STEPHEN B. MILLIKENAND RUSSELLH. JOHNSEN
2116
in methanol as tested through light-scattering measure water to methanol, ethanol, and acetone and does ments. not exist in the latter. From Table I it is seen that the slope of S, of nAcknowledgments. The authors wish to thank Dr. is higher than those Of the Other alkylammo(7. Cohen for the light-scattering measurements and nium salts.32 It seems that the large size of the ion Miss E. Halperin for technical help. is responsible for the steeper increase of with concentration. This might possibly be due to a stronger (32) Measurements carried out on dilute solutions of (CHa)rNBr a~coho~phobic effectif i t may -be said so, since io seem to indicate that the slope Sv for this salt is higher than that for evidence could be found for the formation of micelles (C&)rNBr.
Ultraviolet Photolysis of X-Irradiated Methanol at 77°K'
by Stephen B. Milliken and Russell H.Johnsen Department of Chemistry, Floriah State Uniueraitv, Tallahassee, Florida 3.2306 (Received November 29,1966)
X-Irradiated methanol was photolyzed at 77°K in the epr cavity and in bulk samples with selected wavelengths. Wavelengths shorter than 3100 A lead to a continuing production of .CHO and a steady-state concentration of .CH, in the matrix; hence the production of CH, and CO from the thawed material is proportional to the duration of photolysis time. Wavelengths between 3300 and 4000 A yield an amount of .CHO trapped in the matrix equal to the original concentration of spins produced by X-irradiation. A study of the hydrogen and methane yields was made in both the glassy and the polycrystalline solids.
Introduction A useful feature of the radiolysis of solid organic materials at low temperatures is that intermediates, reactive a t room temperature, can often be trapped and studied at leisure. Radical intermediates can be identified by electron paramagnetic resonance and correlated with the yields from the thawed material. When relating such intermediates to radiolysis mechanisms it is necessary to know whether these trapped species are produced by decomposition of excited molecules, by ion molecule reactions, or by radical precursors which are still able to react with the substrate a t the low temperature. For this reason reactions of radicals in the matrix a t low temperatures relate directly to radiation chemistry. In the irradiated aliphatic alcohols such radical reactions may be initiated by the photolysis of the irradiated material by The J o u r 4
of Physical Chemistry
visible and ultraviolet light. It is possible to photolyze the alcohol samples in an epr cavity and directly observe the radical reactions during and after the photolysis. Investigations of this type have been made,2-s but the question of radical reactivity a t 77°K has not been answered fully and study is continuing. This particular work is concerned with methanol. Previously it has been ~ h o w n ~that - ~ aCH20H and e- are the paramagnetic entities trapped in irradiated (1) This material was abstracted in part from the thesis of S. B. Milliken and was supported in part by U. 8. Atomic Energy Commission Contract AT-(40-1)-2001. This is AEC Document ORO2001-3. (2) R. S. Alger, T. H. Anderson, and L. A. Webb, J. Chem. Phys., 30,
685 (1959). (3) C. Chachaty and E. Hayon, Nature, 200, 59 (1965). (4) E. Hayon and C. Chachaty, J . Chim. Phya., 61, 1115 (1965). ( 5 ) D. R. Smith and J. J. Pieroni, Can. J. Chem., 43, 876 (1965).
ULTRAVIOLET PHOTOLYSIS OF X-IRRADIATED METHANOL
methanol a t - 196”. Ultraviolet light transforms the .CH20H radical into .CHO. This .CHO radical is further decomposed by visible light into CO and Ha. Visible light also removes the trapped electron and simultaneously produces CH20H. Chemical analysess of the products of the photolyzed radicals have indicated that the transformation of the CH2OH by ultraviolet followed by visible light gives a yield of CO equal to the original yield of radicals. This chemical analysis corroborated the results of Anderson, Alger, and Webb,2 who found that each .CH20H photolyzed produced a .CHO radical. It was in disagreement with the results of Tepley and Dainton,’ who observed that a steady-state concentration of eCH20H was produced by a “cyclic” process propagated by the ultraviolet light. In the present study both the photolysis experiments and the chemical analyses were repeated, using optical filters to control the wavelength of the photolyzing light. In the course of the investigation a dependence of the yields upon the nature of the solid was found.
