Research highlights

Very complex organic molecules are detected in cold molecular clouds, such as Taurus Molecular Cloud (TMC-1). This shows that such species are already synthesized in the earliest stages of star formation. In this study published in Nature Astronomy we show that ortho-benzyne, a cyclic molecule with biradical character that has recently been detected in the cold molecular cloud TMC-1, plays a critical role in the bottom-up formation of complex hydrocarbons. To achieve this, we recorded threshold  photoelectron spectra at the Swiss Light Source to identify the products formed in a pyrolysis microreactor from the methyl + ortho-benzyne reaction. In conjunction with quantum chemical calculations we could assing the ethynylcyclopentadienes (1- and 2-) as well as fulvenallene that were recently found in TMC-1.  Also see this press release on the paper.

Threshold photoelectron spectroscopy of quinoxaline, quinazoline and cinnoline

Nitrogen-containing Polycyclic Aromatic Hydrocarbons (PAHs) and possibly also PAHs with multiple nitrogen atoms in their carbon skeleton are thought to be abundantly present in various environments, ranging from planetary atmospheres, such as that of Saturn's largest moon Titan, to the interstellar medium. Little is known about the actual reaction mechanisms by which these species form. One commonly applied laboratory method for identification of such products in reactive environments is threshold photoelectron spectroscopy, but for this, accurate reference data are needed to assign species. In this work, we present the threshold photoelectron spectra of three isomers of C8H6N2 composition (cinnoline, quinoxaline and quinazoline) to investigate their electronic structure and the role of isomerism in their spectroscopic appearance. We identify a total of 12 adiabatic ionization energies based on quantum chemical computations in combination with Franck-Condon simulations. This work is published in the Journal of Physical Chemistry A - 10 Years of the ACS PHYS Astrochemistry Virtual Special Issue.  Credits for this work go to Dr. Domenik Schleier, who has done a fantastic job analyzing the complicated electronic structure of these species.

Formation of pentalene from the dissociation of naphthalene

The loss of an acetylene unit constitutes one of the major dissociation pathways of irregular PAHs, yet the structures resulting from this unimolecular dissociation havd remained elusive for a long time. Using Infrared Multiphoton Dissociation (IRMPD) on mass selected species trapped in a quadrupole ion trap apparatus we have studied the dissociative ionization of naphthalene. From the spectra, we concluded that the pentalene cation is formed upon dissociative ionization of the smallest PAH, naphthalene. The underlying hexagon to pentagon formation may play a crucial role in the formation of interstellar buckminsterfullerenes by photodissociaion of large PAHs. The work is published in Chemical Communications.

Five birds with one stone: photoelectron photoion coincidence unveils rich phthalide pyrolysis chemistry

Molecules with a mass-to-charge ratio of 90 amu are commonly detected as reaction products in combustion and flame experiments. The pentagon-bearing molecules fulvenallene, 1-ethynyl-cyclopentadiene, 2-ethynylcyclo-pentadiene, and 5-ethynyl-cyclopentadiene are four energetically favorable isomers of C7H6 composition that can account for this signal. The nature of the m/z 90 species formed in combustion, however, is unknown due to the fact that the ionization energies of the potential carriers have not been determined experimentally. Moreover, the computed ionization energies of fulvenallene and 1-ethynylcyclo-pentadiene are too close in energy to make a conclusive assignment. In this work, we pyrolyze phthalide and record the photoion mass-selected threshold photoelectron spectra (TPES) of the product species at two temperatures. Based on Franck-Condon simulations of the TPE spectra we were able to assign the ionization energies of the four isomers as well as identify electronically excited ionic states. The C7H5 fulvenalleny radical forms at higher temperatures and its ground- and excited states have also been characterized from the TPES. The ionization potentials and excited state features allow for these species to be identified in flame experiments to assess their role in the growth of polycyclic aromatic hydrocarbons and soot formation. The work is published in the Journal of Physical Chemistry A, but unfortunately without the marvelous artwork made by Zoey Sophie Bouwman. 

Formation of cyclopentadiene in the allyl+acetylene reaction

The formation of PAHs in combustion environments has been extensively studied, yet the chemical mechanisms leading to the formation of the first aromatic rings is still not completely understood. It has been proposed that resonantly stabilized radicals play a pivotal role in the formation of aromatics. One of these radicals, the allyl (C3H5) radical, has been proposed to yield a cyclic hydrocarbon by reaction with acetylene. Direct spectroscopic evidence for this mechanism is largely lacking from the literature. Using threshold photoelectron spectroscopy on the iPEPICO machine at Vacuum Ultraviolet Beamline of the the Swiss Light Source (SLS) we found that cyclopentadiene is indeed formed from this reaction. The results have been published in Physical Chemistry Chemical Physics 

Structural investigation of doubly-dehydrogenated pyrene cations

In interstellar regions that are illuminated by strong UV fields, PAHs can ionize and even dissociate. One common PAH decomposition mechanism is the loss of molecular hydrogen. We set out to reveal the molecular structure of the dissociation product resulting from H2 loss from a prototypical pericondensed PAH species, pyrene (C16H10). We employed infrared predissociation spectroscopy in a cryogenic 22-pole ion trap instrument at the free electron laser for infrared experiments (FELIX) to record a well-resolved infrared fingerprint spectrum of the mass-isolated (neon-tagged) dissociation products. Comparison with Density Functional Theory (DFT) computed IR spectra revealed that the 1,2- and 3,4- doubly dehydrogenated Pyrene (ddPy+) isomers are abundantly formed and that a contribution by the 1,3-ddPy isomer cannot be ruled out. The study shows that hydrogen roaming from one ring to an adjacent aromatic ring prior to dissociation is inhibited. Furthermore, the study sheds light on the potential role that small PAHs have on hydrogen formation in strongly illuminated interstellar objects. The work is available as a HOT article from Physical Chemistry Chemical Physics. Figure courtesy of first author Sanjana Panchagnula.