1642
Communications to the Editor
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9 eV. The cross section curve is not symmetrical about its peak. Small spikes and nonzero cross section between 9 and 17 eV are not seen. However, if we go to a higher accuracy; e.g., 1in lo8, we do see the spikes and nonzero cross section. This kind of accuracy is beyond the experimental measurements in these sort of experiments. However, when we plot the log of the cross section vs. energy we obtain Figure 3. The following conclusions are drawn: (1)Elementary theory does not support a measurable cross section in the energy range 9-17 eV. It is possible that more sophisticated theories might give a satisfactory explanation. (2) In at least one case, the observed cross section was the effect of three-body or other processes not accounted for by the reaction e-
+ 0 2 + 0- + 0
It is not clear whether this is true in all cases. 1
ENERGY ( c V
ELECTRON
Figure 2. Cross section for negative ion formation in 0 2 by electron impact vs. the incoming electron energy.
Acknowledgment. I wish to acknowledge the support and help of Professor D. Rapp and Mr. Y. Jani of The University of Texas a t Dallas.
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References and Notes
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(1) W. W. Lozier, Phys. Rev., 48, 268 (1934). (2) W. L. Fite and R. T. Brackmann, General Atomics Report No. GA-4313, June 1963; also Proceeding of the 7th Conference on Ionized Gases, Paris, France, July, 1963. (3) D. Rapp and D. D. Briglia, J. Chem. Phys., 43, 1480 (1965). (4) G. J. Schulz, Phys. Rev., 128, 178 (1962). (5) W. R. Henderson, W. L. Fite, and R. J. Brackman, Phys. Rev., 183, 157 119691. (6) ?.F-.-O'Maliey, Phys. Rev., 155, 59 (1967). (7) D.Spence and G. J. Schulz, Phys. Rev., 188, 280 (1969). (8) H. D. Hagstrum, Rev. Mod. Phys., 23, 185 (1951). (9) P. J. Chantry, Phys. Rev., 172, 125 (1968). ( I O ) P. J. Chantry and G. J. Schuiz, Phys. Rev., 156, 134 (1967). (11) P. J. Chantry, Phys. Rev., 55, 185 (1971). (12) R. K. Curran, Proceeding of the Mass Spectrometry Conference, New Orleans, 1962 (unpublished). (13) G. Herzberg, "Spectra of Diatomic Molecules", Wiley, New York, N.Y. (14) J. M. Jacksonand N. F. Mott, Proc. R. SOC.London, Ser. A, 137,703(1932). (15) H. Bateman, "Higher Transcendental Function", Vol. 2, California institute of Technology, 1953, p 87. (16) D.Rapp and D.D. Brigiia, Lockheed Missiles and Space Co., Report No. LMSC 6-74-64-65 (1964). (17) Address correspondence to the Department of Physics, York University, Downsview, Ontario, M3J 1P3 Canada.
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(3) Hagstrum's observation that the cross section is zero in the energy range 9-17 eV is in conformity with elementary theory.
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Departmentof Physics The University of Texas at Dallas Richardson, Texas 75080
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M. Yaqub Mlrzai7
Received July 3, 1975 I
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ELECTRON ENERGY ( e V 1 Figure 3. Log of the cross section for negative ion formation in 02 by electron impact vs. the incoming electron energy.
For the ground state a simple harmonic oscillator solution of the Schrodinger equation for the 0 2 molecule was used. The spline fit method was used to calculate the overlap integrals with a 0.001-A resolution. The square of the overlap integrals which is proportional to the cross section are calculated for an energy from 4.32 to 12.60 eV. The values of cross section vs. the energy are plotted as shown in Figure 2. It can be seen that the value of cross section rises sharply at 4.7 eV, peaks at 6.69 eV, and falls off rather slowly at about The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
Anisole Radical Cation Reactions in Aqueous Solution Publication costs assisted by the Danish Atomic Energy Commission
Sir: Recently O'Neill, Steenken, and Schulte-Frohlindel published a paper in which they discuss the formation and properties of radical cations of methoxylated benzenes in aqueous solution.
