0. AMICHAIAND A. TREININ
830 which gives a bad result for A reflects the changes rather well. For the carbonyl 170the experimental shift of 57.0 ppm to a higher field when an H bond is formed is also found in the calculations. Thus here we find a shift to a higher field despite the fact that no appreciable increase of the electron density on oxygen is predicted. Unfortunately, no experimental 170chemical shifts are available for protonated acetone. A shift to a lower field is normal for H-bonded prot o n ~ .For ~ ~ the water protons in acetone-water we have two competing types of H bonds, water-acetone and water-water. The shift to a lower field with increasing water concentration (Figure 1) suggests that the H bonds water-water are stronger than water-acetone.53 The near constancy of the acetone lH chemical shift when an H bond is formed is probably due to a cancellation of several factors. As in both calculations the charges on the methyl protons decrease when an H bond is formed (and even more for protonation), we can expect a decreased diamagnetic screening. This is more or less compensated by a greater neighbor anisotropy effect. For the acidic solutions the changes in the ‘H chemical shift of the solvent (Table IV) are more difficult to explain. We observe a weighted average of the protons in the species HzS04, HSOd-, H30+, H20, (CH&CO+H, and higher order complexes. The deviation linearity in the relation between the changes in carbonyl 8(170) and S(’%C> when an H
from
bond is formed (Figure 2) can be attributed to the fact that the carbon atom has an opportunity to compensate for the loss of electrons at the cost of the methyl groups. Therefore the change in S(W) is not as rapid as in S(170),whereby we get the curve. The linear relation between the changes in ‘JCHand carbonyl S ( W ) (Figures 3 and 4) can be understood by the direct way in which both depend on the increased electron attracting power of the carbonyl oxygen when an H bond is formed and for protonation. In the light of the more compli~ relation between cated mechanisms for 4 J the~ linear ~ S(13C)when an H bond the changes in 4 Jand~ carbonyl is formed (Figure 3) must be more or less fortuitous. This is confirmed by the fact that this linearity does no longer hold in the case of protonation (Figure 5 ) .
Acknowledgment. The author wishes to express his gratitude to G. P. Beneder for carrying out the measurements on acetone in sulfuric acid-water, and to Professor L. L. van Reijen (Chemistry Department) and Professor J. Smidt and several other members of the group of magnetic resonance (Physics Department) for valuable discussions and technical assistance. The CNDO program was kindly provided by G. A. Segal (Pittsburgh). (63) J. A.Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y.,1959, 400.
On the Oxyiodine Radicals in Aqueous Solution by 0. Amichai and A. Treinin Department of Physical Chemistry, Hebrew University, Jerusalem, Israel (Received July 16, 1969)
hu
+
The flash photolysis of I03-and IO- provides evidence for the following primary processes: 1 0 3 - +1 0 2 0 hu and IO- +I 0-. These are followed by secondary reactions involving the parent ions. 0 - (or OH) reacts with 1 0 8 - and IO- to yield IO3 and IO, respectively; the rate constants of these reactions could be determined by studying the 03-decay in alkaline solutions containing Oz. 102 and Iz- (produced from I in presence of I-) react with IOs- and IO-, respectively, to yield IO. The spectra and decay kinetics of the oxyiodine radicals were also studied by pulse radiolysis. The two techniques lead to the same conclusions.
+
The flash photolysis and pulse radiolysis techniques have yielded some basic information on the oxybromine radicals BrOz and BrO in solution.’t2 The analogous chlorine radicals are well known. On the other hand, very little is known on the oxyiodine radicals. Such information is for the photochemistry of the oxyiodine anions. The Journal of Physical Chemistry
The oxybromine anions were shown to undergo two radical types of photo-dissociation to yield 0- and 0 atom, respectively, which can react with the parent (1) G.V, Buxton and F. S. Dainton, Proc. Roy. ~ O C . A3049 , 427, 441 (1968). (2) 0 , Amichai, G. Ceaps,i, and A, Treinin, Israel J . Chem., 7, 361 (1969).
