Photochemistry of ~ e t ~ a ~ e t h i o l
295
f Methanethiol at 254 and 214 n d ridges and John M. White" Deparrment of Chemistry, The University of Texas at Austin, Austin, Texas 78772 (Received August 28, 1972) Publication costs assisted by the University et Texas
Quantum yields for the production of hydrogen, methane, and dimethyl disulfide by the photolysis of methanethiol at 254 and 214 nm have been measured. When photolyzing pure methanethiol, no pressure dependence i s observed. The quantum yields of hydrogen and methane are, however, wavelength dependent; the hydrogen quantum yield drops from 0.83 at 254 nm to 0.66 at 214 nm while the quantum yield of methane increases. Adding n-butane reduces the yield of methane and increases the yield of hydrogen. The results are interpreted in terms of a mechanism involving hot hydrogen atom reactions.
~ ~ t r ~ ~ u ~ i i ~ n In a continuation of studies of the photochemistry of thiolsza we have invlestigated the photolysis of methanethi01 at 254 and 214 nm. Previous work includes that of Steer and Knight2 who carried out an extensive study of the photolysis of methanethiol at 254 nm. They reported a pressure-dependent quantum yield of hydrogen with a value of 1.0 0.06 when extrapolated to zero pressure of methanethiol. They concluded that the only important primary process is scission of the S-H bond. S t ~ r m in , ~a short set of photolyses at 254 nm, concluded that the sum of @(H2) m d @(CH*)is constant and independent of methanethiol pressure. He also found the yield of CH4 relative to W2 yield decreased with the addition of Dz or He. Dxantiev, et el.,* in a less detailed study than Steer and Knight suggested that scission of the C-S bond is also important and occurs with a quantum yield on the order of 0.1. Callear and Dicksons in a flash photolysis study at .-I95 nrn reported the two primary processes with the ratio of C-S to S-$1 bond cleavage being 1:1.7. We report here the results of our investigations and suggest a mechanism with wavelength-dependent quantum yields of 9-W and C-S bond rupture which is analogous to, but dightly simpler than, the mechanism proposed for the photolyqis of ethanethiol.2a
Experimental Section Most of the experimental techniques and conditions were identical with ithose we reported previously.za A few variations and additions are outlined here. The light source filters were the same except that a 10-cm cell with about 200 Torr of acetone was used along with the cis-2butene filter at 214 nm. Methanethiol was purified by degassing and distilling under vacuum from -98 (methanol slush) to -3.17" (ethanol slush). Mass spectral analysis for products CH4 and M2 was performed. In a few experiments the change in optical density induced by photolysis was measured over the range 270-300 nm as a method of determining C H3SS CI-43 production. Gas chromatography was used to identify products condensible at - 196". Actinometry used HEr and HI as outlined previously.2a The extinction coeffkieiits for CH3SI.I are involved in the actinometric considerations at each wavelength. The values measured at several wavelengths are listed in Table I. It IS well known that dimethyl disulfide, a major product in the photolysis of methanethiol, absorbs at 254 and
214 nm.6 Therefore, in order to minimize errors in the quantum yields of the products, photolysis times were chosen so that decomposition of methanethiol was less than 1%.
