OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study

The reaction between the ground-state hydroxyl radical, OH(²Π), and ethylene, C₂H₄, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH₂CHOH (vinyl alcohol) + H, CH₃CHO (acetaldehyde) + H, and H₂CO (formaldehyde) + CH₃ product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH₂CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H₂CO + CH₃ channel for ∼58%, while the CH₃CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Fragmentation of interstellar methanol by collisions with He˙+: an experimental and computational study

Methanol is a key species in astrochemistry as its presence and reactivity provides a primary route to the synthesis of more complex interstellar organic molecules (iCOMs) that may eventually be incorporated in newly formed planetary systems. In the interstellar medium, methanol is formed by hydrogenation of CO ices on grains, and its fate upon collisions with interstellar ions should be accounted for to correctly model iCOM abundances in objects at various stages of stellar evolution. The absolute cross sections (CSs) and branching ratios (BRs) for the collisions of He˙⁺ ions with CH₃OH are measured, as a function of the collision energy, using a Guided Ion Beam Mass Spectrometer (GIB-MS). Insights into the dissociative electron (charge) exchange mechanism have been obtained by computing the entrance and exit multidimensional Potential Energy Surfaces (PESs) and by modelling the non-adiabatic transitions using an improved Landau-Zener-Stückelberg approach. Notably, the dynamical treatment reproducing the experimental findings includes a strong orientation effect of the system formed by the small He˙⁺ ion and the highly polar CH₃OH molecule, in the electric field gradient associated to the strongly anisotropic intermolecular interaction. This is a stereodynamical effect that plays a fundamental role in collision events occurring under a variety of conditions, with kinetic energy confined within intervals ranging from the sub-thermal to the hyper-thermal regime.