-
Experimental Section The methanol was Baker Analyzed reagent grade used without further purification, since it was found that ordinary chemical means produced no measurable improvement. Samples were handled in sealed containers or in a drybox to control their water content. The amount of water added regulated the phase of the ’ solid. It was found that with less than 2 mole % of water the methanol froze in a white, opaque, polycrystalline solid. Greater than 2% water content produced a clear, glassy solid. When observing the effect of the state (glassy or polycrystalline) on the radiolysis yields it was noted that no change appeared until sufficient water had been added to produce the glassy solid. Additional water caused no further change in yields. After it had been shown that it is the type of solid and not the water which changes the yields, a concentration of 5 mole % was used in the glassy samples t o make the samples insensitive to the method of freezing. Degassing was effected by the technique of repeated freezing and thawing with pumping. Samples to be examined in the epr apparatus were prepared either in thin-walled Thermosil tubes 3 mm in diameter or by freezing drops in liquid nitrogen. The drops were frozen by injecting a 2 0 4 aliquot of degassed methanol under the surface of liquid nitrogen by means of a Hamilton syringe. This minimized the oxygen content. Methanol containing water formed glassy spheres which could be transfered to the dewar in the epr cavity after irradiation and held in position with
2117
a quartz fork. The polycrystalline material was fragile and required the use of the Thermosil tubes to contain it. Irradiation of these tubes generated some epr signal which, however, was small compared with the signal from the irradiated alcohol and could easily be subtracted from the spectrum of the alcohol radicals. Irradiations were carried out with the Florida State University Van de Graaff accelerator using 3-Mv peak X-rays. The epr spectrometer was a Varian Model V-4502. Ultraviolet photolyses were carried out with a General Electric A-H6 lamp. The total dose varied from 10lg to 1020 ev/g. I n general, 1020 ev/g were required for accurate determinations of molecular products; however, 1019 ev/g produced a more than sufficient radical concentration for epr spectra to be taken. The effects of photolysis of the radicals were not dependent on total dose and the lower concentration permitted a more nearly complete photolysis of sCH20H radicals in a given period of time. Except for a lower G(radica1s) in the polycrystalline material a t the higher dose which is discussed later, none of the yields of radicals or products appeared to be dose dependent. Dosimetry was by the Fricke method,* using a G(Fea+) = 15.5. Gas yields were determined by gas chromatography using a 1-m charcoal column at room temperature and a thermal conductivity detector. Corning Glass filters and Baird Atomic interference filters were used to study the effect of different wavelengths. The Corning Glass filters, with broader band pass and greater transparencies than the interference filters, were used for the study of the concentration as a function of time during photolysis. The Baird Atomic interference filters were used to help fix the wavelength region between that which produced a net increase in radical concentration and that which did not. The filters were inserted into the path of light collimated and focused from the AH-6 source by two quartz lenses. By using a combination of filters, fairly narrow wavelength regions could be investigated a t the expense of decreased intensity. Transmission regions of the optical filters are defined arbitrarily as the region where the transmittance is greater than 10% of the peak transmittance. (Thus, Corning Glass filter KO. 5860 transmits approximately 25% of the light a t 3600 A. It transmits 2.5% of the light a t 3300 and 3825 A. Its transmittance region is then defined as being approximately 3300-3800 A.) The (6) R.H.Johnsen, J. Phya. Chem., 65, 2144 (1961). (7) F. S. Dainton, G. A. Salmon, and J. Tepley, PTOC.Roy. SOC. (London), A286, 27 (1965). (8)A. Weiss and H. Swarc, PTOC.Intern. Conf. Peaceful Uaea At. Energy, Geneva, 1066, 14, 179 (1956).
