Reactions of thiyl radicals. IX. Sensitized photolysis of methyl sulfide. Pressure dependence of methylthiyl radical disproportionation-combination

Abstract

The triplet mercury photosensitized decomposition of methyl sulfide vapor has been investigated to determine the effect of reactant and deactivating gas pressure on products from thiyl radical reactions. The ratio of yields of CH3SH and CH3SSCH3 is found to be remarkably pressure sensitive, particularly at low-sulfide concentrations. With 68 µ of CH3SCH3, the ratio passes through a maximum at about 20 Torr of added N2 or CF4 or increases to a constant, pressure independent, value at 20 Torr of He pressure. The maximum results from the competing effect of diffusion of CH3S to the wall, favoring recombination, which rate decreases with pressure, and collisional stabilization of CH3SSCH3* molecules in the vapor phase, favored by higher pressure. The results show that observed #(CH3SH)/<£(CH3SSCH3) values are indicative of the actual disproportionation-combination ratio only at pressures sufficiently large for complete deactivation of hot-combination products and negligible wall reactions. The limiting value for kdiaJkComb determined for CH3S radicals here is 0.04. Ethane yields follow the same trends as those of CH3SSCH3 indicating the pressure effects may be characteristic of other radicals whose combination product requires stabilization. Kinetic analysis yields a number of rate constant values and indicates a lifetime of 10~9 sec for CH3SSCH3* in agreement with an RRK calculation. The quantum yields of products from the photosensitized decomposition at 200 Torr of pressure are C2H6,0.47; CH3SSCH3, 0.37; CH4, 0.07; and CH3SH, 0.01.

Reactions of Thiyl Radicals. X. Photolysis of Ethyl Sulfide Vapor

“…Figure 3 shows the initial rate to be 0.50, and thus, using the R70 minimum value of 0.27, (R, ,O + R, , O) I 0.23. On the basis of the known small contribution of the disproportionation mode to the total rate of thiyl radical reactions of the radical-radical type that has been rationalized principally on the basis of the tendency of organosulfur compounds not to undergo reactions in which C-S bonds are converted to C=S bonds (4,12), it would be expected that Rl3/R,, is quite minute and therefore that the 0.23 likely represents essentially the maximum value of R,,.…”

“…Methyl sulfide vapor has been studied in direct photolysis (1,2) and in mercury photosensitization (3,4). The 'From the Ph.D. Thesis of C. S. Smith, to be submitted to the University of Saskatchewan, Saskatoon, Saskatchewan.…”

“…In each case the products of the reaction can be rationalized on the basis of a C-S bond cleavage in the primary process, followed by reactions of the alkyl and thiyl radicals formed thereby. It has been shown (4) that the thiyl radical disproportionation :combination ratio determined from thiol and disulfide yields is pressure sensitive and that only at high total pressures are the observed yields of these products indicative of the actual relative contribution of the two processes.…”

Reactions of thiyl radicals. XII. Triplet mercury photosensitized decomposition of ethyl sulfide in the gas phase

“…Methyl sulfide has been studied in both direct (1) and Hg-sensitized decomposition (2), but only the direct photolysis of ethyl sulfide has been investigated (3).…”

“…The apparatus and procedures ill general have been particularly as affect product yields described previously (2,3). In the sensitized experiments fro111 which h.,~isl,ropor~iona~iOII/h.ConI~~I1a~ioI, values the light source was a Hanovia 68SA45 low pressure for thiyl radicals can be con~puted.…”