J. Phys. Chem. 1982, 86, 962-966
962
Photoellmlnation of H from Radicals in CH, and Xe Matrices. Photodecomposition of CH302 Debasls Bhattacharya and John E. Willard' Department of Chemistfy, Universtty of Wisconsin, Madison, Wisconsin 53706 (Received: August 14, 198 1; In Final Form: November 9, 198 1)
The photolytic decomposition of free radicals trapped in solid matrices has received little study. This paper reports on such decomposition of nine simple radicals in CHI, CD,, and/or Xe matrices at 5 K. The reactions observed include CH3 + hv (185 nm) CH, + H; CD, + hv (185 nm) -+ CD, + D; CzH5+ hv (185 nm) CzH4 + H; CZH3 + hv (185 nm) C2Hz + H; HCO + hv (-500 nm) -+ H + CO; DCO + hv (-500 nm) D + CO; HO, + hv (254 nm) H + 02; DOz + hu (254 nm) D + 0,; CH302+ hv (254 nm) CH, + 0,; CH302+ hu (254 nm) HCO + (evidence for OH + H). The production of CzH3by both the radiolysis and the near-UV photolysis of C2H4 in Xe at 5 K is also reported and the use of the photolysis of (CH3)zNzas a source of CH, radicals in solid CH4is demonstrated.
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Introduction It has recently been shown that near-W photoactivation of C2H5 radicals (formed by the photodecomposition of dipropionyl peroxide in Ar at 8 K) produces CZH4,l implying photoelimination of H atoms. This was of particular interest to us because we had found that D atoms are produced when HBr is photolyzed in CD4 at 5 K, and attributed them to H + CD4 CD3 + D displacement by hot H atoms. Test experiments in which the CD3 in Xradiolyzed CD4was exposed to 185-nm light showed that part (but not all) of the D observed in the HBr experiments was due to the reaction CD3 + hv CD2 + D., To the best of our knowledge, such H and D elimination reactions have not been recognized prior to the examples noted, although they may play a significant role in the solid-state photochemistry of many systems containing radicals. To determine something about the generality of this type of process we have investigated the photoelimination of H(D) from CH3, CD3, C2H5,CZH3, HCO, DCO, HOz, DO,, and CH3O2 in CHI, CD4, and/or Xe at 5 K. In each case, H or D production occurs at near-UV or visible wavelengths absorbed by the radical. The major process in the photolysis of CH302is CH3O2 CH, + O2and another mode of decomposition produces HCO. In general, the radicals used in the study have been produced by X radiolysis of the appropriate matrix at 5 K, followed, in the case of the oxygen-containing radicals, by warming to allow diffusion and reaction of the H or CH3 with Ozor CO solute. CzH3 has been produced by both the radiolysis and the photolysis of C2H4in Xe, and the photolysis of (CH3)2N2has been shown to be effective in producing CH3 in CH4 at 5 K.
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Experimental Section The sources, preparation, and X irradiation of CHI, CD4, C2H6, and Xe samples have been Matheson CP CzH4 and Merck Sharp Dohme (CH3)zNzwere used as received. All ESR measurements were made with a Varian E-15 spectrometer, in the X band with 100-kHz modulation. The first-derivative signals were stored in tape cassettes, using a Tracor Northern NS-111 tape unit, and were doubly integrated with the aid of a Tracor Northern NS-570 signal averager. After normalization for the in(1) Pacansky, J.; Coufal, H. J. Chern. Phys. 1979, 71, 2811. (2) Bhattacharya, D.; Wang, H. Y.; Willard, J. E. J.Phys. Chern. 1981,
85, 1310.
(3) Bhattacharya, D.; Willard, J. E. J. Phys. Chern. 1981, 85, 154.
