A search for a gas-phase free-radical inversion displacement reaction at a saturated carbon atom

Abstract

Five thermal and photochemical reaction systems have been examined in an attempt to detect or to place upper limits on the rates of the simple inversion reaction, CH3 + CH4 → C2H6 + H, and its reverse. No positive evidence was obtained for the occurrence of these reactions. An upper limit for the rate constant for the forward reaction of 0.063 M−1s−1 at 802 K was obtained from the thermal decomposition of methane, and upper limits of 0.044 and 0.14 at 823 and 983 K respectively were estimated from the thermal decomposition of ethane. Other systems examined were the thermal decomposition of azomethane in the presence of methane, and the mercury-photosensitized decompositions of methane and ethane, which were complicated by unexpected side reactions.

Accurate Coupled Cluster Calculations of the Reaction Barrier Heights of Two CH3• + CH4 Reactions

“…They are also depicted in Figure , which not only shows the calculated rate constants at 300, 350,..., 1500 K but also the curves that are obtained by fitting expressions of the type of eq to the calculated points in the range of 600−1500 K. In Figure , the solid line corresponds to the reaction rate constants used in ref , that is, using eq with the parameters of Tabayashi and Bauer . Our rate constants are below the upper limit of Back at 802 K and below the rate constant expression of Skinner and Ruehrwein …”

“…For comparison, the rate constants used in ref for the latter reaction are shown as a solid line. The upper limit of Back at 802 K is marked as +. The rate constants of Skinner and Ruehrwein are indicated as a dotted line.…”

“…On the basis of this estimate, these authors concluded that the reaction can be neglected in a mechanism for the pyrolysis of methane. 4 Furthermore, more than 25 years ago, Back 5 derived an upper limit for the reaction rate constant of k ) 63 cm 3 mol -1 s -1 at 802 K, about 16 times below the value of Tabayashi and Bauer for that same temperature. Hence, the role of this reaction in the mechanism of Norinaga and Deutschmann was considered to be suspicious, and we decided to reinvestigate it by means of modern, highlevel ab initio quantum chemical methods.…”

Formation of HCOH + H2 through the reaction CH3 + OH. Experimental evidence for a hitherto undetected product channel

“…A heating power of about 40 W was needed to obtain temperatures around 1200 K. The performance of this pyrolytic source is shown in Figure 1: Between 1150 and 1170 K a typical dissociation yield of 80% was achieved. This yield could be slightly increased above 1170 K to 82% and under these conditions, due to the reaction (see [241) CH3NNCH3 + CH3 CH3NNCHz + CH4 small amounts of CH4 were formed. At even higher temperatures this latter reaction took over resulting in a severe decrease of the CH3 yields.…”

Chemical Reaction Mechanisms of Diamond Growth

“…43 Such relatively large barriers are typical for the addition of a primary hydrocarbon radical to a hydrogensaturated carbon atom. 44 , 45 When both methyl groups undergo H abstraction, the C-C bond should be formed quickly and irreversibly as is typical of radical combination. 4 3 However, the produced -CH 2 -CH 2 -adspecies is kinetically unstable under the conditions of diamond CVD.…”