Volume 71,Number 7 June 1967
2118
STEPHEN B. MILLIKENAND RUSSELLH. JOHNSEN
respective upper and lower wavelength limits for the visible and ultraviolet light which photolyzes radicals were determined approximately by the combination of Corning Glass filters No. 9863 and No. 3850. Light in this region of transmission, approximately 36504050, affected neither the .CHO nor the -CH2CH radical. Relative concentrations reported in the tables were determined by numerical integrations of the esr spectrograms and in the graphs from the product of peak height and line width. These methods were used only in those cases when spectra did not show appreciable overlap or when only total concentration of spins was of interest. Ultraviolet photolysis of an unirradiated sample generated a small concentration of radicals when the light source was intense and included short wavelengths. The concentration of these radicals after several hours of photolysis did not exceed 10% of the radicals generated by X-irradiation, indicating that the effects seen are due to the photolysis of the radicals and not the bulk material.
Results It was found that the effects of photolysis depend on the wavelengths employed. Ultraviolet light in the approximate range 33W3600 A transforms the CH2OH radical into .CHO with no change in the total radical concentration as reported earlier2 (Figures 1 and 2). Photolysis with light that includes ultraviolet of shorter wavelengths (3100 A) but excludes visible light produces a net increase in the radical concentra-
1
TIME
Figure 1. Esr spectra: a, X-irradiated methanol; b, X-irradiated methanol after 35-min ultraviolet photolysis (A 3300-3800 A). Arrow indicates g of free electron.
The Journal of P h y h l Chemdstry
MINUTES
Figure 2. Relative concentration of CHzOH and GHO during ultraviolet photolysis. Regions A, B, and C refer to the band pass of filters during a given period of photolysis: region A, 3300-3800 A ; region B, 4500-7000 A; region C, 3000-3900 A. Slight increase in CH20H concentration in region B also occurs in the dark and is not related to photolysis by visible light.
tion, as found by Dainton.7 This can be seen from an examination of experiments 1 and 2 in Table I. When a filter transparent over a broad region is used the intensity of light is sufficient to establish a steady-state concentration of .CH20H radicals in 20-30 min. This is the case in experiment 1 in Table I; the last column represents the approximate steady-state concentration of .CH20H relative to the original concentration. Light filtered through narrow band-pass filter combinations took somewhat longer to establish the steady-state concentration. However, as long as light with wavelengths shorter than 3100 A is present a net increase in radicals is actually seen. The results in Table I are supported by the chemical analysis of bulk samples which showed a continuing production of CO and CH, with photolysis by light below 3100 A. This effect is shown in Figure 3. On the other hand, an ultimate yield of CO equal to the original concentration of CH20H was found previously by Johnsen: using thin-walled Pyrex ampoules which were appreciably transparent only above 3300 A. In this work an attempt was made to duplicate the effects of the thin-walled Pyrex tubes by using an optical filter system, but the intensity limitations precluded the complete destruction of the . CHzOH within practicable time limits. The results are shown in Table 11. It can be observed that methane has reached a limit and the rate of production of carbon monoxide and hydrogen has slowed after 6 hr of photolysis with the filter systems No. 5850 and 0160 (3100-4800 A). Photolysis with filter No. 5860, which is less trans-
-
1
IN
2119
ULTRAVIOLET PHOTOLYSIS OF X-IRRADIATED METHANOL
Table I : Effect of Ultraviolet Photolysis on Radical Concentration Expt no.
Filtera
1
9863
2
9863 7740 9863 Pyrex 5860
Transparent region, A
24004000 29004000 34004000 33003800
+ +
3
4
Relative transparency
Relative yields of radioala (total dose 1010 ev/g)-Initial-6min photolysisLimiting values CHtOH * CHO * CHI * CHIOH * CHO CHs CHzOH CHO CHI I
*
a
9
.
