3196
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979
Yokota and Strausz
Reaction of Hydrogen Atoms with Dimethyl Sulfidet T. Yokota and 0. P. Strausz* Hydrocarbon Research Centre, Department of Chemistiy, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 (Received May 14, 1979) Publicatlon costs assisted by the University of Alberta
Hydrogen atoms produced by the photolysis of HzS in the presence of a large excess of inert gas were reacted with CH3SCH3.A mechanism involving a short chain and the two novel elementary reactions H + CH3SCHS CH3SH+ CH3and HS + CH3SCH3 CH3SSH+ CH, adequately explains all experimental observations. The rate coefficient of the former reaction was determined in the temperature range of 27-199 "C to have the value (1.71 f 0.26) X 1013exp[-(2621 f 88)/RT] cm3mol-1 s-l relative to the literature value for the H + H2S SH + Hz reaction of (7.77 f 0.90) X 10l2exp[-(1709 f 60)/RT] cm3 mol-'^-^,
-
-+
-
Introduction As part of our continuing effort to unravel the mechanism of the reactions of hydrogen atoms and alkyl radicals with organosulfur molecules, we have reported in earlier publications from this laboratory on the reactions H + COS,' H + thiirane,2 CH3 thiirane,3 and CHB+ methylthiirane., In all these reactions the sulfur atom was abstracted in a concerted, single-step attack on the substrate to yield SH or CH3S. In the present study we have extended these investigations to the reaction of hydrogen atoms with dimethyl sulfide. The main aim of the work was to identify the elementary reactions involved, establish the overall reaction mechanism for the system, and determine the Arrhenius parameters of the individual steps. Unlike our previous studies where the source of hydrogen atoms was the Hg(3P1)-photosensitizeddecomposition of H2,in the present study it was found more advantageous to generate H atoms by the photolysis of hydrogen sulfide.4-7 The hydrogen atoms produced from this source possess excess translational energyS-l3which is removed when the photolysis is carried out in the presence of large concentrations of an inert gas.14-16 In the present study helium and carbon dioxide were used to this end.
+
Experimental Section The same apparatus was used as in previous studies from this laboratory1i2with minor modifications. Hydrogen atoms were produced by the photolysis of H2S in the presence of nearly 1 atm of He or C02. A Hanovia 30620, Type SH, 140-Wmedium-pressure Hg lamp was employed as the light source, with the radiation filtered through a 5 X 5 cm Hg-vapor filter and a 2-mm Corning 7910 filter positioned between the lamp and the 5 X 10 cm quartz reaction vessel, in order to eliminate any Hg-resonance radiation and radiation below 210 nm, giving an effective output in the range 236-270 nm. The reaction vessel was surrounded by an aluminum block furnace heated by pencil heaters and connected to a circulatory system with an Hg reservoir having a total volume of 510 cm3. Circulation maintained a constant mercury concentration and also minimized the accumulation of a black sulfide film on the reaction vessel window; it was effected by a fan constructed of glass and Teflon and driven by an external induction motor. The incident light intensity was attenuated by wire mesh screens of different mesh sizes. The light intensity absorbed by H2S was determined by measuring the hydrogen produced in the absence of di+Presented at the 176th National Meeting of the American Chemical Society, Miami Beach, Fla., Sept. 1978. 0022-365417912083-3196$01.0010
methyl sulfide and assuming the quantum yield of H2 to be ~ n i t y . ~ - ~ CH3SCH3 (Terochem) was purified by trap-to-trap distillation at -140 " C and HzS (Matheson) was distilled at -160 "C. High-purity COz (Airco) was used without purification after thorough degassing at -196 OC. He (Union Carbide) was slowly introduced into the reaction system through a molecular sieve column, 1X 100 cm, kept at -196 "C to eliminate impurities. In preliminary runs, the major products were found to be H2, CHI, and CH3SH. In the system with added He, only CH4 and CH3SH were measured because of the difficulty in separating small quantities of H2 from a large excess of He. On completion of irradiation, the noncondensables at -196 "C which consisted of Hz, CH4, and a large excess of He were introduced into a 1 X 100 cm molecular sieve 3A column at -196 "C, where CHI was trapped and He and Hz were passed through. After removal of He and Hz by pumping, the column was heated to 100 "C to recover the trapped CHI. The amount of CH4 was measured by a gas burette as well as by gas chromatography using a 0.5 X 180 cm molecular sieve 13X column at 60 "C. All condensables at -196 "C, consisting of HzS, CH3SH,and CH3SCH3,were injected directly into the GC and separated on an 80-100 mesh Poropak Q 0.5 X 60 cm column temperature programmed from 50 to 130 "C. In the system with added COz, only H2 and CHI were measured because of the difficulty in separating small amounts of CH3SH from large quantities of COD On completion of a reaction, noncondensables at -196 "C, consisting of H2 and CHI, were collected by a Toepler pump and measured in a gas burette. The mixture was then brought in contact with copper oxide at 300 "C to oxidize the H2 to H20. After completion of the oxidation and condensation of H20 in a trap cooled by liquid Nz, CH4 was again collected and measured by the gas burette, followed by GC analysis using the molecular sieve column. The front window of the reaction vessel was baked between each run to remove sulfur-containing polymers and maintain a constant transparency. Throughout the experiments, concentrations of He or C02 (-29 pmol cm-S) ( ~ 5 5 torr 0 at 27 "C) and HzS (0.275 pmol cm-,) ('5.12 torr at 27 "C) were maintained. The experiments with added He were performed only at room temperature, while the runs with added COz were performed at 27,91,151, and 199 "C.