1643
Communications to the Editor Pulse radiolysis work in our laboratory on radical cations of methylated benzenes2 led us to make studies of radical cations of methoxylated benzenes, especially anisole, in order to compare their properties with those of the methylated compounds. For anisole, the absorption band of the OH adduct is ,A,, 320 nm, the extinction €320 3400 M-l cm-l, and the decay constant for the bimolecular reaction 2k = 1.4 X lo9 M-l s-l. The H adduct absorbs a t the same wavelength with 6320 4000 M-l em-1 and 2k = 3.8 X lo9 M-l s-l. We measured the spectrum of the anisole radical cation produced from the OH adduct in acid solution (pH 0-3) and in neutral (pH 6-10) produced by SO4- formed in the reaction of 2-10 mM K2S20~ with the solvated electron. The spectrum consists of two bands X 280 nm, e280 7400 M-l cm-l, and X 430 nm, €430 3200 M-l cm-l. These data are in good agreement with ref 1. The assignment of this transient to the anisole radical cation is additionally confirmed by its unreactivity toward oxygen and the reactions with Fe2+and OH-. Oxygen saturation of the acid solution of the anisole removes the H adduct absorption band from the transient spectrum by scavenging the H atom forming HO2, but has no influence on the decay kinetics of the 280- and 430-nm bands, which is a characteristic cationic b e h a ~ i o r The .~~~ radical cation reacts with ferrous ions (eq 1)with a rate constant lzl OCHB I
-0q 0
,
,
0.20
0.30
,
010
yVy1 +&"
1
Flgure 1. The effect of ionic strength, 1,on the rate of anisole radical cation reactions: 0, reaction with Fez+,eq 1; 0 , reaction with OH-,
eg 2.
OCH3
= 6 X lo8 M-l s-l at pH 1.0. In the Fenton reagent system Jefcoate and Norman4 found a drastic decrease in the yield of hydroxylated anisoles (the yield decreases from 14.6 to 0.018) when going from pH 3.6 to 0.8. This is to be expected as the OH adduct in strong acid solution is converted into the radical cation and the reaction of the radical cation with ferrous regenerate the parent molecule, so that the net reaction in the system is an oxidation of the ferrous ions. In alkaline solution (pH 8-11) containing 5 mM K2S208, the anisole radical cation reacts with OH- (eq 2) with a rate
I
OCH3 I
I
OCH3 I1
sorption bands at 380-420 nm (Figure 2) with ,A, 405 nm and €405 2500 M-l cm-1. The product decays in a second-order reaction with a rate constant 2k = 2 X lo9 M-l s-l. We assume the absorption at 380-420 nm to be the abOCH, OCH, sorption of I1 on the basis of its second-order decay and the I I similarity to the absorption spectrum of the biphenyl H adduct.3 An indication of the formation of such dimeric products is found in the nitromethane-boron trifluoride system, where Allara et a1.6 concluded that the dimeric radical cation is derived from a reaction between the monomeric radical cation constant 122 = (1.0 f 0.2) X IO9 M-l s-l. The SO4- radical and the substrate. They observed the same ESR spectrum for reacts with the anisole forming the radical cation with a rate the radical cation for anisole and 4,4'-dimethoxybiphenyl and constant of 4.9 X lo9 M-l s-l.l Even at pH 11with M found that the latter compound was a product of the reaction anisole about 90% of the SO4- radicals will react with the substrate and not with OH- to form OH radicals ( k s o 4 - + 0 ~ - of anisole. = 6.5 X lo7 M-l s - ~ ) .The ~ rate constant for reaction 2 was An electrophilic attack by the radical cation on the elecdetermined by the decay rate of the radical cation at 430 nm tron-rich parent compound molecule seems to be an important as function of the OH- concentration. The product of reaction ability of radical cations of aromatic compounds. It was observed in anodic oxidation of mesitylene7and suggested as an 2 was identified by the buildup of a spectrum similar to and important process in oxidative substitution of arenes by cowith the same extinction as that measured in N2O saturated balt(III).g However, it was not observed for radical cations of solution for the OH adduct. The kinetics of reactions 1 and methylated benzenes,2 but conditions with low solubility of 2 are ionic strength dependent and the plot log k l k o vs. the substrates and low stability of the corresponding radical k1/2/(p1/2 1) (Figure 1)is consistent with the species having cations are unfavorable in the water system. A fast reaction a unit positive charge. with the solute confirms the assignment of the radical cation In a saturated anisole solution (14 mM) the anisole radical cation reacts with the solute, eq 3 (eq 1 in ref 1).The rate to a monomer species. constant for this reaction measured in acid solution (pH 0-2) As pointed out by O'Neill et al.,l the anisole radical cation is k 3 = 1 X lo7 M-l s-l. The product of this reaction has abin dilute anisole solution M) decays in a second-order
+
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
1644
Communications to the Editor OD
I
I
t
t
P
0'06 0.05
360
400 440 Wavelength(nm) Flgure 2. Transient absorption spectra of the product from the reaction of the anisole radical cation with the solute molecule, eq 3.