Results Photolysis of Pure Methanethiol. The products noncon-
densible at - 196", hydrogen and methane, were measured mass spectrometrically. Hydrogen sulfide was the only other product noncondensible at - 117" (ethanol slush temperature). In the few cases where W2S production was measured quantitatively, its yield equaled that of methane. The only products detected by gas chromatographic analysis of the - 196" condensibles were hydrogen sulfide and dimethyl disulfide. The dimethyl disulfide yields were determined assuming the increase in optical density in the 300-270-nm region upon photolysis of thiol was due to dimethyl disulfide only (see Table I). Using the change in optical density a t 290 and 285 nm along with the cell length and corresponding extinction coefficient,6 the yield of dimethyl disulfide was calculated at both these wavelengths. The average of these two values was used in calculating the quantum yield of the disulfide. Figures 1 and 2 give the quantum yields of hydrogen, methane, and dimethyl disulfide over a range of methanethiol pressures at 254 and 214 nm. The points marked by open symbols represent quantum yields determined by HI or HBr actinometry. The 214-nm values were obtained using the acetone filter described in the Experimental Section. A check showed no significant difference in quantum yields when the filter was removed. The quantum yields of hydrogen and methane are independent of substrate pressure and, within experimental uncertainty, have a sum equal to unity at both wavelengths. The quantum yield of dimethyl disulfide is also independent of thiol pressure at both wavelengths. In Figures 1 and 2, Supported in part by the Robert A. Welch Foundation. (a) L. Bridges, G. L. Hemphill, and J. M. White, J . Pbys. Chem., 76, 2668 (1972); (b) R. P. Steer and A. R. Knight, ibid., 72, 2145 (1968). 6.P. Sturm, Jr., doctoral thesis, University of Texas, '1969, pp 7585. B. G. Dzantiev, A. V. Shishkov, and M. S. Unukovich, Khim. Vys. Energ.. 3, 111 (1969). A. B. Callear and D. R . Dickson, Trans. Faraday Soc.. 66, 1987 (1970). J. G. Calvert and J. N. Pitts, Jr., "PhotocheMistry,': Wiley, New York, N. Y., 1966, p 490. The Journal of Physical Chemistry, Vel. 77, No. 2, 7973
296
Lamar Bridges and John M. White
TABLE I: Extinction ~ 0 e f f ~ ~ i(Ton-' e n ~ s cm-') 290 nm
Molecule
254 nm
285 nm
274 nm
I _ _
(2.40i 0.04)X
CH3SW W Br
(1.56f 0.03))< (6.57i 0.04)X 10-3
(5.82i 0.02) x 10-3
HI 3.35 x 10-3
CH$33CI-13'
4.67 x 10-3
a Reference 6.
TABLE II: Average Quantum Yields Wavelength, nm
254 214
t
t Q
50
IO0
150
PRESSURE
250
200 CH3SH
300
350
(torr)
Figure 1. Quantum yields, 4, of products from the photolysis of pure methanethiol at 254 nm vs. pressure of methanethiol: hy; methane, 0 and HI; and dimethyl disulfide, A . The open symbols represent quantum yields based on HI actinometry. The filled symbols represent quantum yieids based on relative yields and a(1-12)C a(CH4) = 1.
6%
0.8
(P 0.6
0.4
0.2
0
50
io0
150
200
250
300
350
PRESSURE CH3SH (torr)
Figure 2. Quanturn yialcls, 4 , of products from the photolysis of
pure methanethiol at 214 nm vs. pressure of methanethiol: hy; methane, 0 and HI; and dimethyl disulfide, A. The open symbols represent quantum yields based on HBr actinometry. The filled symbols represent quantum yields based on relative yields and Q~(y'12) 4- G(CH4) -. 1. each quantum yield represented by a filled symbol was determined from the ratio of the yields of hydrogen and methane along with the relation G(H2) G(CH4) = 1. The average values of the quantum yields are listed in Table 11. Earlier investigations7 in our laboratory have resulted
@(H2)
0.83 0.66
* 0.03 0.03
@(CH4)
G(CH3SSCHs)
0.16 i 0.03 0.35 A-0.03
0.99 f 0.1 1.03 f 0.1
in relative yields of Ha and CH4 at 254 and 214 nm which are in good agreement with these results. Results at 229 nm were very nearly the same as those for 254 nni. A neutral density filter was used to decrease the light intensity as a means of determining any possible intensity dependence of the product, quantum yields. The lower intensity was determined, using HBr photolysis a t 214 nm, to be 0.363 of the original intensity. The quantum yields a t the lower intensity were calculated using the above factor and the relative yields of products a t high and low intensities taking values of @(Hz)and @(CW4)at high intensity from Table 11. In these experiments the per cent, decomposition of the thiol was kept below 0.2% except at the lowest pressure of methanethioi where decomposition approached 1%. No corrections were made for the slight variations in per cent decomposition. We conclude that the quantum yields of hydrogen and methane are independent of light intensity in the range covered by our experiments. Photolysis of Methanethiol-Thermalizer Mixtures. To evaluate the role played by excited atoms and other free radicals, we conducted a series of photolyses with n-butane as an added thermalizer. The use of n-butane as a thermalizer has been discussed in a previous paper.2a Figure 3 summarizes the results obtained at 254 and 214 nm. The addition of n-butane results in an increase in the quantum yield of hydrogen with accompanying decrease in the quantum yield of methane. At each pressure of added n-butane, the sum of the quantum yields of these two products is, within experimental uncertainty, equal to unity. Photolysis of CH3SSCH3 in the Presence of CH3SH. In a different type of experiment dimethyl disulfide was photolyzed in the presence of methanethiol at 300 nm using a high-pressure Hg arc-monochromator arrangement. At this wavelength the disulfide absorbs while the thiol is transparent. Using a reactant ratio [C)HsSSCHsJ/ [CHaSH] = 0.16 the photolysis products were analyzed in the same manner as in the photolysis of pure methanethi01. No hydrogen or methane was observed.