-
Total
1.0
1.0
0
0
0.70
0.22
0.22
0.45
0.67
0.16
1.30
0.98
1.0
0
0
0.95
0.05
0
0.75
0.30
0.05
1.10
Unknown
1.0
0
0
1.0
0
0
...
...
...
...
0.36
1.0
0
0
0.92
0.08
0
0.8
0.20
0
1.00
Corning stock number.
18
Table II: Product Yields Resulting from Ultraviolet Photolysis
-
Hours photolyzed-6 3 6 3 No. of molecules/100 ev (total dose 1020 ev/g) -COY -CHt---.
3
-H-
5850+ 0160” (3100-4800 A)
8.8
10.0
2.4
3.7
1.3
1.3
4.0
6.3
1.0
1.8
0.4
0.8
(3300-3800 A) Reference 6
0
a
6
Corning stock number.
23. Ob 6
6. 6b
11.06
Limiting value.
an increase in radical concentration, while a similar filter centered on 3300 A does not. Both have a onetenth band width of about 300A. Other observations follow which are indicative of a more complex system than has been suggested pre-
A G(C0) 2
4
6 8 IO HOURS PHOTOLYZED
12
Figure 3. Product yield from ultraviolet photolyzed X-irradiated methanol; X 2400-4000 A; dose 10%ev/g.
parent, has not appreciably used up the aCH20H (G = =6) even after 6 hr. The long-wavelength limit of light of sufficient energy to bring about a net increase in radicals during photolysis lies somewhere between 3100 and 3300 A. It cannot be fixed more exactly because of the band width of the filters necessary to transmit sufEcient light to cause an observable change. It was noted that a Baird Atomic interference filter centered on 3130 A permits the passage of light which generates
1. Whenever there is a net production of radicals the presence of .CH3 is observed (Figures 4 and 5 ) . When short-wavelength light is excluded there is no evidence of methyl radicals (Figure lb). With light of wavelength less than 3100 A the concentration of * CH20H drops rapidly, with the initial production of .CHs, then reaches a steady-state concentration. The concentration of methyl radicals passes through a maximum, then ultimately also approaches a steady state. The production of CHO continues indefinitely. This behavior is shown in Figure 6. 2. The ’CH3 disappears in the dark by a process which is approximately first order a t least in the initial stage (Figure 7) and which is accompanied by a net production of aCH20H. Double integration of the four-line spectrum was not conclusive, but it appears that at least three sCH20H radicals are produced by every four CHI radicals that disappear.
-
Volume 71, Number 7 June 1967
STEPHEN B. MILLIKENAND RUSSELLH. JOHNSEN
2120
I
Figure 4. Dry methanol photolyzed 5 min with ultraviolet light (23004000 A): 1, methyl quartet; 2, CH20H triplet; 3, CHO doublet. Arrow indicates g of free electron. TIME
(minutes)
Figure 7. Decay of the methyl radical in the dark.
:1,,,,,,,,,,,,,,. a
5
2
4
6
8
12
IO
14
TIME (minutes)
Figure 5. Same sample as Figure 4 after 35 min of photolysis.
m
*CH20H GHO
Figure 8. Behavior of the central esr peak of the . CHtOH spectrum during ultraviolet photolysis with 24004000-A light.
light above 3300 A over an extended period of time. When most of the ‘CH20H is gone, continued photolysis by ultraviolet light from 2400-4000 A does not materially affect the concentration of CHO. These results appear to be in contradiction with the observation made by Dainton.’ A steady-state concentration of .CH20H unrelated to the .CHO concentration can be established a t any concentration of .CHO by preceding the short-wavelength photolysis with photolysis by light above 3300 A. While the complications cited above seemed primarily related to the choice of wavelength of the photolyzing light, other organic systems have shown yields which varied as a function of phase.e Thus, solids which are not homogeneous can exhibit varying yields. lo Since the radiolysis yields from solid methanol previously reporteds were not as reproducible as those found for liquid methanol, an investigation of the effect of phase seemed relevant.
-
TIME
(MINUTES)
Figure 0. Changes in radical concentration with photolysis below 3100 A.
