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J . Phys. Chem. 1989, 93, 2262-2266
Photochemistry and Radiation Chemistry of Semiconductor Colloids. 27. Preparation of Colloidal PbOp and Various Electron-Transfer Processes Ani1 Kumar, Arnim Henglein,* and Horst Weller Hahn- Meitner-Institut, Berlin GmbH, Bereich Strahlenchemie, 1000 Berlin 39, Federal Republic of Germany (Received: April 4 , 1988; I n Final Form: August I , 1988)
Colloidal PbOz (mean particle size 3.6 nm) was prepared by radiolytic oxidation of lead ions in alkaline solution, sodium hexametaphosphate acting as stabilizer. The Pb02 particles dissolved cathodically upon attack by hydroxy alkyl radicals, which is explained by electron transfer from the radicals to the colloidal particles. This transfer is accompanied by a blue-shift in the absorption spectrum of Pb02 (bleaching between 300 and 600 nm) and a weak absorption at long wavelengths (600-730 nm). One colloidal particle can accept a large number of electrons to yield Pb(I1) centers. Further reduction by radicals leads to Pb(0) which, however, rapidly conproportionates with Pb(IV) in the colloidal particles. Under illumination Pb02 particles are more oxidizing than in the dark. Bleaching and absorption signals were also observed in the laser flash photolysis of colloidal Pb02 solutions; the rapid decay of the signals indicated that the generated charge carriers were very short-lived.
Introduction During the past IO years an increasing amount of attention has been paid to the photochemistry and radiation chemistry of colloidal semiconductors.' Light absorption in these materials leads to the formation of electrons and positive holes, which can rapidly move to the surface of the colloidal particles to initiate redox processes. Absorption of ionizing radiation leads to free radicals in the bulk solution, which subsequently transfer an electron or inject a positive hole into the particles. Manganese dioxide and lead dioxide constitute special cases as these materials are strong oxidants and are often used for oxidations in preparative chemistry. M n 0 2 in the colloidal state was studied recently, and it was shown there that its catalytic action in redox processes is due to an intricate reaction cycle involving manganese surface states of different valencies.2 In the present paper, the preparation and some reactions of colloidal P b 0 2 are described. P b 0 2 has a bandgap of 1.4 eV,3which corresponds to an absorption threshold of 883 nm. The colloidal solutions of PbOz sometimes absorb at shorter wavelengths, an effect that has been observed for many other materials and explained by size-dependent quantization of the electronic energy levels in extremely small particle^.^
Experimental Section The colloid was prepared through oxidation of lead ions in alkaline solution by hydroxyl radicals generated radiolytically. The solution contained Pb(C104)2and sodium hexametaphosphate, HMP, to avoid precipitation of Pb(OH), before and PbOz after irradiation (commercial H M P consists mainly of polyphosphate chains up to a chain length of 450; molarities refer to the formula (NaP03),). Each time freshly prepared solutions were used. Exposure to y-rays under an atmosphere of nitrous oxide led to the formation of Pb02. Nitrous oxide was used to scavenge the hydrated electrons formed by irradiation: eaq- + N 2 0 + H 2 0 Nz OH- O H . Figure 1 shows the absorption of the solution before and after various periods of irradiation. The numbers on the curves give the irradiation time and the percentage conversion of Pb(I1). The conversion could be calculated knowing the absorbed dose and
+
+
-
(1) Henglein, A. Top. Curr. Chem. 1987, 143, 113, and many references therein. (2) (a) Lume-Pereira, C.; Baral, S.; Henglein, A,; Janata, E. J . Phys. Chem. 1985,89, 5772. (b) Baral, S.; Lume-Pereira, C.; Janata, E.; Henglein, A . J . Phys. Chem. 1985, 89, 5779. (c) Baral, S.; Lume-Pereira, C . ;Janata, E.; Henglein, A . J . Phys. Chem. 1986, 90, 6025. (3) (a) Mindt, W. J . J . Electrochem. SOC.1969, 116, 1076. (b) Lappe, F. J. Phys. Chem. Solids 1962, 23, 1563. (4) (a) Brus, L. E. J. Chem. Phys. 1983, 79, 5566; 1984,80,4403; J . Phys. Chem. 1986, 90, 2555; Nouu. J . Chim. 1987, 11, 123. (b) Fojtik, A,; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 969. (c) Henglein, A.; Fojtik, A,; Weller, H. Ber. Bunsen-Ces. Phys. Chem. 1987, 91, 441.