Discussion The following mechanism is proposed as an explanation of the above results.
+
The Journal of Physicai Chemistry, Voi. 77, No. 2, 1973
(7) G. P. Sturm, F. Growcock, J. Mena, and J. M. White, unpubkhed results.
Photochemistry of Methanethioi
CH3SH
+
h,~
297
-+
+
CHR (or CH,”)
W*
-1- CH3SH
€3” -I- CH3SH
-4-
CH,
(H” t- CH3SH CH, (orCH*) -f CH3SH *) -4- CH,SH
SH (or SH”) (2) CHBS + El2 H,S and/or -+
CHd
-+
CH,S
--+
CH3S
+
+ + +
H” 4- l\ix --+ H M W 4. CH3SH ----t CHp9 Hz CH3S 3- CHZS -+ CH3SSCHS
+
I .0
0.8
+
0.6
4, 7 , and 8 offer the most logical explanation of the observed increase in @(Hz) and decrease in @(CHI) with added n-butane, i e . , reaction 4 for thermal I-I atoms is insignificant compared with reaction 8, but becomes significant for hot H atoms. We now have other experimental evidence from the photolysis of HI-CH3SH mixtures that reaction 4 occurs.12 Since ethane was not observed as a product, the only fate of CH3 radicals seems to be the abstraction of hydrogen atoms from the thiol. Inaba and DarwentlO observed CH3D as the only methane species when they photolyzed CH3SD at 254 nm indicating abstraction of only the sulfhydral hydrogen by CH3 radicals. The fact that we did not observe 1,2-ethanedithiol as a product is in agreement with the observation of Inaba and Darwent. Steer and Knight2b reported a pressure dependent @(&) with a value of 1.00 & 0.05 when extrapolated to zero methanethiol pressure. These authors concluded that reaction 1 is the only significant primary process. The extra hydrogen produced at higher thiol pressures was attributed to reaction between methyl thiyl radicals and the substrate, reactions 10 and 11. Reaction of excited dimethyl disulfide, formed by combination of methyl thiyl radicals, with the substrate was proposed as the source of CHI and H2S.
0.4
I 0.2
+
+
CHSS CHSSH + P (10) CW,SH -+ C€&S + H, -5 CH,SSCH, (11)
CH3SSCH,*
0
SO
IQQ
l50
200
2 5 0 300 350 400 450
PRESSURE n-C4HI0 (torr)
Quantum yields, 4 , of hydrogen 0 and methane as a function of the pressure of added n-butane for the photolysis of -40 Torr methanethrol at 254 nm. Filled symbols are for photolysis at 2’14 rim Figure 3.
