J. Php. Chem. 1082, 86, 967-970
967
Reactions of CH, with CH, and CH, in CH,, and Xe Matrices Debasls Bhattacharya and John E. Wlllard' Department of Chemlsby, Unlversiry of Wlsconsin, Madison, Wlsconsin 53706 (Recehred: August 14, 1981; In Final Form: November 9, 1981)
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ESR evidence is presented for the reactions CH2 + CH2 C2H3 + H and CH2 + CH3 C2H5 in Xe and CH, matrices at cryogenic temperatures. Stabilization of the C2H3 and C2H5 contrasts with their reported instability in gas-phase reactions of CH2. The CH2has been produced by the CH3+ hv (185 nm) CH2+ H and c-CH2N2 + hv (185 nm) CH2 + N2 photolyses. Additional photolytic pathways for c-CH2N2,and evidence on the photolysis of CH,CO, in Xe and CHI at 5 K is reported.
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Introduction While investigating the CH, + hv (185 nm) CH2 H, reaction in CHI by monitoring the CH3 and trapped hydrogen atom (H,) concentrations by ESR at 5 K,' we have noted that C2H5 and C2H3 were formed when the sample was warmed to 35 K, where CH2 radicals can diffuse and encounter each other and CH,. We report here experiments done to further elucidate such CH2reactions in matrices at cryogenic temperatures. Determinations have been made on CH2produced by the photolysis of (a) CH3 in CH,; (b) CH, in Xe; (c) diazarine (CH2N=N, hereafter designated as c-CH2N2)in CH,; (d) c-CH2N2in Xe; (e) c-CH2N2in Xe with alkane additives. Following photolysis and recording of the ESR spectra at 5 K, where trapped radicals and H atoms do not diffuse or react in either CH, or Xe, the CHI samples were warmed to 35 K and the Xe samples to 70 K to allow the CH2to diffuse2 and react. The radicals produced by such reactions were then determined by ESR.
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and for controlling sample temperatures are described in the preceding paper in this issue of the journal' and in earlier paper^.^^^ ESR Saturation. In earlier work we have found that the ESR power thresholds for saturation of the signals of H, and radicals in CH4 and CD, vary widely, depending on the method of production of the trapped species, the extent of their decay, the matrix, and the t e m p e r a t ~ r e Some .~~~ of the thresholds were -4000 G,6 if it were not too severely broadened to detect. When the sample of Figure l b was warmed to 35 K the spectrum changed to that of Figure IC. A replication of this experiment gave essentially identical results. When CHI which had been radiolyzed at 6 K was exposed to light from the Suprasil lamp through a Vycor filter to exclude the 185-nmline, while passing the 254-nm line, no Ht was produced, there was no decrease in the CH, signal, and no
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(1) Bhattacharya, D.; Willard, J. E. J.Phys. Chem. Preceding article in this issue. (2) In CH,, H diffuses and reacts in minutes at > 12 K, CH2 at > -30 K, and CH3 at > -45 K. In Xe the corresponding temperatures are H, -45 K;CHI, -55 K;and CH3, 85 K. (3) (a) Bhattacharya, D.; Wang, H. Y.; Willard, J. E. J.Phys. Chem. 1981,85, 1310. (b) Bhattacharya, D.; Willard, J. E. Ibid. 1981,85, 154. (4) (a) Graham, W. H. J. Org. Chem. 1966,30,2108. (b) Wood, L. S. Jr., Ph.D. Thesis, The Pennsylvania State University, 1972. Available from University Microfilms Ann Arbor, MI. (c) Jenkins, A. D. J. Chem. SOC.1952, Part 111, 2563.
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0022-3654/82/2086-0967$01.25/0
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(5) It should be noted that there is a typographical error in the legend of Figure 3 of ref 3a. The relation used to obtain the percent saturation Wa8
100 x [(sample)o/(IS)o - (sample),/(IS),] /(sample)o/(IS)o rather than [(sampled/(ISh - (sample),/(IS),l /(I% (6) Bernheim, R. A.; Chien, S. H.