the yield of OH radicals, the latter being 5.5 radicals per 100 eV of absorbed radiation energy and assuming the reaction 2 0 H Pb(I1) Pb(IV) + 2 0 H - to occur quantitatively. The solution obtained after 100 min was titrated with 0.1 M Na2S03at 80 OC and under argon. The amount of oxidizing equivalents found was 1.8 X M, as expected from practically 100% oxidation of Pb(I1) initially present. The product formed during the first 10 min of irradiation was colorless. At longer periods of irradiation yellow to brown solutions were obtained. The absorption spectrum after 100 min resembled that of authentic lead d i ~ x i d e .This ~ solution was stable for weeks. The solution obtained after a IO min of irradiation changed its absorption spectrum upon aging. As can be seen from Figure 2, the spectrum became red-shifted. Pulse radiolysis experiments were also performed to elucidate the mechanism of lead oxidation. The details of this study will be reported elsewhere. It might be mentioned briefly that no reaction between PbZf ions and OH radicals was found in acidic solution, but a fast reaction was observed in alkaline solutions where the lead ions are complexed by OH- anions. At pH = 10.9, a rate constant of 5.5 X los M-' s-' was found by following the absorption increase due to the first product of O H attack. This product is believed to be a hydroxilated Pb(II1) species which subsequently forms Pb(IV) via disproportionation and dehydration. Under our preparation conditions, the lead ions are mainly present as Pb(OH)z stabilized by polyphosphate. This is concluded from the observation that addition of Pb(C104)2to an alkaline polyphosphate solution resulted in a decrease in pH roughly corresponding to the pick-up of two OH- anions by each Pb2+ion. During the irradiation the pH slightly decreased (corresponding to a consumption of OH- anions of several M). We formulate the oxidation as
-
Pb(0H)Z
+
+ 20H
----*
Pb(OH)4
PbO(OH)2
-H20
(1) the slight decrease in pH possibly being due to adsorption of OHanions on the colloid formed. Results Radiolysis and Photolysis of Colloidal PbOz. In the radiolysis experiments an alcohol was added to the colloidal solution, and the irradiation occurred under N 2 0 . Under these circumstances, hydroxyalkyl radicals are formed as is known from general radiation chemistry. 1-Hydroxyalkyl radicals are strongly reducing. For example, the standard redox potential of the hydroxy methyl radical, which is formed in the presence of methanol, is -1.0 V ( C H 2 0 H 6 CHzO + H+ + e-);5 in alkaline solution, where the ~~
( 5 ) Butler, J.; Henglein, A. Radial. Phys. Chem. 1980, 15, 603
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PbO2
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2263
Chemistry of Semiconductor Colloids
photolysis
radiolysis
I
-6
\
c
01
61
n
L v) 0
n
300
m
300
400
500 600 hlnml Figure 1. Absorption spectrum before and after various periods of yirradiation at a dose rate of 2 X IO5 rad/h. Solution: 1 X Pb(C104)2r 4 X lo4 M HMP, 2 X M N,O; pH = 11.0.
periods of aging.
400
500
Figure 4. Ratio of absorbancies as a function of wavelength for photolyzed (left) and irradiated (right) solutions of Pb02 (derived from Figures 3 and 5 , respectively).
V
A [nml Figure 2. Spectrum of a solution irradiated for 10 min after various
500 300 X [nml
400
400 X[nml
300
500
600
Figure 5. Illumination under argon with 366-nm light of 6 X lo4 M PbOl solution containing 4 M methanol. Spectrum after various periods of illumination. 0.03 1
O.O1
1
t -/
&"" " 0 0
"
"
"
'
5 [ C H 3 0 H I [MI
10
Figure 6. Quantum yield of PbOl consumption as a function of methanol concentration.
A[nml Figure 3. y-irradiation of a solution containing 6 X lo4 M Pb02, 4 M methanol, and 2 X M N20; pH = 10.5. Spectrum after various
irradiation times.
