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 kinetics and mechanism of the reaction of OH with furan have been theoretically studied. The potential energy surface for each possible pathway has been investigated by employing DFT, G3MP2, and CCSD methods. The potential energy surface consists of one hydrogen-bonded complex and two energized intermediates. Three different pathways are suggested to be possible for the title reaction. The most probable channel is the hydroxyl radical addition to the C(2) position on the furan ring to cause the ring-opening process. The two other pathways are hydrogen abstraction from one of the C(2) or C(3) position on furan and hydroxyl radical substitution at the C(2) or C(3) position on furan. Abstraction and substitution channels are minor paths at low temperature, but they become comparable with addition channels at high temperature. Addition and substitution reactions proceed via formation of two energized intermediates, Int(1) and Int(2). Multichannel RRKM-TST calculations have been carried out to calculate the individual and overall rate constants for addition and substitution reactions. Direct-dynamics canonical variational transition-state theory calculations with small curvature approximation for tunneling were carried out to study hydrogen abstraction pathways.

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 .

The kinetics and mechanism of the reaction between amidogen radical and hydroxyl radical have been theoretically investigated on the lowest singlet and triplet surfaces. The singlet surface consists of two long-lived chemically activated NH(2)OH* and NH(3)O* intermediates with 10 different channels. A hydrogen abstraction channel on the triplet surface proceeds through van der Waals complex in both reactant side and product side to produce NH(3) + O((3)P). The effect of multiple reflections of the van der Waals complexes on the rate constant is investigated. Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for the formation of those products that proceed through activated adducts on the singlet surface. The rate constants for direct hydrogen abstraction reactions were calculated by using direct-dynamics canonical variational transition-state theory with small curvature approximation for tunneling.

Hydrogen abstraction reactions of methyl radicals or hydroxyl radicals with hydrogen sulfide are studied over the temperature range of 200−3000 K from a theoretical point of view. Potential energy surfaces are explored at the MP2/6-311++G(d,p) level. Values of 17.5 and 4.2 kJ mol-1 were found for the barrier height of reaction CH3 + H2S at the MP4 = full/6-311++G(3df,3pd) level and of reaction OH + H2S at the QCISD = full/aug-cc-pvtz level, respectively. Rate constants of the two reactions are calculated according to generalized transition-state theory and also canonical variational transition-state theory (CVTST). According to generalized transition-state theory, both reactions showed non-Arrhenius behavior at lower and higher temperatures. The tunneling factors for both reactions are calculated at different temperatures. Characteristic tunneling temperature for reactions CH3 + H2S and OH + H2S were found to equal 277 and 340 K, respectively. The full width of the barrier at half its height (Δs 1/2) were found to equal 0.37 and 0.14 Å for reaction of hydrogen sulfide with CH3 or OH, respectively. According to CVTST, we have found the Arrhenius parameters for the reaction of CH3 + H2S, k 1 = 6.8 × 104 T 1.2 exp(−6.0 kJ mol-1/(RT)) L mol-1 s-1, and for the reaction of OH + H2S, k 2 = 9.7 × 109 exp(−4.5 kJ mol-1/(RT)) L mol-1 s-1.