Differential adsorption of complex organic molecules isomers at interstellar ice surfaces

Abstract

Context. Over 20 of the ∼150 different species detected in the interstellar and circumstellar media have also been identified in icy environments. For most of the species observed so far in the interstellar medium (ISM), the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle -MEP) with few exceptions such as, for example, CH 3 COOH/HCOOCH 3 and CH 3 CH 2 OH/CH 3 OCH 3 , whose formation is thought to occur on the icy mantles of interstellar grains. Aims. We investigate whether differences found in the compositions of molecular ices and the surrounding gas phase could originate from differences between the adsorption of one isomer from that of another at the ice surface. Methods. We performed a coherent and concerted theoretical/experimental study of the adsorption energies of the four molecules mentioned above, i.e. acetic acid (AA)/methyl formate (MF) and ethanol (EtOH)/dimethyl ether (DME) on the surface of water ice at low temperature. The question was first addressed theoretically at LCT using solid state periodic density functional theory (DFT) to represent the organized solid support. The experimental determination of the ice/molecule interaction energies was then carried out independently by two teams at LPMAA and LERMA/LAMAp using temperature programmed desorption (TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Results. For each pair of isomers, theory and experiments both agree that the most stable isomer (AA or EtOH) interacts more efficiently with the water ice than the higher energy isomer (MF or DME). This differential adsorption can be clearly seen in the different desorption temperatures of the isomers. It is not related to their intrinsic stability but instead to both AA and EtOH producing more and stronger hydrogen bonds with the ice surface. Conclusions. We show that hydrogen bonding may play an important role in the release of organic species from grains and propose that, depending on the environment, differential adsorption should not be rejected as a possible way of interpreting MEP exceptions.

Nitrile versus isonitrile adsorption at interstellar grains surfaces

“…This concerted approach allows for determining values of adsorption energies, obtained with two independent methods, whose reliability is supported by thorough cross-checking between experiments and numerical simulations. The validity of such an approach has already been illustrated in the study of the adsorption of complex organic molecules on water ice (Lattelais et al 2011). In the present work, the adsorption of the two nitrile and isonitrile isomers has been studied on a silica surface, as a model for bare interstellar grain surface, and on a water ice surface, as water is the most abundant constituent of the icy mantles recovering the dust grains.…”

“…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.…”

Differential adsorption of CHON isomers at interstellar grain surfaces

“…Experimentally, the temperatures at which various species desorb from the ice have been characterized using the temperature programmed desorption (TPD) technique (see, e.g., Collings et al 2004). Recently, we reported a joint theory/TPD experiment (Lattelais et al 2011b) focused on the adsorption of two couples of isomers: acetic acid (AA)/methyl formate (MF) and ethanol (EtOH)/di-methyl ether (DME). Comparison between the theoretical adsorption energies E ads on ice with those derived from the TPD experiments showed that both agree whenever measurement is possible.…”

“…A possible explanation for these exceptions is that adsorption on the grain surfaces plays a crucial role. In particular, Lattelais et al (2011b) carried out a study of the adsorption of the couple methyl formate and acetic acid and found that the relative adsorption energies may be at the origin of the exception to the MEP. Alternatively, isomerization by UV photolysis may have timescales that induce the observed MEP exception for the acetic acid -methyl formate couple (Puletti et al 2010).…”

Homochirality in space – Selective enrichment of chiral molecules on chiral surfaces

“…Here we address the question of how much the adsorption energy differs from one enantiomer to the other. To this end we rely on numerical simulations based on the methods of quantum chemistry, namely Density Functional Theory (DFT) that proved efficient in the super-molecule (cluster) and periodic approaches to this category of phenomena [4].…”