Laboratory photo-chemistry of covalently bonded fluorene clusters: observation of an interesting PAH bowl-forming mechanism. (arXiv:1901.03325v1 [astro-ph.IM])

Laboratory photo-chemistry of covalently bonded fluorene clusters: observation of an interesting PAH bowl-forming mechanism. (arXiv:1901.03325v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_W/0/1/0/all/0/1">Weiwei Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Si_Y/0/1/0/all/0/1">Yubing Si</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhen_J/0/1/0/all/0/1">Junfeng Zhen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_T/0/1/0/all/0/1">Tao Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Linnartz_H/0/1/0/all/0/1">Harold Linnartz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tielens_A/0/1/0/all/0/1">Alexander G. G. M. Tielens</a>

The fullerene C$_{60}$, one of the largest molecules identified in the
interstellar medium (ISM), has been proposed to form top-down through the
photo-chemical processing of large (more than 60 C-atoms) polycyclic aromatic
hydrocarbon (PAH) molecules. In this article, we focus on the opposite process,
investigating the possibility that fullerenes form from small PAHs, in which
bowl-forming plays a central role. We combine laboratory experiments and
quantum chemical calculations to study the formation of larger PAHs from
charged fluorene clusters. The experiments show that with visible laser
irradiation, the fluorene dimer cation –
[C$_{13}$H$_{9}$$-$C$_{13}$H$_{9}$]$^+$ – and the fluorene trimer cation –
[C$_{13}$H$_{9}$$-$C$_{13}$H$_{8}$$-$C$_{13}$H$_{9}$]$^+$ – undergo
photo-dehydrogenation and photo-isomerization resulting in bowl structured
aromatic cluster-ions, C$_{26}$H$_{12}$$^+$ and C$_{39}$H$_{20}$$^+$,
respectively. To study the details of this chemical process, we employ quantum
chemistry that allows us to determine the structures of the newly formed
cluster-ions, to calculate the hydrogen loss dissociation energies, and to
derive the underlying reaction pathways. These results demonstrate that smaller
PAH clusters (with less than 60 C-atoms) can convert to larger bowled
geometries that might act as building blocks for fullerenes, as the
bowl-forming mechanism greatly facilitates the conversion from dehydrogenated
PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment
that – in principle – allows to search for such species using radio astronomy.

The fullerene C$_{60}$, one of the largest molecules identified in the
interstellar medium (ISM), has been proposed to form top-down through the
photo-chemical processing of large (more than 60 C-atoms) polycyclic aromatic
hydrocarbon (PAH) molecules. In this article, we focus on the opposite process,
investigating the possibility that fullerenes form from small PAHs, in which
bowl-forming plays a central role. We combine laboratory experiments and
quantum chemical calculations to study the formation of larger PAHs from
charged fluorene clusters. The experiments show that with visible laser
irradiation, the fluorene dimer cation –
[C$_{13}$H$_{9}$$-$C$_{13}$H$_{9}$]$^+$ – and the fluorene trimer cation –
[C$_{13}$H$_{9}$$-$C$_{13}$H$_{8}$$-$C$_{13}$H$_{9}$]$^+$ – undergo
photo-dehydrogenation and photo-isomerization resulting in bowl structured
aromatic cluster-ions, C$_{26}$H$_{12}$$^+$ and C$_{39}$H$_{20}$$^+$,
respectively. To study the details of this chemical process, we employ quantum
chemistry that allows us to determine the structures of the newly formed
cluster-ions, to calculate the hydrogen loss dissociation energies, and to
derive the underlying reaction pathways. These results demonstrate that smaller
PAH clusters (with less than 60 C-atoms) can convert to larger bowled
geometries that might act as building blocks for fullerenes, as the
bowl-forming mechanism greatly facilitates the conversion from dehydrogenated
PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment
that – in principle – allows to search for such species using radio astronomy.

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