First spectroscopic evidence for the formation of pentalene in the dissociation of gaseous naphthalene
Polycyclic aromatic hydrocarbons (PAHs) are perfused throughout chemistry and play an important role in combustion, atmospheric science, but also in the interstellar medium. Acetylene loss is well known to constitute one of the main breakdown pathways of gaseous PAHs, but what the molecular structures of the reaction products are, remains poorly understood. In a publication in Chemical Communications, Jordy Bouwman, Arjen de Haas and Jos Oomens from the FELIX Laboratory at Radboud University provide the first spectroscopic evidence for pentalene formation in the dissociative ionization of naphthalene.
Naphthalene (C10H8) is the smallest PAH having the structure of two fused 6-membered benzene rings. When the naphthalene ion loses an acetylene unit (C2H2), a product ion of C8H6 composition is formed. Several C8H6 isomers are conceivable, but the exact structure of this product ion had never been experimentally established. One of these possible isomers is pentalene, which consists of two fused 5-membered rings and which as a bulk neutral species is very unstable due to its anti-aromatic character.
Using a combination of tandem mass spectrometry and infrared laser spectroscopy at the FELIX Laboratory, Jordy Bouwman, Arjen de Haas and Jos Oomens now confirmed experimentally that the acetylene-loss product of naphthalene is indeed pentalene.
The structural change that naphthalene undergoes to form pentalene may provide important clues to understand the formation of fullerenes, a class of carbonaceous molecules with 5- and 6-membered rings leading to cage-like structures. Chemists hypothesize that processing of large PAHs is responsible for the formation of fullerenes. The current publication shows how facile 6-memebered ring species convert to 5-membered ring species, possibly linking PAHs to fullerenes. Further studies will address structural conversion in larger PAHs, to confirm the relation between PAHs and fullerenes.
At the heart of the FELIX Laboratory are three free-electron lasers that produce very intense, short-pulse infrared and terahertz (THz) light. The interaction of this light with molecules and materials provides unique information about the 3D structure, electronic properties and dynamics of matter.
Spectroscopic evidence for the formation of pentalene in the dissociative ionization of naphthalene
Jordy Bouwman, Arjen J. de Haas en Jos Oomens
Chem. Commun. 2016, 52, 2636-2638 (DOI: 10.1039/c5cc10090a)
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Dr. Jordy Bouwman