3. Under intense ultraviolet light in the 24004OOO-A region the central peak of the eCH20H triplet does not exhibit the monotonic decay of the side peaks but passes through a maximum during the first 5 min of photolysis. This is shown in Figure 8. 4. Formyl radicals do not appear to be destroyed by ultraviolet light. Most of the aCH20H can be transformed into CHO by photolysis with ultraviolet
-
The Journal of Phyaical Chemistry
(9) R. M. A. Hahne and J. E. Willard, J . Phy8. C h m . , 68, 2582 (1964). (10) H. Swarc, J . Cham. Phya., 59, 1067 (1962).
ULTRAVIOLET PHOTOLYSIS OF X-IRRADIATED METHANOL
As mentioned above, the type of solid obtained by freezing is a function of the water content. The independence of product yields and water content has been reported by Tepley, et al." The yields obtained from the two different solids are shown in Table 111. They are as reproducible as the yields from liquid methanol providing the solid is homogeneous. The yield of hydrogen is greater in the glass than in the crystal while the yield of methane is lower. The initial yields of -CH20Hare apparently the same in both solids, but after a dose of 1019 ev/g is exceeded more spins seem to be trapped in the glass. The photolysis of the CH20His unaffected by the nature of the solid.
Table 111: Effect of Phase at -196" -No.
of molecules/100 ev-
Phase
Ha
co
CHI
Glassy Crystal Reference 6 (possibly a mixture)
3.4 2.8 3.6
0.06 0.08 0.45
0.50 1.60 0.57
Discussion The effect of ultraviolet light with wavelengths greater than 3300 A is to transform the *CHIOHinto .CHO. This may be a two-photon process
That the first step is probably reversible is suggested by a slight increase in the * CHzOH concentration after ultraviolet photolysis is stopped (region B of Figure 2), Figure 6. If ultraviolet photolysis is followed by exposure to visible light
H.
CO
+ H-
+ H . +H2
(2) (3)
takes place. The photolysis of .CHO with visible light does not result in the continuing formation of radicals. Photolysis with light in the ultraviolet region has no effect on the CHO radical Concentration. It would thus seem that the H . atom so produced is of thermal energy. This is in contrast with the proposal of Tepley and Dainton,' who suggested that CHO was destroyed by ultraviolet light with the production of a hot hydrogen atom *CHO +CO
+ Ha*
According to these workers, this hot He atom could then abstract to produce aCH2OH CHaOH
+ H**
4
mCH20H
(4)
+ H2
(5)
However, the effect of ultraviolet light in the absence of visible light was evidently never investigated. Moreover, the postulated cyclic process involving the regeneration of CH2OH through the photolysis of .CHO should determine a value for the relative concentration of .CH20H and .CHO. However, we have observed that the steady-state concentration of -CH20H is independent of the concentration of .CHO. This is further evidence that no process involving hot He atoms from the photolysis of .CHO is involved with wavelengths between 3300 and 4000 A. Light above 4000 A simply removes the .CHO without the generation of more radicals in agreement with the results obtained by Johnsena and those of Anderson, Alger, and Webb.2 The effect of light with wavelengths shorter than 3100 A cannot be explained in a simple manner. The lowering of the *CH20H concentration by light in this region is always accompanied by the appearance of the .CH3 radical, implicating the .CH20H as the source of the methyl radicals, perhaps in the fashion indicated by eq 6-8.
-
,--
+ .OH :CH2 + CHaOH + CH3 + CHzOH aCH3 + CHaOH + *CH20H + CH, .CH2OH
:CH2
*
+ H. :CHOH 2.CHO + H .