The observation that the quantum yield of hydrogen at both 254 and 214 nrn i s less than unity suggests the possibility of at least two primary processes. An examination of the absorption spectrum of methanethi018 indicates there are no long-lived electronically excited states. This indication is confirmed by the fact that the quantum yields of hydrogen and methane are independent of thiol pressure. The additional facts that the sum of @(H2)and @(CHI)is unity both for the photolysis of the pure thiol and with each pressure of added n-butane and that the yield of CHI i s significant at large pressures of added thermalizer point to reactions 1 and 2 8 s 1he primary processes. The hydrogen atoms produced in reaction 1 are hot9 and in the proposed mechanism can undergo reactions 3, 4, or 7 Inaba and Darwentlo observed Dz as the only hydrogen product when CH&D was photolyzed at 254 nm showing that the deuterium atom produced in the primary p iocess abstract sulfhydral deuterium atoms but not methyl hydrogen atom3. Steer and Knight,2b considering only thermal hydrogen atoms, ruled out reaction 4 as playing any significant role in the photolysis of methanethiol, These authors based the exclusion of reaction 4 on the data of Greig and Thynnell for the abstraction of the SH group us the abstraction of sulfhydryl hydrogen from by CDs radicels. In view of the fact that hot hydrogen atoms arc produced in reaction 1, we feel reactions
+
-
CH,SH C&SSCH,
=t CH3 -I- H2S (12)
The fact that we observed no hydrogen or methane when a mixture of dimethyl disulfide and rnethanethiol was photolyzed at 300 nm shows reactions 10, 11, and 12 are not of significant importance in the photolysis of methanethiol. The major primary process in the photolysis of dimethyl disulfide at 254 nm has been shown to be the scission of the S-S bond with recombination as the main fate of the resulting methyl thiyl radlcals.l3
+
-
CH3SSCH3 hv 2CH3S 2CH3S == CH3SSCH;*
(13) (14)
Therefore, the photolysis system of disulfide and thiol a t 300 nm should contain all the reactants for reactions 10 and 12 and for reaction 11if reaction 10 occurred. Reactions 10, 11, and 13 were included by Steer and Knight in the mechanism for the photolysis of methanethi01 at 254 nm to also explain an observed very weak decrease in @(H2)with light intensity. Our results suggest that @(H2)does not depend on light intensity. However, it should be noted that Steer and Knight varied their intensities by a factor of 10 while our variation was only a factor of 3. The uncertainty in our results would allow for a small variation with intensity but we observed no trends in that direction. The light source used by Steer and Knight was an unfiltered mercury source that has a spectral distribution concentrated a t 254 nm but has some in(8) Reference 6, p 489. (9) (a) G. P. Sturm, Jr., and J. M. White, J. Phys. Chern., 72, 3679 (1968); (b) R. P. Steer and A. R. Knight, Can. J. Chern., 46, 2878 (1968); (e) J. M.White and G. P. Sturm, Jr.. ibid., 47, 357 (1969). (10) T. lnaba and B. deB. Darwent, J. Phys. Chern., 64, 1431 (1960). (17) G. Greig and J. C. J. Thynne, Trans. FaradaySoo., 62, 379 (1966). (12) L, Bridgesand J . M. White, unpublished results. (13) P. M. Rao, J . A. Copeck, and A. R. Knight, Can. J . Chern.. 45, 1369 (1967). The Journal of Physical Chemistry, Vol. 77, No. 2, 1973
298
Lamar Bridges and John M. White
tensity at longer wavelengths near 280 nm. Even if the 254-nm light was completely absorbed for all nonaxial paths the light at longer wavelengths because of decreased extinction coefficient would show a pressure-dependent absorption and thus misleadingly high quantum yields would be observed unless properly accounted for in the actinometry. This effect would be compounded if nonaxial path absorption a t 254 nm were present and incomplete. In our method the nonaxial light absorption a t 254 nm is properly accounted for in the actinometry. Our lamp has some intensity in the 280-300-nm region (about 2.5% of the 254-rim intensity) and could give rise to pressuredependent quantum yields due to weak long-wavelength absorptions. Taking account of this long-wavelength light, calculations indicate our quantum yields at 250 Torr would be low by 2.2% since HI absorbs more strongly in this region than the thiol. A steady-stste treatment of the proposed mechanism, reaction 1-9, furnishes the following relationships.