0022-365418212086-0962$01.25/0
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strument settings and sample tube diameters, the integral values allowed calculation of the unpaired spin concentrations. The free radical and trapped electron signals produced in 3-methylpentane glass by a known dose of y radiation were used as the spin concentration standard, assuming G(R) = 3.0 and G(e;) = 0.70. A strong pitch sample calibrated against the 3-methylpentane was used as a reference. Further details of the ESR measurements, including power saturation measurements and control and measurement of the sample temperature, are given in ref 2 and 3. A qualitative determination of the relative photolytic effectiveness of different wavelength regions was made by use of different light sources. These included (1)for X > 380 nm, a 2500-W high-pressure Hg lamp filtered by a Corning CS 3-73 filter; (2) for X > 290 nm, the 2500-W Hg lamp with 5-mm thick Pyrex filter; (3) for X > 220 nm (primarily 254 nm), a Suprasil spiral low-pressure Hg lamp with Vycor filter; (4) for X = 185 nm, the Suprasil spiral Hg low-pressure lamp with an interference filter with an OD of 3 at 254 nm and 0.6 at 185 nm.4 The light from the 2500-W Hg lamp was focused on the sample in the ESR cavity by a quartz lens. For illuminations with the Suprasil spiral, which was mounted concentric with the chimney of the He gas flow ESR dewar, the sample could be raised above the cavity to a position in the center of the lamp, while continuing to be cooled by the cold He from a Helitran LTD-3-110 transfer unit. The temperatures during illumination were measured at the position of the sample. The total light intensities on the portion of sample tubes exposed to the light from the sources described were, in watts ern-,, (1)2.6, (2) 3.5, (3) 0.05, (4) 0.02.
Results Absorption Spectrum of CH3 i n CH, ut 5 K. Preparatory to investigating the photolysis of CH3 in CH, we have determined its absorption spectrum at 5 K from 185 to 400 nm. Two samples of CHI were y irradiated at 5 K in 5 mm X 5 mm square Suprasil cells and their spectra at 5 K determined with a Cary 14 R spectrophotometer, using equipment described earlier.5 After recording the 5 K spectrum each sample was held at 70 K for 5 min to allow the CH3to decay and the spectrum was again recorded at 5 K. Subtraction of the second spectrum from the first (4) Takacs, G. A., Ph.D. Thesis, University of Wisconsin, 1972. Available from University Microfilms, Ann Arbor, MI. (5) Paraszczak, J.; Willard, J. E. J. Chern. Phys. 1979, 70, 5823.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982 963
Photoelimination of H from Radicals in Matrices
o,llllILJ 0180
190
200
210
220 2 3
240
260
X,nm
Flgure 1. Optical absorption spectrum of 1.35 X at 5 K, with a 5-mm light path.
lo-,
M CH, in CH,
gave the absorption shown in Figure 1and attributed to CH,. The irradiation time for the sample of Figure 1was 1h at a dose rate of 7.74 X 10'' eV g min-'. The more intense spectrum from the second sample, which received a 1.5-h y exposure, was similar, with identical A- at 192.5 nm. On the basis of G(CH3) = 3.32 the extinction coefficient of CH3 at 192.5 nm is 1 X lo3 M-l cm-'. These results indicate a large broadening and blue shift of the spectrum of CH3 in CH, at 5 K relative to its spectrum in the 215-217-nm range in the gas phase.6 Photolysis of CH, in CH,. Concentrations of -1.05 X lo-, M CH3 radicals were produced in two samples of CHI in 3-mm i.d. quartz ESR tubes by 30 min, 3.6 X 1019eV g-l, X irradiations at 30 K. The ESR spectra were free of the doublet signal of trapped hydrogen atoms (HJ, since H, diffuse and combine at temperatures > 15 Ka2 Exposure of one of the samples at 5 K for 60 min to 254 nm from the Suprasil spiral lamp filtered by Vycor caused no change in the CH3 concentration and produced no H,, indicating that CH3 is not photolyzed by 254 nm, consistent with the absence of absorption in this region indicated by the spectrum of Figure 1. Exposure of the samples to the unfiltered light of the same lamp for 15 rnin reduced the intensity of their CH3 ESR signals by 80% and generated a strong H, signal.' Evidence for the presence of CH2,the second product of the photolysis of CH,, is given by the production of C2H3 and C2H5when samples such as the above are warmed to >30 K where CH2 can diffuse.