Results In preliminary studies hydrogen atoms were produced by the Hg(3P1)-photosensitizeddecomposition of H2.The 0 1979 American Chemlcal Society
The Journal of Physical Chemistry, Vol. 83,No. 25, 1979 3107
Reaction of Hydrogen Atoms with Dimethyl Sulfide I
1
I
1-
1
vi
a F
Exposure Time, min
Exposure
Figure 3. H, (0) and CHI (V)yields as a function of exposure time in the presence of C02. T = 27 OC, [H2S] = [CH3SCH3] = 0.275 pmol ~ m - ~ .
Time, min
Figure 1. CH4 (0) and CH3SH (--V--) yields as a function of exposure time in the presence of He. T = 27 OC, [H2S] = [CH3SCH3] = 0.275 pmoi ~ m - ~ .
1 I
0
I
I
I
4 [ C H ~ S C H ~1]0,- 7 ~ cm-3 ~1 2
3
I
I
I
I
05
10
15
20
3
[CH 3 SCHJ 1[HGI
I
5
1
Figure 2. CH4 (0) and CH3SH ('I yields ) as a function of CH3SCH3] in the presence of He. T = 27 OC, [H2S] = 0.275 pmol cm , exposure time = 30 min.
reaction of hydrogen atoms with dimethyl sulfide yielded CH4, C2Hs,and CH3SH as major products along with some CH3SSCH3. It soon became evident, however, that the system is complex and that the primary reactions are obscured by an extensive array of secondary processes. The photolysis of H2S as a source of H atoms appeared to be a reasonable alternative, since the methyl radicals produced in the primary reaction will react rapidly with H2S forming methane, thereby eliminating the complicating secondary reactions of methyl radicals. As seen from the plots shown in Figure 1the yields of the principal products, CH, and CH3SH, were equal and linear with exposure time. At a fixed concentration of H2S and He, both yields showed a nonlinear dependence on the concentration of CH3SCH3while maintaining R(CH4) = R(CH3SH), Figure 2. It is also apparent that the yields of each product significantly exceed the total number of light quanta absorbed, 1.47 beinstein. The yields of H2 and CH4 were then measured in systems with added COz instead of He. As illustrated by the plots in Figure 3, the yields were proportional to the exposure time up to 60 min. Next, the rates of hydrogen and methane formation were measured as a function of CH3SCH, concentration at fixed concentrations of H2S and C02 at three different light intensities, and the results are presented in Table I. When the reciprocal quantum yields of hydrogen formation were plotted against [CH3SCH3]/[H,S], a straight-line relationship was obtained with an intercept
Figure 4. 4(H&' as a function of [CH,SCH,]/[H,S]. = 4.17 X 10- einstein s-' ~ m - ~ . 9
T = 27
OC,
I,
7
[CH3SCH3], pmol cm-3
+
Figure 5. Rates of H, ('I CH4 ),( O ) , and H, CH, (0)formation as a function of [CH3SCH3] in the presence of COP. T = 27 OC, I, = 4.17 X lo-'* einstein s-' ~ m - ~ .
