Dissociation of polycyclic aromatic hydrocarbons: molecular dynamics studies

Abstract

We present dynamical studies of the dissociation of polycyclic aromatic hydrocarbon (PAH) radical cations in their ground electronic states with significant internal energy. Molecular dynamics simulations are performed, the electronic structure being described on-the-fly at the self-consistent-charge density functional-based tight binding (SCC-DFTB) level of theory. The SCC-DFTB approach is first benchmarked against DFT results. Extensive simulations are achieved for naphthalene [Formula: see text], pyrene [Formula: see text] and coronene [Formula: see text] at several energies. Such studies enable one to derive significant trends on branching ratios, kinetics, structures and hints on the formation mechanism of the ejected neutral fragments. In particular, dependence of branching ratios on PAH size and energy were retrieved. The losses of H and H (recognized as the ethyne molecule) were identified as major dissociation channels. The H/CH ratio was found to increase with PAH size and to decrease with energy. For [Formula: see text], which is the most interesting PAH from the astrophysical point of view, the loss of H was found as the quasi-only channel for an internal energy of 30 eV. Overall, in line with experimental trends, decreasing the internal energy or increasing the PAH size will favour the hydrogen loss channels with respect to carbonaceous fragments.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

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

“…Statistical convergence on averaged properties can be reached taking into account explicitly the electronic structure for quite large systems. We can cite for instance the unimolecular reactivity of isolated molecular systems in the gas phase such as the isomerisation [177] and dissociation at high energy [346][347][348] of PAH radical cations. MD/DFTB simulations provide insights into statistical dissociation branching ratios and pathways.…”

“…Finally, MD/DFTB simulations at Figure 10. Snapshots retrieved from MD/DFTB simulations describing the evolution of cationic perylene [C 20 H 12 þ at high energy (,24-26 eV of internal energy): the formation of a fulvenetype isomer was observed, as well as losses of H, H 2 and C 2 H 2 , the expected statistical dissociation pathways for PAH radical cations [346].…”

Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces

“…Another example of gas-phase reactivity is given in the contribution by Simon and coworkers [53]. They studied the decomposition of a particularly interesting class of molecules, the polycyclic aromatic hydrocarbons, which are relevant in astrophysics and astrochemistry.…”

Dissociation of polycyclic aromatic hydrocarbons at high energy: MD/DFTB simulations versus collision experiments

“…A way to achieve extensive simulations introducing explicitely the electronic structure is to use an approximate DFT-scheme such as Density Functional based Tight Binding (DFTB) approach [33,41,11]. The limitation of such an approach is that simulations can hardly be achieved for more than one nanosecond for a PAH as large as coronene (C 24 H 12 ) for instance [45]. As a result, events occurring at longer timescale are unfortunately ignored.It may be noted that in the case of PAHs for which a variety of isomerisation reactions and dissociation pathways are expected [51,52], even at low energy, MD/DFTB simulations allow to provide insight into reaction mechanisms [35] that can be further probed with more accurate approaches.…”