0 1982 American Chemical Society
J. Chem. Phys. 1977, 66, 5703.
068
Bhattacharya and Willard
The Jownal of phvsicel Chemlsby, Vol. 86, No. 6, 1982
approx A H total, kcal mol-' CH,
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CH,
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31/
C2H:
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'ZH5
'
C,H3
C+H4
40 G
1
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Figuo 1. (a) ESR spectmn from CH, sample at 32 K folbwlng 1.08 X 10'@eVg-'X ray doeeat 6 K mocluletkn ampltude, 0.5 0; power, 150 NW; gain, 630. (b) ESR spectrum at 6 K from sample of (a) fdlowlng 15-mkr exposve to MI light of SuprasH spiral bw-pfeawre Hglamp: moduletbnemp#de,59power,1~Wgah,630. (c)ESR Spectnm Of end c& redlcak produced kl Mmpk Of (b) when it was warmed to 35 K moduletion ampllhde, 0.5 0; power 150 F W galn, 2500.
c&
change in the initial signal of the radiolyzed sample (Figure la) occurred when the sample was warmed to 30 K. This indicates that the intermediate (CHJ responsible for the generation of the new radicals (Figure IC)at 35 K is produced by 185 nm, but not by 254 nm. The spectrum of Figure ICis the 12-line spectrum of C2H6radicals, with added lines (1, 2, 3, 4) due to C2H3 radicals. The C2H6spectrum is essentially identical with that observed by Fessenden and Schuler7 during the steady-state radiolysis of liquid C2& at 93 K, and to the spectra we observe when H from the photolysis of HI in CHI at 5 K abstracts H from C2H,solute, or adds to C2H4 solute, on warming. The total spectrum of Figure ICis essentially identical with that obtained' during steady-state radiolysis of a 35-65% C2H4-C& mixture at 93 K. The reasoning7 that lines 1, 2, 3, and 4 are the outer lines of two seta of four resulting from unequal coupling of the unpaired electron with the three H atoms of the C2H3 radical is compelling.' The absence of a detectable signal from the additional lines, which have been observed by Cochran et al.8 following photolysis of HI in an Ar matrix containing acetylene at 4 K, and by us following CH2 CH2 reaction in Xe (see Below) and following radiolysis or photolysis of C2H4 in Xe is attributed to their broadening as the result of rapid inversion of two forms of the radical.' It appears that similar broadening must occur in the CHI matrix at 30 K. That CH2in ite triplet ground state is inert with respect to reaction with CH, is demonstrated by (1) the evidence given above that it is stable in CHI at 6 K, and undergoes reaction only when it is allowed to encounter other radicals by the diffusion which occurs on warming; (2) the finding that in the gas phase it does not react in at least los collisions with CHb9 On the basis of the results above, and those reported in later sections, it appears probable that reaction 1 is responsible for production of the C2H6and that the CzH3 is produced by one or more of the reactions 2 , 4 , and 6. All of the reactions are exothermic. Reaction 3 occurs in
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(7)Feesenden, R.;Schuler, R. H. J. Chem. Phys. 1963,39, 2147. (8) Cochran, E.L.;Adrian, F. J.; Bowers, V. A. J. Chem. Phys. 1964, 40,213. (9)Lee,P.S.T.;Russell, R. L.; Rowland, F. 5.Chem. Commun. 1970, 18.
+ +
H2 H
- 99 - 13 - 61
(1 (2)
(3)
the gas phase?l0 where there is sufficient time between collisions to allow decomposition of the C2H6*. It is not surprising that in the solid-phase stabilization of the C2HS by loss of energy to the matrix should compete with decomposition. Reaction 4 seems to be the only mechanism for production of C2H3 from CH2 produced by the photolysis of c-CH2N2in Xe (see below), and may be assumed to occur in CHI also, with the possibility of added contribution from (2). Additional C2H3 may be formed by addition of H from (3) and (4) to C2H2from (5). Similar addition of H to C a 4 from (3) may contribute to the C2H6yield. Reactions of CH2 Produced by CH3 + hu (185 nm) CH2 + H in Xe. A sample of 0.1 mol% CH4 in Xe was radiolyzsd to a dose of 3.3 X lOlS eV g-l at 7 K, and warmed to 48 K for 5 min to remove He Subsequent photolysis at 5 K for 75 min with the fulllight from the Supraail spiral produced Ht and reduced the CH3 signal. No evidence of the triplet CHz signal between 4000 and 11OOO Gs could be detected, even at high sensitivity settings. When the sample was then held at -50 K for 15 min the outer lines of the C2H6signal with 127-G splitting and the C2H3 lines corresponding to 1,2,3, and 4 of Figure IC appeared, while a strong four-line CH3 signal, with superimposed unresolved structure, remained. Photolysis of C - C H ~ Nand , Subsequent Reactions of CH2 in Xe. Samples of 0.1 mol % c-CH2N2in Xe and in Xe containing alkane additives were photolyzed with three goals: (1)to determine whether we could observed the 3CH2 