of unity, as illustrated in Figure 4 for I , = 4.17 X 10-l2 einstein s-l ~ m - ~ The . values of the intercepts and slopes are independent of light intensity, Table 11, at the three light intensities measured. The rates of H2 and CHI formation and their sum as a function of CH3SCH3concentration were also studied, and the results obtained at I, = 4.17 X einstein s-l cmJ
3198
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979
Yokota and Strausz
TABLE I: Rates of Formation of H, and CH, as a Function of Dimethyl Sulfide Concentration and Light Intensitya 101'Ia,einstein s - ' cm-, 4.17
[CH,SCH,], pmol cm-' 0.055 0.068 0.137 0.203 0.274 0.348 0.414 0.542 0.138 0.205 0.276 0.344 0.414 0.138 0.207 0.275 0.412 0.548
1.79
1.16
a
[CH,SCH,]/ [HZSI 0.199 0.250 0.500 0.749 1.00 1.28 1.50 2.00 0.498 0.748 1.00 1.25 1.50 0.500 0.750 1.00 1.50 2.00
[H,S] = 0.275 pmol cm"; [CO,]
=
101'(rate), mol s - ' cm-, _ l _ _ l _ l _ _
H2 3.78 3.68 3.40 3.09 2.81 2.61 2.42 2.15 1.44 1.33 1.22 1.12 1.07 0.952 0.842 0.774 0.674 0.595
CH, 0.83 1.02 1.86 2.60 3.44 4.12 4.88 6.26 0.80 1.12 1.35 1.80 2.05 0.477 0.723 0.893 1.32 1.62
@(H,)-'
H, t CH, 4.61 4.70 5.25 5.69 6.25 6.73 7.30 8.41 2.24 2.45 2.57 2.92 3.12 1.43 1.57 1.67 1.99 2.21
.I___
1.10 1.13 1.23 1.35 1.48 1.60 1.72 1.94 1.24 1.35 1.47 1.60 1.67 1.22 1.38 1.50 1.72 1.95
29 bmol ~ m -photolysis ~ ; time, 30 min.
TABLE 11: Intercepts and Slopes of the Plots @(H2)-'vs. [ CH,SCH, ] / [ H,S] at Three Different Light Intensities
10I2Iaa
10'' (intercepta )
slope
1.16 1.79 4.17
1.01 i: 0.02 1.00 i 0.02 1.00 i: 0.00
0.475 i 0.013 0.476 i: 0.022 0.471 i 0.003
In units of einstein s - ' cm-'.
TABLE 111: Intercepts and Slopes of the Plots of Figure 6, R(H, + CH,) vs. [CH,SCH,] 1.16 1.79 4.17 a
1.17 i 0.03 1.80 f 0.02 4.17 i 0.03
1.92 i 0.07 3.22 i. 0.05 7.65 t 0.10
In units of einstein s - ' ~ m - ~b In . units of s - ' .
0
1
2
3
4
L,,Io-'~ einstein s-1 crn-3 Flgure 7. Slopes of the plots in Figure 6 vs. 0' 0
I
0.1
I
0.2
I
0.3
I
I
I
0.4
0.5
0.6
O0
+
Flgure 0. Rate of H2 CH, formation as a function of [CH3SCH,] for 1, = 4.17 X 10-12(0), 1.79 X IO-'* (0),and 1.16 X 10-12(0)einstein s-' ~ r n - ~T. = 27 OC.
are plotted in Figure 5 which shows a gradual decrease in R(H2), a relatively steep increase in R(CH4),and a linear increase in R(H2+ CHJ with increasing [CH3SCH3].The intercept of the R(H2+ CH4) vs. [CH3SCH3]plot is equal to the absorbed light intensity, as seen from the data summarized in Table I11 and plotted in Figure 6, while the slope is a linear function of the absorbed light intensity, Figure 7. Similar studies in which the yields of H2 and CH4 were measured as a function of substrate concentration were also carried out at 91, 151, and 199 "C and the results obtained at the four temperatures are summarized in Table IV.
I,.
t
I '
WT), I O - ~ K
Figure 8. In (k3/k2) as a function of I / T .
