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A consensus has progressively developed that the path towards complex organic molecules could be favoured by the icy mantles of interstellar dust particles. However, the question of whether activation energy is required to promote those reactions is open. This work deals with the simplest process thought to increase complexity i.e. successive additions of a single atom (H). Two situations can be considered, the direct additions of H in the gas phase and the atomic hydrogenation processes on water ice surfaces available in the interstellar medium (ISM). The synthesis of methanol, largely discussed in the literature is revisited as a case study. Computational investigations of the reactions leading from CO to CH3OH show that, with [H...(H2O)n] complexes as hydrogenation vectors, all steps of the process are barrier-less, contrary to the gas phase process and this, whatever the level of theory considered. The key parameter is the position of the CO + [H...(H2O)n] initial system on the global energy scale. At the present level of theory, when H is pre-adsorbed on the ice, the system happens to be high enough above the reaction path, namely, above any intermediates and possible transition states. This is true whatever the dimension of the ice support, even for the simplest model of one H2O molecule. Application of such a simplified synthetic approach, here validated for the synthesis of methanol, could be generalized, providing a simple way to get a fair insight into the important class of atomic hydrogenations on ices in the ISM.

International audience; Following a previous study by de Courcy et al. ((2009) Interdiscip. Sci. Comput. Life Sci. 1, 55-60), we demonstrate in this contribution, using quantum chemistry, that metal cations exhibit a specific topological signature in the electron localization of their density interacting with ligands according to its "soft" or "hard" character. Introducing the concept of metal cation subvalence, we show that a metal cation can split its outer-shell density (the so-called subvalent domains or basins) according to it capability to form a partly covalent bond involving charge transfer. Such behaviour is investigated by means of several quantum chemical interpretative methods encompasing the topological analysis of the Electron Localization Function (ELF) and Bader's Quantum Theory of Atoms in Molecules (QTAIM) and two energy decomposition analyses (EDA), namely the Restricted Variational Space (RVS) and Constrained Space Orbital Variations (CSOV) approaches. Further rationalization is performed by computing ELF and QTAIM local properties such as electrostatic distributed moments and local chemical descriptors such as condensed Fukui Functions and dual descriptors. These reactivity indexes are computed within the ELF topological analysis in addition to QTAIM offering access to non atomic reactivity local index, for example on lone pairs. We apply this "subvalence" concept to study the cation selectivity in enzymes involved in blood coagulation (GLA domains of three coagulation factors). We show that the calcium ions are clearly able to form partially covalent charge transfer networks between the subdomain of the metal ion and the carboxylate oxygen lone pairs whereas magnesium does not have such ability. Our analysis also explains the different role of two groups (high affinity and low affinity cation binding sites) present in GLA domains. If the presence of Ca(II) is mandatory in the central "high affinity" region to conserve a proper folding and a charge transfer network, external sites are better stabilised by Mg(II), rather than Ca(II), in agreement with experiment. The central role of discrete water molecules is also discussed in order to understand the stabilities of the observed X-rays structures of the Gla domain. Indeed, the presence of explicit water molecules generating indirect cation-protein interactions through water networks is shown to be able to reverse the observed electronic selectivity occuring when cations directly interact with the Gla domain without the need of water.

International audience