Marie-Christine Bacchus-Montabonel

Negative ion formation in electron transfer experiments from fast neutral potassium (K) atom collisions with neutral tetrachloromethane (CCl4) molecules has been investigated in the laboratory frame range of 8–1000 eV. Comprehensive calculations on the electronic structure were performed for CCl4 in the presence of a potassium atom and used to help analyse the lowest unoccupied molecular orbitals participating in the collision process. Additionally, K+ energy loss produced in the forward direction has served to further our knowledge on the electronic state spectroscopy of CCl4. A vertical electron affinity of (–0.79 ± 0.20) eV has been obtained and assigned to a purely repulsive transition from CCl4 ground state to the 2T2 state of the temporary negative ion yielding Cl– formation. Other features in the energy loss spectrum were observed for the first time and related to Cl2–, CCl2– and CCl3– formation. A special attention is also given to the unresolved feature corresponding to a positive electron affinity of (0.24 ± 0.2) eV, assigned to a vibrationally hot transition from CCl4 ground state into the triply degenerate 2T2 excited state of the negative ion. The combined time-of-flight mass spectrometry together with K+ energy loss data represents the most comprehensive assignment of the tetrachloromethane anion yields and the role of CCl4 electronic states in collision induced dissociation to date.

The behaviour of potential prebiotic species in space is of main concern in the chemistry at the origin of life. Their reactivity or stability in spatial conditions, under strong UV radiations or ion bombardments remains an open question and needs wide investigations. As protons are by far the most abundant ions in space, we focus presently on proton-induced collisions on imid-azole and 2-aminoimidazole evidenced as important prebiotic RNA intermediates. Uncon-strained full optimization of the structures was performed with B3LYP/cc-pVTZ model chem-istry. The calculations were performed in a wide collision energy range in order to model vari-ous astrophysical environments, from eV in the interstellar medium, up to keV for solar winds or supernovae shock-wave protons. Such study drives for the first time a theoretical insight on the influence of the amino substituent on the proton-induced charge transfer. We evaluated besides the role of icy grain environments through a cluster approach modelling the effect of a stepwise microhydration on the process. Comparisons with oxygenated and sulfurated ana-logues address further qualitative trends on the respective stability or reactivity of such heter-ocycles which may be of tremendous interest in prebiotic chemistry. Charge transfer appears effectively quite efficient for imidazole compounds and their sulfurated analogue compared to the oxygenated heterocycle.

Electron transfer induced decomposition mechanism of nitroimidazole and a selection of analogue molecules in collisions with neutral potassium (K) atoms from 10 to 1000 eV have been thoroughly investigated. In this laboratory collision regime, the formation of negative ions was time-of-flight mass analyzed and the fragmentation patterns and branching ratios have been obtained. The most abundant anions have been assigned to the parent molecule and the nitrogen oxide anion (NO₂^–) and the electron transfer mechanisms are comprehensively discussed. This work focuses on the analysis of all fragment anions produced and it is complementary of our recent work on selective hydrogen loss from the transient negative ions produced in these collisions. Ab initio theoretical calculations were performed for 4-nitroimidazole (4NI), 2-nitroimidazole (2NI), 1-methyl-4- (Me4NI) and 1-methyl-5-nitroimidazole (Me5NI), and imidazole (IMI) in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process.

The rate constants of H2 formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg4Si4O12)(H2O)N, were computed over a wide temperature range [180-2000K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculat-ing reaction energies and barrier heights, and computed the minimum energy H + H → H2 reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semi-classical approaches that take into account tunneling and non-classical reflection effects by means of the Zero Curvature Tunneling (ZCT), the Small Curvature Tunneling (SCT) and the one-dimensional Semi Classical Transition State Theory (SCTST) methods, which all provided comparable results. Our investigations show that the H2 formation process following the Langmuir-Hinshelwood (LH) mecha-nism is more efficient on the hydroxylated (N = 1 - 4) nanoclusters than on the bare (N = 0) one due to the barrier height being higher than for the hydroxylated nanoclusters. H2 formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg4Si4O12)(H2O)2 nanocluster for all nine considered methods. Overall, we conclude that all the con-sidered nanoclusters are very efficient catalyzing grains for H2 formation in the physical conditions of the Interstellar Medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