.CH20H -% :CHOH
*CHO
2121
(6)
(7) (8)
The abstraction of a hydrogen atom by CHa. (eq 8) in the cage to produce .CH20H would result in the net increase in the radical concentration, such as is observed. This increase continues after the light source is removed and the sample allowed to remain in the dark, with the concomitant disappearance of the methyl radicals by what would be interpreted as a process first order in that radical. The half-life for this disappearance is of the order of 600 sec. This is much faster than one would predict on the basis of the extrapolation of rate constants measured in the gas or liquid phase.12-14 An alternate source for the methyl radicals would involve the breakage of a carbon-oxygen bond in a (11) J. Tepley, A. Habersbergerova, and K. Vacek, Collection Czech. Chem. Commun., 30, 793 (1966). (12) T. W. Shannon and A. G. Harrison, Can. J . Chem., 41, 2455 (1963). (13) R. Shaw and J. C. J. Thyne, Trane. Faraday Soc., 62, 104 (1966). (14) M. Cher, J . Phy8. Chem., 67,605 (1963).
Volume 71, Number 7 June 1967
2122
neighboring methanol owing to a sensitization by the CH20H radical. This is not as an attractive hypothesis because of the immediate decrease in the alcohol radical concentration during the early stage of the photolysis. The abstraction reaction involving methylene radicals is made plausible by the fact that such reactions have recently been observed in i s o b ~ t a n eand ~ ~ by the observed behavior of the central peak shown in Figure 7. This temporary increase of the signal intensity in the center of the spectrum can be explained by the transient presence of a singlet. The methoxy radical spectrum is such a singlet; the indiscriminate abstraction by :CHz would be expected to produce some methoxy radicals. It is here, where the interpretation of the epr spectra becomes conjectural, that the implications for the radiolysis mechanism become most important. It is uncertain whether the .CH20H is the initial radical in the radiolysis process or is preceded by CH30.l‘ The proposed abstraction in the dark of hydrogen atoms by .CH3 suggests that CH30., which has similar reactivity, might also participate in an abstraction reaction at 77°K. The behavior of the central peak, if it is indeed due to .CH30, indicates that this species does abstract. Since there is evidence for alkoxy radicals in the liquid and gas phase radiolysis of ethanol and m e t h a n ~ l l ~it* seems ’~ probable that they are also present in the solid. The implication is, then, that the radical .CHzOH does have a radical precursor at least in some cases. The hydroxyl radical is not observed in the methanol matrix under any of the experimental conditions. It seems likely that it is not trapped, but diffuses through the matrix and reacts by the more rapid radicalradical process as does the hydrogen atom. The change in the nature of the solid (glassy or microcrystalline), while having no apparent effect on the photolysis of .CH20H or .CHO, gave the interesting change in radiolysis yields shown in Table 111. No peak attributable t o electrons is seen in the epr spectrum of the polycrystalline material. While the yield of vCH20H is the same in both glass and crystalline material at low doses, after a total dose of about 1019 ev/g the concentration of the ‘CH20H in the polycrystalline material is less than in the glass. Upon warming, the polycrystalline material gives a greater yield of CH, and a lower yield of Hzthan the glass. Changes i n radiolysis yields with phase, both molecular and radical, have been observed in other organic systems but no general explanation has been offered.lo Here the absence of electrons in the polycrystalline material and the increase of CH, yield are regarded The Journal of Physical Chemistry
STEPHEN B. MILLIKENAND RUSSELLH. JOHNSEN
as related. The sequence of events in the glass may be considered to be similar to that in the liquid,lg which has been proposed
+ eCH30H+ + CH3OH +CH30H2++ CH30. CH30. + CH30H +CH3OH + *CH20H CH30H +CH30H+
(9)
(loa) (lob)
and the electron is solvated sCH30H
+ e- +(CH,OH),-
(11)
