No abstract

We have combined the AMOEBA09 polarizable force field with the ONIOM(QM:MM) method to rationalize binding energies and binding preferences of the OH, HCO, and CH 3 radicals on crystalline water ice (I h ). O N I O M ( M 0 6 2 X : A M O E B A 0 9 ) a n d O N I O M -(wB97XD:AMOEBA) calculations suggest that the dangling hydrogen (d-H) or dangling oxygen (d-O) on the binding sites play an important role on the binding energies. Depending on the dangling nature at the binding site, a range of binding energies is found for the OH radical (0.67−0.20 eV), HCO radical (0.42−0.12 eV), and CH 3 radical (0.26−0.11 eV). The binding energies of these radicals are larger in the presence of both d-H and d-O at the binding site. On the other hand, binding energies are weaker in the presence of only d-H or d-O at the binding site. The ONIOM(QM:AMOEBA09) methodology is found to be a useful approach to calculate binding energies of atoms, radicals, and molecules on I h .

Binding energies of the CH3O radical on hexagonal water ice (I h) and amorphous solid water (ASW) were calculated using the ONIOM­(QM:MM) method. A range of binding energies is found (0.10–0.50 eV), and the average binding energy is 0.32 eV. The CH3O radical binding on the ASW surfaces is stronger than on the I h surfaces. The computed binding energies from the ONIOM­(wB97X-D/def2-TZVP:AMBER) and wB97X-D/def2-TZVP methods agree quite well. Therefore, the ONIOM­(QM:MM) method is expected to give accurate binding energies at a low computational cost. Binding energies from the ONIOM­(wB97X-D/def2-TZVP:AMBER) and ONIOM­(wB97X-D/def2-TZVP:AMOEBA09) methods differ noticeably, indicating that the choice of force field matters. According to the energy decomposition analysis, the electrostatic interactions and Pauli repulsions between the CH3O radical and ice play a crucial role in the binding energy. This study gives quantitative insights into the CH3O radical binding on interstellar ices.

Heterogeneous radical processes on ice surfaces play a vital role in the formation of building blocks of the biologically relevant molecules in space. Therefore, quantitative mechanistic details of the radical binding and radical reactions on ices are crucial in rationalizing the chemical evolution in the Universe. The radical chemistry on ice surfaces was explored at low temperatures by combining quantum chemical calculations and laboratory experiments. A range of binding energies was observed for OH, HCO, CH3, and CH3O radicals binding on ices. Computed reaction paths of the radical reactions on ices, OCS + H and PH3 + D, explained the experimentally observed products. In both radical reactions, quantum tunnelling plays a key role in achieving the reactions at low temperatures. Our findings give quantitative insights into radical chemistry on ice surfaces in interstellar space and the planetary atmospheres.