OXYIODINE RADICALS IN AQUEOUS SOLUTION i ~ n . ~These t ~ findings have refuted the previous interpretation of their phot~chemistry.~J'The present work shows that the 0 - type of dissociation is a major primary process also in the case of IO3- and IO- and that some of the subsequent secondary processes involve the parent ion.
Experimental Section
0-
+ O2
-3
OS-;kz
= 2.5 X lo9M-' sec-'
the following rate constants could be determined
03-+[02 108-
+ OH
+ 0-; --j
k3
IO3
=
(5.0 f 0.4)X lo3 sec-l
+ OH-;
k4 = (9.2
+
+
+
+
f 0.8)
X lo8 M-1 sec-'
IO3- 0- 2 IO3 20H-; The flash photolysis setup was as described elsewhere16but the energy of the flash was increased to ks = (2.0 ik 0.3) X lo8 M-l sec-l 600 J. The monitoring light source was a 120-W tungsten projecting lamp, and the optical path length IO0- % IO 20H-; 5 cm. For pulse radiolysis a Varian linear accelerator ka = (4.6 f 0.6) X lo9 M-l sec-l was used which delivered pulses of 5 MeV, 200 mA, 1.5-psec duration. Full details of the experimental (The reaction of IO- with OH could not be studied conditions will be published in "Pulse Radiolysis at the because the hypoiodite solutions were unstable below Hebrew University Accelerator, Internal Report, Dept. pH -13.5.) The value of k3 is in good agreement with of Physical Chemistry, The Hebrew University." previous data (summarized in ref 2). R 136 and IP 28 photomultipliers were employed with Flash photolysis of oxygen containing solutions did two Bausch and Lomb monochromators, Type 567 AB. not give rise to the 260-nm band of 0 3 . This was done By multiple reflections the monitoring light passed with comparable concentrations of IO,- and 0 2 so that 0 2 through effective optical path lengths of 6 or 12 cm. could effectively compete with the oxyiodine anions3on The mean dose per pulse was determined with airO(3P). We conclude that IO3- and IO- do not photosaturated solution of K4Fe(CN)Gas dosimeter, taking dissociate to yield O(3P)at X above -200 nm. e s e ( C ~ ) s 3 - = lo3 M-' cm-' at 420 nm and G F ~ ( c N ) ~ =~ Some other transients were observed but they could 2.7. All the spectra recorded were measured at interbe better investigated in absence of 02 with neutral vals of 5 nm; splitting of the monitoring light beamg solutions. Figure 2 shows the results of such experiwas used to overcome irreproducibility of the flashes ments with 1 0 3 - solutions. Three absorption bands and pulses. (The experimental points are scattered within less than 5% from the recorded absorption curves .) The materials used were of Spectrograde or analytical grade. Dilute solutions of NaIO (lo+ to l o W M 4 ) were prepared by rapidly dissolving solid I2 in solutions containing 0.4 M NaOH. (Under these conditions their a I disproportionation rate was relatively slow.) Matheson's O2 N2 mixtures, He, or N20, were bubbled through the solutions. K 2 0 was purified by passing through solutions of pyrogallol in alkali. Unless otherwise stated the solutions were neutral, air-free (saturated with He), at 23 f 2",
