Reactividad en fase gaseosa del radical OH con moléculas orgánicas oxigenadas estudiada en condiciones del medio interestelar (T=21-107 K) con una nueva técnica CRESU pulsada

The interstellar medium (ISM) is defined as the space between stars (formed by 99% gas and 1% dust), where complex and extraordinary astronomical environments as dense molecular clouds are present. Until 1937, the interstellar gas was thought to be constituted only by H2 and He. However, when the methylidyne radical (CH), the first polyatomic molecule, was detected by Swings and Rosenfeld (1937) there was a paradigm shift and Astrochemistry was born. Since then, around 200 chemical species (molecules, ions and radicals) have been detected in the ISM or circumstellar shells using the recent advances of radio astronomy. Among them, this PhD thesis is focused on a ubiquitous molecule in astrochemical sources, the hydroxyl radical (OH), which plays an important role as intermediate in the interstellar chemistry (Acharyya et al., 2015). More than 70 of these 200 chemical species are considered by the astrochemical community as complex organic molecules (COMs), concretely, molecules which are formed by 6 or more atoms containing C, H, N, P, S or O (Herbst and van Dishoeck, 2009).
\nIn the inner conditions of the IS molecular clouds, where the temperature is as low as 10 K and the molecular density as low as 104 cm?3 (Millar, 2015), the gas-phase chemistry is mainly controlled only by bimolecular mechanisms, since the probability of molecular collisions in reactions with higher molecularities is extremely low. In particular, the knowledge of the formation/destruction rates of reactions involving prebiotic COMs like formaldehyde (H2CO), ethanol (CH3CH2OH) or methanol (CH3OH), which are considered as potential precursors of biological molecules, has important implications in the interpretation of their observed abundances in different regions of the ISM (Ehrenfreund et al., 2002). For that purpose, astrochemical models include the bimolecular rate coefficients at a temperature, T, (k(T)) and the branching ratios of reaction products for all potential formation and depletion processes of each species. Nevertheless, for most gas-phase reactions involving neutral-neutral species, scarce kinetic information is currently available at the temperatures of the molecular clouds in the ISM (10 - 100 K). Hence k(T) values used in astrochemical modelling are often estimates or extrapolations from temperature dependencies reported at T > 200 K. This procedure, however, is not valid for many radical-molecule reactions, especially when a hydrogen abstraction is involved due to the observed enhancement of k(T) at low temperatures (Heard, 2018; Cooke and Sims, 2019). Chapter I is dedicated to the chemistry of the ISM.
\nThe OH-reactivity with the oxygenated organic molecules object of this PhD thesis (H2CO, CH3CH2OH and CH3OH) has been previously investigated in the gas-phase in a different temperature range. For the H2CO + OH reaction, the kinetic behaviour has never been investigated at T < 200 K. Concerning the CH3CH2OH + OH reaction, there is only one previous study at T = 54 - 91 K and at T= 91 - 146 K (Caravan et al., 2015) and for the CH3OH + OH reaction, although there are 3 different studies at low temperatures, the available kinetic data is scattered and the kinetic behaviour is unclear. The first one in the range of T = 63 ? 82 K (Shannon et al., 2013) announced the dramatic enhancement of k(T) at low temperatures, the second one consists in two measurements at 56 and 88 K and in the range between 123 and 295 K (Gómez Martín et al., 2014), and the last one, performed by our research group, between 22.4 and 64.2 K (Antiñolo et al., 2016).
\nTherefore, the goal of this PhD thesis is to determine the rate coefficients for the reaction of OH radicals with H2CO, CH3CH2OH and CH3OH between 21.1 K and 107.0 K in order to confirm the previous trend and filling the existing gaps in k(T) to provide to the astrophysical community reliable k(T) at interstellar temperatures. A newly developed pulsed CRESU (French acronym to "Cinétique de Réaction en Ecoulement Supersonique Uniforme", that stands for Reaction Kinetics in a Uniform Supersonic Flow) technique, based on an aerodynamic chopper, has been employed to achieve the objectives on the present and future works and it is described in detail in Chapter III. This technique is based in the generation of a uniform supersonic flow as a consequence of the isentropic, rapid and controlled expansion of a gas from a pre-expansion chamber known as reservoir at a relative high pressure to a vacuum chamber through a Laval nozzle, which is the heart of this technique (Canosa et al., 2008). In combination with the isentropic gas expansion, the pulsed laser photolysis (PLP) of a suitable precursor has been used to generate OH radicals in the supersonic jet and the laser induced fluorescence (LIF) from electronically excited OH radicals has been employed to monitor the OH temporal evolution. The PLP-LIF technique and the main components of the experimental set-up are described in detail in Chapters II and III. Chapter IV is dedicated to the characterization of five Laval nozzles specifically designed by Dr André Canosa (CNRS- University of Rennes 1) to cover the 21.1 ? 107.0 K temperature range, allowing to explore new frontiers at very low temperatures in chemical kinetics.