* Photolysis of CH, in Xe. CH3 radicals trapped in Xe were produced by a 6 X 1019eV g-' X radiolysis of 0.5 mol % CHI in Xe and a 1.2 X 1019eV g-' X radiolysis of 0.1 mol % CHI in Xe at 8 K, producing CH3 concentrations of 9.6 X an 2.1 X M, respectively. Exposure of the first sample to the light of the high-pressure Xe-Hg lamp with Vycor filter for 45 min produced no change in either the CH, or H, concentration, indicating again that wavelengths >220 nm do not decompose CH,. Each sample showed a decrease in the CH, signal of >1% min-'
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(6) (a) Herzberg, G. h o c . R. SOC. London, Ser. A 1961,262,291. (b) Parkes, D. A.; Paul, D. M.; Quinn, C. P.; Robaon, R. C. Chem. Phys. Lett. 1973, 23, 425. (7) The ratio of the double integral of the H, signal to the decrease in the double integral of the CH, signal was in each case 0.6 rather than the value of 1.0 expected for the reaction CH8+ hv CH2+ H. This implies partial saturation at 5 K of either the signal of the H, formed by the photolysis, or the CHI remaining, or both. The measurement of the CH, signals at 30 K prior to photolysis were made at 100 pW and the measurements at 5 K after photolysis at l pW. It is known*that neither the signal of CH, nor that of the H, present in equal amount immediately after radiolysis of CH, is saturated at these conditions. (8) Bhattacharya, D.; Willard, J. E. J.Phys. Chem. Following article in this issue.
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40 G
Figure 2. ESR spectra at 30 K: (a) CH , , produced in Xe-0.1 mol % CH , , by 1.8 X 10'' eV g-' radiolysis at 5 K; power, 10 pW; modulation amplitude 0.5 0; gain, 1250. (b) C&13produced in Xe-0.5 mol % C&14 by 30-min exposure to 185 nm from Suprasil spiral at 5 K; power, 10 pW, modulation amplitude 0.5 G, gain 1600. (c) Sample of (a) after 2.5-min exposure to 254 nm from Vycor-filtered Suprasil spiral; power 10 pW, modulation amplitude 0.5 0,gain 1600.
TABLE I: Photolysis of C,H3 in Xe Matrix at 5 K extm-
0 0.5 2.5 14.5
1.81 1.49 1.17 1.17
4.90 5.22 5.60 5.60
-0.32 -0.32 0
0.32 0.38 0
Total time of exposure to 254-nm light.
during exposure to 185-nm light from the unfiltered Suprasil spiral lamp, and concurrent production of H,. About 81% of the CH3 was removed during 75 rnin of illumination of the second sample. Photolysis of CD3 in CD4and in Xe. CD, for the photolysis tests was produced at 1 X M by radiolysis of neat CD4 and of Xe containing 0.5 mol % CD,. No decrease in CD, concentration was caused by exposure of either sample to >290-nm light or 254-nm light. Following 85-min illumination of the neat CD4sample with 254-nm light from the Vycor filtered Suprasil spiral, a 20-min exposure to 185 nm from the same lamp with interference filter reduced the CD3 concentration by -90% and produced D, equivalent to 85% of the CD3 decrease. Similarly, a relatively short illumination of the X 4 D 4sample with 185 nm caused a large decrease in [CD,] and a large increase in [D,]. These results c o n f m the earlier evidence2 for the CD, hv (185 nm) CD2 + D reaction and indicate that the photoelimination does not occur at 254 nm. Radiolysis and Photolysis of C2H4 and Photolysis of C&, in Xe. Figure 2a is the ESR spectrum at 30 K of the species produced by a 1.8 X 1019eV g-I radiolysis at 5 K of Xe containing 0.1 mol % C2H4.9a Figure 2b is the spectrum at 30 K of the species generated in Xe containing 0.5 mol % C2H4by a 30-min exposure to the 185-nm light of the Suprasil spiral. Prolonged exposure to 254 nm prior to the 185-nm exposure produced no paramagnetic species. The spectrum of Figure 2a is nearly identical with that of the species produced by the CH2 + CH2 C2H3 + H