+
Results A . Flash Photolysis. The flash photolysis of alkaline solutions of 1 0 3 - and IO- in presence of O2gave rise to transient absorption peaking at 430 nm, identical with that of 0 3 - (Figure 1). Its decay rate obeyed the law
which has the same form as the corresponding law for 03produced from the oxybromine systems.2 This suggests a similiar mechanism for the generation and decay of 0 3 - . From the experimental values of p and q, the dependence of q on pH, and the known2 rate constant of the reaction
Figure 1. Absorption spectra (a) and kinetics of decay (b and c) of Oa- produced in alkaline solutions of 1 0 3 - and IOcontaining 02. Curve (a) dashed curve, spectrum of OS-(ref 2); 0, 10-3 M IO3-, 10-3 M 02,pH 12.4, 1 msec after flash; 0,4.2 X 10-4 M IO-, 2 X M 02, pH 13-6, 0.6 msec after flash. ko is the pseudo-first-order rate constant, of the ozonide decay. (3) 0. Amichai and A. Treinin, Chem. Phys. Lett., 3,611 (1969). (4) L.Farkas and F. S. Klein, J . Chem. Phys., 16,886 (1948). (5) N. K. Bridge and M. S. Matheson, J . Phys. Chem., 64, 1280 (1960). (6) D.Behar and G. Caapski, Israel J . Chem., 6,43 (1968). Volume 74,Number
4
February 19, 1970
0. AMICHAIAND A. TREININ
832 Table I: Rate Constants for the Reactions of the Oxyiodine Radicals at 23 4 2‘ Transient
A(I0a)
C(I0n) C(IO2) B(I0)
Rate constant M -1 sec -1 a
Proposed reaction
+ 1 0 8 prod. IO2 + 1 0 8 IO f 1 0 4 + ethanol +. IO + prod. prod. IO + IO IO3
+
+
102
-c
Conditions and methodb
(5.1 f 0.5) X 106 X emax ( 1 0 8 ) fp (0.07-5.0) X lO-’M 108M 102(4.6 i 0.3) X 106 X emax(IOS) pr (1.0-8.0) X pr (1$0-8.0) X 10-2 M IOa(3.5 zk 0.5) X 10‘ fp 10-2 M IO*M 103(0.5-17.0) X 10-2 M ethanol (1.6 i 0.3) X lo6 fp 2 X pr 10-2 M 1 0 3 (3.5-17.0) X 10-2 M ethanol M IO-, pH 13.6 (4.4 i 0.2) X 106 X emax (IO) pr 1.2 X 1M 1 0 3 (4.8 f 0.5) X lo6 X emax (IO) pr (1.0-8.0) X M 10sethanol (up to 0.17 M ) (4.3 zk 0.2) X lo6 X emax (IO) pr 1.1 x M IO-, pH 13.6 (4.5 i 0.4) X lo6 X emax (IO) fp 1.1 X (4.2 f 0.5) X lo6 X emax (IO) fp (0.8-5.0) X lo-’ M IOjethanol (4.3 f 0.3) X 106 X emax (IO) fp (0.8-5.0) X 10-2 M 1 0 3 (up to 0.17 M ) (5.0 zk 0.6) x 107 fp (1.0-3.0) X M IO-, pH 13.6 M IO(1.0-3.0) X fp (1.0-3.0) X 10-4 M I-, PH 13.6
-
+
+
+
+
D(L-)
12-
+ IO- + IO + 21-
+
2k is recorded for reactions involving two radical molecules. From our work the values of cmex (M-l cm-1) obtained were 900,220, and 400 for IO, 1 0 2 , and 108, respectively. * fp and pr stand for “flash photolysis” and “pulse radiolysis,” respectively.
X,nm
’I‘
C
f
It
n
0 W
9 m
85 v)
m 4
0
I
15
25
20
3, kK
20 t
, pscc
LO
0.1
0.15 1 , rnsec
Figure 2. Absorption spectra of the transients produced by flash photolysis in solutions of IOg-, alone and in presence of ethanol. The absorbance was measured 110 psec after start of flash. Resolution of the spectra to component bands (Gaussian type of analysis) is shown by the dashed curves.
