Atomic hydrogen interactions with gas-phase coronene cations: hydrogenation versus fragmentation

Addressing electronic and dynamical evolution of molecules and molecular clusters: DFTB simulations of energy relaxation in polycyclic aromatic hydrocarbons

We present a review of the capabilities of the density functional based Tight Binding (DFTB) scheme to address the electronic relaxation and dynamical evolution of molecules and molecular clusters following energy deposition via either collision or photoabsorption. The basics and extensions of DFTB for addressing these systems and in particular their electronic states and their dynamical evolution are reviewed. Applications to PAH molecules and clusters, carbonaceous systems of major interest in astrochemical/astrophysical context, are reported. A variety of processes are examined and discussed such as collisional hydrogenation, fast collisional processes and induced electronic and charge dynamics, collision-induced fragmentation, photo-induced fragmentation, relaxation in high electronic states, electronic-to-vibrational energy conversion and statistical versus non-statistical fragmentation. This review illustrates how simulations may help to unravel different relaxation mechanisms depending on various factors such as the system size, specific electronic structure or excitation conditions, in close connection with experiments.

Low-Energy Transformation Pathways between Naphthalene Isomers

The transformation pathways between low-energy naphthalene isomers are studied by investigating the topology of the energy landscape of this astrophysically relevant molecule. The threshold algorithm is used to identify the minima basins of the isomers on the potential energy surface of the system and to evaluate the probability flows between them. The transition pathways between the different basins and the associated probabilities were investigated for several lid energies up to 11 eV, this value being close to the highest photon energy in the interstellar medium (13.6 eV). More than a hundred isomers were identified and a set of 23 minima was selected among them, on the basis of their energy and probability of occurrence. The return probabilities of these 23 minima and the transition probabilities between them were computed for several lid energies up to 11 eV. The first connection appeared at 3.5 eV while all minima were found to be connected at 9.5 eV. The local density of state was also sampled inside their respective basins. This work gives insight into both energy and entropic barriers separating the different basins, which also provides information about the transition regions of the energy landscape.

Formation of the acenaphthylene cation as a common C2H2-loss fragment in dissociative ionization of the PAH isomers anthracene and phenanthrene

Polycyclic aromatic hydrocarbons (PAHs) are thought to be a major constituent of astrophysical environments, being the carriers of the ubiquitous aromatic infrared bands (AIBs) observed in the spectra of galactic and extra-galactic sources that are irradiated by ultraviolet (UV) photons. Small (2-cycles) PAHs were unambiguously detected in the TMC-1 dark cloud, showing that PAH growth pathways exist even at low temperatures. The processing of PAHs by UV photons also leads to their fragmentation, which has been recognized in recent years as an alternative route to the generally accepted bottom-up chemical pathways for the formation of complex hydrocarbons in UV-rich interstellar regions. Here we consider the C12H8+ ion that is formed in our experiments from the dissociative ionization of the anthracene and phenanthrene (C14H10) molecules. By employing the sensitive action spectroscopic scheme of infrared pre-dissociation (IRPD) in a cryogenic ion trap instrument coupled to the free-electron lasers at the FELIX Laboratory, we have recorded the broadband and narrow line-width gas-phase IR spectra of the fragment ions (C12H8+) and also the reference spectra of three low energy isomers of C12H8+. By comparing the experimental spectra to those obtained from quantum chemical calculations we have identified the dominant structure of the fragment ion formed in the dissociation process to be the acenaphthylene cation for both isomeric precursors. Ab initio molecular dynamics simulations are presented to elucidate the fragmentation process. This result reinforces the dominant role of species containing a pentagonal ring in the photochemistry of small PAHs.

Collision-induced dissociation of protonated uracil water clusters probed by molecular dynamics simulations

Collision-induced dissociation experiments of hydrated molecular species can provide a wealth of important information. However, they often need a theoretical support to extract chemical information. In the present article, in order to provide a detailed description of recent experimental measurements [Braud et al., J. Chem. Phys., 2019, 150, 014303], collision simulations between low-energy protonated uracil water clusters (H₂O)_1-7,11,12UH⁺ and an Ar atom were performed using a quantum mechanics/molecular mechanics formalism based on the self-consistent-charge density-functional based tight-binding method. The theoretical proportion of formed neutral vs. protonated uracil containing clusters, total fragmentation cross sections as well as the mass spectra of charged fragments are consistent with the experimental data which highlights the accuracy of the present simulations. They allow to probe which fragments are formed on the short time scale and rationalize the location of the excess proton on these fragments. We demonstrate that this latter property is highly influenced by the nature of the aggregate undergoing the collision. Analyses of the time evolution of the fragments populations and of their relative abundances demonstrate that, up to 7 water molecules, a direct dissociation mechanism occurs after collision whereas for 11 and 12 water molecules a statistical mechanism is more likely to participate. Although scarce in the literature, the present simulations appear as a useful tool to complement collision-induced dissociation experiments of hydrated molecular species.

Density-functional tight-binding: basic concepts and applications to molecules and clusters

The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation, treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, non-adiabatic dynamics. A number of applications are reviewed, focusing on -(i)- the variety of systems that have been studied such as small molecules, large molecules and biomolecules, bare orfunctionalized clusters, supported or embedded systems, and -(ii)- properties and processes, such as vibrational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given.

Ion Dissociation Dynamics of 1,2,3,4-Tetrahydronaphthalene: Tetralin as a Test Case For Hydrogenated Polycyclic Aromatic Hydrocarbons

The unimolecular dissociation of ionized tetralin was probed by tandem mass spectrometry, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy, and theory. The major reactions observed were the loss of the hydrocarbons CH₃^•, C₂H₄, and C₃H₅^• together with H^•-atom loss. RRKM modeling of the iPEPICO data suggested a two-well potential energy surface. Ionized tetralin can lose all four neutrals via H-shift and ring-opening reactions or CH₃^• and C₂H₄ after interconversion to the 1-methylindane ion, a process similar to that found for ionized 1,2-dihydronaphthalene (isomerizing to form the 1-methylindene ion structure). This was confirmed at the B3LYP/6-31+G(d,p) level of theory, and potential mechanisms for all reactions are described. The ionization energy of tetralin was established from the threshold photoelectron spectrum to be 8.46 ± 0.01 eV.