Plots of $(H2)-l vs. [CH3SCH3]/[H2S]at the four temperatures were all linear, having the same intercept but different slopes. These values are listed in Table V. The temperature dependence of the values for the slopes con-
The Journal of Physical Chemistry, VoI. 83, No. 25, 1979 3189
Reaction of Hydrogen Atoms with Dimethyl Sulfide
TABLE IV:
o ( H 2 ) - 'and @(H,+ CH,) as a Function of [CH,SCW,] and Temperaturea
____._
T , "C
no. of runs
27
1 1 4 3 6 2 2 2 1 1 1 1 1
[CW,SCH,
[CH,SCH,I, @molcm-' 0.053 0.068 0.138 0.205 0.275 0.346 0.413 0.54 5 0.100 0.138 0.192 0.243 0.277 0.344 0.415 Q.lOO 0.136 0.201 0.273 0.341 0.409 0.101 0.136 0.203 0.275 0.304 0.376 0 411
I/
ow,)-'
[FI2SI 0.199 0.250 0.500 0.749 100 1.27 1.50 2.00 0.364 0.500 0.692 0.876 1.01 1.24 1.50 0.367
1 1 1 2 1 1 1 1 1 1 1 1 1 1 1
151
199
[HIS] = 0.275 @mol~ r n - [CO,] ~ ; = 29 pmol cm-j; I ,
1.10 1.13 1.23 1.36 1.49 1.60 1.72 1.95 1.22 1.30 1.43 1.53 1.61 1.73 1.95 1.26 1.37 1.53 1.74 1.92 2.10 1.30 1.41 1.61 1.84 1.92 2.10 2.25
0.500
0.735 1.00 1.25 1.50 0.368 0.495 0.734 1.00 1.10 1.37 1.49 =
4.17
TABLE V : Intercepts and Slopes of the Plots q(H,)-' vs. [CH,SCH,]/[H,S] at Four Different Temperatures
X
10.'' einstein s-' ~ m - ~ .
14.5
_ _ l _ _ _ l _
T , "C 27 91 151 199
+ C"l) 1.10 1.13 1.25 1.36 1.48 1.62 1.73 1.97 1.22 1.30 1.42 1.49 1.61 1.76 1.91 1.26 1.37 1.55 1.73 1.89 2.05 1.30 1.41 1.60 1.78 1.86 2.08 2.1 2
G I 2
~ _ _ _ _
_ l l _ _ l l
91
a
~
___-l_--l_-__-
_ I
intercept 1.00 * 0.99 * 0.99 i 1.00 f
V
0.00 0.02 0.01 0.01
slope 0.474 2 0.620 t 0.741 t 0.828 *
1-
0.004 0.023 0.008 0.008
I
'3'L-;?51 ).
215
,
L
30
35
A
(I/T), IO+ K-'
Figure 10. In (k7/~lkB-l[X,]) as a function of 1 / T .
The plots of 4(Hz + CH4) vs. [CH3SCH3]at the four temperatures are also linear, again having the same intercept and different values for the slopes, Table VII. The Arrhenius plot of the values of the slopes is a good straight line, Figure 10. The uncertainties quoted with the numbers in the text and in the tables are the standard errors which are calculated from a least-squares analysis of corresponding experimental data. [CH3SCH3], pmol cmm3
Flgure 9. Rates of CHI ( 0 ,V) and I-1, -t CH., (0, e) formation as a function of [CH,SCH,] in the presence of He (open symbols) and CO, (solid symbols).
forms to the Arrhenius relationship, and when plotted accordingly a good straight line is obtained, Figure 8. The rates of CH4 and H2 -t CH4 formation in the two systems are compared in Table VI and in Figure 9 from which it is seen that R(CHJHe > R(CH4)C02and R(Hz + CH4)He> R(Hz + CH4)C02.
