Differential adsorption of CHON isomers at interstellar grain surfaces

Abstract

Context. The CHON generic chemical formula covers different isomers such as isocyanic acid (HNCO), cyanic acid (HOCN), fulminic acid (HCNO), and isofulminic acid (HONC); the first three have been identified in a large variety of environments in the interstellar medium (ISM). Several phenomena could be at the origin of the observed abundances, such as different pathways of formation and destruction involving gas phase reactions with different possible activation barriers and/or surface processes depending on the local temperature and the nature of the support. Aims. The scope of this article is to shed some light on the interaction of the CHON isomers with interstellar grains as a function of the nature of the surface and to determine the corresponding adsorption energies in order to find whether this phenomenon could play a role in the abundances observed in the ISM. Methods. The question was addressed by means of numerical simulations using first principle periodic density functional theory (DFT) to represent the grain support as a solid of infinite dimension. Results. Regardless of the nature of the model surface (water ice, graphene, silica), two different classes of isomers were identified: weakly bound (HNCO and HCNO) and strongly bound (HOCN and HONC), with the adsorption energies of the latter group being about twice those of the former. The range of the adsorption energies is (from highest to lowest) HOCN > HONC > HNCO > HCNO. They are totally disconnected from the relative stabilities, which range from HNCO > HOCN > HCNO > HONC. Conclusions. The possibility of hydrogen bonding is the discriminating factor in the trapping of CHON species on grain surfaces. Whatever the environment, differential adsorption is effective and its contribution to the molecular abundances should not be ignored. The theoretical adsorption energies provided here could be profitably used for a more realistic modeling of molecule-surfaces interactions.

Nitrile versus isonitrile adsorption at interstellar grains surfaces

“…for two substrates favouring the formation of OH bonds. A similar result, with the same origin, was obtained in a preceding study on the HCNO and HNCO couple of isomers whose attachments to the silica surface were found to be of ∼130 and 240 meV, whereas they are of ∼460 and 600 meV on the ice, respectively (Lattelais et al 2015). In any case, the different adsorption behaviour between HCN and HNC on naked grain surfaces or on water ice would likely induce different abundance ratios in different environments, which is well in line with their observed fluctuating abundance ratios in different regions of the ISM.…”

“…As periodic calculations were based on the replication of the unit cell along the three directions of space, the size of the unit cell is a critical parameter whose dimensions were determined here to avoid any spurious lateral and vertical interactions between successive slabs. The experience drawn from preceding studies (Lattelais et al 2011(Lattelais et al , 2015 showed that a distance of ∼14 Å between the adsorbed species is necessary to avoid the interactions generated by lateral translations. The energy minima were then obtained by full optimizations starting from most of the chemically reasonable positions (over atoms and depressions) and orientations of the adsorbates over the host surfaces.…”

Reconstruction of water ice: the neglected process OH + OH → H2O + O

“…To avoid these limiting factors, a solution is to set up the icy environment with a proper solid surface of infinite dimension. Based on our preceding works on molecular adsorption on water ices (Lattelais et al 2011(Lattelais et al , 2015Bertin et al 2017), we took a periodic representation of solid hexagonal ice I h , composed of bilayers of water molecules. The ice surface was described by the (0001) Miller indices.…”

Origin of Molecular Oxygen in Comets: Current Knowledge and Perspectives

“…To address this issue, Mousis et al (2016b) investigated the question of the stability of the radiolytically produced O 2 molecules in the water matrix of ice grains. To do so, a sampling of the representative structures of O 2 in solid water ice has been obtained using a strategy based on first principle periodic density functional theory quantum calculations (Lattelais et al 2011(Lattelais et al , 2015Ellinger et al 2015). These calculations, performed in the case of hexagonal ice Ih using the Vienna ab initio simulation package (VASP) Hafner 1993, 1994), show that when 2, 3, or 4 adjacent H 2 O molecules were removed from the hexagonal lattice, well defined cavities form with shapes depending on the positions of the dismissed entities.…”