Communicatians to the Editor
133
Fe(CN)d- 4- hr
-+
In n
[(CN)6FeCN---HOMe]3-
--+
+
Fe(CN),'-
MeOH'
or +
(CNXFeCNH3-
+
Me6
MI& + MeOH --+ MeOH + H260H' The HCO radicals, whose relative yield increases with time, are known5 to be formed by incidental photolysis of the resulting H:&@H radicals H,bH
-%- H
b
+
H2
though in this case they could also be formed from Fe(CN)5MeOH2-. The FdI* species must be formed from the symmetrical parent Fe(CN:)s3- ion by some sort of ligand modification. (The parent ions display no detectable resonance a t 77 K because of their high symmetry and efficient spin relaxation..) Possible candidates are Fe(CN)53-, RNCI"e(C:N)$ (where R = H or solvent radical), and Fe(CN)5MeOH2- . Clearly, departure from axial symmetry (gl = 2.354, gz = 2.170, g3 = 1.928) rules out Fe(CN)s3-, which has probably been detected in y-irradiat'ed alkali halide lattices containing the Fe(CN)s*ion.8 This species was clearly axial, with g, 2.13 and g , , 2.000. It is difficult to see why a species of the type RNCFe(CN)52- should form on photoexcitation, and we are drawn to the conclusion that the nonaxial species is Fe(CN)sMeOH2--. Since we were not able to detect Fe(CN)53- radicals, it seems that, if the process is S N 1 in type (FE(CN);-)*
--+
Fe(CN)j'-
+
CN-
Fe(CN);- + MeOH + Fe(CN),MeOpthen the second step must be extremely efficient even at 77 K. These studies, which are clearly of significance to the study of reaction mechanisms, are being extended to other complexes.
References and Notes (1)
(2)
(3) (4)
(5) (6)
(7) (8) (9)
G. V. Buxtori, F. S. Dainton, and J. Kalecinski, Int. J. Radiat. Phys. Chem., 1, 87 (1969). See, for example, U. Klaning and M. C. R. Symons, J. Chem. SOC., 3239 (1959); 977 (1960). J . J. Alexander and H. E. Grey, J. Amer. Chem. SOC., 90, 4260 (1968). ,J. F. Gibson, M. 6.R. Symons, and M. G. Townsend, J. Chem. SOC., 269 (1959). J. A. Brivati, K. D.J . Root, M. C.R. Symons, and D. J. A. Tinling, J. Chem. SOC.A, 1942 (1969). K. DeArmond and W. Halper (J. Phys. Chem., 75, 3230 (1971)), studied the photolysis of I R h ( ~ h e n ) ~ ] 3and + related complex cations in glassy ethanolic solutions and detected a poorly resolved "free radical" signal in the g = 2, region which was tentatively assigned to a mixture of radicals (CH3CHOH, CH3CH2, CH3C0, and CH3). However, their solvent gave the same esr spectrum on photolysis in the absence of the complex ions, and they were clearly studying a double absorption process. Since our reactions do not have the characteristics of a double photon process, there is no direct analogy. M e 0 radicals do not give detectable esr spectra under these conditions, but they are known to attack neighboring solvent molecules most efficiently to give HzCOH radicals. M. C. R. Symons and J. G. Wilkinson, J. Chem. SOC.,Dalton Trans., 14 (1973). On sabbatical leave from Central Michigan University, Mount Pieasant, Mich.