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(9) (a) At 30 K this spectrum was unsaturated at powers up to at least 10 pW, the highest power used, while at 5 K it was saturated at 290-, 254-, and 185-nm light at 5 or 35 K, with warming to 50 K between exposures to allow any H, to decay. The 185-nm exposure (but not the >290 or 254 nm) produced a major decrease in C2H5 and a larger increase in H, implying the C2H5 + hv C2H4 + H reaction and one or more other reactions such as C2H4 + hv (185 nm) C2H3 + H, C2H4 + hv (185 nm) C2H2 2H, and C2H3 hv (254 nm) C2H2 H, involving products of the radiolysis. Some H, was also produced during the >290- and 254-nm exposures, implying the presence of a species which absorbs at these wavelengths. The absorption spectrum of C2H5 in Xe is unknown, but it would be surprising if it extended to wavelengths this long. CzH3 produced by the X radiolysis of C2H6,with its ESR spectrum obscured by the more intense broad line C2H5 spectrum, may have been responsible. The CzH5 ESR spectrum in Xe consists of six broad lines with some superimposed structure which is temperature dependent. The breadth between the centers of the outermost peaks is 125 G, equivalent to that of the well-resolved 12-line spectrum of CzH6 in CHI at 5 K8 and in liquid C2HG at 93 K.l0
Bhattacharya and Willard
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(IO) Fessenden, R.; Schuler, R. H. J. Chem. Phys. 1963,39, 2147.
i/
I/ I(\- - iiv
,40G
~
Flgure 3. Successive ESR spectra at 5 K of sample of CH,-0.1 mol % O2 irradiated (3.6 X I O ' @ eV g-') at 5 K: (a) (a) after warming to 40 K; (b) after warming to 46 K; (c) after I-min exposure at 5 K to 254 nm from Vycor-filtered Suprasil spiral; (d) after 10min exposure at 5 K to >290 nm from Xe-Hg lamp. The radical concentrations are given in Table 11.
Photolysis of H02, CH302, and HCO Formed in Radiolyzed CH,. When the ESR spectrum of CHI containing dissolved O2 is examined at 5 K following radiolysis at 5 K the signals are identical with those from radiolyzed neat CH4, i.e., the four-line spectrum of CH,, with a line splitting of 23 G; the Ht doublet with 504-G splitting; and the four quartets of the exchange coupling signal which appear between the H, and CH3 signals.l' When such a sample is warmed to 40 K, and again examined at 5 K, a new signal is superimposed on the low-field side of the CH3 spectrum (Figure 3a). If the sample is held for a few minutes at 46 K, where CH, diffuses, and returned to 5 K, the ESR spectrum changes to the form of Figure 3b. The spectral changes are those to be expected from the growth of HOz and CH3O2, which have indistinguishable ESR spectra.12 It is probable that the predominant species formed at 40 K, where Ht diffuse readily,2is H 0 2 (a small proportion of CH302 may be formed from CH, radicals born in close proximity to O2molecules). This conclusion is consistent with the observation that the intensities of the high-field CH3 lines are not significantly affected by the 40-K exposure. The dramatic change to a spectrum of the type of Figure 3b caused by warming to 46 K is undoubtedly due to conversion of most of the CH3 to CH302. Evidence on the photolysis of H 0 2 has been obtained from illuminations of CH4-0.1 mol % O2 samples which have been warmed only to 30 K following radiolysis at 5 K. Evidence on the photolysis of CH302 has come from similar samples warmed to 46 K. In both cases light of 254 nm from the Vycor-filtered Suprasil lamp was used. Both HO?, and CH30tbhave gas-phase absorption spectra (11)Gordy, W.;Morehouae, R. Phys. Reu. 1966,151,207. (12) Bennett, J. E.; Mile, B.; Thomas, A. Int. Symp. Znt. Combust. 1967,11,853. (13)(a) Hochanadel, C. J.; Ghormley, J. A.; Ogren, P. J. J. Chem. Phys. 1972,56,4426.(b) Pankert, T. T.;Johnston, H. S.Ibid. 1972,56, 2824.