Ultimately neutralization occurs between the protonated species and the electron to produce hydrogen. Reaction 10a has been observed t o occur in the gas phase on the first collision; it might also be expected to occur rapidly in the solid. Whether the radical produced is CH3O or CHzOH cannot be settled conclusively with the available evidence; both, in fact, may be produced. From the epr evidence it is clear that the net effect is the production of .CH20H. The plausibility of an abstraction reaction by CH3O has been discussed above. Reaction 11 is postulated to occur exclusively in the glass. Evidence from saturation studies4 indicates that solvation takes place at an average distance of 40 A from the parent positive ion. This close proximity of the electron to a positive ion indicates that neutralization would be the favored reaction once the electron was released from the trap by thawing or exposure to visible light. In crystalline methanol the pattern of hydrogen bonds extends in one dimensionlZ0linking molecules into endless chains between which van der Waals forces alone are operative. These chains are arranged laterally in an approximately close-packed arrangement with each chain symmetrically surrounded by six neighbors. In the glass, with added water, while the hydrogen bonding can be assumed to persist, one would expect a disordered three-dimensional system of shortened chains. This difference probably accounts for the observation that neither color nor the esr spectra of the trapped electron is seen in crystalline material. Solvation of the electron necessary for trapping is prohibited (15) M. L. Halberstadt and J. R. McNesby, J . Chem. Phys., 45, 1666 (1966). (16) R. A. Basson, Nature, 211, 630 (1966). (17) J. J. J. Myron and G. R. Freeman, Can. J . Chem., 43, 1484 (1965). (18) K.R. Ryan, L. W. Sieck, and H. J. Futrell, J . Chem. Phycr., 41, 111 (1964). (19) L. M.Theard and M. Burton, J. Phys. Chem., 67, 59 (1963). (20) R. C. Evans, “An Introduction to Crystal Chemistry,” Cambridge University Press, London, 1964,p 363.
ULTRAVIOLET PHOTOLYSIS OF X-IRRADIATED METHANOL
in the one-dimensional array of hydrogen bonds. Similarly one would not expect the counterion CH3OH2+ to be as extensively solvated in the crystalline material as in the glass. In the polycrystalline material one would expect a competition between reactions 12 and 13 in which unsolvated electrons react with unsolvated ions almost immediately following their respective formation.
+ CH30H2++ e-
CHBOHZ+ e-
+ H2 ---+ CH3. + H20
----t
CH20H
(12)
(13)
These reactions would be expected to be rapid and have relatively small activation energies. It would not be unreasonable to expect them to occur with approximately equal probability. Experimentally one finds that G(e-) is -3 and that the yield of CH4 is -1.5, which is consistent with this formulation. On the other hand, in glassy methanol the reactions of the electrons take place only upon untrapping by bleaching and would thus be expected to occur with a highly solvated species.
+
.CH20H
(CK30H2+),o~v e-
+
(CH~OHZ+),,I, e-
----+
.CH3
+ Hz
+ H2O
(14) (15)
Reactions such as these might be expected to have appreciable energies and entropies of activation and, furthermore, differ significantly in this regard. If this is true then one might expect reaction 14 to be favored under these circumstances, leading to dimin-
2123
ished yields of methane in the case of glassy material. Furthermore, a yield of water has been noted in solid radiolysis but not in dry liquid in which solvated electrons have been observed. 21
Conclusions The photolysis of trapped aCH20H shows a wavelength dependence indicating more than one mode of decomposition of CH20H. The subsequent reactions do not involve a hot hydrogen but may involve a methylene biradical. Thermal atoms that are free to move do not abstract a t . 77"K, but trapped radicals such as 'CH3 apparently do abstract, at a rate somewhat faster than predicted by extrapolation of the rate in fluid media at higher temperatures. The reactivity of 'CHI suggests that 'CH20H may be preceded by another radical, most probably CH30.. A change of yields is observed with a change in the nature of the solid. Explanation remains speculative, but the coincidence of the lack of electron trapping with the changed yield suggests that a change in the neutralization step may account for the observations.
Acknowledgments. Appreciation is expressed to the Atomic Energy Commission for its support of this work, to Mr. D. Pritchett for help with the irradiations, and to Mr. K. Morgan for assistance with the analyses. (21) A. H. Samuel and J. L. Magee, J . Chem. Phys., 21, 1080 (1953).
Volume 71, Number 7 June 1967