D e p a r t m e n t of Chemistry The University Leicester L E I ' R f f , United K i n g d o m
Martyn C. R. Symons" Douglas X. Westg James G. Wilkinson
Received October 18, 1973; Revised Manuscript Received April 1, 1974 The Journal of Physicai Chemistry. Vol 78, No. 13. 1974
Electron Spin Polarization Effects in a Study of Transient Hydrogen Atoms in Acidic Ices under Electron Irradiation Publication costs assisted by the Faculty of Engineermg, University of Tokyo
Sir: The phenomenon of the chemically induced dynamic electron polarization (CIDEP) has recently attracted much attention,I since it presumably comes from some elementary process of the radical reaction. Its mechanism, however, is still far from complete understanding because of the scarcity of the experimental results. The phenomenon of CIDEP on hydrogen atoms (H atoms) in acidic aqueous solution was investigated by Fessenden, et ~ l . , ~ and by Smaller, et uL3 As the phenomenon is considered to be sensitive to variation of the time scale of the events, measuring in different phases and at irarious temperatures seems helpful for its elucidation. It is known that H atoms are stably trapped at -196" in y-irradiated acidic ices, and that they disappear rapidly above -165".* We have measured esr spectra of H atoms formed in acidic ices in the temperature range from -160 to -50", and have observed the CLDEP phenomenon of H atoms in the higher temperature region (see below) together with the motional narrowing of their line width. The measurements were carried out during continuous electron irra accelerator.5 Here a preliminary account of the experimental results is given, which revealed rather different aspects of the CIDEP phenomenon of H atoms. Solutions of sulfuric acid and h ~ ~ r o c ~ i l o acid r i c were used. Since this seems to be the first observation of narrowing of H atom signals, the change of spectra with temperature is first described, taking 8.5 M sulfuric acid solution as an example. A t -160", the spectrum of transient H atoms was observed. It had the same characteristics as the so-called trapped H atoms at -196".* temperature, the signals become less intense due to the increase in the detrapping rate. The narrowing of the line width begins at about --130", where the signal intensities are extremely low. Above -110", there appears an anomaly which is characteristic of the CIDEP phenomenon; the low-field line (LF line) is now in emission and the highfield line (HF line) in absorption. The apparent signal intensities in the derivative representation increase toward higher temperatures, with the line width showing further narrowing. With constant dose rate the minimum around -130", while the EF Line disappears around -130" and at higher temperature reappears as the emission line. Approximate line width a t maximum slope is 3.8 G a t -155", 3.3 G at -139", 1.6 G at -121", 0.5 G at -96", and 0.4 G at -74". There is no phase transition in a usual hexagonal ice. Since the narrowing of the proton nmr line width begins above -60",6 the narrowing of the esr line width observed here should he ascribed not to the tumbling motion of the water molecules surrounding the trapped H atoms, but to the translational diffusion of H atoms. This almost free diffusion of 14 atoms is considered to be a necessary condition for the observation of the CIDEP phenomenon. It must be mentioned here that in the higher temperature region, namely, above -130", the system may well be regarded to be in the steady state, since the accumulation of other radicals is very small.7 One important difference from CIDEP in the liquid phase was found in the dose rate dependence of the signal intensity in the higher tem-
1337
Communicaticm to the Editor
r
e,
B. Smaller, E. C. Avery, and ,I. R. Remko, J. Chem. Phys., 55, 2414 (1971). See for review, L. Kevan in "Radiation Chemistry of Aqueous Systems," G . Stein, Ed., Weizman Science Press of israei, Jerusalem, 1968, p21. Far the experimental setup see, H. Shiraishi, H. Kadoi, K. Hasegawa, Y . Tabata, and K. Oshima, Bull. Chem. Soc. Jap., 47, 1400 (1974). K. Kume, J. Phys. SOC.Jap., 15, 1493 (1960). ATthe hi her temperature, OH and SO4- radicals which accumulate at -196 decay so rapidly that they give hardly any spectra. There remain some unknown species whose G value is very small. The situation is similar for HCI solution. In fact, the intensity of H atom signal changed very little during the course of irradiation P. W. Atkins, R. C. Gurd, K. A. Mclauchlan, and A. F . Simpson, Chem. Phys. Lett., 8, 55 (1971). Department of Polymer Technology, The Royal Institute of Technology, Stockholm, Sweden, an leave from the University of Tokyo. Address correspondence to the Nuclear Research Laboratory, Faculty of Engineering, University of Tokyo 7-3-!, Hongo, Bunkyo-ku, Tokyo, Japan,
>
"!-
Dose Rate, Mradlmin
.(i
9
-= 8
4
12
16
20
Dose'Rate, Mradlmln
0
-"-, \\
I-
m-
Department of Nuclear Engineering University of Tokyo Tokyo, Japan
' '\
Hirotsugu Shiraishig Hajime Kadoi Vosuke Katsumura Yoneho Tabata* io Keichi Oshima
\ '\
\
',
Received October 29, 1973; Revised Manuscript Received Apnl29, 1974
A\ r a t e dependence of H atom signals in
(a) 0.5 M
H2S04solution at --77', (b) 1 M HCI solution at -78': field line; 0, low-field line.
