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This work ia a study of chemistry in molecular clouds. I begin by describing the improvements made to gas phase chemical reaction data in the recent release of the UMIST database for astrochemistry (Rate 12). Improvements to the reaction network include the addition of anions, new reaction rate coefficient and branching rate measurements across all reactions types and newly calculated photodissociation and photoionisation rates.

This thesis addresses the problem of complex molecule formation in the prestellar and hot core stages of interstellar star formation. We have enhanced a modified rates chemical code to include calculations of physisorption and chemisorption on interstellar grains consisting of amorphous carbon, graphite PAH particles, para-site PAH particles, and silicates as well as calculations of non-thermal desorption via cosmic ray heating, H2 desorption, and cosmic ray-induced photodesorption. We incorporate a time-dependent, warm-up parameter in order to self-consistently treat the chemistry of our dark cloud and hot core models. We find that different dark cloud species achieve better observational agreement for different desorption conditions and that molecules which are inefficiently destroyed in the dark cloud tend to demonstrate an enhanced presence in the early chemistry of the hot core. During the warm up, we observe temporary enhancement of a number of species on the grain surface and • suggest this is due to the physical cycling of molecular material between the gas and solid states as well as the presence of fast barrier-possessing reactions which occur between particles on the grain surface. The presence of a time-dependent warm up also allows a transient period of N-rich chemistry to occur in the gas phase and indicates a number of possibilities which may ::allow the simultaneous occurrence of cyanide and complex organic species in star-forming molecular clouds. We argue that for high NH3 abundances, alkyl-cation transfer reactions may proceed sufficiently rapidly that complex organic species and nitriles can coexist in both compact and ultra-compact hot molecular cores.

L'objectif de ce travail est de caractériser par des méthodes de chimie quantique de haute qualité les processus réactionnels impliquant H, CO, H2CO, H3COH et H2O à la surface des grains interstellaires. En effet, H2CO et H3COH sont parmi les molécules organiques les plus abondantes dans les nuages moléculaires interstellaires. Il s’agit de la première étude décrivant les étapes conduisant au méthanol par hydrogénations successives de CO. En phase gazeuse, les méthodes multi-références ont été employées. Les calculs montrent que certaines étapes cruciales de la synthèse (formation de HCO) mettent en jeu des barrières d'activation significatives dans les conditions interstellaires. L’effet tunnel a aussi été pris en compte. Les mêmes réactions ont ensuite été étudiées sur des agrégats d'eau ou de CO. Ceux-ci ont été choisis comme surfaces modèles pour les grains interstellaires. Compte-tenu de la taille du système, les calculs ont été menés en utilisant la théorie DFT et la méthode MP2. Pour la formation de HCO, la présence d'un agrégat ne modifie pas l'énergie d'activation. En revanche, pour la formation de COH et H3CO, l'énergie d'activation est réduite et l'endo/exothermicité change. L'isomérisation HCOH-H2CO devient possible dans des agrégats d'eau. Ces résultats indiquent que l'état de surface et le flux d'atomes H peuvent avoir une influence notable, ouvrant d'autres voies non envisageables en phase gazeuse. Enfin, ces résultats seront intégrés dans le modèle astrophysique GRAINOBLE, ce qui permettra de tenir compte des différents intermédiaires qui pourraient potentiellement participer à la synthèse de ces deux molécules ainsi que d’autres plus complexes. / Throughout this work high level quantum chemistry methods have been used to investigate reactive processes involving: H, D, CO, H2CO, H3COH and H2O at model interstellar grains surfaces. This study has mainly focused upon the formation of the two most abundant Complex Organic Molecules (COMS), H2CO and H3COH. For the first time, all of the hydrogenation steps have been considered and treated with reliable methods in the gas phase, and in particular making use of multi-reference approaches such as MRCI+Q and MRMP2. Following the characterization of all the reactions in the gas phase, the same processes have been investigated within small molecular clusters using various density functionals and MP2. This was done as a preliminary attempt to model the icy grain mantles of interstellar dust grains. For some of the steps, such as the formation of HCO, the activation energy does not vary significantly between the gas phase and the clusters. In contrast, for other processes, such as the formation of COH, and H3CO, the activation energy is lowered and the exothermicity/endothermicity of the reaction changes. In addition, the isomerizations of some species, as for instance HCOH to H2CO, are also strongly affected by the presence of water. From the cluster calculations, we conclude that the arrangement of the surface molecules and the H flux may have a significant influence on the chemical routes leading to H2CO and H3COH. Finally, we have also discussed how these results may be incorporated into astrophysical models, as our results suggest that the current route, that is considered, may not include all of the possible steps which may contribute to the actual formation of these COMs.

L’azote et les gaz rares présents dans les astéroïdes, les comètes et les atmosphères planétaires sont piégés dans de la matière organique et ont des compositions chimiques qui sont différentes de celle du Soleil, représentatif du gaz primordial à partir duquel les différents objets du système solaire se sont formés il y a 4,5 milliards d’années. Au cours de cette thèse, des synthèses de matière carbonée à partir d’un mélange de gaz ont été réalisées dans un plasma appelé le Nébulotron, afin de mieux comprendre les processus à l’origine des compositions de l’azote et des gaz rares présents dans les météorites. Les caractéristiques de la matière organique ainsi que la composition des gaz rares piégés dans les météorites sont relativement bien reproduites dans les expériences, mais pas celle de l’azote. Ces résultats expérimentaux permettent de proposer des mécanismes clé à l’origine des compositions des éléments volatils présents dans les objets du système solaire. / Nitrogen and noble gases present in asteroids, comets or planetary atmospheres are trapped in organic matter and bear a composition that is different from the composition of the Sun, which is representative of the primordial gas from which the different objects in the solar system were formed 4.5 billion years ago. During this thesis, experimental syntheses of organic matter from gas mixtures in a plasma setup called the Nebulotron were performed in order to better understand the processes responsible for this chemical difference between the meteorites and the Sun for nitrogen and noble gases. The characteristics of the organic matter and the signature of the noble gases trapped in meteorites are relatively well reproduced in the experiments, whereas the composition of nitrogen is not. These experimental results give hints about the key mechanisms that are responsible for the variations of the volatile elements composition in the solar system objects.