Combination Of Methyl Radicals Research Articles

What Do We Know About the Iconic System CH3 + CH3 + M ↔ C2H6 + M?

Abstract A comparison of representative experimental data [296<T/K<1920; 103<P/atm<101] and “theory” reveals some of the strengths and weaknesses of current understanding of the title reaction. The available data has been reconciled with a theoretically derived value of the high-pressure rate constant for methyl radical combination using a hindered-Gorin transition state using the Multiwell suite of codes.

Methylation of 2'-deoxyguanosine by a free radical mechanism.

The mechanistic aspects of the methylation of guanine in DNA initiated by methyl radicals that are derived from the metabolic oxidation of some chemical carcinogens remain poorly understood. In this work, we investigated the kinetics and the formation of methylated guanine products by two methods: (i) the combination of *CH3 radicals and guanine neutral radicals, G(-H)*, and (ii) the direct addition of *CH3 radicals to guanine bases. The simultaneous generation of *CH3 and dG(-H)* radicals was triggered by the competitive one-electron oxidation of dimethyl sulfoxide (DMSO) and 2'-deoxyguanosine (dG) by photochemically generated sulfate radicals in deoxygenated aqueous buffer solutions (pH 7.5). The photolysis of methylcob(III)alamin to form *CH3 radicals was used to investigate the direct addition of these radicals to guanine bases. The major end products of the radical combination reactions are the 8-methyl-dG and N2-methyl-dG products formed in a ratio of 1:0.7. In contrast, the methylation of dG by *CH3 radicals generates mostly the 8-methyl-dG adduct and only minor quantities of N2-methyl-dG (1:0.13 ratio). The methylation of the self-complementary 5'-d(AACGCGAATTCGCGTT) duplexes was achieved by the selective oxidation of the guanines with carbonate radical anions in the presence of DMSO as the precursor of *CH3 radicals. The methyl-G lesions formed were excised by the enzymatic digestion and identified by LC-MS/MS methods using uniformly 15N-labeled 8-methyl-dG and N2-methyl-dG adducts as internal standards. The ratios of 8-methyl-G/N2-methyl-G lesions derived from the combination of methyl radicals with G(-H)* radicals positioned in double-stranded DNA or that with the free nucleoside dG(-H)* radicals were found to be similar. Utilizing the photochemical method and dipropyl or dibutyl sulfoxides as sources of alkyl radicals, the corresponding 8-alkyl-dG and N2-alkyl-dG adducts were also generated in ratios similar to those obtained with DMSO.

Pressure Effect on CH3 and C2H3 Cross-Radical Reactions

The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH3COC2H3) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C3H6), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C2H6), the methyl radical combination product. At 27 kPa [C3H6]/[C2H6] = 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to about 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C2H3 + CH3 + M → C3H6 + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C2H3 + CH3 → C3H5 + H, becomes available, producing a host of allyl radical reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C3H6. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C−C bond as contrasted to the weaker C−C bond formed from combination of saturated hydrocarbon radicals. This C−C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C−H bond or isomerize to cyclopropane.

Etching of GaAs (100) with gaseous H/CH3 mixtures

AbstractGaAs (100) wafers were etched in mixtures of hydrogen atoms and methyl radicals. The atoms were formed in a remote hydrogen plasma, and a fraction of these were converted into methyl radicals by introducing methane into the flow system upstream from the semiconductor surface. The flux of hydrogen atoms into the reaction chamber was determined by isothermal calorimetry. The methyl radical flux passing over the substrate was then calculated using previously determined rate parameters for the reaction between atomic hydrogen and methane, and a simple modeling program. The GaAs etch rates were about an order of magnitude faster when methyl radicals were present in the hydrogen atom stream, and were found to follow a first‐order dependence on the partial pressure of methyl radicals. Absolute rate constants were determined and an Arrhenius activation energy of 1.2 kcal mol−1 was calculated. The values of k and Ea are consistent with a diffusion‐controlled process. SEM photographs of the surface revealed small crystallographic features that made the surface appear very rough. XPS analysis indicated that these surfaces were arsenic deficient. A mechanism is proposed for the etching of GaAs by a combination of methyl radicals and hydrogen atoms. © 1994 John Wiley & Sons, Inc.

ChemInform Abstract: REACTION OF METHYL RADICALS WITH HALOALKANES

The kinetics of reactions involving halogen atom abstraction from haloalkanes by methyl radicals have been studied in the gas phase. Arrhenius parameters for halogen atom transfer were determined relative to those for methyl radical combination: Text Table Text. RX log10A2(L/mol · s) E2(kcal/mol) CFCl3 8.3 ± 0.2 10.7 ± 0.4 CF3CCl3 7.9 ± 0.3 9.7 ± 0.6 CF2Cl2 9.1 ± 0.4 11.3 ± 0.7 CF3Cl 8.8 ± 0.5 11.8 ± 1.0 CF3CF2Cl 8.3 ± 0.3 10.9 ± 0.7 CF3Br 8.6 ± 0.2 9.3 ± 0.5 CF3I 8.1 ± 0.1 4.3 ± 0.2 CH3CH2I 8.9 ± 0.3 7.4 ± 0.6 The rate data obtained are used to provide information on the importance of polar effects for halogen abstraction processes.