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\nChapters V, VI and VII are dedicated to the kinetic study of the OH-reactions with H2CO, CH3CH2OH, and CH3OH, respectively. The T-dependence of k(T) is commonly described in astrochemical models by a three-parameter expression:
\nk(T) = ?· (?exp (? ) Eq. 1 
\nThe best fit to the T-dependence of k(T) observed in this PhD thesis has been found when ? = 0. Our kinetic results have been combined with the rate coefficients reported in previous studies on the reaction of H2CO, CH3CH2OH and CH3OH towards OH radicals (Sivakumaran et al., 2003; Shannon et al., 2013; Gómez Martín et al., 2014; Caravan et al., 2015; Antiñolo et al., 2016). The resulting T-dependencies of k(T) can be expressed as:
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\nH2CO:\tk(21.1 - 250 K) = (7.73 ± 1.82)·10?12 ·(T/300 K) (?1.03 ± 0.09)\tEq. 2
\nCH3CH2OH:\tk(21.1- 148 K) = (1.97 ± 0.39)·10?11 ·(T/300 K) (?0.74 ± 0.10)\tEq. 3
\nCH3OH:\tk(21.1- 107.0 K) = (1.68 ± 1.21)·10?11 ·(T/300 K) (?0.53 ± 0.34)\tEq. 4
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\nthe units of k(T) are: cm3 molecule?1 s?1. Errors are the combination of statistical (± ?) and a confident interval at a 95% level of confidence.
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\nThe results obtained in this PhD thesis show that the OH-reactivity of H2CO, CH3CH2OH and CH3OH is greatly enhanced at T ?21 K compared with the value of the rate coefficient at room temperature, k(298K). Table 1 shows the enhancement factor using k(298 K) recommended by the International Union of Pure and Applied Chemistry (IUPAC) (Atkinson et al., 2006). The enhancement of the OH-reactivity by more than two orders of magnitude reveals that it is extremely important to measure accurate rate coefficients at temperatures below 200 K in order to provide useful kinetic data at interstellar temperatures to be integrated into astrochemical models.
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\nUnderstanding the reasons for the reported enhancement of k(T), implies to comprehend which physicochemical processes can occur during the supersonic expansion to evaluate the reliability of the kinetic results. As can be concluded from the preceding state of the art, the enhancement of k(T) in exothermic reactions in which a hydrogen abstraction occurs can be explained by the formation of a pre-reactive complex, which evolves to products, through the barrier (if it'd exist), facilitated by quantum mechanical tunnelling (Shannon et al., 2013; Sims, 2013; Heard, 2018). In a recent theoretical study, Siebrand et al. (2016) suggested that the enhancement reported in the rate coefficient for the CH3OH + OH reaction at very low temperatures was due to the higher reactivity of the methanol dimers formed, according to those authors, along the supersonic flow. This controversial issue was faced by experts in Laval nozzle kinetics. To corroborate the assumption of Siebrand et al. (2016), I spent, as part of my PhD formation, three months in the University of Rennes 1 (France) under the supervision of Dr Ludovic Biennier. The aim of that work was to detect and quantify the formation of aggregates of methanol at T = 14.4 - 123.0 K along the supersonic jet in a continuous CRESU coupled to a time of flight mass spectrometry (TOF-MS). Despite the aggregates were detected, it was not possible to quantify the formation rate coefficient. Chapter VIII is dedicated to the description of the TOF-MS experimental set-up employed and the results obtained.
\nThe results derived from this PhD thesis have motivated many research groups of theoreticians around the world. Some of them have collaborated closely with our research group in order to explain from a theoretical point of view the kinetic behaviour reported. In particular, Dr Octavio Roncero's group (CSIC-Madrid) performed a computational study on the H2CO + OH reaction by using quasi-classical trajectories (QCT), that included quantum effects, to reproduce the kinetic trend measured in the laboratory. QCT calculations were performed at zero pressure. The recent established collaboration with Dr Luc Vereecken from the Forschungszentrum Jülich (Germany) allowed to investigate the kinetics of the CH3OH + OH reaction using a E,J-resolved microvariational Transition State Theory (E,J-µVTST) in a Rigid Rotor Harmonic Oscillator (RRHO) approximation for the formation rate of the pre-reactive complex and the Multi-Conformer Canonical Transition State Theory (MC-CTST) in a RRHO approximation to calculate the high- pressure rate coefficients and the low-pressure rate coefficients in the temperature range between 20 and 400 K. The branching ratio of reaction products was also computed at the same temperature range. The theoretical results revealed the two possible

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