to superoxide. Generally speaking the results presented in this paper appear self-consistent and in good agreement with previous studies They satisfactorily support the hypotheses made on the existence of catalytic effect, by 0 z 2 - and 0 2 - , on rea:tion 6 even if some phenomena could be explained only on a semiquantitative basis because of system complexity. On the other hand, this is the limit often found for molten salt studies because of the few analytical techniques which can be conveniently employed and because of several experimental restrictions which characterize the use of these high-temperature solvents.
Bonneau, and S. R. Logan
References and Notes (1) (a) P. G. Zambonin and J. Jordan. J. Amer. Chem. Soc., 89, 6365 (1967); (b) ibid., 91, 2225 (1969); (6) P. G. Zambonin and A. Cavaggioni, ibid.. 93, 2854 (1971); (d) P. G.Zambonin, F. Paniccia, and A. Bufo, J. Phys. Chem., 76, 422 (1972). (2) J. M. Schlegel and D. Priore, J. Phys. Chem.. 76, 2841 (1972). (3) H. Lux, R. Kuhm, and T. Niedermeier, 2.Anorg. Alig. Chem.. 286 (1959). (4) J. Goret and B. Tremillon, Bull. SOC.Chim. F r . , 67 (1966) (5) E. P. Mignosin. L. Martinot, and G. Duyckaerts. lnorg. Nucl. Chem. Lett., 3, 511 (1967). (6) F. L. Whiting, G. Mamontov. and J. P Young, J. lnorg. Nucl. Chern., 34, 2475 (1972). (7) F. Paniccia and P. G. Zambonin. J. Phys. Chem., 77, 1810 (1973). (8) P. G. Zambonin, J. Electroanal, Chem., 24, 365 (1970). (9) P. G. Zambonin, J. Electroanal, Chem., 33, 243 (1971). (10) P. G. Zarnbonin, Anal. Chem.. 41, 868 (1969) (11) E. Desimoni, F. Paniccia, and P. G. Zarnboriin, J. Electroanal. Chern., 38, 373 (1972). (12) P. G. Zambonin, V . L. Cardetta, and G. Signori!e, J. Electroanal, Chem., 28, 237 (1970). (13) W. E. Triacaand A. J. Arvia, Electrochim. Acta, 9, 1055 (1969). (14) It is to be noted that the present experiments cannot give sufficient information to distinguish (on the basis of the overall stoichiornetries)between mechanisms I l l and I V or any other reaction path which can be written to rationalize period CD. This is rnainiy because of the difficulty in estimating (at various times of interval CD) the extract contribution of mechanisms i I and V to the total oxygen consumption.
Laser Flaish Photolytic Study of Mercury( I I ) Iodide in Aqueous Solution Philippe Fornier de Violet,*” Roland Bonneau, and Samuel R. Logan Laboratoire de Chimie Physique A, Universitb de Bordeaux I , 33405 Talence, France (Received February 4 , 1574) Pubiicatlon costs assisted by Centre National de ia Recherche Scieniifique
Absorption spectra and kinetics of formation of transient species produced by laser flash photolysis of mercury(II) iodide in aqueous solutions are reported. An analysis of the results shows that the primary process in the photolysis of HgI2 is the formation of atomic iodine and of a transient species absorbing a t 340 nm which has been tentatively assigned to Hgl. In the presence of small quantities of I- ions, the 12radical anion is formed as a result of a secondary dark reaction. A previously speculated mechanism i s hereby directly confirmed. The primary quantum yield of photodissociation has been evaluated as 0.04 and the rate constants of the equilibrium reaction I I- 2 12- as k l 2 x 1 O l o M - l sec-I and k z 1.7 x l@sec-l. These data are obtained from a simplified kinetic scheme.
+
Introduction Studies of the Dhotochemistry of mercury(II) halides, IlgX2, and of the related complexes, HgX42-, were previously undertaken by Langmuir and Hayon2 using a flash photolysis system capable of following the events occurring subsequent to 20 psec after the photolytic pulse. They found that on photolysis HgC12, HgC142-, HgBr2, WgBrs2-, and ETgI,~][.I2gave rise to the corresponding radical anions, Clz-. Brz-, and 12-. The only transient detected in the photolysis of HgI2 absorbed a t 330 nm and it was not identified. 1 2 - , with Xmax a t 385 nm, was detected when small quantities of I- ions were added. Several possible primary processes were considered by Langmuir arAdHayon2 The participation of hydroxyl radicals, formed by charge transfer from a water solvent molecule to a mercuric ion, was discounted because of the abThe Journal of Physical (?hemisfry, Vol. 78, No. 17, 1574
sence of an effect by OH radical scavengers and because of the slight effect of pH on the yields of Xz-. Two other mechanisms were considered: (I) an intramolecular rearrangement of the electronic charge followed by direct formation of X2HgXz
in,
{HgtX2-}
-
X2-+ product
and^ (11)the dissociation of a mercury-halogen bond HgXz
hy
HgX
-+
X
followed by the reaction of a halogen atom with a halide ion
x + x-
-+
x2-
However, it was not found possible to distinguish between these two mechanisms.