+
radical is partly dissociated ( C H 2 0 H C H 2 0 - H+, pK = 10.7),6 the redox potential is even more negative (-1.66 V at pH = 11). The absorption of colloidal Pb02decreases upon irradiation of such a solution as is seen from Figure 3. PbOz is reduced as could readily be shown by titrating the irradiated solution with Na2S03. At irradiation times much longer than in Figure 3 the reduction proceeds beyond the state of two-valent lead as a precipitate of the metal appears. From the initial decrease at 400 nm and assuming that the product of reduction does not absorb, one calculates a radiation chemical yield of 10.4 P b 0 2 molecules consumed per 100 eV of absorbed radiation energy, although C H 2 0 H radicals are known to be formed with a yield of only 6.0 radicals/100 eV. The yield of PbOz consumption was independent of the dose rate (investigated range 2.4 X 104-6.0 X lo5 rad/h) and of the methanol concentration (0.5-6 M). Rapid degradation with a yield of 7.0 PbOz molecules per 100 eV also took place ( 6 ) Asmus, K.-D.; Henglein, A.; Wigger, A,; Beck, G. Ber. Bunsen-Ges. Phys. Chem. 1966, 70, 756.
in solutions containing 2-propanol, in which (CH3)2COHradicals are formed. However, when tert-butyl alcohol was used, the yield was only 2.9 Pb02 molecules per 100 eV. The 2-hydroxy radical, CH2C(CH3)20H,produced in the presence of this alcohol is known to be less reducing than the 1-hydroxy radicals produced from the above alcohols.' A closer look at Figure 3 reveals that the absorption spectrum not only decreased in intensity but also changed slightly its shape. This can best be recognized from the plot at the right side of Figure 4. The ratio A/Ao is plotted here versus wavelength, A. being the absorbance before irradiation and A the absorbance after 5 or 10 min. This ratio should be independent of wavelength, if the shape of the spectrum of P b 0 2 did not change during its cathodic dissolution. However, a substantial decrease at longer wavelengths occurs. It should be emphasized that all these solutions were clear, Le., nonopalescent; changes in the ratio A / A o cannot be attributed to scattering effects. One has to conclude that the smaller particles formed during the dissolution start to absorb at shorter wavelengths than the initial material. When the colloidal solution was illuminated with 366-nm light under argon, the spectrum very slowly faded away. Titration with (7)Henglein, A. Electroanal. Chem. 1976, 19, 163.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
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Kumar et al.
P
-,b, -1
" 250
350 400 h [nml
300
a
io0
5bO 700 h [nml Figure 9. Difference spectrum observed in the pulse radiolysis of a PbOz solution (formed after 100 min in Figure 1) in the presence of 10 M methanol. The radical concentration per pulse was 5 X lo-' M.
150
1100
Figure 7. Photodissolution of the colloid formed after I O min in the experiment of Figure 1. Illumination at 305 nm. Spectrum after various illumination times.
ms
0
20
10
LO
30
10 m
;
n L
0
o:TG;*--j
4a -5
[PbOzl , S,lO-'M [CHjOHI : 10M
o/o-o-o
1
0
m
e
-____ -
A--&-Aa-*7+4
-30
W
u
5
::-200
'Q4-a0
oP0
L-
I
0
a n
-
-20
I
,
-800' 0
'
10
I
'
20
a
'
30
'
'
LO
S
Figure 10. Irradiation of a colloidal Pb02 solution with trains of electron pulses; 450-nm absorbance as function of time. Upper: the effect of 12 pulses. Lower: the effects after application of up to 12000 pulses. Solution: 1 X M Pb02; 1 M methanol; 2 X M N20;pH = 11. Radical concentration per pulse: 1 X lod M.
absorption coefficient at 460 nm, which amounts to 4800 M-I cm-I. The colloid absorbed here with only 600 M-' cm-'. This means that one C H 2 0 H radical upon its attack on a colloidal particle decreased the absorption of several P b 0 2 molecules at 460 nm. One could argue that the formaldehyde molecule formed upon the oxidation of a hydroxymethyl radical can further reduce lead dioxide. However, addition of 1 X lou3M formaldehyde to the colloidal solution did not result in a decrease in PbO, absorption. Pulse radiolysis can be used to determine the size of colloidal particles.]-* From the half-life time of 30 ps with which the signals in Figure 9 were built up and the P b 0 2 overall concentration of M, one calculates a rate constant of 2.3 X lo7 M-I s-' 1X for the reaction of C H 2 0 H radicals with the colloid. This value in turn was used to calculate a mean particle diameter of 3.6 nm,I** assuming that the radicals react with the colloidal particles at a diffusion-controlled rate and assuming a diffusion constant of 1 X cm2 s-l of the radicals. Experiments were also carried out with solutions that did not contain an additive and were under argon atmosphere. In this case, the reaction of hydrated electrons with the colloidal particles could be observed by recording the decay of the 700-nm absorption of the hydrated electron. Using cm2 s-l of the hydrated the diffusion coefficient of 4.9 X e l e ~ t r o n a, ~mean size of 3.8 nm of the colloidal particles was obtained. A mean agglomeration number of 600 is calculated from these size determinations. At an overall concentration of P b 0 2 of 1 X M the particle concentration is 1.6 X 10" M. Multipulse Radiolysis. In the experiments of Figure 10 the P b 0 2 solution was irradiated with a train of pulses, each pulse being 0.5 p s long and the interval between pulses being 3.3 ms; 1X M C H 2 0 H radicals were produced in each pulse. The (8) Henglein, A,; Lilie, J. J . Am. Chem. SOC.1981, 103, 1059. (9) Hart, E. J.; Anbar. M. The Hydrated Efecfron; Wiley-Interscience: New York, 1970.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2265
Chemistry of Semiconductor Colloids 01
I
under the influence of CHzOH radicals xCH20H
-200
x
-600
z -800 C
-200
-400
/
1
---___, ,
- 6 0 0 / b ~, ~ , -BOOo
-
x/2Pb(OH)2
" -400
z
+ (PbOJ,
10
20
30
40
S
Figure 11. Irradiation with pulse trains; 450-nm absorbance as function of time. Upper: application of 974 pulses (-) and afterward (---). Lower: application of 1700 pulses (-) and afterward (---), Solution as in the experiments of Figure 10.
