Methyl and trifluoromethyl radical attack on methyl formate: isotope effects and reactivities at different sites

Abstract

Isotopic labelling has been employed to determine the position and extent of hydrogen abstraction from the formyl and methoxyl sites in methyl formate by methyl and trifluoromethyl radicals. Attack is principally at the formyl group, but at 182" a significant proportion of abstraction occurs from the methoxyl ; 15 % for attack by CH3 and 25 % for attack by CF3. Values for rate constants (at 182") and for Arrhenius parameters are as follows : reaction pre-exponential velocity log10 log10 activation factor constant at 182OC (A/cn$ mole-1 sec-1) (k/cm3 mole-1 sec-1) energy CH3+HC02CH3 +CH4+[CzOzH3]*

Variational transition state theory rate constants and H/D kinetic isotope effects for CH3 + CH3OCOH reactions

“…Comparison of calculated values k H / k D for R1a, R1b, and R1c (Table ) show clearly the importance to consider the multidimensional tunneling corrections in order to achieve accurate values of this molecular property. At 455 K, the ratio k H / k D computed with CVT/SCT method is 6.7, which is in excellent agreement with 6.2 measured by Arthur and Gray . The values of k H / k D for R2 are smaller or close to one because they correspond to a secondary isotope effect…”

“…CVT/μOMT rate constants for the temperature range of 250–2000 K are also in reasonable agreement with previous theoretical results, and in particular, for the temperature range of 382–500 K in the present results, CVT/μOMT rate constants are in excellent agreement with the experiment. There are few experimental results for the k H / k D and in the case of the isotopomer reactions CH 3 + CH 3 OCOH → CH 4 + CH 3 OCO and CH 3 + CH 3 OCOD → CH 3 D + CH 3 OCO, the value of KIE is 6.7 ( T = 455 K), which is in good agreement with the experimental value of 6.2, and is an indication the electronic and vibrationally adiabatic PESs are well described by the MP2/aug‐cc‐pVDZ method. Finally, improved values of thermal rate constants of R1 and R2 calculated in function of temperature would be useful in modeling the submechanisms of oxidation of alkyl esters which are simple models of biofuels compounds.…”

“…Table gives calculated global (overall) ( k 1 + k 2 ) rate constants experimental, previous theoretical studies, and calculations performed with the Chemkin code for the temperature range of 250–2000 K, and Figure shows corresponding Arrhenius plots of rate constants. At the temperature of 455 K, the CVT/μOMT rate constant of 4.8 × 10 −18 cm 3 /molecule / s is in excellent agreement with the experimental value of 5.6 × 10 −18 cm 3 /molecule / s measured by Arthur and Gray …”

“…Table gives the values of activation energies for all isotopomers computed as local two‐point slopes of Arrhenius for R1 and R2 paths. There are few experimental measurements of the kinetics for comparison and consequently for the estimation of activation energy. In general, there is a smooth reducing of the activation energies in CVT, CVT/ZCT, and CVT/SCT tunneling calculations.…”

“…There are few experimental studies of the above elementary reactions including photolysis measurements in the small range of temperatures 393–523 K, and at the 455 K, the overall rate constants are equal, respectively, to 5.6 × 10 −18 and 8.3 × 10 −18 cm 3 /molecule/s . These studies also show that the hydrogen abstraction from alkyl formates occurs predominantly at the formyl hydrogen with a ratio R1/R2 ~ 6 in relation to the methyl site.…”