Figure 3. The pseudo-first-order decay of transient C (10%) in presence of loa-; a (flash) and b (pulse), the absorbance A a t 710 nm us. time for various 1 0 8 concentrations; c, the dependence of the first order rate and pulse constant on (108-) as derived from the flash (0) ( 0 )experiments. In the pulse experiments (b) the optical path lengths were 12 cm (left scale) and 6 cm (right scale).
emerge, designated A, B, and C, which peak at 380 nm, 490 nm, and 715 nm, respectively. (A, was determined by Gaussian type of analysis; see dashed curves in Figure 2 ) . Transient A undergoes a second-order decay which does not depend on 10%-concentration (Table I). The decay of C appeared t o be first order with respect to IOa-, which suggests a reaction of C with 1 0 3 - (Figure 3). (More accurate kinetic data on this reaction were derived from the pulse experiments; see later.) This reaction appears t o produce transient B, since shortly after flash the ratio between the intensities of bands B and C, respectively, markedly increased with 1 0 8 - (Fig-
ure 2). At 1 0 3 - 2 8 X 10-*M band C could hardly be detected. At (IOa-) 2 0.1 M the generation of B was relatively fast ( ~ 5~201psec), ~ so its decay could be readily measured starting 100 psec after flash. It was found t o be second order with k independent of (loa-) (Table I). I n presence of C the decay of B became slower. This is probably due to the parallel generation of B by the reaction of C with Io3-. No effect of NzO was obtained by flashing 2.7 X 10-3 M IO31.5 X 10-2 M NzO. (Under these conditions NzO should effectively compete with 1 0 3 on any solvated electrons if present.’) On the other
The Journal of Physical Chemistry
+
OXYIODINERADICALS IN AQUEOUS SOLUTION
1
833
C
l
a
0
1
0
2
3
( 10-).104, M
L
Figure 4. The pseudo-firsborder decay of transient C ( 1 0 2 ) in presence of ethanol; a (flash), the absorbance A a t 710 nm us. time for solutions containing 2 x 10-8 M 1 0 8 - and various concentrations of et!hanol; b (pulse, 12 cm optical path length), the same with 1.1 x 1.0-2 M IO8-; c, the dependence of the first order rate cmstant on ethanol concentration as derived from the flash ( 0 )and pulse ( 0 )experiments. A,
r~
7 50
500
450
1
I
1
t.Z.lO-'
400
'
I /
0-' M
0.2
IO- + 3.6 ' 16' M 1-
0.3
t , msec
350
I
M IO-
-
after ZOy,sec, 1 - 6 cm
Figure 6. The pseudo-first-order decay of transient D (12-) in the presence of IO- (with equivalent amount of I-, unless otherwise stated) p H 13.6.
k being proportional to (IO-) (Figure 6 and Table I). In the presence of D the decay of B became slower and
' 1
I
15
21 0.1
nm
600
(1
4: CD +
= 370 nm
25
20
30
J,kK
Figure 5. Absorption spectra of the transients produced by flash and pulse in solutions of IO- with equivalent amount of I-, p H 13.6.
deviated from second order in a way which suggests further generation of B after the flash. B. Pulse Radiolysis. The pulse radiolysis of M 1 0 s - solution clearly showed bands A, B, and C (Figure 7). Ethanol had the same influence as in the flash photolysis experiments, but now NzO also had a pronounced effect: in the presence of 1.5 X M NzO, band A intensified whereas bands B and C became weaker. (NzO and 10s- scavenge eaq- at comparable rates.' To obtain detectable yields of radicals we could not make ( 1 0 3 - ) much lower than the concentration of
NzO.) hand, ethanol had a pronounced effect. Addition of ethanol to IOa- led to suppression of band A and to weakening and intensification of bands C and B, respectively (Figure 2). The decay of C became faster; a pseudo-first-order reaction involving C and ethanol appeared to occur (Figure 4), with transient B being the product of this reaction too. I n air-free solutions of 1.1 X 10-3 M IO- the flash produced a transient band peaking a t 490 nm (Figure 5). This and the decay kinetics (Table I) prove its identity with transient B. On lowering the concentration of IO- a new band grew a t 370 nm, designated by D (Figure 5 ) . It showed a first-order decay rate with
With IO- solutions the pulse produced band B (Figure 5 ) , but band D could not be detected. The identity of the three transients produced by pulse with A, B, and C was verified by detailed study of their kinetics. The pseudo-first-order decay of C, with and without ethanol, is illustrated in Figures 4 and 3, respectively. Table I records all the rate constants obtained by the two techniques. In the case of IO-, since transient D was not present in the pulsed solution, no deviations from the normal decay of B were observed. (7) M. Anbar and P.Neta, Intern. J . A p p l . Radiat. Isotopes, 18, 493 (1967). Volume 74, Number .4 February 19,19YO