$(HJ= [(k,[(%SJ-Il $(CW
= 4)z
-+
+
h7[M])/((h3
hdCH3SHI [k,[B=H,SW((h, hdC&SHI
+
+
+ bCMI)I4i
(15)
+ h[Ml>14i
(16)
@(H#J) = $(CHA +(CH$SCHJ = 1 &(-CH,SH) := 2
(17) (18)
(19) $1 $2=1 (20) In the above expressions, $I1 and & are the quantum yields for the primary processes 1 and 2, respectively. Equations 35 and 16 predict that when [MI is large (M other than CB3SEI in this case) the quantum yield of hydrogen approaches $11 while the quantum yield of methane approaches $2. Using this information along with data from Figure 3 we estimate & is about 0.93 at 254 nm and about 0.75 at 214 nm. The estimates for & are 0.07 and 0.25 at 254 and 214 nm, respectively. Using the above estimates along with eq 16, estimates for the fraction X of hot H atoms undergoing reaction 4 in the photolyfiis of pure methanethiol can be obtained.
+
X;-h,/(hs
+
/C,
+
h7)= [@(CH,J - @ ~ 2 ] / @ ~
(21)
For 254 and 214 11x1 photolysis, X is 0.098 and 0.133, respectively. Sirwe the average translational energies of the hydrogen atoms a t these two wavelengths are known to be different,14 the above results inclicate that the ratio k4/k3 varies slowly with hydrogen atom energy in the range 0.89-1.40 eV. The value of near unity for the quantum yield of disulfide production at the two wavelengths indicates that recombinatioii (reactioii 9) rather than disproportionation is
The Journal of Physical Chernisfry, Vo/. 77, No. 2, 7973
TABLE Ill: Quantum Yields for the Photolysis of Methanethiol and Ethanethiol 254 nm
214 nm
CH3SH
@Z
0.83 f 0.93 0.16 f 0.03 0.93 0.07
QI.(HZ) @(C2H6)
CzHsSH 0.82 f 0.02 0.16 f 0.02
@I
0.9
@Z
0.09
@(Hz) @ (CH4) @I
0.66 f 0.03 0.35 f 0.03 0.75 0.25
0.75 f 0.03 0.28 f 0.03 0.8 01 :9
the main fate of methyl thiyl radicals. These results are in agreement with recent studies of the photolysis of dimethyl sulfide15 and dimethyl disulfide12vapors. Two groups have reported a quantum yield value of 1.7 for the disappearance of methanethiol at 254 nm. The first group16 carried the photolysis to large per cent decompositions of thiol and there is no indication that corrections were made for absorption by the disulfide. The second group4 used the production of hydrogen from the photolysis of H2S as a chemical actinometer. The value 1.2617 was used for the quantum yield of €32 production. If the more recent value of 1.018 is used, a @p(-C!H3SH)= 2.1 is obtained. The above results and mechanism for the photolysis of methanethiol me not much different from those obtained with ethanethiol.2* Both exhibit wavelength-dependent quantum yields for primary processes and both mechanisms involve reactions of hot hydrogen atoms with the substrate. Table I11 contains results from the present study along with those from the ethanethiol study.2a The absorption spectral9 of these two thiols are very similar with two different electronic bands which appear to be important in the wavelength region 214-254 nm, There is a small shift in the methanethiol spectrum toward longer wavelength which could possibly place the absorption at 214 nm more into the wing of the 200-nni band as compared to the corresponding absorption by ethanethiol. Considering the experimental uncertainties, no conclusion can be made as to the significance of this small shift on the photochemistry of the two thiols. (14) G. P. Sturm, Jr., and J. M. White, J. Chern. Phys., 50, 5035 (1969). (15) P. M. Rao and A. R. Knight, Can. J. Chern.. 50, 844 (1972). (16) N. P. Skerrett and N. W. Thompson, Trans. faraday SOC.. 37, 81 (1941). (17) B. deB. Darwent and R. Roberts, Proc. Roy. SOC., Ser. A, 216, 344 (1953). (18) B. deB. Darwent, R. L. Wadlinger, and Sr. M. J, Allard, J . Phys. Chern., 71,2346 (1967). (19) L. B. Clark and W. T. Simpson, J. Chern. Phys., 43, 3666 (1965).