Discussion Since the yields of H,, CH4, and CH3SH are proportional to exposure time, as seen in Figures 1 and 3, all three products appear to be of primary origin. The following simple mechanism should then apply: HZS + hv H + HS (1) H + HZS H2 + HS (2) H + CH3SCHs CHSSH + CH3 (3) CH3 + H2S CH4 HS (4) -+
+
-+
+
3200
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979
HS
+ HS 4 H 2 S + S
M
HzSz (5) (64
HS + Xi XiSH where Xi may be a radical and/or the wall of the reaction vessel. Application of the usual steady-state assumptions yields the following simple relations: 4
(.b(H2)-’ = 1 + k3[CH3SCH,]/h2[HZS] (1) R(CH4) = R(CH3SH) (11) R(H2 + CH4) = I , (111) It is seen from Figures 1 and 2 that eq I1 is obeyed. The linear relationship between 4(HZ)-’and [CH,SCH]/ [HzS] with an intercept of unity and a slope independent of the absorbed light intensity, as seen in Figure 4 and Table 11, satisfies eq I. This suggests that H abstraction from dimethyl sulfide H + CH3SCH3 H2 + CH3SCHz +
is unimportant. The plots of the data in Table I, Figures 5 and 6, however, show that R(Hz + CHJ > I,. It is seen that with increasing [CH,SCH,], R(HJ decreases gradually, owing to increased competition from step 3, while R(CH4) increases very rapidly, becoming greater than I,. It would therefore appear that the simple mechanism consisting of steps 1-64 is not entirely satisfactory and that some other CH4-producingreaction is operative in this system. As will be shown, the novel elementary displacement reaction HS + CH3SCH3 -.+ CHBSSH + CH, (7) followed by wall disproportionation of the resultant disulfide wall
CH3SSH CH3SH + (S) (8) in conjunction with steps 1-6-i can adequately account for the observed relation R(H2 + CH4) > I , and eq I and 11. The overall mechanism proposed consists of a chain carried by HS and CH, radicals, initiated in steps 1-3 and propagated in steps 4 and 7. There are three possible chain terminating cases: Case I: termination by HS, steps and 6 and 6-i. Case 11: termination by CH, ___+
M
CHB + CH3 CzH6 CH3 + XI XiCH, +
4
(9) (10)
Case 111: termination by CH3 and SH CH3 + HS -”, CH3SH (11) and steps 5, 6-4 9, and 10. Kinetic treatment of the reaction sequence 1-4,7, and 8 followed by each case of chain termination separately shows that only the sequence with case I, i.e., termination by SH radicals, predicts R(CH4)= R(CH3SH). This is also supported by the apparent absence of C2H6among the products. The new rate equation for R(Hz + CHI) then becomes R(H2 + CHd) = I , + k,[HS][CH:,SCHJ (IV) which predicts a linear relationship between R(H2+ CHI) and [CH,SCN,] and an intercept equal to 2,. The experimental results presented in Table I and plotted in Figure 6 indeed obey this relationship at all three light intensities studied.
Yokota and Strausz
The HS radical concentration is derived from the equation 2hS[HS12 + ck6.i[xi][Hs] - 21, = 0 i
If termination takes place solely by step 5 , then [HS] = Ia1/z/k51/2, but if only steps 64 are involved, then [HS] = 21a/CikGi[Xi].In order to differentiate between these two possibilities, the slopes of the plots in Figure 6, which are tabulated in Table 111, were plotted against I,, Figure 7. Clearly, the chain-terminating reaction in this system consists of the steps of eq 64, although the true nature of the termination reactions cannot be considered as being clearly established at this stage. Equation IV can be rewritten as
and 4(H2 + CH4) = 1 +
2121 [CHBSCHBI (VI) Ck,.i[XiI