upper part of the figure shows the bleaching at 450 nm by the first 12 pulses. Each pulse produced practically the same effect. The lower part of the figure shows the bleaching during application of 12000 pulses. After 974 pulses, which corresponds to about M radicals generated, maximum bleaching was reached. During the following pulses the bleaching became weaker again, which indicates that the radicals now produced an absorbing species. The bleaching signal did, however, not decay to zero but only to about 50% of its maximum value. Bleaching and disappearance of bleaching are rather complex phenomena as can be seen from the experiments in Figure 11. In the upper part of this figure, the solution was irradiated with 974 pulses to produce maximum bleaching. It turns out that the bleaching signal remains unchanged after this irradiation, which proves that further free radical attack is indeed necessary to make the bleaching signal fade away. In the lower part of the figure, 1700 pulses were applied, which caused maximum bleaching and partial decay as in the lower part of Figure 10. However, when the irradiation was over, the bleaching recovered to a certain degree, which indicates that a thermal process took place in which P b 0 2 is consumed and/or an absorbing species produced by irradiation disappears.
Discussion Absorption and Size of P b 0 2 Particles. Hydroxyl radicals oxidize lead ions in alkaline solution. In the presence of polyphosphate the precipitation of the P b 0 2 colloid formed is prevented. The absorption spectrum of the product of oxidation present after 100% conversion is similar to that of commercial lead dioxide. However, at lower conversions the product has an absorption threshold at shorter wavelengths (Figure 1). There could be two reasons for this phenomenon. It could be that dehydrated P b 0 2 is not yet formed during the first stages of O H attack, Le., the product is Pb(OH)4 or PbO(OH), (eq 1). On the other hand, it could be that Pb02 particles formed initially are extremely small and show the typical size quantization effects that are known for other semiconductor materials. A few other observations make us believe that size quantization changes certain optical properties of very small lead dioxide particles. When P b 0 2 colloids of larger size are dissolved either by light or radiation (Figures 3-5), the remaining particles absorb at shorter wavelengths with increasing degree of dissolution. This effect has also been observed in the cases of CdS,4a,CZnS,l0 andCd3Pz,l' where it was proven that the effect was due to increased size quantization in the smaller particles. In the dissolution ~~
(10) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 649. (1 1) Weller, H.; Fojtik, A.; Henglein, A. Chem. Phys. Lett. 1985, 117, 485.