0 , high-
Figure
I. Dose
perature regiton (Figure 1). Neither the H F line, nor t,he LF line is proportional to the dose rate in contrast to the observation by :Fessenden, et aL2 Still more important is the difference in dependence of the LF and HF line intensity upon the change of dose rate. In liquid-phase experiments, both iincs are reported always to appear in about the same intensjty."3 Increasing the dose rate results in a decrease of the life time of H atoms due to the increase of their concentration.. Tlnus, the weakness of the LF line relative to the :HF line at low dose rates is attributable to the relaxation process that effects significantly when the life time is long. On the other hand, the difference a t the higher dose rate., or, more explicitly, the fact that the LF h e far exceeds .the HF line in the case of 1 M hydrochloric acid solution a t igh dose rates (Figure l b ) has an important implication to the theory of the phenomenon. This cannot be ascribed to a difference between the reaction schemes in the d i d and liquid state, and must be due to the polarization process itself. Interpreted from the standpoint of the radical pair theory,l this phenomenon cannot be explained only by the mixing of singlet S and triplet To, since -1; predicts equal polarization for both lines. As was proposed by Atkins, et U L . , ~ in the quite different case with radicals that have excited triplet precursors, the mechanism of mixing of triplet T-1 with S must be contributing :it least to a comparable order, though it is difficult to explain at the present stage why the latter mechanism sh.ould be emphasized in the present experiment. :Further experiinents and analysis are necessary to clarify the polarization mechanism, and we think this system is a good example for such study. References and PlJoltes (1) See for review, P. W. Atkins and K. A. Mciauchlan in "Chemically Induced Magnetic Po!arization," A . R. Lepley and G. L. Cioss, Ed., Wilsy, New York, N. Y . , 1973, p 42. (2) (a) P. Neta, R . \N. Fessenden, and R. H. Schuler, J. Phys. Chem., 75, 1654 (1971) (b) N. 12, Verma and R. W, Fessenden, J. Chem. Phys., 58, 2501 11973).
Reaction of the Nitrate Radical with Acetaldehyde and Propylene Publication costs assisted by the f o r d Motor Company
Sir: Acetaldehyde is known to react rapidly1 with a mixture of 0 s and NO2 but rather slowly2 with O3 or NO2 alone. NzO5 and NO3 are produced in the 03-NO2 system by the reaction^^,^ 0,
+ NO,
NO3
+ NO,
-
NO,
+ 0,
k , = 4.4 x IO-''
cm3 molecule-' sec-'
(1)
N,O, K = k-.Jh2 = 0.80
X
10" molecule/cm3
(2)
It has also been reported5 that a mixture of N205 and CH3CHO gives a high yield of peroxyacetylnitrate (PAN). Thus either Nz05 or NO3 is reacting with acetaldehyde. Stephens1 has observed that the PAN yield in the O3 NOz -t- CH3CHO system falls off sharply when [ N 0 ~ ] / [ 0 3 ] 9 2. Since reactions 1 and 2 require a stoichiometry of two molecules NOpper molecule of 0 3 , excess NO2 reduces the NO3 concentration by shifting reaction 2 toward Nz05. Hanst6 has analyzed these data and calculated the NO3 concentration using reactions 1 and 2 . From the linear relation between PAN yields and NO3 concentrations, he concludes NO3 is involved in PAN formation. As suggested by various author^,^ a plausible mechanism for PAN formation is given by NO, + CH,CHO --+HNO, + CW,CO~ (3)
+
CH,CO.
+ 0, CH,COOO, + NO, PAN
CH,C000*
+
-
(4) (5)
In view of the potential role of these reactions in photochemical smog formation, the kinetics and mechanism of the Nz05 + CH3CHO system have been studied in some detail. Since the reaction of propylene with N02-03 mixtures also yields PAN as a product, the N20&3& system has also been investigated. The concentrations of reactants and products were determined using a 40-m path length infrared cell. A deThe Journal of Physical Chemistry, Vol. 78, No. 73, ?974