Laser Flash Photolytic Study of Mercury(l1)Iodide
1699
The main purpose of' the present study has to determine whether Xz- was a primary product of the photolysis of these compound3 (mechanism I) or produced by a secondary reaction (niechanism II) as suggested in ref 2. The technique of lases flash photolysis was applied in a previous study3 to the photochemistry of 12 in water arid the time resolution was fully adequate to distinguish the initial product, the charge transfer complex IqH20, from the 12- radical ion formed subsequently by a thermal reaction with I-. In this work we seek to make a similar distinction for ElgI2.
~ x ~ e p ~ Soction ~ e n ~ a ~ The experimental se t-up. using a collinear arrangement, has been described el~ewhere.~ The flash excitation is provided by a Q switched neodymium glass laser (C.G.E. instrument) associated with two KDP frequency doublers, delivering approximately 50 m J at 265 nm in 30 nsec. The analyzing light murce is a pulsed xenon discharge lamp giving pulses having a flat part of 10 psec. The cell has a 5-cm path length and aqueous Hg12 solutions in the range M were used, giving an optical density at 265 of 6 X nm of approximately unity. Transient absorption spectra were obtained point by point a t a given time after the laser pulse. When I- ions were required in the HgI2 solution, measured qirantities of standard KI were added. The resulting concentration of free I- ions was then deduced, taking account of the equilibrium
esults A I aqueous solutions of HgIz were excitWhen 6 X ed with laser pulses of 265-nm light, a transient absorption was detected with1 a maximum a t 340 nm. The spectrum, calculated from the transmittances observed just after the laser pulse, is given in Figure 1, spectrum 1.This transient decays uniformly and by a first-order process, as if due to a single species. At both 340 and 385 nm, the rate constant was, found to be 2.5 X lo5 sec-I. The addition of small quantities of iodide ion causes both the spectral and the kinetic characteristics to be modified. Tmmediately after the laser pulse, the extent of the transient absorption is considerably diminished in the spectral regions below 350 nm and below 335 nm the measured A(0D) becomes negative. However, l psec after the laser pulse the transient absorption has changed so that it now shows a maximum at 385 nm (Figure 1, spectra 2 and 3) In Figure 2 , we compare the changes in the OD at 385 nm over the 1.5 pusec following the laser pulse for HgI2 alone (curve 1) with those for various amounts of added KI (curves 221, Zb, 2c, and 2d). In the presence of I-, we observe the same initial transient absorption as for HgI2 alone, Collowed ty c'i slow increase, the extent and the rate of which depend on the iodide concentration. 1
Interpretation an The transient absorption spectrum shown in Figure 1 curve 1 is similar to that previously reported but is not positively identiiled; it is most probably a primary photochemical product since i t is not observed to grow subsequent to the laser pulse. This transient absorption could be attributed to HgI since, in the gas phase, this species has an electronic transition around 300 nm.5 The formation of lHg& obwrved as an end product in this system is
300
400 X n m
350
Figure 1. Transient optical density changes produced by laser flash photolysis of (1) [Hgl?] N 6 X 10-5 M immediately after the laser pulse, ( 2 , 3) [Hgln] 6 X M in presence of [I-] M, (2) immediately after the laser pulse and (3) 1 psec after the laser pulse
I/------
O.D. 385nm
nnc
as
20
t
(ps)
Figure 2. Transient optical density at 385 n m vs time: (1) [Hg12] = 6 X 10-5 M ; (2) [Hg12] = 6 X M in presence of (a) [ I - ] = 2 X M , (b) [I-] = 3.8 X M , (C) [ I - ] = 5.4 X M , and ( d ) [ I - ] = 7.7 X M.
in support of such an assignment. The transient absorption appearing in the presence of I- in the spectral region 380-400 nm is assigned to the well-known diiodide ion I z - . ~In the spectral region 320-360 nm, the negative A(0D) does not necessarily imply that 12- or I- reacts with the product absorbing at 340 nm as has been previously suggested.z An alternative possibility is a decrease of the concentration of Hg142- (which absorbs in the spectral region in question with A,, 325 nm and emax 20,000 M-1 cm-I) by direct photolysis since this species is reported2 to have a large absorption at 265 nm. Since this spectral region is extremely complex to analyze, we examined only the kinetic behavior of the transient absorption a t 385 nm. In Figure 2, we measured the difference in optical density, A(QD), between the curves 2a, b, c, d, and curves 1,that i s
A(OD) = OD2a,bPc The increase in A(OD) is clearly attributable to the formation of I2-, and this quantity may be taken as a measure of 12- concentration, subject to the assumption that the rate of disappearance of the initial transient, having a Xmax at 340 nm, is unaffected by the presence of I- ions. If the only reactions involving 12- that need be considered are The Journal of Physical Chemistry, Vol. 78 No. 17. 1974
P. F. de Violet, R . Bonneau, and S. R. Logan
1700
UoD)]
log [A(OD3--
05
005
Since in these experiments [Iz-]
