Methyl radical recombination rate constant

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... 

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) ... 

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... 

Contrary to the dissociation rate constant, the recombination rate constant exhibits a very high chain-length dependence where fee for the macroradical is lower than fee for the model radical. For instance, fee for styryl radical and SGI is 4.6 X 10 Lmol s at 120 °C, whereas a 10-fold decrease was observed for the polystyryl radical (5.3 x 10 Lmol" s at 120 "C). The same trend was found for acrylate/SGl sys-tem. In the case of poly(methyl methacrylate) (PMMA) macroradicals and SGI, the fee value decreased by 2 orders of magnitude compared to the model radical. This result was assigned to a strong penultimate effect which also increased the dissociation rate constant but to a lower extent. ... 

Figure B2.5.7 shows the absorption traces of the methyl radical absorption as a fiinction of tune. At the time resolution considered, the appearance of CFt is practically instantaneous. Subsequently, CFl disappears by recombination (equation B2.5.28). At temperatures below 1500 K, the equilibrium concentration of CFt is negligible compared witli (left-hand trace) the recombination is complete. At temperatures above 1500 K (right-hand trace) the equilibrium concentration of CFt is appreciable, and thus the teclmique allows the detennination of botli the equilibrium constant and the recombination rate [54, M]. This experiment resolved a famous controversy on the temperature dependence of the recombination rate of methyl radicals. Wliile standard RRKM theories [, ] predicted an increase of the high-pressure recombination rate coefficient /r (7) by a factor of 10-30 between 300 K and 1400 K, the statistical-adiabatic-chaunel model predicts a...

Ayscough56 also studied the recombination of trifluoromethyl radicals by the rotating sector technique and found a rate constant of 2.34 X 1013 (mol./cc.)-1sec.-1, which is close to that for methyl radical combination obtained in an analogous manner from acetone. 

Termination rate constants for alkyl and benzyl radicals in solution range between 109 and 1010 M 1 sec-1.85 These rates correspond quite closely to that calculated for a diffusion-controlled reaction, about 8 x 109 M x sec-1 for the common solvents at room temperature.86 Gas-phase rotating sector results are similar a newer method, however, shows that in the gas phase the rotating sector technique overestimates termination rates. Recombination is fastest for methyl radicals (1010,5 M-1 sec-1) and slower for others (-CF3, 109-7 at 146°C ... 

The kinetics of the bimolecular decay of poly(vinyl alcohol) (Ulanski et al. 1994) and poly(vinyl methyl ether) radicals (Janik et al. 2000b) have been studied in some detail (cf. Fig. 9.2). The OH radicals formed during the pulse generate on the (coil-shaped) polymer a non-random distribution of radicals. First, the radicals which are very close to one another recombine. The intrinsic bimolecular rate constant for such a process can be much faster than that of the decay of an equal concentration of randomly distributed low molecular weight radicals. As the number of close-by radicals decreases, the intrinsic rate constant drops, and the lifetime of the polymer radicals increases considerably. Now, the bimolecular decay of the polymer radicals becomes much slower than that of the corresponding low molecular weight radicals. While in the case of low molecular weight radicals the bimolecular rate constant is independent of the... 

Thus the raw data are given in the form AjaAb == Ry where R is an experimental quantity, A b is the specific rate constant for the recombination of methyl radicals, and Aja is the specific rate constant for the reaction in question. It then follows that Ek — J obs + where we have assumed Eb = 0 in making our calculations of... 

All values for rate constants k are calculated from the relation k = Rkst where R is an observed quantity and ks is the rate constant for the recombination of methyl radicals, llie value used here is from Gomer and Kistiakowsky (see Table XII.5, footnote e). The E listed is in reality E — HEb, and we have taken Eb = 0. For the reactions of CDs we have assumed that fcs = (mb/mb ) = 0.91fcB> or Jcb = 0.96. ... 

In the case of vitamin B12, eq 19 is well investigated and its rate constant k = 4 x 109 L/mohs for primary radicals.203 Comparison of this value with the rate constant for methyl radical dimerization of 1010 L/mohs191 indicates that eq 19 proceeds at diffusion-controlled rates. Newer approaches may provide greater accuracy in the measurement of rates of recombination of free radicals, providing a better understanding of hydrogen atom abstraction (eq 18).205... 

In RCs of Rb,capsulatus wild-type, mutation at amino acid residue TRP M250 was introduced by oligonucleotide-mediated mutagenesis, according to methods described in [4,5]. Quinone-titrations for determining the apparent dissociation constants in the mutant was performed as in [4]. Measurements of kg and the P Qi recombination rate k were done on RCs reconstituted with 2-methyl-1,4-naphthoquinone (NQ) in contrast to recombination studies on P H where the RCs were quinone-free. All experiments were performed on RCs in aqueous buffer/glycerol(60%) solution. Picosecond [10], nanosecond [6,7,11] and millisecond [12] transient absorption spectrometers were used for the measurements of kg, radical pair recombination parameters and k. . 

Recombination of alkyl radicals, as that of atoms, occurs practically without an activation energy. In the gas phase at a sufficiently high pressure the recombination of methyl radicals is bimolecular with the rate constant close to (l/4)Zo (where Zo is the frequency factor of bimolecular collisions, and the factor 1/4 reflects the probability of collisions of particles with the antiparallel orientation of s ins). The theoretical estimation of the constant at a collision diameter of 3.5-10 m agrees with the experimental value = 2-10 ° l/(mol-s) (300 K). This k value agrees with the estimation by the theory of absolute reaction rates under the assumption that the free rotation of methyl groups is retained in the transition state. In the liquid the recombination of meth)d radicals is bimolecular with the rate constant of difiiision collisions (see Qiapter 5). For example, in water 2k = 3.2-10 l/(mol s) (298 K). Ethyl radicals react with each other by two methods recombine and disproportionate... 

The bulky ter/-butyl substituent loses the free rotation of methyl groups during aminyl radical recombination. This results in the loss of entropy during the formation of the transition state and a decrease in the rate constant. Two isopropyl groups in the corresponding aminyl radical sterically hinder recombination and, therefore, disproportionation occurs. The transition state is rather compact in this case, and its formation is also accompanied by the entropy loss, which explains the low value of kj. 

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