An absolute measurement of the rate constant for t-butyl radical combination

The rate constant for tert-butyl radical recombination has been measured near 700°K by the very-low-pressure pyrolysis (VLPP) technique and was found to be 10^8.8±0.3 M^?1·sec^?1 with neglibible temperature dependence. The thermochemical parameters for tert? butyl radicals were varied within reasonable limits to bring into agreement the data for the decomposition of 2,2,3,3-tetramethyl butane and the recombination of tert-butyl radicals. The revised thermochemistry also makes the gas-phase results and liquid-phase results compatible.     

An indirect measurement of the absolute rate constant of the self-reaction of tert-butoxy radicals

No reliable rate constant is available for the self-reaction of tert-;butoxy radicals. We have set up a competition between hydrogen abstraction and self-reaction of tert-butoxy radicals in a flash photolysis electron spin resonance study to extract this information. Experimental values of hydrogen abstraction product radical concentrations under various hydrogen donor concentrations were then compared with theoretically calculated values with different values of 2k₄ to obtain the best fit. Hydrogen donors such as cyclopentane, anisole, methyl tert-butyl ether, and methanol were chosen for the study. A value of (1.3 ± 0.5) × 10⁹M^?1 sec^?1 for the rate constant of the self-reaction of tert-butoxy radicals has been determined at 293°K.     

Study on improving the turbidity measurement of the absolute coagulation rate constant

The existing theories dealing with the evaluation of the absolute coagulation rate constant by turbidity measurement were experimentally tested for different particle-sized (radius = a) suspensions at incident wavelengths (lambda) ranging from near-infrared to ultraviolet light. When the size parameter alpha = 2pi a/lambda > 3, the rate constant data from previous theories for fixed-sized particles show significant inconsistencies at different light wavelengths. We attribute this problem to the imperfection of these theories in describing the light scattering from doublets through their evaluation of the extinction cross section. The evaluations of the rate constants by all previous theories become untenable as the size parameter increases and therefore hampers the applicable range of the turbidity measurement. By using the T-matrix method, we present a robust solution for evaluating the extinction cross section of doublets formed in the aggregation. Our experiments show that this new approach is effective in extending the applicability range of the turbidity methodology and increasing measurement accuracy.     

The hydroxyl radical reaction rate constant and products of ethyl 3-ethoxypropionate

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH₃CH₂(SINGLE BOND)O(SINGLE BOND)CH₂CH₂C(O)O(SINGLE BOND)CH₂CH₃). EEP reacts with OH with a bimolecular rate constant of (22.9±7.4)×10⁻¹² cm³ molecule⁻¹s⁻¹ at 297±3 K and 1 atmosphere total pressure. In order to more clearly define EEP's atmospheric reaction mechanism, an investigation into the OH+EEP reaction products was also conducted. The OH+EEP reaction products and yields observed were: ethyl glyoxate (EG, 25±1% HC((DOUBLE BOND)O)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl (2-formyl) acetate (EFA, 4.86±0.2%, HC((DOUBLE BOND)O)(SINGLE BOND)CH₂(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl (3-formyloxy) propionate (EFP, 30±1%, HC((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₂(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl formate (EF, 37±1%, HC((DOUBLE BOND)O)O(SINGLE BOND)CH₂CH₃), and acetaldehyde (4.9±0.2%, HC((DOUBLE BOND)O)CH₃). Neither the EEP's OH rate constant nor the OH/EEP reaction products have been previously reported. The products' formation pathways are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry. © 1997 John Wiley & Sons, Inc.     

The methyl radical combination rate constant as determined by kinetic spectroscopy

The rate constant for the reaction has been determined by means of vacuum ultraviolet flash photolysis and time-resolved kinetic spectroscopic observations of the 1504-Å absorption band of CH₃. The measurements made using three different sources of methyl radicals (azomethane, dimethylmercury, and ketene-hydrogen) were in accord and yielded a value for the rate constant of k₁ = (9.53 ± 1.17) × 10^?11 cc molec^?1 sec^?1. A detailed error analysis is presented. The f-value for the 1504-Å band of CH₃ is determined to be (2.5 ± 0.7) × 10^?2.     

Termination rate constant in radical polymerization

On the basis of the assumption that the primary radical immerses into a polymeric radical, the rate constant of primary radical termination was derived. By applying and developing further the same treatment, the rate constant of chain termination was derived. This rate constant was experimentally confirmed. It was found that the rate constant of chain termination for the polymerization of some methacrylates depends on the distance of translation of radical chain end per collision, solvent viscosity, and the Taft polar constant for ester group.     

Absolute rate constant for the reaction of Phenyl radical with Acetylene

The absolute rate constant for the reaction of phenyl radical with acetylene has been measured at 20 torr total pressure in the temperature range of 297 to 523 K using the cavity-ring-down technique. These new kinetic data could be quantitatively correlated with the data obtained earlier with a relative rate method under low-pressure (10^?3–10^?2 torr) and high-temperature (1000–1330 K) conditions. These kinetic data were analyzed in terms of the RRKM theory employing the thermochemical and molecular structure data computed with the BAC-MP4 technique. The calculated results reveal that the total rate constant for the C₆H₅ + C₂H₂ reaction (k_t) is pressure-independent, whereas those for the formation of C₆H₅C₂H (k_b) and the C₆H₅C₂H₂ adduct (k_c) are strongly pressure-dependent. A least-squares analysis of the calculated values for 300–2000 K at the atmospheric pressure of N₂ or Ar can be given by and all in units of cm³/s. The latter equation effectively represents the two sets of experimental data. © 1994 John Wiley & Sons, Inc.     

