Absolute measurement of the rate constant for isopropyl radical combination

Abstract

Absolute values of the rate constant for isopropyl radical combination have been determined over the temperature range 683-808°K. The value log kr (M~1 sec-1) = 9.5 ± 0.2 fits all the data, leading to the conclusion that the temperature dependence, if any, is slight. The compatibility of this value with other values determined either directly or from the reverse process via thermochemical data depends on the particular model chosen to represent the transition state since the temperatures of the various determinations differ. It has been found that

Predictive theory for the combination kinetics of two alkyl radicals

“…For clarity the experimental data prior to 1976 95,[114][115][116][117][118][119][120][121][122] are not plotted. Again, the dynamically corrected VRC-TST predictions are in quantitative agreement with the direct experimental observations from Shafir et al 113 Plots of the theoretical predictions and experimental measurements of the capture rate for the iso-propyl 96,103,119,120,[123][124][125][126] and tert-butyl 96,103,110,119,120,123,[127][128][129][130][131] self combination reactions are provided in Fig. 10 and 11.…”

Very low‐pressure pyrolysis (VLPP) of pentynes. III. Pent‐2‐yne. Heat of formation and resonance stabilization energy of the 3‐methylpropargyl radical

“…When the ion source energy was lowered to 10 eV for the examination of parent peaks only, the mass spectrum of this reaction mixture clearly indicated the presence of methyl radicals at rnle = 15 and molecular products at rnle = 66, 54, 52, and 2. The 3-methylpropargyl radicals from C4-CS bond breaking may subsequently abstract hydrogen atoms from the walls (inside or outside the reactor [9]) to yield but-2-yne (mle = 54), or they may also decompose to butatriene (mle = 52) according to the reaction C H~C E C C H~ -CHz=C=CCH3 -CH2=C=C=CHZ + H The mass-spectral observations a t mle = 52 and 54 were consistent with the existence of these molecular products.…”

Rate of decomposition of hexafluoroazomethane and the absolute rate of recombination of trifluoromethyl radical at higher temperatures

“…The external rotation around the figure axis was made active to allow this rotational mode to share in the random distribution of molecular energy. Four rocking modes of the CF3 fragments in C2F6 had to be replaced by unusually low-frequeney bending modes (15 cm-l) in order to yield an Arrhenius A factor for decomposition of C2FG which would correspond to the measured rate of recombination of the CF3 radical around room temperature [1-41 with the aid of the equilibrium constant and relation (13). The critical energy for the bond-breaking process E$ was set equal to -aEt for reaction (8), assuming zero activation energy for recombination E, at 0 K [S].…”

“…Unfortunately no experimental or group additivity value [14] of for hexafluoroazomethane could be found in the literature. The rate constants for recombination of CF; radicals were calculated using RRKM theory [19] for the unimolecular decomposition of hexafluoroethane making use of the thermodynamic relation (13) k r = K r , d x k d where K r , d is the equilibrium constant for reaction (8) and k d characterizes the reverse of reaction (8).…”

“…That the straight lines in Figure 2 have the common intercept 1.0 is indicative of the fact that CF; FL2~s. A plot of the massspectrometric intensity a t m/e = 119 versus flow rate of CzF6 out of the reactor gave straight lines. radicals do not undergo a homogeneous disproportionation reaction, unlike ethyl, isopropyl, and t-butyl radicals [13]. This was supported by the absence of any mass-spectrometric signals attributable to CFI, which means also that the reaction characterized by k , produces no CF4 in the wall catalyzed reaction, although the disproportionation reaction of CF, to CF2 and CF4 is exothermic by some 40 kcal/mol [14].…”