Equations V and VI indeed obey the experimental results at the four temperatures, as presented in Tables IV and VII. It could perhaps be argued that the following alternative reactions, which are all energetically feasible, are taking place: H + CH3SCH3 CH4 + CH3S (3’) HS + CH3SCH3 -.+ CH3SH + CHSS (7’) However, substitution of steps 3 and 7 by 3’ and 7’ or 3 and 7’ cannot account for the chain production of CH4. The last alternative, 3’ and 7, requires the slightly endothermic reaction CHBS + H2S CH3SH + HS in order to satisfy the relation R(CH4) = R(CH,SH). Moreover, displacement reactions such as step 3’ must be negligibly slow and can be disregarded. According to eq I, the values of k3/k2 are determined from the slopes of the linear plots of 4(H2)-’ vs. [CH3SCH3]/[H2S].The values obtained at the four temperatures are presented in Table V. The Arrhenius plot of k3/kz gives a good straight line as seen from Figure 8, from which E3 - Ez = 912 f 28 cal mol-’ and A3/A2= 2.19 f 0.08. Accepting the value kz = (7.77 f 0.90) X 10l2 exp[(-1709 f 60)/RT]cm3 mol-l s-l,16 the value of k3 is calculated to be (1.71 f 0.26) X 1013exp[(-2621 f 88)/RT] cm3 mol-l s-’. For comparison, the kinetic data on the formation of CH4in the presence of both moderating gases are collected in Table VI, where it is seen that R(CH4)He> R(CH4)Co2. R(HJHe was calculated from the data obtained in the system with added COP Plots of eq V, illustrated in Figure 9, are linear but have different slopes, due to the different R(CH4). The initial surmise that hot hydrogen-atom participation in the reaction scheme could explain this difference was rejected on two grounds. First, they would have to be more active in the system with added C02 and consequently R(CH4)C02> R(CH4)Hesince E3 > E2 and, second, they could not affect the rate of chain production of CH4since they are not involved in the propagation steps of the chain. Since k,I, is not affected by the nature of the moderator gas, the difference must lie in the values of Cik,,[Xi] in equation VI, i.e., in different rates of chain termination. There are two possible cases for the chain termination step 6-i, namely, a simple addition reaction -+
-+
The Journal of Physical Chemistty, Vol. 83, No. 25, 1979 3201
Reaction of Hydrogen Atoms with Dimethyl Sulfide
TABLE VI: R(CH,) and R(H, [CH,SCH, pmol
as a Function of [CH,SCH,] in the Presence of He and CO,
10IZ(rate)uin He
I,
0.069 0.137 0.206 0.275 0.344 0.412 0.481 a
+ CH,)
CH4
H,
1.11 2.09 3.17 3.94 4.91 5.63 6.42
3.73 3.37 3.07 2.83 2.61 2.44 2.27
In units of mol s-' cm-'.
H,
+ CH,
CH,
H,
H, t CH,
0.055 0.068 0.137 0.203 0.274 0.348 0.414 0.542
0.83 1.02 1.86 2.60 3.44 4.12 4.88 6.26
3.78 3.68 3.40 3.09 2.81 2.61 2.42 2.15
4.61 4.70 5.25 5.69 6.25 6.73 7.30 8.41
4.84 5.46 6.24 6.77 7.52 8.07 8.69
Calculated from R(H,) in the system with added CO,.
-
TABLE VII: Intercepts and Slopes of the Plots @(H, + CH,) vs. [ CH,SCH,] a t Four Temperatures T , "C intercepts 10-6( sl opeu) 27 91 151 199
1012(rate)uin CO,
[CH,SCH,I, pmol cm-,
1.00 t 0.01 1.00 f 0.01 1.02 i 0.02 1.02 i. 0.02
1.76 i. 0.02 2.21 i. 0.02 2.57 i. 0.08 2.78 f 0.08
In units of cm3 mol-'.
assisted by a third body or a diffusion-controlled reaction. The rate for both cases depends on the nature of the moderator, COz or He in these experiments, when the concentration of moderator is constant. To conform with the experimental results the requirements for Xi are that its concentration must be independent of both light intensity and concentration of CH3SCH3. Hg atoms satisfy both requirements, since they are constantly replenished from the Hg reservoir located in the circulatory system, and they appear to be the best candidate for Xi in this system. A similar conclusion as to the fate of HS radicals was reached in the earlier study of the H + COS system.' There are report^'^-^^ that S1, Sz..., HSz, HSs..., etc. exist in similar Hg-free systems. However, the S(3P)atoms that are the source of Sa, S3..., HSz, HS3..., etc. do not exist in the present system. I t would also be difficult to explain why the concentration of these species should be independent of the light intensity and concentration of CH3SCH3. A brown/black solid, deposited on the wall of the reaction system, suggests the formation of such species as HgS, (HgSH)z, and Hg(SH)z, supporting the suggestion that the most likely chain-termination step is the reaction between SH radicals and Hg atoms, assisted by the third body COz or He. The efficiency of the third body must be C 0 2 > He, and consequently R(CH4)He> R(CH4)Co~. On the other hand, if [SH] is diffusion controlled and Xi is the wall of the reaction vessel, then R(CH4)C02> R(CH4)He,since D(HS)He> D(HS)C02,21inbut this is contrary to the experimental results. Relative Arrhenius parameters for steps 6 and 7 can be obtained assuming that step 6-i is