Reaction of methyl radicals with haloalkanes

AbstractThe kinetics of reactions involving halogen atom abstraction from haloalkanes by methyl radicals have been studied in the gas phase. Arrhenius parameters for halogen atom transfer were determined relative to those for methyl radical combination: equation image RX log10A2(L/mol · s) E2(kcal/mol) CFCl3 8.3 ± 0.2 10.7 ± 0.4 CF3CCl3 7.9 ± 0.3 9.7 ± 0.6 CF2Cl2 9.1 ± 0.4 11.3 ± 0.7 CF3Cl 8.8 ± 0.5 11.8 ± 1.0 CF3CF2Cl 8.3 ± 0.3 10.9 ± 0.7 CF3Br 8.6 ± 0.2 9.3 ± 0.5 CF3I 8.1 ± 0.1 4.3 ± 0.2 CH3CH2I 8.9 ± 0.3 7.4 ± 0.6 The rate data obtained are used to provide information on the importance of polar effects for halogen abstraction processes.

ChemInform Abstract: RATE CONSTANTS OF THE COMBINATION OF METHYL RADICALS WITH NITRIC OXIDE AND OXYGEN

Rate constants for the combination of methyl radicals with NO and O2 have been measured by flash photolysis of azomethane coupled with product analysis by gas chromatography. Values of the rate constants have been obtained over the pressure region from 50 to 700 torr with He, N2, and Ar as quenching molecules. The high-pressure limits were obtained through an RRKM model calculation and were found to be The rate constants were measured relative to the methyl combination reaction k1 with k1 = 9.5 × 10−11 cm3/molec · sec. The RRKM model suggests D0(CH3O2) = 32 ± 3 kcal/mole.

Rate constants of the combination of methyl radicals with nitric oxide and oxygen

AbstractRate constants for the combination of methyl radicals with NO and O2 have been measured by flash photolysis of azomethane coupled with product analysis by gas chromatography. Values of the rate constants have been obtained over the pressure region from 50 to 700 torr with He, N2, and Ar as quenching molecules.The high‐pressure limits were obtained through an RRKM model calculation and were found to be The rate constants were measured relative to the methyl combination reaction k1 with k1 = 9.5 × 10−11 cm3/molec · sec. The RRKM model suggests D0(CH3O2) = 32 ± 3 kcal/mole.

The methyl radical combination rate constant as determined by kinetic spectroscopy

AbstractThe 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.

High temperature decomposition of ethane

AbstractMeasurements of the rate of formation of methane in the thermal decomposition of ethane in dilute mixtures with argon were made by the shock tube technique. Derived values of the rate constant of the dissociation reaction are compared with earlier data of the same type and with recent shock tube data on the combination of methyl radicals. An RRKM calculation correlating all the data is described, from which an Arrhenius equation for the range 1000–1500°K, log k∞ = 16.9 − 89,500/2.3RT, is obtained.

Kinetics of the Mercury-Photosensitized Decomposition of Neopentane. Part II. Reactions of the Methyl and Neopentyl Radicals

The results of Part I are further analyzed with reference to certain of the elementary free-radical processes occurring. A fall-off in the methyl radical combination is observed at low pressures. Comparison of this process with the CH3 + neopentane abstraction yields for the latter an activation energy of 11.5 kcal/mol and a frequency factor of 4.9 × 1011 cc mol−1 s−1. The relative importance of CH3 + neopentyl and neopentyl + neopentyl is compared. The decomposition of the neopentyl radical into i-C4H8 + CH3 shows a fall-off at low pressures; the limiting activation energy at high pressures is 29.0 kcal/mol, while that at low pressure is 17.1 kcal/mol. The former value leads to 6.7 kcal/mol for the heat of formation of the neopentyl radical at 25 °C, to 21.3 kcal/mol for the heat of its dissociation into i-C4H8 + CH3, and to 98.5 kcal/mol for the heat of dissociation of neopentane into neopentyl + H. Entropy values are also calculated in an approximate manner.

Unimolecular isomerization of chemically-activated methylcyclopropane formed by cyclopropyl plus methyl radical combination

Unimolecular isomerization of chemically-activated methylcyclopropane formed by cyclopropyl plus methyl radical combination

Spectroscopic measurement of the rate of the gas-phase combination of methyl radicals with nitric oxide and oxygen at 295 K

The rates of combination of CH3 with NO and O2 were measured by flash spectroscopy at 295 K. The high-pressure rate coefficient for CH3+ NO(+M)→CH3NO(+M) was recorded as 1.0(±0.15)× 1013 cm3 mol–1 s–1. The low-pressure fall-off was interpreted with the Rice-Marcus model for D0(CH3—NO)= 46.8 kcal mol–1. The high pressure rate coefficient for CH3+ O2(+M)→CH3O2(+M) was deduced to be approximately 1.1 × 1012 cm3 mol–1 s–1.