We report novel data on ion-pair formation in hyperthermal (30–800 eV) neutral potassium collisions with neutral pyrimidine (Pyr, C₄H₄N₂) molecules. In this collision regime, negative ions formed by electron transfer from the alkali atom to the target molecule were time-of-flight mass analyzed and the fragmentation patterns and branching ratios have been obtained. The most abundant product anions have been assigned to CN⁻ and C₂H⁻ and the electron transfer mechanisms are comprehensively discussed. Particular importance is also given to the efficient loss of integrity of the pyrimidine ring in the presence of an extra electron, which is in contrast to dissociative electron attachment experiments yielding the dehydrogenated parent anion. Theoretical calculations were performed for pyrimidine in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process. In order to further our knowledge about the collision dynamics, potassium cation (K⁺) energy loss spectrum has been obtained and within this context, we also discuss the role of the accessible electronic states. A vertical electron affinity of (−5.69 ± 0.20) eV was obtained and may be assigned to a π3*(b₁) state that leads to CN⁻ formation.

Charge transfer theoretical treatment of proton-induced collisions on the sulfur-containing cyclic RNA precursor, 2-aminothiazole. Role of sulfur in the process and implications in prebiotic selective synthesis.

We report on a combined experimental and theoretical study of electron-transfer-induced decomposition of adenine (Ad) and a selection of analog molecules in collisions with potassium (K) atoms. Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6–68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine (Pu), adenine (Ad), 9-methyl adenine (9-mAd), 6-dimethyl adenine (6-dimAd), and 2-D adenine (2-DAd). Following our recent communication about selective hydrogen loss from the transient negative ions (TNIs) produced in these collisions [T. Cunha et al., J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH₂ loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K⁺ cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process.

The role of sulfur in proton-induced charge transfer is analyzed through an ab initio theoretical study of mercaptoacetonitrile HSCH₂CN with related HCN oligomers.

Recent photochemistry experiments provided evidence for the formation of hydantoin by irradiation of interstellar ice analogues. The significance of these results and the importance of hydantoin in prebiotic chemistry and polypeptide synthesis motivate the present theoretical investigation, in which we analyzed the effects of stepwise hydration on the structures, electronic and thermodynamical properties of microhydrated hydantoin using a variety of computational approaches. We generally find microhydration to proceed around the hydantoin heterocycle until 5 water molecules are reached, at which stage hydration becomes segregated with a water cluster forming aside the heterocycle. The reactivity of microhydrated hydantoin caused by an impinging proton was evaluated through charge transfer collision cross sections, for microhydrated compounds but also for hydantoin on icy grains modeled using a cluster approach mimicking the true hexagonal ice surface. The effects of hydration on charge transfer efficiency are mostly significant when few water molecules are present, and progressively weaken and stabilize in larger clusters. On the ice substrate, charge transfer essentially contributes to a global increase in the cross sections.

Context. Helium, the second most abundant element in the Universe, with a relative abundance of He/H ~ 1/10, has never been observed in any other form than that of a neutral atom (He) or an ion (He⁺) in the interstellar medium. Since He is a noble gas its non-observation as part of neutral molecular systems is understandable, but it is very surprising for a positively charged species such as HeH⁺ that is a stable diatomic ion whose spectral signatures are well known in the laboratory. Aims. This non-observation, even in hydrogen rich regions, could imply that HeH⁺ is not a proper target and that alternatives have to be considered, such as small HeH_n⁺ clusters. The present study aims at finding whether the leading term HeH₃⁺ fulfills the conditions required. Methods. We addressed the question with state-of-the-art numerical simulations. We determined a two-dimension ab initio potential energy surface (PES) of the HeH₃⁺ cluster along the He...H₃⁺ and HeH⁺...H₂ reaction coordinates. The calculations rely on complete active space configuration interaction followed by a second order perturbation treatment (CAS-PT2). This surface was used for the evaluation of the two radiative associations rate constants by means of a quantum treatment of the collision between the interacting fragments. Results. These calculations show unambiguously that HeH₃⁺ is the most stable point on the corresponding global PES. Then, we determined the rate constants of the radiative associations HeH⁺ + H₂ and He + H₃⁺ leading to HeH₃⁺. Conclusions. Significative values were obtained that reach up to 2 × 10^-18 cm³ s^-1, which should stimulate new tentatives to detect molecular helium in astrophysical objects.