peaks in the 4000-11000-G region previously reported,B'll but not found by us following photolysis of CH3 in CHI or Xe; (2) to observe the spectrum of the radical producta formed by the reaction of CH2with CHz, in the absence of the overlapping spectra of CH3and the products of the CH2 + CH3reaction; (3) to determine whether WH2 reacts with alkane solutes in the Xe matrix. Four wavelength ranges were tested for dissociation of 0.1 mol % c-CH2Nzin Xe. A 30-min exposure at 5 K to the 2500-W Hg-Xe lamp through a Coming CS 3-73 filter, which is opaque at 1380 nm, produced no species observable by ESR,consistent with the onset of strong absorption only at 290 nm for 5 min from the same lamp passed by a 5mm thick pyrex filter, produced strong ESR signals of the high-field CH2 triplet: accompanied by a very weak broad singlet with -200 G width between extremes centered at -3200 G. These signals disappeared when the sample was heated to 30 K but reappeared quantitatively on returning to 5 K. This temperaturereversible removal of the triplet is consistent with its disappearance above 10 K, as a result of line broadening,
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(10) Laufer, A. H.; Bass, A. M. J. Phys. Chem. 1975, 79, 1635. (11) Bicknell, B. R.; Graham, W. R. M.; Weltner, W., Jr. J. Chem. Phys. 1976,M, 3319. (12)Graham, W.H.J. Am. Chem. SOC.1962,84, 1063.
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982 969
Reactions of CH, and CH, in CH, and Xe Matrices
TABLE I: Summary of Radicals Produced by the Photolysis of CH, and c-CH,N, in CH, and Xe at Different Wavelengths and Temperaturesa ~
CH, matrix A,
nm
CH, 5K
Xe matrix c-CH,N,
35 K
CH3
5K
35 K
> 380 > 290
5K
c-CH,N, 70 K
5K
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-
70 K
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CH,, R.‘
a Photolysis at the indicated wavelength was carried out at 5 K following which the CH, samples were warmed to 35 K and the Xe samples to 70 K. The long wavelength absorption limit of CH, in CH, at 5 K is 240 nm; that of c-CH,N, in the gas phase is 330 nm. The blank spaces in the table are for wavelengths longer than these, at which no tests were made. The. horizontal lines indicate that no paramagnetic species was formed by illumination. ti The samples gave no detectable CH, ESR signal but its presence is implied by the formation of C,H, and C,H, on warm-up. Weak signal. R. indicates uncharacterized radicals, with signals centered at 3200 G.
40G
I
I
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Flgure 3. ESR spectrum of C2H, radicals produced by CH, CH, reaction on warming Xe-c-CH,N, sample to 77 K following photolysis at >290 nm at 5 K: modulation amplitude, 0.5; power, 100 pW, gain, 2500.
s
40 G Flgure 2. ESR spectrum of Xe-c-CH,N, sample following 185 nm exposure at 5 K: modulation amplitude, 5 0;power, 5 p W gain, 12.5.
reported earlier.6 No Ht signals were produced by the >290-nm photolysis. The results of a photolysis with light confined to the >220-nm 290 nm. A 5-min photolysis of c-CH2N2in Xe at 5 K with 185 nm, from the Suprasil low-pressure Hg spiral with interference fiter,’ produced CH2 at 116 of the concentration from the above >290-nm exposure, accompanied by a species with a 3200-G spectrum (Figure 2) completely different from that obtained by the >290-nm exposure, and by Ht. Although, the spectrum of Figure 2 is not unequivocally identified, the results suggest that it is due to c-CHN2formed by the c-CH2N2+ hv (185 nm) c-CHN2 + H reaction. When any one of the samples in which the CH2 signal was observed following c-CH2N2photolysis was warmed to 77 K, where CH, diffuses readily, and returned to 5 K for monitoring, the spectrum of Figure 3 appeared in the 3200-G region, and the CH2 signal was gone. The 3200-G signal, which is that of C2H3, was measurable at a sensitivity 1% of that required for the weak signal present in this region before cycling to 77 K following >290-nm photolysis. The assignment to C2H3was first made on the basis of the correspondence of the splittings of the pairs of lines on the extremes of Figure 3 (115 G between extremes, 14.5 G within pairs) to those of lines 1, 2, 3, and 4 of Figure IC,which, in turn, correspond to the C2H3lines in irradiated liquid C2H4-C2HJ and to those of C2H3 radicals produced by the addition of H to C2H2in Ar.8 The assignment is further confirmed by comparison with the ESR spectrum of C2H3 produced by X irradiation of 0.1 mol % C2H4 in Xe (Figure 2a of ref 1). It is noteworthy that when 0.1 mol % c-CH2N2in Xe containing 0.1 mol 3‘% CHI, C2&, C2H4, or CD3CH3 was photolyzed with >290-nm light, followed by warming to 77 K, the ESR spectrum taken at 5 K was in every case the C2H3 spectrum, indistinguishable from that obtained in the experiments without alkane additive.