+ (Pb02),,/2 + xCH20 (2)
the initial yield was 10.4 P b 0 2 molecules/lOO eV, although only 6 radicals were generated per 100 eV. This yield was calculated assuming that the product of reduction did not absorb at 400 nm and that the remaining smaller (Pb02),/2 particles had the same specific absorption per Pb02 unit as the starting material (Pb02),. However, if the smaller particles had a smaller absorption due to size quantization, a seemingly larger radiation chemical yield would be obtained. (The only other possibility to explain the large yield of 10.4 molecules/lOO eV would consist in postulating a chain mechanism for the cathodic dissolution of eq 2. However, this can be ruled out as the yield did not change with either methanol concentration or dose rate and as C H 2 0 was found to be nonreactive toward colloidal Pb02.) Mechanism of Cathodic Dissolution by Free-Radical Attack. 1-Hydroxyalkyl radicals are electron-transfer agents. For example, electron transfer to colloidal TiOz,I2w o 3 , I 3 CdS,I4 and ZnOIS has been observed. In most metal oxide semiconductors, the lower edge of the conduction band is positioned close to 0 V vs NHE,I6 Le., there is a lot of driving force for electron transfer from 1 -hydroxyalkyl radicals, the reduction potentials of which are more negative than -1.0 V.5 The fact that even a less reducing 2hydroxyalkyl radical, Le., CH2C(CH3),0H, can dissolve P b 0 2 cathodically is an indication for the conduction band in Pb02 being located at a more positive potential than in other oxide semiconductors, such as T i 0 2 and ZnO. The reaction between the hydroxymethyl radical and colloidal P b 0 2 is formulated as electron-transfer process: CHZOH
+ (PbOZ),, + OH-
-
CH2O + (PbOz);
+ H20
(3)
This reaction is accompanied by a decrease in absorption between 300 and 600 nm and a slight increase at longer wavelengths (Figure 9). At first sight one might be inclined to explain the decrease by a consumption of Pb(IV) in the colloidal particles. The negative absorption coefficient should then be equal to the absorption coefficient of one PbOz molecule in the colloid. The latter was 600 M-I cm-' at 460 nm. However, a bleaching coefficient of 4800 M-' cm-I was found, which is 8 times greater. The effect indicates that many Pb0, molecules are affected when a small colloidal particle picked up an electron. In the cases of CdSI7 and ZnO,I5 this effect has also been observed and explained by the influence exerted by the excess electron on the optical transition to the excitonic state in the colloidal particle. A similar explanation is invoked in the present work. The increased absorption at longer wavelengths is possibly due to the absorption of the stored electron. Reactivity of Colloidal PbO,. The oxidation power of colloidal PbO, is increased by illumination. Reactions become possible that do not occur in the dark, such as the oxidation of primary and secondary alcohols, and thermal reactions are accelerated, such as the oxidation of 2-aminoethanol. The action of light is explained by the formation of electrons and positive holes, the latter being able to oxidize dissolved substrates. The electrons generated remain on the particles and corrode them by reducing Pb(IV) to Pb(I1). However, these photoeffects are not very efficient, the highest quantum yields observed being only a few percent (Figure (12) Henglein, A. Eer. Bunsen-Ges. Phys. Chem. 1982, 86, 241. (13) Nenadovic, M. T.; Rajh, T.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1984.88, 5827. (14) Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 474. (IS) (a) Koch, U.; Fojtik, A,; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985,122,507. (b) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J . Phys. Chem. 1987, 91, 3789. (c) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 482. (16) Memming, R. Prog. Surf. Sci. 1984, 17, 7. (17) Henglein, A,; Kumar, A,; Janata, E.; Weller, H. Chem. Phys. Letf. 1986, 132, 133.
J . Phys. Chem. 1989, 93, 2266-2275
2266
6). Obviously, most of the charge carriers generated rapidly recombine. It was found in the laser flash experiments (Figure 8) that the signals decayed very rapidly when no oxidizable substrate was present. This is also understood in terms of a rapid recombination of the charge carriers. Probably most of the charge carriers recombined already during the 20 ns laser flash. In the presence of 10 M methanol, the signals were stronger and decayed more slowly. This is understood in terms of scavenging of a small percentage of the holes, the remaining electrons being longer lived than in the presence of the substrate. These electrons produce a bleaching effect (Figure 8) similar to that produced by excess electrons transferred from free radicals (Figure 9). Multielectron Deposition. A colloidal particle can be attacked by a large number of organic radicals to produce more and more bleaching as is shown by the multiple-pulse radiolysis experiments in Figure 10, upper part. In the maximum of bleaching (Figure 10, lower part) roughly as many electrons are on a particle as it contains P b 0 2 molecules. It does not seem possible that all these electrons are present in traps. We rather suppose that they reduce Pb(1V) to Pb(I1) in the particles: Pb(IV)
