QM/MM Study of the H2 Formation on the Surface of a Water Ice Grain Doped With Formaldehyde: Molecular Dynamics and Reaction Kinetics

Formaldehyde has been widely observed in the icy mantle of interstellar grains. H 2 CO may be formed from successive hydrogenations and further contribute to chemical complexity Interstellar medium (ISM) participating heterogeneous reactions with colliding gas phase atoms. Within this context, Eley-Rideal Langmuir-Hinshelwood rate constants formation on a formaldehyde doped amorphous water ice grain model ISM, were computed over wide temperature range [15–2000 K]. We used classical molecular dynamics (MD) simulations build surface. Then we studied theoretically by means hybrid QM/MM ab initio mechanics methodology (ONIOM) atoms abstraction molecules formation. Specifically, investigate reactivity atom toward one molecule lying at slab surfaces. The reaction path energetics are predicted, mechanism is found exothermic 14.89 kcal/mol barrier 6.75 QM level CBS/DLPNO-CCSD(T)//ONIOM/aug-cc-pVTZ. employ two approaches that take into account tunnelling non-classical reflection effects Zero Curvature Tunnelling (ZCT), Small (SCT) which all provided comparable results predict kinetics path. show important quantum low temperatures when compared rates obtained purely transition-state theory (TST) canonical variational transition state (CVT). Corner cutting highlighted SCT calculations 4 5 orders magnitude respect ZCT temperatures.