Photoeliminationof H from Radicals In Matrices
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982
965
TABLE 11: Formation and Photolysis o f HO,, CH,O,, and HCO in CH,-0.1 m o l % 0, Matrix
[CH,l, M X lo4
treatment
(1) 3.6 x l O I 9 e V g-' X-ray dose at 5 K ( 2 ) warmed t o 30 K, cooled t o 5 K ( 3 ) 254 n m at 5 K 1 min
CH,
+
HO,.
CH,O,.
M
X
lo4
[Htl, M X lo4
2.06
A[RI,
Mx
1.95 0.07
2.76a 2.62a 2.48' 2.17' 2.52a,C
3 min total 1 3 min total ( 4 ) warmed t o 30 K, cooled t o 5 K ( 5 ) unfiltered Xe-Hg lamp, 30 K, 1 5 min ( 6 ) 1 0 rnin at 46 K, cooled t o 5 K ( 7 ) 254 n m , 5 K, 1 rnin ( 8 ) > 2 9 0 nm, 8 K, 10 rnin a
[mixed radicals],
0.23 0.38 0.65 0 0 0 0.22
2.06 1.04b 1.04c 0.68
A[Htl,
lo4
M x lo4
reactions
0.7
-1.88
1
-0.14 -0.14 -0.31 0.35 -0.46 -1.02 0 -0.36
0.16 0.15 0.27 -0.27 0 0 0.22
4 4 4 1,2 4, 6 2, 3 5 a-d 6
CH, t HCO.
TABLE 111: Formation and Photolysis o f HO,, CH,O,, and HCO in CH,-0.1 m o l % 0, Matrix
[CH,l, M X104
treatment
(1) 3.0 x
eV g-' X-ray at 5 K, warmed t o 46 K, cooled t o 5 K ( 2 ) 254 n m , 5 K, 1 min ( 3 ) > 2 9 0 nm, 5 K, 1 0 min ( 4 ) warmed t o 47 K, cooled t o 5 K ( 5 ) 254 nm, 5 K, 1 rnin ( 6 ) > 2 9 0 n m , 5 K, 1 0 rnin a
HO,
[mixed radicals],
CH,
+
1.33b 1.19 0.97' 1.23b 0.97
2
-+
CH3 + CH3 hu
HO2
CH3O2
(2)
C2HG
(3)
H +0
2
+ H2O + OH + H CHz02 + H HCO -&+H + co OH + CH4 CH3 + H20 HCO HCO
(5b) (5c) (54
(7) Warming the X-irradiated sample of Table I1 to 30 K produced an increase in radical concentration of 0.70 X lo4 M, attributable predominantly to HOz formation by reaction 1. Of the H, which did not react with 02,part was lost by the CH3 + H CH4 geminate combination (8% of the total H,) and part by the H + H H2 reaction.2 Exposure of the sample to successive 254-nm illuminations removed H 0 2 and produced an equivalent amount of H, by reaction 4. Warming to 30 K (step 4) removed the H,, +
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A[Rl,
M x104
0.23 0.36
0.03 0.14 -0.36 0.26 -0.26
a[Htla
MxlO
reactions
0.21 0.46
0.23 -0.13 -0.36 0.21 0.25
5 a-d 6 2, 3 5 a,b,d 6
HCO.
which extend from 260 nm. For HCO, which was found as a product of the photolysis of CH302,light of >290 nm from the Pyrex-filtered Xe-Hg lamp was used to provide high intensities in the 460-860-nm region of ita absorption spectrum.14 The results are illustrated by the data from two experiments (Table I1 and 111),which we interpret in terms of reactions 1-6, and the knowledge that H atoms in CHI diffuse and react readily at 120 K while CH3 radicals do not do so below 45 K. H + 02 H02 (1) CH3 + 0
[&I,
M Xl o 4
1.30a
lOl9
+ CH,O,.