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The results of this section confirm that the ESR signal of 3CH2produced by the photolysis of c-CH2N2in Xe6 is readily observed, in contrast to that of the CH, presumed to be formed from the photolysis of CH, in CHI or Xe, and they show that when Xe containing CH, at 5 K is warmed to 77 K the CH2 + CH, C2H3 + H reaction occurs with a high probability per collision relative to reaction with any of the three alkanes or the alkene tested. The observation of the C2H3 ESR signal indicates that energy loss by rupture of a single C-H bond in the activated C2H4* of reaction 4 competes favorably in the Xe matrix with the loss of H, or 2H, processes which have been postulated to explain C2H2formation in the gas pha~e.~JO Photolysis of c-CH2N2in CH,.Samples of 0.1 mol % c-CH2N2in CHI were illuminated with >290-, 254-, and 185-nm light at 5 K and warmed to 35 K in order to compare the reactions of CH2 formed from c-CH2Nzin CHI with those of CH2 from the photolysis of CH3 in CHI and from the photolysis of c-CH2N2in Xe. In contrast to the c-CH2N2in Xe, described above, the c-CH2N2in CH, did not give a CH2 signal after exposure to 30-min illumination from the >290-nm source, nor did it give a radical signal in the 3200-G region either before or after warm-up from 5 to 35 K. This implies that cCH2N2which has absorbed a photon of >290 nm in a CH, matrix is deactivated by energy transfer to the matrix without decomposition whereas decomposition forming CHz is probable in Xe. After 20-min exposure to the 254-nm light of the Suprasil low-pressure Hg spiral lamp filtered with Vycor, the sample again gave no CHz signal and no evidence of growth of CzH3 and C2H5on warming to 35 K. It gave a weak CH3 signal at 5 K. The results suggest that at 254 nm, as at >290 nm, there is major stabilization of the c-CHzN2 without decomposition, but that same abstraction from CHI by CH2 occurs, giving the low yield of CH3. Exposure of 0.1 mol % c-CH2Nzin CHI at 5 K to 185-nm radiation from the spiral lamp for 1h produced -2 X lo4 M CH, and the same order of concentration of H,. Underlying the CH3 signal was a much weaker broader
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970
J. Phys. Chem. 1982,86,970-972
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spectrum. On warming the sample a 20-line spectrum 170 G wide appeared which included lines attributable to C2H3 and C2H5. These results imply that the 185-nm photolysis of c-CH2N2in CHI produces CH,, some of which abstracts from CHI (as translationally hot CHz or as the singlet before relaxation to the stable triplet ground state), and some of which is stabilized until the sample is warmed allowing 3CH2+ 3CH2and ,CH2 + CH3 encounters which result in reactions 1and 2 and/or 4. No CH2 ESR signal was observable in the CHI matrix. Photolysis of CHzCO in X e and in CH,. The photolysis of (CHzCO)has been extensively used as a source of CH2 radicals in gas-phase studies.'% By contrast to the results in the gas phase, CHzCO does not yield CH2 when photolyzed in an N2matrix.14 This was attributed to a prompt back-reaction of CH2with CO, or reaction of the CH2with the N2 matrix to form CH2Nz. We have sought evidence for CH2 formation from the photolysis of CH2C0 in the inert matrix Xe and also in CHI, and have looked for evidence of CH&O decomposition to form species other than CH2 and CO. When either Xe or CHI containing 0.1 mol % CH2C0 was photolyzed with the full light of the Suprasil lowpressure Hg lamp for 30 min at 5 K no CH2 signal was detectable in the sample even at very high sensitivity. In both matrices a broad (120 G) ESR line centered at 3200 G, with a peak-to-peak l i e width of -30 G appeared with some superimposed, partially resolved line structure. This signal did not change significantly on warming to 35 K, in either matrix. The concentrations achieved were of the (13) (a) See for examples and references: O h b e , H."Photochemistry of Small Molecules";Wiley-Interscience: New York, 1978. (b) Laufer, A. H.;Keller, R. A. J.Am. Chem. SOC.1971,93,61. (c) Braun, W.; Bass, A. M.; Pilling, M. J. Chem. Phys. 1970, 52, 5131. (14) Moore, B. C.; Pimentel, G. C. J. Chem. Phys. 1964, 41, 3504.
