Search

The reaction rates of O((super 3)P) + CH4 -> OH + CH3 and O((super 3)P) + CD4 -> OD + CD3 were investigated over a range of temperatures. The dual-level dynamical calculations of the reaction rates were based on variational state theory in curvilinear coordinates with transmission coefficients determined by the microcanonical optimized multidimensional tunneling approximation. Results were found to be consistent with experimental rate constants.

Deuterium and carbon-13 kinetic isotope effects for the reaction Cl + CH4 were investigated using dual-level direct dynamics calculations with the MORATE computer program. Low-level measurements of the potential energy surface were performed at the NDDO-SRP level using two different sets of specific reaction parameters. The dual-level calculations were conducted using the ICL-Eckart improved interpolated corrections model. Forward and reverse reaction rates and kinetic isotope effects were calculated using variational transition state theories.

The reaction of methanol (CH 3 OH) with atomic nitrogen was studied considering three elementary reactions, the hydrogen abstractions from the hydroxyl or methyl groups (R1 and R3, respectively) and the C-O bond break (R2). Thermochemical properties were obtained using ab initio methods and density functional theory approximations with aug-cc-pVXZ (X = T and Q) basis sets. The minimum energy path was built with a dual-level methodology using the BB1K functional as the low-level and the CCSD(T) as the high-level. This surface was used to calculate the thermal rate constants in the frame of variational transitional state theory considering the tunneling effects. Our results indicate the dehydrogenation of the methyl group (R3) as the dominant path with k R3 = 7.5 × 10 -27 cm 3 ·molecule -1 ·s -1 at 300 K. The thermal rate constants were fitted to a modified Arrhenius equation for use in mechanism studies of the methanol decomposition.

The reaction system formed by the methanethiol molecule (CH 3 SH) and a hydrogen atom was studied via three elementary reactions, two hydrogen abstractions and the C-S bond cleavage (CH 3 SH + H → CH 3 S + H 2 (R1); → CH 2 SH + H 2 (R2); → CH 3 + H 2 S (R3)). The stable structures were optimized with various methodologies of the density functional theory and the MP2 method. Two minimum energy paths for each elementary reaction were built using the BB1K and MP2 methodologies, and the electronic properties on the reactants, products, and saddle points were improved with coupled cluster theory with single, double, and connected triple excitations (CCSD(T)) calculations. The sensitivity of coupling the low and high-level methods to calculate the thermochemical and rate constants were analyzed. The thermal rate constants were obtained by means of the improved canonical variational theory (ICVT) and the tunneling corrections were included with the small curvature tunneling (SCT) approach. Our results are in agreement with the previous experimental measurements and the calculated branching ratio for R1:R2:R3 is equal to 0.96:0:0.04, with k R1 = 9.64 × 10 -13 cm 3 molecule -1 s -1 at 298 K.

Thermochemical and kinetics properties of the hydrogen abstraction from the hydrazine molecule (N2H4) by an oxygen atom were computed using high-level ab initio methods and the M06-2X DFT functional with aug-cc-pVXZ (X = T, Q) and maug-cc-pVTZ basis sets, respectively. The properties along the reaction path were obtained using the dual-level methodology to build the minimum energy path with the potential energy surface obtained with the M06-2X method and thermochemical properties corrected with the CCSD(T)/CBS//M06-2X/maug-cc-pVTZ results. The thermal rate constants were calculated in the framework of variational transition-state theory. Wells on both sides of the reaction (reactants and products) were found and considered in the chemical kinetics calculations. Additionally, the product yields were investigated by means of a study of the triplet and singlet surfaces of the N2H4 + O → N2H2 + H2O reaction.