Analyse topologique de quelques interactions métal-ligand (mX (M = Sc, Ti, ..., Cu ; X = CO, N2, CN-, H2O, NH3 et F-))

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Etude théorique de processus radicalaires sur les glaces interstellaires

Aujourd hui le rôle potentiel de la chimie interstellaire dans l apparition des molécules du vivant est considéré attentivement. Synthèse et destruction des molécules organiques complexes dans l espace constituent un sujet brûlant, expérimentalement comme théoriquement. En particulier, pour ces molécules dont la formation en phase gaz dans les conditions usuelles du milieu interstellaire est difficile, tous s interrogent sur le rôle de la glace d eau, constituant solide omniprésent du milieu. Cette thèse traite du rôle multiple de la glace d eau, protecteur, catalyseur ou destructeur au travers des processus suivants : la formation du méthanol par hydrogénations radicalaires successives de CO à la surface d un grain interstellaire glacé, les déshydrogénations de la méthylamine, produit de fragmentation primaire de la glycine, et du méthanol par les radicaux OH issus de l irradiation cosmique de la glace. Les méthodes employées sont celles de la chimie théorique (ab initio, fonctionnelle de la densité). La glace est considérée soit en tant que surface modélisée par un cluster cristallin, soit en tant que volume modélisé avec un continuum polarisable. Le rôle de la glace apparaît complexe et antagoniste. Côté formation, l hydrogénation de CO est indéniablement facilitée par la présence de glace d eau mais très dépendante des conditions environnementales. Côté destruction, la glace impose une structure zwitterionique à la glycine dont l irradiation conduit à une fragmentation partielle (70%) en méthylamine et CO2. La méthylamine, comme le méthanol, est détruite dans une série de réactions exothermiques par les radicaux OH dont le rôle a jusqu ici été peu considéré.The role possibly played by interstellar chemistry in the emergence of the molecules of life is currently investigated with great attention. The synthesis and destruction of complexes organic molecules in space are a permanent challenge for both theory and experiments, especially for these molecules whose formation in the interstellar gas phase is highly questionable. Consequently, chemistry at the interface of dust and gas has now gained the status of a major actor in astrochemistry. This thesis deals with the multiple aspects under which water ice, ubiquitous in space, can act, namely as catalyst, molecular shield or destructive medium. The following processes have been considered: the formation of methanol by successive radical hydrogenations of CO on the surface of water ice, the destructions of methylamine, primary product of the glycine fragmentation, and methanol by the OH radicals created when the icy mantles of the grains are irradiated by the cosmic radiations. The methods used are those of theoretical chemistry (ab initio post Hartree-Fock and Density Functional Theory). The ice surface is modeled by a crystalline cluster and the water ice bulk by a polarizable continuum medium. The role of the ice is complex and antagonist. On the formation side, the CO hydrogenation is undeniably aided by the water ice presence but very dependent of the environment. On the destruction side, the ice forces a zwitterionic structure of glycine whose irradiation leads to a partial fragmentation (70%) into methylamine and CO2. Methylamine as well as methanol is destroyed, in a series of exothermic reactions by the OH radicals whose role has been little considered as of today.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Introducing the ELF topological analysis in the field of quasirelativistic quantum calculations

We present an original formulation of the electron localization function (ELF) in the field of relativistic two-component DFT calculations. Using I2 and At2 species as a test set, we show that the ELF analysis is suitable to evaluate the spin-orbit effects on the electronic structure. Beyond these examples, this approach opens up new opportunities for the bonding analysis of large molecular systems involving heavy and super-heavy elements

QTAIM Analysis in the Context of Quasirelativistic Quantum Calculations

International audienceComputational chemistry currently lacks ad hoc tools for probing the nature of chemical bonds in heavy and superheavy-atom systems where the consideration of spinorbit coupling (SOC) effects is mandatory. We report an implementation of the Quantum Theory of Atoms-In-Molecules in the framework of two-component relativistic calculations. Used in conjunction with the topological analysis of the Electron Localization Function, we show for astatine (At) species that SOC significantly lowers At electronegativity and boosts its propensity to make charge-shift bonds. Relativistic spin-dependent effects are furthermore able to change some bonds from mainly covalent to charge-shift type. The implication of the disclosed features regarding the rationalization of the labeling protocols used in nuclear medicine for At-211 radioisotope nicely illustrates the potential of the introduced methodology for investigating the chemistry of (super)heavy elements

Design Of Next Generation Force Fields From AB Initio Computations: Beyond Point Charges Electrostatics

International audienc

Design Of Next Generation Force Fields From AB Initio Computations: Beyond Point Charges Electrostatics

International audienc

Possible survival of simple amino acids to X-ray irradiation in ice: the case of glycine

International audienceContext. Glycine, the simplest of amino acids, has been found in several carbonaceous meteorites collected on Earth, though its presence in the interstellar medium (ISM) has never been confirmed as of today. It is now considered that its synthesis took place in the icy mantles of interstellar grains, but it remains unclear how glycine, once synthesized and trapped in interplanetary particles, survives during the transfer to the Earth.Aims. Assuming that glycine was effectively formed in the ice, we address the question of its resistance to a solar-like radiation field and look for the possible molecular remnants that would be useful tracers of its former existence.Methods. The search was conducted using an interdisciplinary approach that mixes, on the one hand, irradiations in ultra high vacuum at 30 K on the TEMPO beam line of the synchrotron SOLEIL, simultaneously with near-edge X-ray absorption spectroscopy (NEXAFS) measurements, and on the other hand, quantum calculations to determine the energetics of the fragmentations and the relative stability of the different byproducts. The last points were addressed by means of density functional theory (DFT) simulations followed by high-level post Hartree-Fock calculations when more accurate relative energies were necessary. The constraints of an icy environment deserved special attention and the ice was modeled by a polarizable continuum medium that relies on the dielectric constant of water ice at 10–50 K.Results. Destruction of glycine is observed in the first seconds of irradiation, and carbon dioxide (CO2) and methylamine (CH3NH2) are formed. Carbon monoxide (CO), methanimine (CH2NH) and hydrogen cyanide (HCN) are also produced in secondary reactions. The amino acid destruction is the same for pure glycine and glycine in ice, indicating that the OH radicals released by the water matrix is barely involved in the photolytic process; however, these radicals are involved in the production of the secondary byproducts through dehydrogenation reactions as shown by ab initio quantum chemical simulation presented in this article along with the experimental results.Conclusions. The experiments show that glycine is only partially destroyed. Its abundance is found to stay at a level of ~30% of the initial concentration, for an irradiation dose equivalent to three years of solar radiation (at a distance of one astronomical unit). This result supports the hypothesis that, if trapped in protected icy environments and/or in the interior of interplanetary particles and meteorites, glycine may partly resist the radiation field to which it is submitted and, accordingly, survives its journey to the Earth

Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association

International audienceThe deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H+3H3+ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H+3H3+, especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H+3H3+ only, considering that the protonated ions produced in the destruction of H+3H3+ by its main competitors present in the primitive nebula, i.e., H2O, CO, and N2, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H2O, CO, and N2, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10−16 to 10−19 cm3 s−1, which is reasonable for such reactions and has to be compared to the rates of 10−16 to 10−18 cm3 s−1, obtained with H+3H3+. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe