title: Cationic Methylene-Pyrene Isomers and Isomerization Pathways: Finite Temperature Theoretical Studies.
abstract: This paper provides spectral characterizations of the two isomers of the 1-methylenepyrene cation, namely, the 1-pyrenemethylium and a pyrene-like isomer owing a tropylium cycle. Both are possible photodissociation products of the 1-methylpyrene cation and were proposed as potential contributors to the diffuse interstellar bands. In that respect, vibrational and electronic spectra are computed for the optimized structures at the density functional theory (DFT) and time-dependent (TD-)DFT levels. Finite temperature effects on these spectra are estimated from molecular dynamics simulations within the density functional-based tight-binding (DFTB) and TD-DFTB frameworks, these methods being first benchmarked against DFT and TD-DFT calculations. The computed spectra allow discrimination of the two isomers. When the temperature increases, bands are observed to redshift and merge. The isomerization mechanism is investigated with the metadynamics technique, a biased dynamics scheme allowing to probe reaction mechanisms with high energy barriers by investigating the free energy surface at various temperatures. Four pathways with similar barrier heights (3.5-4 eV) are found, showing that the interconversion process would only occur in interstellar clouds under photoactivation. The present study opens the way to simulations on larger methyl- and methylenePAHs of astrophysical interest and their experimental investigation.
authors.affiliation: Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS , 118 Route de Narbonne, F-31062 Toulouse, France.
authors.forename: Mathias
authors.initials: M
authors.lastname: Rapacioli
authors.affiliation: Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS , 118 Route de Narbonne, F-31062 Toulouse, France.
authors.forename: Aude
authors.initials: A
authors.lastname: Simon
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP , 31400 Toulouse, France.|CNRS, IRAP , 9 Avenue du Colonel Roche, BP 44346-31028 Toulouse Cedex 4, France.
authors.forename: Charlotte C M
authors.initials: CC
authors.lastname: Marshall
authors.affiliation: Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS , 118 Route de Narbonne, F-31062 Toulouse, France.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Cuny
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP , 31400 Toulouse, France.|CNRS, IRAP , 9 Avenue du Colonel Roche, BP 44346-31028 Toulouse Cedex 4, France.
authors.forename: Damian
authors.initials: D
authors.lastname: Kokkin
authors.affiliation: Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS , 118 Route de Narbonne, F-31062 Toulouse, France.
authors.forename: Fernand
authors.initials: F
authors.lastname: Spiegelman
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP , 31400 Toulouse, France.|CNRS, IRAP , 9 Avenue du Colonel Roche, BP 44346-31028 Toulouse Cedex 4, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Joblin