Polycyclic Aromatic hydrocarbons Reactions in Cryogenic Solids – PARCS

The PARCS project is built with a double approach, experimental and theoretical, in order to descibe PAH reactions.
The experimental part concerns the study of ternary systems PAHs/Fe/H2O à very low temperature, thanks to a specific technique called matrix isolation spectroscopy.
The theoretical part needs to build a multiscale/multimethod approach. Difficulties arise from the big size of these systems, the complexity of their electronic structure, the nature of their interactions (weak van der Waals bonds, Hydrogen bonds, ...) and from the statistical significance of the molecular dynamics simulations.
Ab-initio methods (DFT type) are used and /or calibrated, with a special address to the description of anharmonic and quantum effects on IR spectra, together with the description of reaction mechanisms thanks to the use of classical and quantum dynamics. The effect of environment (matrix medium, water ice) is studied via combined orbital/force field methods.

Interstellar ice analogs have been studied for twenty years, but the study of PAH doped water ices is quite recent and we have demonstrated that UV-visible irradiation of coronene or pyrene embedded in amorphous water ice at 10K led to the formation of quinones and alcohols very quiclky and at energy levels much lower than the PAH ionization potential values.
We have shown that these reactions occur also in diluted conditions (ie between one PAH molecule and a few water molecules isolated in argon matrices). This is an unexpected result, which opens new questions about environment effects, the properties of water aggregates adsorbed on PAHs, the nuclear quantum effects, the role of PAH excited states and the modelisation of water ices.
First results on organometallic reactions between iron atoms and water have shown a facile insertion of Fe into H2O, leading to HFeOH species. These reactions could play a role in the production of hydrogen in the ISM.

Thanks to the synergy of both approaches, this project will contribute to the assignment of AIBs and DIBs, to the understanding of the role of water and iron in the photo-oxidation of PAHs, with new data for astrophysics and environmental science, as PAHs and oxidized PAHs are also pollutants of our atmosphere.
New experimental and theoretical developments will be also produced, and particularly the development of new calulation codes (implementation of DFTB3 and metadynamics into the deMonNano code).

Five papers have already been published in international scientific journals: J. Chem. Theor. Comput. 2014, A. Scemama et al; J. Phys. Chem. A 2015, A. Simon et al ; J. Phys. Chem. A 2015, M. Lanza et al ; PCCP 2015, L.F.L. Oliveira et al.; J. Phys. Chem A 2015, M. Rapacioli et al.
23 oral communications and 14 posters have been produced in international and national conferences, with 11 invited conferences, and two oral communications at ERC NanoCosmos meetings.
We also have contributed to seven actions for dissemination of science to the public at Toulouse (Novela festival) and Bordeaux (Reseacher European Night).

Polycyclic aromatic hydrocarbons (PAH) are organic macromolecules that have an astrophysical interest, since the discovery of the AIBs (aromatic interstellar bands), assigned to vibration modes of PAH, which emit in the infrared (IR) after absorption of star UV light. PAH are also candidates for diffuse interstellar bands (DIBs), which are weak absorption bands measured in the visible range. However, no specific PAH has been identified yet, and a variety of PAH-derived species (ionized, substituted, coordinated) have been proposed. The ambition of the PARCS project is to bring new elements for the understanding of these assignments through the study of:
1) Reactions of PAH with Fe atoms and aggregates. The hypothesis of the formation of metal complexes Fex(PAH)y was proposed to account for the depletion of Fe from the gaseous phase of the interstellar medium, but no study has validated this hypothesis for neutral Fex(PAH)y complexes yet.
2) Photo-reactions of PAH embedded in or trapped on water ice. In dense molecular clouds, atoms and molecules are condensed on cold dust and ice particles, where they are transformed by thermal and photochemical processes, leading to the formation of more complex molecules, some of them being of astrobiological interest. IR spectroscopy, associated to matrix isolation techniques, is the main tool used to study the composition of interstellar ices, and has allowed to identify simple molecules (NH3, CO2, OCS, CH3OH) and to understand the formation of more complex species. Although IR spectroscopy of ice has been performed for a long time, studies of ices containing PAH have just begun and have shown that neutral PAH may play an important and unexpected role in cosmic ice chemistry. For instance, IR identification of quinones, alcohols and ketones through photo-irradiation of coronene or pyrene on amorphous solid water has been published in 2011 by Partner 1.
The originality of this project is to propose pioneering joint theoretical and experimental treatments of these reactions in order to understand how these molecules are formed. In the experimental part, the challenging novelty is the study of ternary PAH/Fe/H2O systems at cryogenic temperatures, that will be made by means of a specially designed furnace fixed to the cryostat, allowing co-deposition of Fe and PAH, and of two spectrometers, allowing to run both IR and UV spectroscopy on the same sample.
In the theoretical part (Partners 2 and 3), the challenging goal is to describe the dynamic processes that drive these reactions, namely (i) characterize the reaction products, through their IR and electronic spectra, and (ii) achieve molecular dynamics (MD) simulations to characterize the mechanisms, in the ground and excited state after electronic excitation. This implies the elaboration of a multiscale-multimethod approach that uses some of the best current available techniques with various levels of sophistication and efficiency. The difficulty arises from the large size of the systems, their complex electronic structure (open-shell), the nature of the interactions (van der Waals, H-bonds…) and efficiency of the MD schemes to achieve statistical significance. Ab initio wavefunction methods (with local orbitals), DFT and approximate-DFT approaches will be used with cross-benchmarking. Efforts will also be dedicated to describe anharmonic and quantum effects on IR spectra using classical and quantum dynamics, and to the elucidation of reactive mechanisms via MD simulations. Finally, the influence of environment (matrix, ice) will be focused on via combined orbital/force field methods.
Thanks to the synergy of both approaches, this project will contribute to the assignment of the AIBs and DIBs and to the understanding of the role of water-ice in the photochemical reactions of PAH with species of astrophysical and environmental interest (Fe, H2O). New experimental and theoretical developments, mandatory to achieve these goals, will be proposed.