%0 DATA
%A Mathias, Rapacioli
%A Aude, Simon
%A Charlotte
C. M., Marshall
%A Jérôme, Cuny
%A Damian, Kokkin
%A Fernand, Spiegelman
%A Christine, Joblin
%D 2015
%T Cationic Methylene–Pyrene Isomers and Isomerization
Pathways: Finite Temperature Theoretical Studies
%U https://acs.figshare.com/articles/journal_contribution/Cationic_Methylene_Pyrene_Isomers_and_Isomerization_Pathways_Finite_Temperature_Theoretical_Studies/2094820
%R 10.1021/acs.jpca.5b09494.s001
%2 https://acs.figshare.com/ndownloader/files/3728032
%K DFTB
%K density
%K Finite Temperature
%K tropylium cycle
%K energy surface
%K interconversion process
%K StudiesThis paper
%K isomerization mechanism
%K Finite temperature effects
%K photodissociation products
%K optimized structures
%K probe reaction mechanisms
%K dynamics scheme
%K metadynamics technique
%K cation
%K Isomerization Pathways
%K energy barriers
%K temperature increases
%K astrophysical interest
%K DFT
%K TD
%K isomers
%K spectra
%K dynamics simulations
%X 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.