reaction with a rate constant in neutral solution of 2k = 1 x lo9 M-l s-l. We find the kinetics of this reaction strongly dependent on pH, which qualitatively may be explained essentially as an ionic strength effect. The rate constant measured at pH 0 and l is 2k = 1.2 X 1O1O and 4 X lo9 M-l s-l, respectively. The determination was made with and without oxygen which had only a minor influence on the rate, but from the higher yield of the product in oxygen-saturated solutions we estimate that the H adduct reacts with the radical cation a t a rate corresponding to the rate given for the OH adduct.l The product of the bimolecular reaction has a broad weak absorption in the region of 660-850 nm with Am, 810 nm and tg1O e 3 0 0 M-l cm-l taking the yield of the product equal to half of the anisole radical cation yield. The absorption at 810 nm decays by first order with a half-life of 90 ps. This firstorder decay may correspond to the deprotonation reaction that is considered to be a step leading to a biphenyl product. It was found that 2,2',5,5'-tetramethoxydiphenyl was a product of 1,4-dimethoxybenzene radical ~ a t i o n . ~ In the case of methylated benzenes the main decay path for their radical cations is the proton loss from the methyl group leading to the benzyl radical.2 As O'Neill et a1.l we find no indications of a proton split off from the methyl group in the anisole and higher methoxylated benzenes. A pH-independent first-order decay of the radical cation was not observed. This is in good agreement with the molecular orbital calculationslO which show the increase of negative charge on a methyl group under ionization of anisole, whereas the methyl group of toluene becomes more positive. Concerning the formation of radical cations from the cyclohexadienyl radical, O'Neill et a1.l assume that the rate of H-atom abstraction from substituents by the OH radical is two orders of magnitude lower than the rate of addition. This assumption does not apply in the case of methylated benzenes,ll where for toluene the rate for the direct H abstraction is only about one order of magnitude lower than the rate of addition. Furthermore this rate is proportional to the number
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
of methyl substituents which yields an appreciable amount of directly formed methylbenzyl radicals from the higher methylated benzenes. We find strong indications of a direct H abstraction reaction in the methoxylated benzenes too. In a 0.6 M NaOH, NzOsaturated solution of anisole, we observe a transient with a peak at 280 nm that can be ascribed to the phenoxymethyl radical, as 0- is known to abstract an H atom from the methyl group rather than add to the ring.11J2 The spectrum of the OH adduct in neutral solution has a shoulder in this region, which could be characteristic of the OH adduct spectrum, but may as well be explained by an absorption due to a second radical, the phenoxymethyl radical. Further support for a direct H-abstraction reaction by OH radicals is the slightly lower yield of the radical cation in acid solution (5-10% lower) as compared to the yield in neutral solution. The yields of the primary radicals, OH in acid and eaq- (SO4-) in neutral solution, are about equal, which indicates that a small fraction of OH radicals reacts in another reaction if we assume a complete conversion of the OH adduct into the radical cation, which is the case for the methylated benzenes.2 The transient spectrum obtained with 1,3,5-trimethoxybenzene both in neutral NzO-saturated and at pH 1in argon-saturated solutions shows a peak a t 260 nm that can be ascribed to the product of a direct H-atom abstraction from the methyl group. Additionally, phenol is recognized as a product in y-radiolysisl3 and in Fenton reagent hydroxylation4 of anisole. In the last case, the phenol yield is independent of pH, in contrast to the yield of methoxyphenols,and constitutes about 7% of the yield of methoxyphenols at pH 3.6. We consider the H-atom abstraction from the methyl group to be a possible source of phenol, This point may be important to the explanation of the lower yields of radical cations obtained in acid solution in relation to the OM-radical yield. However, it does not seem to explain some very low yields, 45-6096, obtained by O'Neill et a1.l
References and Notes (1) P. O'Neill, S.Steenken, and D. Schulte-Frohlinde, J. Phys. Chem., 79, 2773 (1975). (2) K. Sehested, J. Holcman, and E. J. Hart, to be submitted for publication. (3) K. Sehested and E. J. Hart, J. Phys. Chem., 79, 1639 (1975). (4) C. R. E. Jefcoate and R. 0. C. Norman, J. Chem. SOC.B, 48 (1968). (5) E. Hayon, A. Treinin, and J. Wilf, J. Am. Chem. SOC.,94, 47 (1972). (6) D. L. Allara, 8. C. Gilbert, and R. 0. C. Norman, Chem. Commun., 319 (1965). (7) L. Eberson in "Organic Electrochemistry", M.M. Baizer, Ed., Marcel Dekker, New Yo&, N.Y., 1973, p 462. (8) J. K. Kochi, R. T. Tang, and T. Bernath. J. Am. Chem. Soc., 95,7114 (1973). (9) A. Nlshinaga, H. Hayashi, and T. Matsuura, Btrll. Chem. SOC.Jpn., 47, 1813 (1974). (IO) B. Cantone, F. Grasso, and S . Pignataro, Mol. Phys., 11, 221 (1966). (11) K. Sehested, H. Corfitzen, H. C. Christensen, and E. J. Hart, J. Phys. Chem., 76, 310 (1975). (12) P. Neta and R. H. Schuler, Radlat. Res., 64, 233 (1975). (13) J. H. Fendler and G. L. Gasowski, J. Org. Chem., 33, 2755 (1968). Danish Atomic Energy Commission Research Establishment Risg Accelerator Department OK-4000 Roskilde, Denmark Received March 10, 1976
Jerzy Holcmin' Knud Sehested
