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Context. The hunt for the interstellar complex organic molecules (COMs) supposed to be the building blocks of the molecules at the origin of life is a challenging but very expensive task. It starts with laboratory experiments, associated with theoretical calculations, that give the line frequencies and strengths of the relevant molecules to be identified and finishes with observations at the telescopes. Aims. The present study aims to suggest possible guidelines to optimize this hunt. Levering on the minimum energy principle (MEP) presented in a previous study, we discuss the link between thermodynamic stability and detectability of a number of structures in the important families of amides, sugars and aminonitriles. Methods. The question of the relative stability of these different species is addressed by means of quantum density functional theory simulations. The hybrid B3LYP formalism was used throughout. All 72 molecules part of this survey were treated on an equal footing. Each structure, fully optimized, was verified to be a stationary point by vibrational analysis. Results. A comprehensive screening of 72 isomers of CH 3 NO, C 2 H 5 NO, C 3 H 7 NO, C 2 H 4 O 2 , C 3 H 6 O 3 and C 2 H 4 N 2 chemical formula has been carried out. We found that formamide, acetamide and propanamide (the first two identified in the Inter-Stellar Medium) are the most stable compounds in their families demonstrating at the same time that the peptide bond >N-C=O at the origin of life is the most stable bond that can be formed. Dihydroxyacetone, whose detection awaits for confirmation, is far from being the most stable isomer of its family while aminoacetonitrile, that has been recently identified, is effectively the most stable species. Conclusions. The MEP appears to be a useful tool for optimizing the hunt for new species by identifying the potentially more abundant isomers of a given chemical formula.

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.

Iron phthalocyanine (FePc) is adsorbed to graphene on Ir(111) at cryogenic temperature. In addition to mobile FePc with four lobes, imaging and spectroscopy with a scanning tunneling microscope reveal immobile molecules that exhibit fewer lobes. A reversible transformation between four- and three-lobed molecules has been induced by current injection. The data are consistent with chemical bonding of lobes to graphene on Ir, pinning down the graphene area toward Ir. Similar observations are made from NiPc, CoPc, CuPc, and H2Pc. The experimental findings can be explained by ab initio calculations, which suggest that a Diels-Alder-type reaction may be involved with an allyl unit of graphene in the top-fcc moiré registry.

Context. Glycine, the simplest of aminoacids, has been found in several carbonaceous meteorites. It remains unclear, however, wether glycine is formed in the interstellar medium (ISM) and therefore available everywhere in the Universe. For this reason, radioastronomers have searched for many years unsuccessfully to detect glycine in the ISM. Aims. We provide possible guidelines to optimize the return of these searches. Since, for most of the species observed so far in the ISM, the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle (MEP)), we assess whether neutral glycine is the best molecule to search for or whether one of its isomers/conformers or ionic, protonated, or zwitterionic derivatives would have a higher probability of being detected. Methods. The question of the relative stability of these different species is addressed by means of quantum density functional theory (DFT) simulations within the hybrid B3LYP formalism. Each fully optimized structure is verified as a stationary point by means of a vibrational analysis. A comprehensive screening of 32 isomers/conformers of the C 2 H 5 O 2 N chemical formula (neutral, negative, and positive ions together with the corresponding protonated species and the possible zwitterionic structures) is carried out. In the sensitive case of the neutral compounds, more accurate relative energies were obtained by means of high level post Hartree-Fock coupled cluster calculations with large basis sets (CCSD(T)/cc-pVQZ). Results. We find that neutral glycine is not the most stable isomer and, therefore, probably not the most abundant one, which might explain why it has escaped detection so far. We find instead that N-methyl carbamic acid and methyl carbamate are the two most stable isomers and, therefore, probably the two most abundant ones. Among the non-neutral forms, we found that glycine is the most stable isomer only if protonated or zwitterionic if present in interstellar ices. Conclusions. Assuming that MEP can be applied to optimize our search for glycine, our conclusion is that this search will remain extremely difficult with the present instruments and we propose searching instead for other examples among the most stable isomers.

Finding complex organic molecules in the interstellar medium (ISM) is a major concern for understanding the possible role of interstellar organic chemistry in the synthesis of prebiotic species. The present interdisciplinary report is a prospective study aimed at helping detection of heteroaromatic compounds or at least of some of their isomers in the ISM. The thermodynamic stabilities of the C(4)H(5)N, C(4)H(4)O, C(4)H(4)S families were calculated using density functional theory (DFT). It was found that pyrrole, furan and thiophene are unambiguously the most stable isomers at the 10-50 K temperatures of the ISM. Several of the less stable isomers were synthesized and flash vacuum thermolysis experiments were performed on these species. Although the detection of pyrrole in the pyrolysis of many compounds has been reported in the literature, we observed that none of its isomers led to pyrrole in these conditions, which suggests that other formation routes are to be considered. On the other hand, these three aromatic compounds present a very high stability, few % been decomposed at 1500 K by flash vacuum thermolysis; these experiments also show a great stability of crotonitrile that is the most stable compound that can be formed in these conditions. The rotational constants, dipole moments and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.

Context. Almost 20% of the ∼200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Among these 37 molecules, 30 are nitrile R-CN compounds, the remaining seven belonging to the isonitrile R-NC family. How these species behave in presence of the grain surfaces is still an open question. Aims. In this contribution we investigate whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of different nature and morphologies. Methods. The question was addressed by means of a concerted experimental and theoretical study of the adsorption energies of CH 3 CN and CH 3 NC on the surface water ice and silica. The experimental determination of the molecule -surface interaction energies was carried out using temperature programmed desorption (TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question was addressed using first principle periodic density functional theory (DFT) to represent the organized solid support. Results. The most stable isomer (CH 3 CN) interacts more efficiently with the solid support than the higher energy isomer (CH 3 NC) for water ice and silica. Comparing with the HCN and HNC pair of isomers, the simulations show an opposite behaviour, in which isonitrile HNC are more strongly adsorbed than nitrile HCN provided that hydrogen bonds are compatible with the nature of the model surface.Conclusions. The present study confirms that the strength of the molecule surface interaction between isomers is not related to their intrinsic stability but instead to their respective ability to generate different types of hydrogen bonds. Coupling TPD to first principle simulations is a powerful method for investigating the possible role of interstellar surfaces in the release of organic species from grains, depending on the environment.

Single-molecule chemistry with a scanning tunneling microscope has preponderantly been performed on metal surfaces. The molecule-metal hybridization, however, is often detrimental to genuine molecular properties and obscures their changes upon chemical reactions. We used graphene on Ir(111) to reduce the coupling between Ir(111) and adsorbed phthalocyanine molecules. By local electron injection from the tip of a scanning tunneling microscope the two pyrrolic H atoms were removed from single phthalocyanines. The detachment of the H atom pair induced a strong modification of the molecular electronic structure, albeit with no change in the adsorption geometry. Spectra and maps of the differential conductance combined with density functional calculations unveiled the entire depopulation of the highest occupied molecular orbital upon H abstraction. Occupied π states of intact molecules are proposed to be emptied via intramolecular electron transfer to dangling σ states of H-free N atoms.