M x IO4
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(14) Herzberg, G. "Molecular Spectra and Molecular Structure. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules"; Van Nostrand Princeton, NJ, 1966.
of which 54% formed new radicals. The ESR spectrum at this point consisted of the CH3 quartet with evidence of an underlying peroxy signal and with weak lines of the HCO doublet on the wings. The formation of HCO on warming implies the H + CO HCO reaction.2 The only possible source of CO appears to be reaction 5b or 5c followed by reaction 6, implying that some CH302was formed in step 2 at 30 K from CH3born in close proximity to OF 100% recovery of the 2.06 X lo4 M initial concentration of CH3 was achieved by complete removal of the H 0 2 and HCO, predominantly by reactions 4 and 6, by exposure to the unfiltered light of the Xe-Hg lamp (step 5). All H was removed in this step, by one or more of the reactions H + H Ha, H + H 0 2 H2 + 02,H + HOz H202,H HCO H2 + CO, H + HCO H2C0,the last four of which may also have contributed to the removal of HOP and of HCO. All are exoergic. When the sample was then held at 46 K for 10 min there was a net radical loss by reaction 3 and the remainder of the CH3 was converted to CH3O2 (Figure 3b). Step 7, a 1-min exposure to 254 nm, converted the CH302to CH, + HCO (Figure 3c) with no change in total radical concentration, demonstrating dramatically the occurrence of reactions 5a and 5b and/or 5c. The formation of H, in this step is evidence for reaction 5c or 5d. Step 8 removed the HCO leaving only the four-line CH3spectrum (Figure 3d) and reducing the total radical concentration by 35%. This indicates that the branching ratio between reactions 5a and 5b + 5c in the 254-nm photolysis of CH302is -2/1. In a second experiment 1.30 X lo-" M H02 + CH302was produced by radiolysis of a CH4-0.1 mol % sample at 5 K and holding it for a few minutes at 46 K (table 111). A 1-min exposure to 254 nm converted the spectrum to that of CH3 + HCO with little change in total radical concentration, and produced 0.23 X M H,. Exposure to 1290-nm light removed the HCO signal, reducing the radical concentration by 0.14 X lo4 M and increasing the [H,] by an equal amount. The sample was then held at 47 for a few minutes during which the radical concentration dropped to 0.97, the CH3 was converted to
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The Journal of Physical Chemistry, Vol. 86,No. 6, 1982
CH302, and the H, was removed. The effect of the 254-nm exposure of step 5 of Table I11 on this sample, i.e., an increase in radical concentration by 0.26 X M, accompanied by production of 0.21 X lo4 M H,, requires a mechanism in which the photolysis of one radical yields two radicals. The most plausible seems to be reaction 5c followed by reaction 7. If this mechanism is responsible for the growth, the concentration of HCO produced must equal the total growth in radical concentration. This equality is demonstrated by the effect of >290 rim-light (step 6) which removed HCO, producing an equivalent amount of H,, and returning the radical concentration to that after step 4 before the 254-nm exposure of step 5. I t is to be noted that the 254-nm exposure of step 2 of Table 111, in contrast to step 5, did not produce a significant net increase in radical concentration. This may be due to a higher ratio of H 0 2 to CH302with a consequent balancing of the increase from the photolysis of CH302 (reactions 5c + 7), by a decrease due to reaction 4. Photolysis of CH302Formed from (CH3),N2Precursor in a CH4-Oz Matrix. To avoid the complications noted above, resulting from the presence of H 0 2 while studying the photochemistry of CH302,we have produced CH302 by photolyzing (CH3),N2(1.3 X M) in CHI containing O2 (3.3 X M) at 5 K. The CH, formed from the (CHJ2N2was allowed to react with the O2 by warming to 47 K. Photolysis of (CH,),N2 for 30 min with the unfiltered light of the Xe-Hg lamp at 5 K produced 3.9 X M CH,, which converted to 6.3 X M CH302on the warming. When this was photolyzed with the Vycor-filtered Suprasil spiral for 1 min it yielded CH,, HCO, and H,, confirming the conclusions of the previous section as to the branching mechanisms of the photolysis. Photolysis of DCO,DO2,and CD302. As expected from the results with HCO, H02, and CH302, DCO in CHI at 5 K is readily removed by >290-nm light from the Xe-Hg lamp an DOz is removed by 254 nm, with concurrent production of D,. Similarly, CD, production has been observed from CD302 in samples exposed to 254 nm.
