Methyl radical recombination

The second-order rate law for bimolecular reactions is empirically well confinned. Figure A3.4.3 shows the example of methyl radical recombination (equation (A3.4.36)) in a graphical representation following equation (A3.4.38) [22, 23 and 24]. For this example the bimolecular rate constant is... 

Figure A3.4.3. Methyl radical recombination as a second-order reaction (after [22, 23]).

Hase W L 1972 Theoretical critical configuration for ethane decomposition and methyl radical recombination J. Chem. Rhys. 57 730-3... 

A classic shock-tube study concerned the high-temperature recombination rate and equilibrium for methyl radical recombination [M, Ml- Methyl radicals were first produced in a fast decomposition of diazomethane at high temperatures (T > 1000 K)... 

Glanzer K, Quack M and Troe J 1976 A spectroscopic determination of the methyl radical recombination rate constant in shockwaves Chem. Phys. Lett. 39 304-9... 

The primary step in the photolysis of methylcobalamin is homolytic fission to give the Co(II) cobalamin and methyl radicals. Recombination can occur, i.e., the reaction is reversed, unless the radicals and/or Co(II) are removed by further reactions ... 

Ciawi E, Ashokkumar M, Grieser F (2006) Limitations of the methyl radical recombination method for acoustic cavitation bubble temperature measurements in aqueous solutions. J Phys Chem B 110 9779-9781... 

Of course, all the appropriate higher-temperature reaction paths for H2 and CO discussed in the previous sections must be included. Again, note that when X is an H atom or OH radical, molecular hydrogen (H2) or water forms from reaction (3.84). As previously stated, the system is not complete because sufficient ethane forms so that its oxidation path must be a consideration. For example, in atmospheric-pressure methane-air flames, Wamatz [24, 25] has estimated that for lean stoichiometric systems about 30% of methyl radicals recombine to form ethane, and for fuel-rich systems the percentage can rise as high as 80%. Essentially, then, there are two parallel oxidation paths in the methane system one via the oxidation of methyl radicals and the other via the oxidation of ethane. Again, it is worthy of note that reaction (3.84) with hydroxyl is faster than reaction (3.44), so that early in the methane system CO accumulates later, when the CO concentration rises, it effectively competes with methane for hydroxyl radicals and the fuel consumption rate is slowed. 

The first quantitative calculation of a high collision efficiency for methyl radical recombination was made by Gorin4 who treated the collision pair as being stabilized by a polarization interaction at relatively large distances. From this point of view the transition state for the reaction corresponds to what might be termed a loose transition state in which there is relatively free libration or rotation of the two methyls relative to each other. 

A methyl radical recombines with the ben/oate anion with regeneration of copper(I) bromide. 

A considerable fraction of the methyl radicals, recombining to ethane, is formed by the decomposition of acetyl radicals. Thus, the increase in importance of the... 

The observed pressure dependence of g cannot be attributed to wall reactions, since significant wall effects were observed only at pressures lower than 1 torr. Dodd and Steacie interpreted these results by the third-body restriction of the methyl radical recombination reaction. The scheme... 

The fall-off region of the methyl radical recombination lies between 1 and 100 torr (see the figures). From these data, a minimum and maximum life-time for C2H6 of approximately 10 sec and of about 10" sec, respectively, were estimated . The latter value appears short for a complex with so many degrees of freedom . 

Methyl radical recombination is a special example of the more general recombination,... 

For the methyl radical/hydrogen atom association reaction there are two degrees of freedom that become vibrations as reactants go over to the product. For the methyl recombination there are five such degrees of freedom. It is thus expected that the first factor, the decrease in the density of states as R approaches the product will be greater for methyl radical recombination, and as a consequence G E, R ) will be a weaker function of E. 

In order to explain the data of Aronowitz et al (12) and previous shock—tube and flame data, Westbrook and Dryer (12) proposed a detailed kinetic mechanism involving 26 chemical species and 84 elementary reactions. Calculations using tnis mechanism were able to accurately reproduce experimental results over a temperature range of 1000—2180 K, for fuel—air equivalence ratios between 0.05 and 3.0 and for pressures between 1 and 5 atmospheres. We have adapted this model to conditions in supercritical water and have used only the first 56 reversible reactions, omitting methyl radical recombinations and subsequent ethane oxidation reactions. These reactions were omitted since reactants in our system are extremely dilute and therefore methyl radical recombination rates, dependent on the methyl radical concentration squared, would be very low. This omission was justified for our model by computing concentrations of all species in the reaction system with the full model and computing all reaction rates. In addition, no ethane was detected in our reaction system and hence its inclusion in the reaction scheme is not warranted. We have made four major modifications to the rate constants for the elementary reactions as reported by Westbrook and Dryer (19) ... 

In the following the salient features of the general transition state theory of Sections IIA and IIC, as applied to methyl radical recombination, are summarized, Details, where omitted, are given in Ref. 11c. 

Of particular interest both theoretically and experimentally is the temperature dependence of kMuch of the available experimental data on k is displayed in Fig. 2 of Ref. 11c. These data span the temperature range 250-2500Kand were published over a period from 1951 to 1985. A comprehensive review of pre-1980 experimental work on methyl radical recombination has been published by Baulch and Duxbury.26 The large scatter in the data is evident in Fig. 2 of Ref. 1 lc and some of the experimental uncertainties (not shown in that figure) are large. Nevertheless, it appears that k may exhibit... 

Reactions of Halo Compounds. - Calculations have been carried out to investigate the decomposition paths for methyl fluoride and methyl chloride. Methyl chloride undergoes photodissociation on irradiation at 157.6 nm. Photodissociation of methyl iodide at 266 nm has been studied. The methyl radical recombination has been followed by time-resolved photothermal spectroscopy. Methyl iodide also undergoes photochemical decomposition on a GaAs(llO) surface. " Photolysis of methyl iodide at 236 nm in the gas phase brings about liberation of iodine atoms with a quantum yield of 0.69. ... 

We shall consider in this review mainly the information to be obtained from ka on unimolecular dynamics. We shall in this context first review the ethane dissociation-methyl radical recombination system, to show what can be done for such a well-studied model reaction and what could eventually be done for other reactions as well. We shall then give a short summary for the most recent results on various classes of unimolecular reactions. We are afraid that this enumeration is not complete, and we apologize for any important omissions. Mechanistic and physical organic aspects, which have previously been extensively reviewed, will not be discussed. 

Table 4 Experimental and theoretical results on methyl radical recombination and ethane dissociation...

Tmax—bubble temperature on collapse T , and are solution temperature and pressure, respeetively, Py is pressure inside the bubble and y is heat capacity ratio of the gas inside the bubble. A theoretical temperature of about 12,700 K could be calculated by using y = 1.66 (ideal gas), T , = 298 K, P , = 2 atm, Py = 0.031 atm. Replacing y of an ideal gas by that of water (1.32), the temperature drops to 6150 K highlighting the importanee of the heat capacity ratio of the gas contained in the collapsing bubbles. Susliek and coworkers [46, 47] have used sonoluminescence spectra to calculate bubble temperatures in multibubble systems and found to be in the order of 1000-5000 K. Henglein and coworkers [48] have used methyl radical recombination method and determined the cavitation bubble temperatures to be in a... 

Wardlaw DM, Marcus RA (1986) Unimolecular reaction-rate theory for transition-states of any looseness. 3. Application to methyl radical recombination. J Phys Chem 90 5383-5393... 

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