Determination of absolute rate constants for free radical polymerization of ethyl α-fluoroacrylate and characterization of the polymer

The absolute rate constants for propagation (k_p) and for termination (k_t) of ethyl α-fluoroacrylate (EFA) were determined by means of the rotating sector method; k_p = 1120 and k_t = 4.8 × 10⁸ L/mol.s at 30°C. The monomer reactivity ratios for the copolymerizations with various monomers were obtained. By combining the k_p values for EFA from the present study and those for common monomers with the monomer reactivity ratios, the absolute values of the rate constants for cross-propagations were also evaluated. Reactivities of EFA and poly(EFA) radical, being compared with those of methyl acrylate and its polymer radical, were found to be little affected by the α-fluoro substitution. Poly(EFA) prepared with the radical initiator was characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Although the glass transition temperature obtained by DSC for poly(EFA) resembled that of poly(ethyl α-chloroacrylate), its TGA thermogram showed fast chain de polymerization to EFA that was distinct from complicated degradation of poly(ethyl α-chloroacrylate).     

Determination of the rate constant for ring opening of an alpha-cyclopropylvinyl radical

The rate constant for ring opening of the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4, generated by photolysis of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride method and was found to be (1.6+/-0.2)x10(10) s-1, one order of magnitude smaller than the rate constant for rearrangement of the trans-2-phenylcyclopropylcarbinyl radical.     

An induction period in the pyrolysis of ethane: Determination of individual rate constants for ethyl radical reactions

An induction period, caused by the initial increase in radical concentrations, has been observed in the flow pyrolysis of ethane at 903 K and 13.4 kN m^?2. The data enable calculation of the rate constants for three reactions, including a value of (1.1±0.4) × 10¹⁰ 1mol^?1 s^?1 for the recombination of ethyl radicals.     

Primary radical termination rate constant at high conversion in radical polymerization

A primary radical termination rate constant given by: k_ti = A_1iD_i, where A_1i is a constant and D_i is the diffusion constant of the primary radical, was examined on the basis of the variation of conversion. It was proved that this rate constant is correct at high conversion. A relationship between primary radical termination rate constant and conversion was derived. The effect of variation of conversion on the gel effect is discussed.     

Determination of absolute rate constants of addition of ethyl radicals to olefins

Theoretical and Experimental Chemistry -     

Diffusion-controlled rate constant of termination reaction in radical polymerization

Smoluchowski's theory has been modified and the improved theory was applied to diffusion-controlled polymerization. This application proved that the rate-controlling process is not transrational diffusion but the segmental diffusion. The segmental diffusion-controlled rate constant was derived by the collision theory. This rate constant explains the experimental fact that the diffusion-controlled rate constant of bimolecular termination in radical polymerization of alkyl methacrylate is inversely proportional to solution viscosity and independent of the molecular weight of the polymeric free radical.     

Yet another direct measurement of the rate constant for the recombination of methyl radicals

The decaying absorption of CH₃ radicals at 216.4 nm has been followed over more than three half-lives using a photoelectric split-beam kinetic spectrometer. The rate constant for recombination k_r was found to be (5.60 ± 0.76) × 10^?11 cm³/molecule·s.     

Effect of medium on the rate constant of decarbonylation of phenylacetyl radical

The rate constant k _CO of decarbonylation of phenylacetyl radicals generated by photolysis of dibenzyl ketone was measured by laser flash photolysis technique in six solvents in a wide temperature range. The pre-exponential factors A and activation energies E _a of decarbonylation were found for all solvents. The k _CO value decreases with an increase in the dielectric constant of the solvent, whereas an increase in the ability of the solvent for hydrogen bonding increases k _CO. The results of quantum-chemical calculations confirm the mutual compensation of the contributions of specific and nonspecific solvations to the activation energy of decarbonylation in alcohols.     

A spectroscopic determination of the methyl radical recombination rate constant in shock waves

The reactions CH₃ +

and CD₃ +

have been studied in shoch waves at 1200–1500 K and densities of 2 × 10^?6 ?2 × 10^?4 mol cm^?3 using UV absorption near 216 nm. The rate constants at the highest densities: k_H = (1.7 ± 0.6) × 10^?11 cm³ s^?1 and k_D = (2.2 ± 0.9) × 10^?11 cm³ s^?1 are close to the second order limit. At the lowest densities the rates are lower by a factor of 5. The experimental results agree well with theoretical predictions based on the statistical adiabatic channel model but differ from those of conventional RRKM calculations. A direct observation of the equilibrium C₂H₆ ? 2CH₃ favours the “high” value for ΔH⁰₀ (87.76 kcal/mol).     

Time dependence of the rate constant in the free radical oxidation in solid phase

ESR studies of oxidation kinetics of ethyl and dimethylpentyl radicals have been performed in glassy deuterium-containing alcohols, toluene-d₈ and in glassy 2,4-dimethylpentane-h₁₆, respectively. The kinetics is interpreted by assuming time-dependent reaction rate constant.

---

, -d₈ 2,4--h₁₆. .

     

Relationship between chain termination rate constant and conversion in radical polymerization

At low and high conversions, the chain termination rate constant for bimolecular termination between polymeric radicals given by k_t = A_tD_s, where A_t is a constant and D_s is the diffusion constant of radical chain end, is completely correct. This termination rate constant does not depend on solution viscosity, but conversion.