Discussion Role of Radical Photolysis. The results given above show that a variety of simple free radicals undergo photoelimination of H atoms when activated at wavelengths 2185 nm in matrices at 5 K, and that methylperoxy radicals undergo 02,H, and HCO elimination. We have estimated the quantum yields of H, from CH3 and D, from CD3 to be -0.1. Such reactions may be important in determining the final products in any photochemical systems in which appreciable light absorption by radical intermediates can occur. These include steady-state
Bhattacharya and Willard
photolyses in the solid state, where stabilized radicals may reach significant concentrations, and flash photolyses and laser illuminations in the solid, liquid, and gaseous states. In typical steady-state photochemical investigations in the gas or liquid phase, where radical lifetimes are very short and the light intensities relatively low, photoactivation of the radical intermediates is negligible. Previously Reported Photoeffects on Trapped Radicals. (1) In early studies, the yields of stable products from alcohols radiolyzed in the solid state at 77 K were compared for samples which were maintained in the dark before melting for analysis and others which were exposed to UV illumination before melting.15 Photolysis produced an increase in the yield of CO equivalent to the concentration of radicals present in the solid before illumination. For CH,OH, it was suggested that this could be accounted for by photolysis of the radicals produced by the radiolysis (CH20H hv HCO H,, followed by HCO CO H). This was consistent with the observation that photolysis converts the ESR triplet of CH20Hto the doublet with 134-G splitting attributable to HCO. This photodecomposition of CH20H is interestingly similar to that of CH302 reported in the present work. (2) When protiated alkanes which have been y irradiated at 77 K are exposed to 254-nm light the radical concentration is reduced by about 30%, following which continued illumination has no further effect.16J7 The experiments have been interpreted to indicate that the decrease must result from excitation of radicals in such a way as to promote migration of the vacant bonds by intermolecular or intramolecular hydrogen hopping.17 (3) It has often been observed in our laboratory that the ESR spectra of trapped radicals produced in alkane glasses by radiolysis become better resolved when the sample is exposed to 254-nm light,16-19and that the changes are reversible in the dark. It appears that the changes are due in some cases to shifts between different states of hindered internal rotation, “rotamers”, and in others to isomerization.16 References describing similar changes are included in ref 16 and 20. Acknowledgment. This work was supported, in part, by the U.S. Department of Energy under contract DEAC02-76ER01715-AO05.
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(15) These results and references to earlier related work are given in Johnsen, R. H. J.Phys. Chem. 1961,65, 2144. (16) Sprague, E. D.; Willard, J. E. J. Chem. Phys. 1975, 63, 2603. (17) Wilkey, D. D.; Fenrick, H. F.; Willard, J. E. J.Phys. Chem. 1977, 81, 220. (18) Perkey, L.; Willard, J. E. J . Chem. Phys. 1974, 60, 2732. (19) Neiss, M.; Willard, J. E. J. Phys. Chem. 1975, 79, 783. (20) Fessenden, R. W.; Schuler, R. H. In ‘Advances in Radiation Chemistry”,Vol. 2, Burton, M., Magee, J. L. Ed.;Wiley-Interscience; New York, 1970, Vol. 2.