+ 2e-
-
(4)
Pb(I1)
and that bleaching is due to the consumption of Pb(IV), Le., of absorbing P b 0 2 molecules. In fact, in the maximum of bleaching in Figure 10, lower part, about half of the original P b 0 2 is still present, the rest having been reduced to Pb(I1). The occurrence of reaction 4 is not sufficient to explain the recovery of the bleaching at higher doses (Figure 10, lower part). This recovery can be understood in terms an absorbing species that is formed after a certain degree of reduction (eq 4)has been achieved. We suppose that this species is lead metal:
+
-
Pb(I1) 2ePb(0) (5) The experiment in Figure 11, lower part, would be understood, if the Pb(0) formed is not stable on the colloidal particles. We ascribe its disappearance to a reaction between Pb(0) and Pb(1V) centers in the particles: Pb(0) Pb(1V) 2Pb(II) (6) This reaction is accompanied by bleaching as both Pb(0) and Pb(1V) absorb while Pb(I1) does not. In the experiment in the upper part of Figure 11, Pb(0) had not yet been accumulated to an appreciable extent, therefore no reaction took place after the irradiation. One can readily prove that reaction 6 takes place by dissolving lead power in a colloidal P b 0 2 solution. Reaction 6 can occur as long as Pb(1V) is still present on the particles. Beyond this point, the reduction leads to a metal precipitate. The conproportionation reaction 6 may finally be compared to a similar reaction between metal centers of different valencies in colloidal Mn02.2 An important conproportionation reaction in the latter system is the reaction between di- and tetravalent manganese centers in the colloid, Mn(I1) Mn(1V) 2Mn(III), in which highly reactive Mn(II1) centers are created. These centers on the surface of the colloidal particles are the sites where substrates undergo redox changes; for example, the Mn(II1) center plays a decisive role in the autocatalytic decomposition of hydrogen peroxide. In the case of Pb02, the conproportionation reaction 6 leads to Pb(II), Le., a product of no noteworthy chemical activity. The mechanisms of redox processes on MnOz and P b 0 2 can therefore be expected to be quite different.
+
-
+
-
Acknowledgment. We thank L. Katsikas for valuable discussions and assistance in the laboratory work. Registry No. Pb02, 1309-60-0; CH,OH, 67-56-1; (CH,),COH, 7565-0; (CH3)2CHOH, 67-63-0; N20, 10024-97-2; CH20H, 2597-43-5.
-
Free Jet-Cooled Laser-Induced Fluorescence Spectrum of Methoxy Radical. 2. Rotational Analysis of the k2A, R2E Electronic Transition Xianming Liu, Cristino P. Damo, Tai-Yuan David Lin, Stephen C. Foster,**+Prabhakar Misra, Lian Yu, and Terry A. Miller Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 (Received: May 24, 1988)
-
The rotational structure of the 0; and 3; bands of the A2A, R2E electronic transition of the methoxy radical has been measured. Simultaneous least-squares fits to these data and that from previously reported microwave studies of the ground state have been performed. Twelve molecular parameters for the ground state and six for the excited state, characterizing the rotational and spin structure of methoxy, have been determined. A brief discussion of the implications, in terms of geometries and electronic structure, of these parameters is given.
I. Introduction The methoxy radical C H 3 0 is one of the most interesting of all free radicals. This fact is attested to by the over 75 papers that have been written “recently” about this species.’ The interest is generated in at least two distinct ways. CH,O is known to be a key intermediate in a number of extremely important chemical reactions. For example, it is a free radical formed in the combustion of hydrocarbon fuels. Add to that the role it plays in atmospheric reactions and its existence in interstellar space, and the study of its spectroscopy can easily be justified by the application of that spectroscopy to the elucidation of methoxy dynamics. ‘Present address: Department of Chemistry, Florida State University, Tallahassee, FL 32306.
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On the other hand, methoxy represents a classic spectroscopic challenge. It has an electronically degenerate ground state, 2E, and hence is subject to Jahn-Teller distortion and a concomitant “quenching” of its electronic angular momentum with a corresponding diminution in its spin-orbit splitting. While Jahn-Teller effects are of a vibronic origin, the rotational levels, or rovibronic structure, should also be profoundly affected. Considerable theoretical work2-6 has been expended upon the rovibronic (1) See,for example, the reference given in the following: Brossard, S . D.; Carrick, P. G.; Chappell, E. L.; Hulegaard, S . C.; Engelking, P. C. J . Chem. Phys. 1986, 84, 2459. (2) Child, M. S.; Longuet-Higgins, H. C. Philos. Trans. R . SOC.London, A 1961, 254, 259. (3) Child, M. S . Mol. Phys. 1962.5, 391; J. Mol. Specfrosc. 1963, IO, 357. (4) Brown, J . M. Mol. Phys. 1971, 20, 817. (5) Hougen, J. T. J . Mol. Specrrosc. 1980, 81, 73.
0 1989 American Chemical Society
