Pressure Dependent Low Temperature Kinetics for CN + CH3CN: Competition between Chemical Reaction and van der Waals Complex Formation

International audienceThe gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CN⋯CH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CN⋯CN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed

Gas-phase reactivity of OH radicals with ammonia (NH3) and methylamine (CH3NH2) at around 22 K

IInterstellar molecules containing N atoms, such as ammonia (NH3) and methylamine (CH3NH2), could be potential precursors of amino acids like the simplest one, glycine (NH2CH2COOH). The gas-phase reactivity of these N-bearing species with OH radicals, ubiquitous in the interstellar medium, is not known at temperatures of cold dark molecular clouds. In this work, we present the first kinetic study of these OH-reactions at around 22 K and different gas densities ((3.4-16.7)×1016 cm-3) in helium. The obtained rate coefficients, with ±2 uncertainties, k1(OH+NH3) = (2.7±0.1)×10-11 cm3 s-1 k2(OH+CH3NH2) = (3.9±0.1)×10-10 cm3 s-1 can be included in pure gas-phase or gas-grain astrochemical models to interpret the observed abundances of NH3 and CH3NH2. We observed an increase of k1 and k2 with respect to those previously measured by others at the lowest temperatures for which rate coefficients are presently available: 230 K and 299 K, respectively. This increase is about 380 times for NH3 and 20 times for CH3NH2. Although the OH+NH3 reaction is included in astrochemical kinetic databases, the recommended temperature dependence for k1 is based on kinetic studies at temperatures above 200 K. However, the OH+CH3NH2 reaction is not included in astrochemical networks. The observed increase in k1 at ca. 22 K does not significantly change the abundance of NH3 in a typical cold dark interstellar cloud. However, the inclusion of k2 at ca. 22 K, not considered in astrochemical networks, indicates that the contribution of this destruction route for CH3NH2 is not negligible, accounting for 1/3 of the assumed main depletion route (reaction with HCO+) in this IS environment.Las moléculas interestelares que contienen átomos de N, como el amoníaco (NH3) y la metilamina (CH3NH2), podrían ser precursoras potenciales de aminoácidos como el más simple, la glicina (NH2CH2COOH). La reactividad en fase gaseosa de estas especies portadoras de N con radicales OH, omnipresentes en el medio interestelar, no se conoce a temperaturas de nubes moleculares oscuras y frías. En este trabajo, presentamos el primer estudio cinético de estas reacciones OH a alrededor de 22 K y diferentes densidades de gas ((3.4-16.7) × 1016 cm-3) en helio. Los coeficientes de velocidad obtenidos, con ± 2 incertidumbres, k1 (OH + NH3) = (2.7 ± 0.1) × 10-11 cm3 s-1 k2 (OH + CH3NH2) = (3.9 ± 0.1) × 10-10 cm3 s- 1 se puede incluir en modelos astroquímicos de fase gaseosa pura o de grano de gas para interpretar las abundancias observadas de NH3 y CH3NH2. Observamos un aumento de k1 y k2 con respecto a los medidos previamente por otros a las temperaturas más bajas para las que actualmente se dispone de coeficientes de velocidad: 230 K y 299 K, respectivamente. Este aumento es aproximadamente 380 veces para el NH3 y 20 veces para el CH3NH2. Aunque la reacción OH + NH3 está incluida en las bases de datos cinéticas astroquímicas, la dependencia de temperatura recomendada para k1 se basa en estudios cinéticos a temperaturas superiores a 200 K. Sin embargo, la reacción OH + CH3NH2 no está incluida en las redes astroquímicas. El aumento observado en k1 a ca. 22 K no cambia significativamente la abundancia de NH3 en una típica nube interestelar oscura y fría. Sin embargo, la inclusión de k2 en ca. 22 K

Kinetics and Dynamics of the S(^1D_2) + H_2 \to SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(^1D_2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves

Common and rare variant association analyses in amyotrophic lateral sclerosis identify 15 risk loci with distinct genetic architectures and neuron-specific biology

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with a lifetime risk of one in 350 people and an unmet need for disease-modifying therapies. We conducted a cross-ancestry genome-wide association study (GWAS) including 29,612 patients with ALS and 122,656 controls, which identified 15 risk loci. When combined with 8,953 individuals with whole-genome sequencing (6,538 patients, 2,415 controls) and a large cortex-derived expression quantitative trait locus (eQTL) dataset (MetaBrain), analyses revealed locus-specific genetic architectures in which we prioritized genes either through rare variants, short tandem repeats or regulatory effects. ALS-associated risk loci were shared with multiple traits within the neurodegenerative spectrum but with distinct enrichment patterns across brain regions and cell types. Of the environmental and lifestyle risk factors obtained from the literature, Mendelian randomization analyses indicated a causal role for high cholesterol levels. The combination of all ALS-associated signals reveals a role for perturbations in vesicle-mediated transport and autophagy and provides evidence for cell-autonomous disease initiation in glutamatergic neurons. A cross-ancestry genome-wide association meta-analysis of amyotrophic lateral sclerosis (ALS) including 29,612 patients with ALS and 122,656 controls identifies 15 risk loci with distinct genetic architectures and neuron-specific biology

CRESU Experiments in Astrophysics

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"Laboratory Studies of Chemical Reactions of Astrophysical Interest"

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Collisions moléculaires inélastiques: Méthodes Expérimentales

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Gas Phase reactivity of Complex Organic Molecules at Interstellar Temperatures

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Gas Phase Reaction Kinetics of Complex Organic Molecules at Temperatures of the Interstellar Medium

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Collisions réactives en phase gazeuse, approche expérimentale

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