M
HS 4- Hg HgSH (6) From the plot, Figure 10, of the data in Table VII, E, E6 = 743 f 36 cal mol-' and A7/A6[Hg] = (3.09 f 0.16) X lo6 cm3 mol-'. From the above values we can estimate a rate constant for step 7, if E6 and A, are known. Step 6
could be regarded BS a radical-radical combination reaction with E , 2 kcal mol-' and A in the range of 1013-10'4 cm3 mol-l s-l. Since [Hg] is ca. mol cm-3 throughout all of the experiments, the value of A7 is then in the range 109-10'0 cm3 mol-l s-l, and the value of E7 is less than 2.7 kcal mol-'. No comparison can be made between these estimated rate parameters for reaction 7 and those for similar reactions because of the absence of reported data. These estimates, however, appear to be reasonable when compared with the relatively low activation energy of reactions initiated by S- - -S interaction and terminated by S-S bond formation, for example, S(3P)+ C2H4S(thiirane) S2 + CzH4 ( E 0 kcal mol-l 23) and S(3P)+ COS Sz + CO ( E 5 kcal mol-'3), and also with the A factor of reactions initiated by T-T orbital interaction, such as CH3 + CzH4S CH3S + CzH4 (A = 7.1 X 10'O cm3mol-l s-I3) and CH3S + CzH4S CH3Sz+ C2H4 (A = 3.2 X loll cm3 mol-' s-l 3). The estimates are also supported by the fact that step 7 is the rate-controlling step in the chain propagation, k7 < k4 = 2.51 X 1011 e~p(-2600/RT).~~ The unique feature of the H + CH3SCH3reaction is the absence of hydrogen abstraction. Instead, attack appears to take place exclusively at the sulfur atom, and the complex subsequently decomposes to give a methylthiol and a methyl radical:
-
H
-
- -
- -
+ CH,SCH,
r
1
H
-+ LH,C.
*
" S .. . C H J
-+
CH, t CH,SH (3)
As postulated in the H + C2H4Ssystem,2 the initial interaction probably takes place with the nonbonding 3p orbital of the sulfur atom. Thus, the overall reaction mechanism is kinetically compatible with the following scheme: HZS + hv H + HS (1) H + H2S -+ Hz + HS (2) H + CH3SCH3 CH3SH + CH3 (3) CH, + HZS CH4 + HS (4) HS + CH3SCH3 CHSSSH + CH3 (7) +
--+
CH3SSH HS
wall
+ Hg
CH3SH + (S)
M
(6)
HgSH
TABLE VIII: Arrhenius Parameters and Heats of Reaction for Hydrogen-Atom Reactions with Organosulfur Compounds __ reaction
H t C,H,S H H a
-+
+ CH,SCH, + COS'HS
In units of kcal mol-'.
+ C,H, + CH, + CH,SH + CO
HS
AHa
- 24.3 - 16.5 - 10.4
In units of cm3 mol-' s-].
10-13~b 5.7 1.7 1.7 0.91 0.54
(8)
EaU
ref
1.94 1.88 2.62 3.90 3.85
1 19 this work 2 19
3202
The Journal of Physical Chemistry, Vol. 83, No. 25. 1979 I
I
Yokota and Strausz
reaction in a common class, e.g., the modified Evans-Polanyi relation by Semenov.26Within this very limited series there is a good correlation between the exothermicities and the activation energies of the reactions, shown in Figure 11, from which it is possible to determine
1
Ea:049-35/AH, kcal mol-’
P
H t COS -)HS+CO
E, = 0.489 - 35.1/AH kcal mol-l Unfortunately the numerical constants cannot presently be related to the physical parameters associated with this reaction series since much of the required data is not available.
0
1
I
I
5
10
15
Acknowledgment. The authors thank the National Research Council of Canada for financial support. References and Notes
[l/(-AH)] x lo2, mol kcai-’
(1) S.Tsunashima, T. Yokota, I. Safarik, H. E. Gunning, and 0. P. Strausz, J. Phys. Chem., 79, 775 (1975). (2) T. Yokota, M. 0. Ahmed, I. Safarik, 0. P. Strausz, and H. E. Gunning, J. Phys. Chem., 79, 1758 (1975). (3) E. Jakubowski, M. G. Ahmed, E. M. Lown, H. S. Sandhu, and 0. P. Strausz, J . Am. Chem. Soc., 94, 4094 (1972). (4) N. 0.Steln, Trans. Faraday Soc., 29, 583 (1933).
