The kinetics and mechanism of reaction of a hydroperoxyl radical (HO2) with a hydrogen atom on both singlet and triplet surfaces were studied by employing DFT, CCSD, and G3 methods along with the Aug-cc-pVTZ basis set. MC-SCF and CCD methods were used to explore potential energy surfaces. Major end products from different channels were H2O+O, H2+O2, and OH. Formation of chemically activated hydrogen peroxide HOOH was the most exothermic path in this system that dissociates to the ground state OH(2Π) radicals. Another energized transient species was water oxide H2OO, which has local minimum on the singlet potential-energy surface. The energized water oxide rapidly isomerized to hydrogen peroxide HOOH or dissociated to H2O + O(1D). Transition state theory and RRKM theory were used to calculate the rate constants for different channels.

The thermal decomposition of methyltrichlorosilane (MTS) was studied in a flow system in the temperature range of 825-977 K and pressure range of 10-120 Torr. Yields of products were measured by gas chromatography. The rate constant, k 1 , for the initiation reaction was determined from the sum of the rates of the termination reactions. The Arrhenius expression for this reaction at the high-pressure limit was obtained from a nonlinear least-squares fit to the experimental data using the Troe factorization method, k 1∞ ) (9.6 ( 2.5) × 10 19 exp (-(392 ( 18) kJ mol -1 /RT) s -1 . The rate constants for hydrogen abstraction, k 2 , and chlorine abstraction, k 3 , from MTS by methyl radicals were also calculated on the basis of experimental measurements. The Arrhenius expression for hydrogen abstraction was k 2 ) (5.1 ( 0.4) × 10 8 exp(-( 61( 3) kJ mol -1 /RT) L mol -1 s -1 and for chlorine abstraction was k 3 ) (1.5 ( 0.5) × 10 9 exp(-(72 ( 6) kJ mol -1 /RT) L mol -1 s -1 .

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Theoretical kinetic studies are performed on the multichannel thermal decomposition of acetaldehyde. The geometries of the stationary points on the potential energy surface of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are obtained by single point energy calculations at the CCSD(T,full)/augh-cc-pVTZ+2df, CBS-Q and G4 levels of theory. Here, by application of steady-state approximation to the thermally activated species CHCHO* and CHCHOH* and performance of statistical mechanical manipulations, expressions for the rate constants for different product channels are derived. Special attempts are made to compute accurate energy-specific rate coefficients for different channels by using semiclassical transition state theory. It is found that the isomerization of CHCHO to the enol-form CHCHOH plays a significant role in the unimolecular reaction of CHCHO. The possible products of the reaction are formed via unimolecular decomposition of CHCHO and CHCHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CHCHOH due to the low barrier height for CHCHO → CHCHOH isomerization process. However, at high temperatures, the product channel CH + CHO becomes dominant.

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