Full-dimensional potential energy surface for the H + CH3OH reaction. Theoretical kinetics and dynamics study

Identification and Photochemistry of the Mercaptomethyl Radical

The mercaptomethyl radical (·CH2SH) is a higher-energy isomer of the methylthio radical (CH3S·) that has been proposed as an important intermediate in atmospheric and interstellar sulfur chemistry. Herein, we report the spectroscopic identification of ·CH2SH during the UV (365 nm) photolysis of CH3S· in a solid Ar-matrix at 10 K. Upon subsequent irradiation at 266 nm, the dehydrogenation of ·CH2SH to yield CS via the intermediacy of the elusive thioformyl radical (HCS·) has also been observed. The characterization of ·CH2SH and HCS· with matrix-isolation IR and UV-vis spectroscopy is supported by 13C-isotope labeling and quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction theory (VCI). The disclosed photochemistry of ·CH2SH provides new insight into understanding the chemical evolution of organosulfur molecules in the interstellar medium (ISM).

The CN(X 2Σ+) + C2H6 reaction: Dynamics study based on an analytical full-dimensional potential energy surface

The hydrogen abstraction reaction of the cyano radical with molecules of ethane presents some interesting points in the chemistry from ultra-cold to combustion environments especially with regard to HCN(v) product vibrational distribution. In order to understand its dynamics, a new analytical full-dimensional potential energy surface was developed, named PES-2023. It uses a combination of valence bond and mechanic molecular terms as the functional form, fitted to high-level ab initio calculations at the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ level on a reduced and selected number of points describing the reactive process. The new surface showed a continuous and smooth behavior, describing reasonably the topology of the reaction: high exothermicity, low barrier, and presence of intermediate complexes in the entrance and exit channels. Using quasi-classical trajectory calculations (QCT) on the new PES-2023, a dynamics study was performed at room temperature with special emphasis on the HCN(v1,v2,v3) product stretching and bending vibrational excitations, and the results were compared with the experimental evidence, which presented discrepancies in the bending excitation. The available energy was mostly deposited as HCN(v) vibrational energy with the vibrational population inverted in the CH stretching mode and not inverted in the CN stretching and bending modes, thus simulating the experimental evidence. Other dynamics properties at room temperature were also analyzed; cold rotational energy distribution was found, associated with a linear and soft transition state, and backward scattering distribution was found, associated with a rebound mechanism.

Analytical potential energy surface and dynamics for the OH + CH3OH reaction

Using as functional form a combination of valence bond and mechanic molecular terms a new full-dimensional potential energy surface was developed for the title reaction, named PES-2022, which was fitted to high-level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples-F12 explicitly correlated level on a representative number of points describing the reactive system. This surface simultaneously describes the two reaction channels, hydrogen abstraction from the methyl group [(R1) path] and from the alcohol group [(R2) path] of methanol to form water. PES-2022 is a smooth and continuous surface, which reasonably describes the topology of this reactive system from reactants to products, including the intermediate complexes present in the system. Based on PES-2022 an exhaustive dynamics study was performed using quasi-classical trajectory calculations under two different initial conditions: at a fixed room temperature, for direct comparison with the experimental evidence and at different collision energies, to analyze possible mechanisms of reaction. In the first case, the available energy was mostly deposited as water vibrational energy, with the vibrational population inverted in the stretching modes and not inverted in the bending modes, reproducing the experimental evidence. In the second case, the analysis of different dynamics magnitudes (excitation functions, product energy partitioning, and product scattering distributions), allows us to suggest different mechanisms for both (R1) and (R2) paths: a direct mechanism for the (R2) path vs an indirect one, related with "nearly trapped" trajectories in the intermediate complexes, for the (R1) path.