order of M. The identity of the species responsible for the signal remains in question. Summary The results of the experiments described above in which CH3 and c-CH2N2were photolyzed are summarized in Table I. Photolysis of either CH3 or c-CH2N2in either CHI or Xe at 5 K by 185-nm light produced a trapped species which reacted to form C2H3 (and C2H5 if CH, was present) when the matrix was warmed to a temperature where CH2 can diffuse. Even though the ESR signal of CH2was only observable in the c-CH2N2-Xesystem, there seems to be no other possible precursor for the CzH3 and C2H5 observed in the other systems.15 The results imply that in the solid phase the CH2 + CH2 CzH3 + H and CH2+ CH, CzH5reactions are important relative to the CH2+ CH2 C2H2+ H2 (or 2H) and CH2+ CH3 C2H4 + H reactions commonly postulated to explain the products found in gas phase studies of CH2. Other features of particular interest in the data include the evidence for (1) stabilization of activated c-CH2N2without decomposition in CH4 following 290- and 254-nm photon absorption, in contrast to the decomposition in Xe; (2) the CH2N2 hv (185 nm) H CHN2 reaction in Xe. Similar to the latter is the evidence, not in the table, that 185-nm absorption by ketene in CHI or Xe yields H atoms and radicals.
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Acknowledgment. This work was supported, in part, by the U S . Department of Energy under contract DEAC02-76ERO1715-AO05. (15) The susceptibility of the signal to broadening by small changes in the matrix environment is illustrated by the reversible disappearance of the signal when CH2 in Xe is warmed from 5 to 30 K and returned to 5 K, as observed both in the present and previous work.6
Temperature Effect on Ion-Molecule Reaction of Hydrogen Transfer in ?-Irradiated P,&Dlmethylbutane at 4 and 77 K As Studied by Electron Spin Resonance Spectroscopy Tetsuo Mlyarakl, Haruyukl Tsuruta, Yoshiteru Fujitanl, and KenjI Fuekl Department of Synthetic Chemistry, Faculty of Engineering, Nagoya Universt?y, Chikusaku, Nagoya 464, Japan (Received: August 17, 1981; I n Final Form: October 22, 1981)
Drastic temperature effects on an ion-molecule reaction of H2 transfer in solid hydrocarbon systems were studied at 4 and 77 K by ESR spectroscopy. When a 2,3-dimethylbutane (DMB)-SF6 (0.55 mol %)-i-C4H8(0.55 mol %) mixture is y-irradiated at 4 K, the DMB+ ion in addition to the DMB radical is formed. The tetramethylethylene (TME) cation is produced by warming the irradiated DMB-SF6-i-C4H8mixture from 4 to 77 K. The formation of the TME+ ion observed at 77 K is interpreted in terms of the H2transfer reaction between the DMB+ ion and i-C4Hs. This ion-molecule reaction is completely suppressed at 4 K. The suppression of the ion-molecule reaction is explained by failure of the formation of a reaction complex in the rigid matrix at 4 K. The amounts of TME+ ions in the DMB-SF6-i-C4H8 mixture increase gradually upon storage of the irradiated mixture at 77 K. This result indicates that the H2 transfer reaction occurs slowly at 77 K. Introduction Extensive studies on ion-molecule reactions have been undertaken in the gas phase and have shown that most of the reactions occur quite rapidly. For example, the rate constant for the H2transfer reaction between c-C6H12+and l-C4H8is 2.1 X cm3 molecule-' s-l,l which is much (1) Sieck, L W.; Searls, S. K. J. Am. Chem. SOC.1970, 20, 2937.
0022-385418212O86-0970$0 1.2510
larger than a collision frequency. The Hz transfer reaction in the solid phase at 77 K has been studied previously by analysis of final products2 and ESR spectro~copy.~When 2,3-dimethylbutane (DMB) (2) (a) Scala, A. A.; Lias, S. G.; Ausloos, P. J. Am. Chem. SOC.1966, 88, 5701. (b) Ausloos, P.; Scala, A. A.; Lias, S.G. Ibid. 1967,89, 3677. (3) Saitake, Y.; Miyazaki, T.;Kuri, 2. J. Phys. Chem. 1973, 77, 2418.
0 1982 American Chemical Society