The Effect of Added Gases on the Dimethylketazine Photolysis

AbstractThe role of intramolecular and radical reactions in the vapor phase photolysis of dimethylketazine has been evaluated on the basis of the behavior of photolyses carried out in presence of added nitrogen, n‐butane, cis‐butene‐2, butadiene‐1,3 and oxygen. These gases deactivate ketazine decomposition in both the long wavelength absorption region (π* ← n) and the middle region (probably σ* ← π) but are much more efficient at the longer wavelengths. The ability to suppress CH3CN formation is roughly proportional to their molecular complexity, but deactivation of CH3N  C(CH3)2 (dimethylketimine) and C2H6, the other important products, is highly dependent on the quencher. Butadiene and oxygen strongly inhibit the formation of imine presumably by a reaction with an excited imine molecule produced on intramolecular decomposition of a singlet excited state of ketazine. The C2H6 production is also reduced to a very large extent by these two quenchers, but the most likely explanation involves suppression of methyl radical combination initiated by radicals produced directly in the primary step dissociation. The importance of this direct primary dissociation is either enhanced as the energy of the absorbed quantum is increased, or perhaps other processes are responsible for the relatively greater importance of ethane at short wavelengths. This proposal is in sharp conflict with that postulated by Ogilvie and Horne [2] for the photolysis of formaldazine and by analogy for acetaldazine and ketazine.

Pyrolysis of Azomethane and Combination of Methyl Radicals in Reflected Shock Waves

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A Quantitative study of alkyl radical reactions by kinetic spectroscopy. Part I. Mutual combination of methyl radicals and combination of methyl radicals with nitric oxide

AbstractCombination reactions of the methyl radical have been studied by following the decay of the absorbance of the methyl radical during the course of the reaction by means of kinetic spectroscopy. The limiting values of the second‐order rate constants at high pressure were determined for two reactions at room temperature: The extinction coefficient of the methyl radical was found to have a maximum value of (1.02 ± 0.06) X 104 1 mole−1 cm−1 at 216.4 nm. Integration of the extinction coefficient over the absorption band of the methyl radical gave an oscillator strength of 1.0 X 10−2.

Pressure dependence of methyl radical combination reaction. Part II. Effect of temperature

The study of the pressure-dependence of the methyl radical combination in the gas phase has been extended. The change of the cross-combination ratio of the methyl and isopropyl radicals with pressure has been followed. The results obtained at 94·5°(photolysis of isopropyl methyl ketone) and at 146·5°(pyrolysis of di-t-butyl peroxide in the presence of isobutyraldehyde) are in agreement with previous results obtained by the same method. A discussion is given of the remarkable differences between our results and those obtained by relating the combination to hydrogen abstraction. The data obtained and the most reliable results at high temperature are tested against the Rice–Ramsperger–Kassel–Marcus theory assuming a nearly loose model for the activated complex. The theory reasonably fits the medium fall-off pressure over a temperature range of 600°.

Pyrolysis of biacetyl and the third-body effect on the combination of methyl radicals

Pyrolysis of biacetyl and the third-body effect on the combination of methyl radicals

Cage reactions. II. Combination of methyl radicals and of methyl and trifluoromethyl radicals into 1,1,1, trifluoroethane

Cage reactions. II. Combination of methyl radicals and of methyl and trifluoromethyl radicals into 1,1,1, trifluoroethane

Methyl radical reactions with isopropanol and methanol, their ethers and their deuterated derivatives

The reactions of methyl radicals with three isotopically different isopropanols three isotopically different methanols and the undeuterated ethers (diisopropyl and dimethyl ethers) have been investigated. The photolysis of acetone or acetone-d6 has been used to generate methyl radicals or trideuteromethyl radicals. An isotopic exchange reaction between acetone and alcohols makes it necessary to work with isotopically compatible pairs. Values for Arrhenius parameters, based on Shepp's rate-constant for the combination of methyl radicals, are as follows : [graphic omitted] Primary isotope effects in the alkyl groups of these alcohols are larger than expected solely on the basis of zero-point-energy difference : at 150°, (kCH/kCD) is ca. 8 for methanol and ca. 6.5 for isopropanol. These values, which resemble those for ethanol and for isobutane, taken together with the corresponding activation-energy differences, suggest that tunnelling may again be important. A secondary reaction, disproportionation between the radicals · C(CH3)2OH and CH3, is important in attack on isopropanol. It gives rise to an apparent enhancement of reactivity of the hydroxyl groups, to a spurious secondary isotope effect and, unless allowance is made, to a low value for the A-factor. The corresponding reaction does not appear to be significant in methanol or ethanol systems. In dimethyl ether, the reactivity of each methyl group is the same as in methanol but in diisopropyl ether there is steric hindrance and the alpha hydrogen atoms are less accessible than in the alcohol.