0. AMICHAI AND A. TREININ
834
+ G, since 0- is produced by the
GIO should equal GOH reaction IO-
+ eaq- +I- + 0-
(10)
as the case' is with BrO-. By extrapolating the IO absorbance to zero time and using the dosimetry results, we obtained GIO X E490 = (5.0 =t0.7) X 103, i.e., emax (IO) = 900 130 cm-l. NO12-was formed in the pulsed solution since the reaction of 0- with I- is probably much slower9than reaction 6. IO is also produced from IOs-. However, here it seems to result not from a primary process but from reaction of transient C with IO3-. The following results lead us to identify transients C and A with IOz and 1 0 3 , respectively. (a) Ethanol which scavenges OH and 0- radicals could completely suppress band A both in the pulsed and flashed 1 0 3 - solutions. This shows that transient A results from reactions 4 or 5, and indeed, in the pulse experiment when NzO is present to convert eaq- to OH, the amount of A increases. Thus we identify A with 103. By extrapolating the 1 0 3 absorbance to zero time and using the dosimetry results with GIOa = GOHwe obtained G I O ~X €380 = (0.85 0.12) X lo3; i e . , emax (103) = 400 f 60 144-' cm-'. (b) In the pulsed solutions the reaction of eaq- with IO3- is expected t o yield IOz
*
J' , k K
Figure 7. Absorption spectra of the transients produced by pulse radiolysis in 1.1 x 10-2 M solutions of IOa-, alone (b) M and in presence of 1.5 X 10-2 M X20 (a) and 3.5 X ethanol (c). The absorbance was measured 23 wec after pulse; 6 cm optical path length. Resolution of the spectra to component bands (Gaussian type of analysis) is shown by dashed curves.
Discussion The detection of 0 3 - in alkaline solutions and the analysis of its decay kinetics prove that 0- radicals are produced by photolysis of both 1 0 8 - and IO-. In analogy with the oxybromine anions,z the generation of 0should involve the following primary processes
102
103-
+ 0-
IO--%I+O-
(7) (8)
Thus 1 0 2 and 1 0 3 (the latter from reactions 4 or 5) are expected to result from the photolysis of IO3-, whereas IO- should yield IO radicals through reaction 6. Therefore we assign band B to IO. The absorption spectrum of IO in the gas phase displays a band structure with maximum intensity at -450 nm; environmental shift of -1800 cm-' is not unreasonable. The iodine atom produced by reaction 8 could yield 12-, since the IO- solutions always contained equivalent amounts of I-. Iz- has a very intense band peaking at 370 nm8 and so we identify transient D with 12-. The pseudo-first-order decay of 1%-suggests the reaction 12-
+ IO-
-
21-
+ IO
(9)
which is analogous to the reaction' of Brz- with BrO-. Reaction 9 is responsible for the generation of IO after flash and hence to the apparent decrease in its rate of decay. From our results we could calculate k9 (Table 1). In pulsed IO- solutions IO is produced by reactions 6 . (At pH 13.6 very little OH is present.) However, The Journal of Physical Chemistry
eaq-
+ 10s-
H20
IO,
+ 20H-
(11)
in analogy with the corresponding reaction1 of BrOa-. Since NzO suppresses both bands C and B they should be related to IOz. Thus we are led to assign band C to IO2and attribute the generation of IO to the reaction
+ 104(Compare with the reactionla 102- + Io3-IO2
+
103-
---f
IO
(12)
+
-+ IOIO4-.) Ethanol is also oxidized by 102 and here too IO is the product. (Under the conditions employed, in presence of ethanol the reaction of 1 0 2 with 1 0 3 could be ignored; see Table I.) From the absorbance ratio AIO,/AIO, a t zero time (derived by extrapolation) we obtained from both techniques, flash and pulse, the ratio tmax(IO2)/emax(IOa) = 0.55 0.08, Le., Emax (IOz) = (220 st 30) M-' cm-'. (c) Our identification of the oxyiodine radicals puts their transition energies hv,,, in the sequence 1 0 3 > IO > IOz. A similar order is displayed by the oxychlorine radicals11 and by the known' oxybromine
(8) L. I. Grossweiner and M. S. Matheson, J . Phys. Chem., 61, 1089 (1957). (9) By analogy with t h e behavior of the chloride7 and bromide anions (B.Cercec, M. Ebert, J. P. Keene, and 4.J. Swallow, "Pulse Radiolysis," Academic Press, New York, N. Y . ,1965, p 83). This can also be inferred from other works, e.g., F. S. Dainton and S. A. Sills, Proc. C h e n . Soc., 223, 1962. (10) 0. Haimovich (Amichai) and A. Treinin, J . Phus. Chem., 71, 1941 (1967).
