Iodine catalyzed pyrolysis of dimethyl sulfide. Heats of formation of CH 3 SCH 2 I, the CH 3 SCH 2 radical, and the pibond energy in CH 2 S

A kinetic study has been made of the gas phase, I 2 ‐catalyzed decomposition of (CH 3 ) 2 S at 630–650 K. Some I 2 is consumed initially, reaching a steady‐state concentration. The initial major products are CH 4 and CH 2 S together with small amounts of CH 3 SCH 2 I, CH 3 I, HI, and CS 2 . The initial reaction corresponds to a pseudo‐equilibrium:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 3} {\rm SCH}_{\rm 3} + {\rm I}_{\rm 2} \mathop {\rightleftharpoons}\limits^{\rm A} {\rm CH}_{\rm 3} {\rm SCH}_{\rm 2} {\rm I + HI} $$ \end{document}accompanied by:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 3} {\rm SCH}_{\rm 2} {\rm I}\mathop {\hbox to 25pt{\rightarrowfill}}\limits^{\rm B} {\rm CH}_{\rm 3} {\rm I + SCH}_{\rm 2} $$\end{document}and\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 3} {\rm I + HI}\mathop {\hbox to 25pt{\rightarrowfill}} {\rm CH}_{\rm 4} {\rm + I}_{\rm 2} $$\end{document}which brings (I 2 ) into steady state and a final complex reaction:From the initial rate of I 2 loss it is possible to obtain Arrhenius parameters for the iodination:We measure k 1 , (644 K) = 150 L/mol s and from both the Arrhenius plot and independent estimates A 1 (644 K) = 10 11.2 ± 0.3 L/mol s. Thus, E 1 = 26.7 ± 1 kcal/mol. From the steady‐state I 2 concentration, an assumed mechanism and the known rate parameters for the CH 3 I/HI system. It is possible to deduce K A (644) = 3.8 × 10 −2 with an uncertainty of a factor of 2. Using an estimated Δ S   A °(644) = 4.2 ± 1.0 e.u. we find Δ H A (644) = 7.0 ± 1.1 kcal. With 〈Δ C PA 〉644 = 1.2 this becomes: Δ H A (298) = 6.6 ± 1.1 kcal/mol. Then Δ H   ƒ298 °(CH 3 SCH 2 I) = 6.3 ± 1 kcal/mol. Making the assumption that E −1 = 1.0 ± 0.5 kcal/mol we find Δ H   ° 1(644) = 25.7 ± 0.7 kcal/mol and with 〈Δ C PI 〉 = 1.2; Δ H   1(298) °= 25.3 ± 0.8 kcal/mol. This gives Δ H   ƒ298 °(CH 3 SĊH 2 ) = 35.6 ± 1.0 kcal/mol and D H   298 °(CH 3 SCH 2 H) = 96.6 ± 1.0 kcal/mol. This then yields E π (CH 2 S) = 52 ± 3 kcal. From the observed rate of pressure increase in the system and the preceding data k 3 , is calculated for the step CH 3 SCH 2 → CH 3 + CH 2 S. From an estimated A ‐factor E 3 is deduced and from the overall thermochemistry values for k −3 and E −3 . A detailed mechanism is proposed for the I ‐atom catalyzed conversion of CH 2 S to CS 2 + CH 4 .