Figure 11. Correlation between E, and AHfor H-atom reactions with COS, CH,SCH3, and C2H,S: (0),this laboratory;‘,* (V), Lee, Stlef, and Tirnmon~.*~
The radical-exchange reaction of HS with CH3SCH3,step 7, is a novel reaction which, together with step 4,propagates a chain involving HS and CH3 radicals as carriers, thus explaining the high yield of CH4 produced. The chain length, 1, can be calculated from 1 1 = -3 + kdCH8CH31 2 ~2[H2Sl 2 0 + ~3[CH,SCH,I/~,[H,~l) using 4 (H2)-l 4 (H, + CHI) as seen from Table IV. 1 was found to be a function of [CH,SCH,]/[H,S] and temperature, and increases as both [CH3SCH3]/[H,S] and temperature increase. At [CH,SCH,]/[H,S] = 1.0,l is 1.6 at 27 “C and 2.1 at 199 “ C , indicating a very short chain at moderate temperature. I t is possible to compare Arrhenius parameters of radical-exchange reactions involving a common bond-forming and bond-breaking sequence of the type R1 + X-RZ [R1*.*X***R2]R1-X + R2 In the series H + COS CO + HS, H + C2H4S C2H4 + HS, and H + CH3SCH3 CH3SH + CH,, the common factor is H-atom attack on sulfur, forming the H-S bond with the resulting rupture of the S-C bond. The heats of reaction and Arrhenius parameters of these reactions are listed in Table VIII. The Ea)s previously obtained in this laboratory for H + COS1 and H + C2H4S2are in good agreement with those obtained by Lee et al.,25while the A factors only agree in their order of magnitude. There have been various attempts to interrelate the E,’s and AH‘S of reactions in order to determine the E, of a new
(5) G. S. Forbes, J. E. Cline, and B. C. Bradshaw, J. Am. Chem. Soc.,
-
-+
--
-+
-
a
60,1431 (1938). (6) B. deB. Darwent and R. Roberts, Proc. R . SOC.London, Ser. A , 216, 344 (1953). (7) B. deB. Darwent and R. Roberts, D/scuss. Faraday Soc., 14, 55 (1953). (8) B. deB. Darwent, R. L. Wadlinger, and Sr. M. J. Allard, J. phys. Chem., 71, 2346 (1967). (9) R. G. Gann and J. Dubrin, J . Chem. Phys., 47, 1867 (1967). (10) L. E. Compton, J. L. &le, and R. M. Martln, J. Phys. Chem., 73, 1158 (1969). (11)G. P. Strum and J. M. White, J . Chem. Phys., 50, 5035 (1969). (12) L. E. Compton and R. M. Martin, J . Chem. Phys., 52, 1613 (1970). (13) R. B. Langford and G. A. Oldshaw, J. Chem. Soc., Faraday Trans. 7 , 68, 1550 (1972). (14)T. Yokota, Doctoral Thesis, The Catholic University, Washington, D.C., 1967. (15) G. R. Woolley and R. J. Cvetanovic, J. Chem. phys., 50, 4697 (1989). (16) M. J. Kurylo, N. C. Peterson, and W. Braun, J . Chem. Phys., 54, 943 (1971). (17) P. Fowles, M. deSorgo, A. J. Yarwood, 0. P. Strausz, and H. E. Gunnlna. J . Am. Chem. Soc.. 80. 1352 (1967). (18) 0.P. SGausz, R. J. Donovan, and M. deSor&, Ser: Bunsenges. phys. Chem., 72, 253 (1988). (19) Von D. Perner and Th. Franken, Ber. Bunsenges. phys. Chem., 73, 897 (1969). (20) R. W. Fair and B. A. Thrush. Trans. Faraday SOC..65. 1208 (1969). (21) J. 0.Hirschfelder, C. F. Curtiss, and R. B.-Bird, “Molecular Theoh of Gases and Liquids”, Wiley, New York, 1985,p 539. (22) N. M. Emanuel and D. G. Knorre, “Chemlcal Kinetics”, Halsted Press, Wiley, New York, 1973, p 307. (23) R. B. Klemn and D. D. Davls, Int. J. Chem. Kinet., 5, 149 (1973). (24) N. Imai and 0. Toyama, Bu//. Chem. Soc. Jpn., 33, 652 (1960). (25) J. H. Lee, L. J. Stlef, and R. B. Tlmmons, J. Chem. Phys., 67, 1705 (1977). (26) N. N. Semenov, “SomeProblems of Chemical Kinetics and Reactivity”, Vol. 1 (translated by J. E. S. Bradley), Pergamcm Press, London, 1958, p 27.