OXYIODINERADICALS IN AQUEOUS SOLUTION
0
0
a'd
835
J
From these data we estimate that BrOt and 1 0 2 - both have absorption peaks at -340 nm. For BrOs the (C103) -300 nm in interpolation was based on A,, aqueous solution, assuming that the red shift is close to that displayed by BrO, 1 0 2 , and C102on passing from gas phase to water. The value predicted for Br03 is close to that assigned to it in aqueous s ~ l u t i o n . ~ ~ ' ~ ~ ' ~ As in the case of Br02' and N02,15the decay of IO and IO3 may involve dimerization as the first stage with the dimer further reacting with OH- or water (in the case of 1 0 3 we detected a pH effect on the rate constant but this needs further investigation). The final products of the decay are probably IO102(from IO IO), and IO81 0 4 - (from IO3 IO3). IO-is known to react with 1 0 8 - to produce IO4-. (102probably being the reactive intermediate in this reactionlo). I04-is also produced by reaction 12, so altogether periodate should be a major product of the photolysis of IO3-. Some preliminary experiments have shown that this is really the case. Further work on the steady photolysis of 1 0 8 - will show if a single primary process (reaction 7) can account for all the results.
+
! 10
I
I
I
Br I 2.5
I
Electronegativity
I
I
3.0
Figure 8. Transition energies of the oxyhalogen compounds a, XOZ in aqueous solution (ref 1, l l b and present work); b, XO- in aqueous solution (ref 12); c, XO in the gas phase (ref l l a ) ; d, XOS- in aqueous solutions (ref 13). (In the latter case the transition energies a t = 1 are recorded).
us. electronegativities of the halogen atoms.
radicals BrO and Br02. Another regularity is shown in Figure 8 : the transition energy hv of analogous oxyhalogen compounds increases almost linearly with the electronegativity of the halogen atom. The pairs = 278 nm ll0-in the gas phase-and C108-IOa (A, 380 nm, respectively) and C102--Br02- (Am,, = 26011b and 290 nm,12 respectively) follow the same order.
+
+ +
Acknowledgment. We are indebted to Dr. J. Rabani and Mr. D. Zehavi for their assistance in the pulseradiolysis experiments, and to Professor G. Czapski for valuable discussions. (11) (a) R. A. Durie and D. A. Ramsay, Can. J . Phys., 3 6 , 3 5 (1958); (b) F. Stitt, S. Friedlander, H. J. Lewis, and I?. E. Young, Anal. Chem., 26, 1478 (1954); (c) C, F. Coodeve and F. D. Richardson, Trans. Faraday Soc., 33,453 (1937). (12) Our own data. (13) A. Treinin and M. Yaacobi, J . Phys. Chem., 68, 2487 (1964). (14) M. 9.Matheson and L. M. Dorfman, J . Chem. Phys., 32, 1870 (1960). (15) M. Ottolenghi and J. Rabani, J . Phys. Chem., 72, 593 (1968).
Volume '74, Number .4 February 19,1070
