Abstract
Formaldehyde has been widely observed in the icy mantle of interstellar grains. H2CO may be formed from successive hydrogenations of CO and may further contribute to the chemical complexity of the Interstellar medium (ISM) participating to heterogeneous reactions with colliding gas phase atoms. Within this context, Eley-Rideal and Langmuir-Hinshelwood rate constants of H2 formation on a formaldehyde doped amorphous water ice grain model of the ISM, were computed over a wide temperature range [15–2000 K]. We used classical molecular dynamics (MD) simulations to build the model of the H2CO doped ice surface. Then we studied theoretically by means of hybrid QM/MM ab initio and molecular mechanics methodology (ONIOM) H atoms abstraction from formaldehyde molecules and the H2 formation. Specifically, we investigate the reactivity of the gas phase H atom toward one formaldehyde molecule lying at one of the slab surfaces. The reaction path and the energetics are predicted, the mechanism is found to be exothermic by 14.89 kcal/mol and the barrier is 6.75 kcal/mol at the QM level CBS/DLPNO-CCSD(T)//ONIOM/aug-cc-pVTZ. We employ two approaches that take into account tunnelling and non-classical reflection effects by means of the Zero Curvature Tunnelling (ZCT), and the Small Curvature Tunnelling (SCT) which all provided comparable results to predict the kinetics of the reaction path. The rate constants show important quantum tunnelling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). Corner cutting effects are highlighted in the SCT calculations by 4 to 5 orders of magnitude with respect to ZCT rate constants at low temperatures.
Highlights
H2 molecules are ubiquitous in the Universe and constitute the most abundant species and are the main trigger of dust chemistry which is responsible for the formation of key molecules, such as H2O, NH3, and CH3OH
In order to validate the methods employed in the ONIOM calculations, namely M05-2X/aug-cc-pVTZ and Complete Basis Set (CBS)/ DLPNO-CCSD(T), we have employed them to calculate the gas phase reactions
It should be kept in mind that the CBS single point calculations were performed at the M05-2X geometries and zero-point vibrational energy (ZPE) is evaluated at this DFT level too
Summary
H2 molecules are ubiquitous in the Universe and constitute the most abundant species and are the main trigger of dust chemistry which is responsible for the formation of key molecules, such as H2O, NH3, and CH3OH. Given the very low temperatures ≈10 K in dense regions of space, dust grains are covered with layers of icy mantles (Ehrenfreund and Schutte, 2000) These serve as a reservoir for molecular accretion, that would subsequently allow the reagents to thermalize, diffuse and react (Congiu et al, 2014), and desorb in a later phase during star formation processes. Hydrogen addition and abstraction from formaldehyde have been investigated computationally with DFT by Goumans (Goumans, 2011), and CCSD (T) (by one of the authors) (Siaï et al, 2016) rate constants predictions in gas phase, and on pure water ice (Song and Kästner, 2017). Song and Kästner (2017), employing a QM/ MM model, studied the H2 formation on an amorphous solid water (ASW) upon which one formaldehyde molecule was adsorbed, and rate constants for both Langmuir-Hinshelwood and Eley-Rideal mechanisms were evaluated with the instanton theory, over the temperature range 60–300 K. We use a slab model of formaldehyde doped water ice, and choose a reactive site where an accessible CH2O molecule can be
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