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International audience; We present a study of the O(3P) + C2H4 scattering reaction, a process that takes place in the interstellar medium and is of relevance in atmospheric chemistry as well. A comprehensive investigation of the electronic properties of the system has been carried out based on multiconfigurational ab initio CASSCF/CASPT2 calculations, using a robust and consistent active space that can deliver accurate potential energy surfaces in the key regions visited by the system. The paper discloses detailed description of the primary reaction pathways and the relevant singlet and triplet excited states at the CASSCF and CASPT2 level, including an accurate description of the critical configurations, such as minima and transition states. The chosen active space and the CASSCF/CASPT2 computational protocol are assessed against coupled-cluster calculations to further check the stability and reliability of the entire multiconfigurational procedure.

Successive hydrogenation reactions of isolated CO molecules adsorbed on a bare graphene surface have been studied by density functional theory using a van der Waals functional. Three hydrogenation scenarios, leading to the formation of methanol via the intermediate species: HCO, H2CO, HCOH, H3CO and H2COH, have been considered. Hydrogenation and adsorption energies on the surface have been calculated for all the species. The fractions of molecules released in the gas phase after formation on the surface have been calculated with two different chemical desorption models. Our results show that the fraction of methanol molecules released in the gas phase is low (

We present a study of the O(

International audience; We present a detailed derivation of the generalized coupled-trajectory mixed quantum classical (G-CT-MQC) algorithm based on the exact factorization equations. The ultimate goal is to propose an algorithm that can be employed for molecular dynamics simulations of non-radiative phenomena, as the spin-allowed internal conversions and the spin-forbidden intersystem crossings. Internal conversions are nonadiabatic processes driven by the kinetic coupling between electronic states, whereas intersystem crossings are mediated by the spin-orbit coupling. In the paper we discuss computational issues related to the suitable representation for electronic dynamics and to the different nature of kinetic and spin-orbit coupling. Numerical studies on model systems allow us to test the performance of the G-CT-MQC algorithm in different situations.

International audience; We investigate spin-orbit interactions in ultrafast molecular processes employing the exact factor-ization of the electron-nuclear wavefunction. We revisit the original derivation by including spin-orbitcoupling, and show how the dynamics driven by the time-dependent potential energy surface allevi-ates inconsistencies arising from different electronic representations. We propose a novel trajectory-based scheme to simulate spin-forbidden non-radiative processes, and we show its performance inthe treatment of excited-state dynamics where spin-orbit effects couple different spin multiplets

PAHs are one of the important components of the carbonaceous matter of the Universe. They are not detected in the darkest regions of the Interstellar Medium and one possible reason could be their chemical transformation through gas phase reactions In particular, their oxidation was considered ineffective because the reaction barriers appear to be too high, based on combustion studies conducted at high temperatures. For the first time, we experimentally studied the oxidation of Coronene, a PAH archetype, at low temperature (50 K), as well as the oxidation of hydrogenated Coronenes. It appears that reactivity is higher than expected and that the fragmentation of coronene is a significant channel of the oxidation. Furthermore, hydrogenated coronenes are very reactive to oxygen. To understand the experimental data, DFT calculations were performed. They confirm a low oxidation barrier (0.11 eV) and show that oxygen is preferentially inserted at the periphery of the coronene and propose a reaction mechanism for fragmentation also involving a hydrogen atom. An estimate of the orders of magnitude shows that PAH oxidation may explain part of the decrease in their abundances in warm environments.

The first steps of the hydrogenation of a coronene molecule deposited on a graphene surface is investigated using Density Functional Theory including a van Der Waals functional. Binding energies and geometries are calculated. The role of the surface is determined by comparing hydrogenation reactions in the gas phase and on the surface. The radical product of the H with a closed-shell system is more stable on the surface. The closed-shell product of the H with a radical species is more stable in the gas phase. This can be interpreted in terms of electronic and geometrical properties of these hydrogenated species.

The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2., Comment: Published in Scientific reports from Nature publishing group

Self-assembled monolayers of p-terphenylthiol (TPT, HS-(C6H4)2-C6H5) deposited onto gold can serve as model systems for aromatic lithography resists. Such thin molecular films are suitably probed using high resolution electron energy loss spectroscopy, due to its high surface sensitivity. Extended energy loss spectra were measured at different probing energies. The TPT monolayer overlapping ν(CH) stretching modes could be modelled by a single effective anharmonic oscillator sustained by a Morse potential energy curve, thanks to the resonant excitation of the associated overtone series at 6 eV. A remarkably good agreement was obtained between the TPT monolayer energy loss spectrum and the computer-simulated infrared vibrational spectrum of the isolated TPT molecule. Density Functional Theory calculations for TPT, fully deuterated TPT and benzenethiol isolated molecules were performed with the exchange correlation functional B3LYP and a dispersion correction, using a triple ζ+ polarisation basis set. By comparing the vibrational patterns obtained for these parent systems, (re-)assignments of all the features observed in the TPT self-assembled monolayer energy loss spectrum are discussed. The obtained vibrational assignments can be confidently transposed to other related systems, such as benzenethiol and biphenyl self-assembled monolayers.

Motivated by the problem of H(2) formation in diffuse clouds of the interstellar medium (ISM), we study the effect of including van der Waals-type corrections in DFT calculations on the entrance PES of the Eley-Rideal reaction H(b) + H(a)-GR → H(b)-H(a) + GR for a graphenic surface GR. The present calculations make use of the PBE-D3 dispersion corrected functional of Grimme et al. (2010) and are carried out on cluster models of graphenic surfaces: C(24)H(12) and C(54)H(18). To assess the soundness of the chosen functional we start by revisiting the H-GR adsorption potential. We find a satisfactory on top physisorption well (43-48 meV) correctly located at an H-GR distance of 3 Å. We then revisit the H(b)-H(a)-GR system using both the PW91 and PBE functionals. Our calculations do not reproduce the tiny potential barrier reported earlier for large H(b)distances from the surface. The barrier in the calculations of Sidis et al. (2000) and Morisset et al. (2003, 2004) has been traced to their previous use of an LSDA + POSTSCF PW91 procedure rather than the genuine PW91 one. The new PBE-D3 PES for the H(b)-H(a)-GR system is reported as a function of the H(b) distance to the surface and its impact parameter relative to the H(a) chemisorbate for the so-called "fixed puckered" ("diabatic" or "sudden") approach. The results are discussed in relation to recent experimental and theoretical work.

Received 22 July 2010; revised manuscript received 28 September 2010; published 8 December 2010 First-principles models of the formation of H2 on interstellar media carbonaceous grains are usually concerned with processes occurring on ideal graphenic surfaces. Until now these models are unable to explain the formation of molecular hydrogen due to the presence of absorption barriers that cannot be overcome at the low temperatures of the interstellar media. We propose an approach emphasizing the role of specific topological defects for molecular hydrogen catalysis at interstellar dust grain models. Using the nudged elastic band method combined with density-functional techniques, we obtain the full catalytic cycle for the formation of the H2 molecule on complex carbon topologies involving the presence of pentagonal rings and C adatoms. Depending on structures, reaction paths can be barrierless or have adsorption barriers as low as 10 3 ‐10 2 eV, which might be easily overcome at the temperatures of the interstellar medium. Such low adsorption barriers indicate that specific carbon grains topological defects are preferential sites for the molecular hydrogen formation in the interstellar medium.