Coincidence mass spectrometry study of double ionization of pyrene by 70 eV electron impact

We have performed coincidence mass spectrometry of fragmentation of pyrene molecules by 70 eV electron impact. Ionized fragments have been mass selected using a reflectron time-of-flight mass spectrometer, and a field programmable gate array has been used for the timing of the electron and ion extraction pulses and for the event-by-event detection of the ions. Double ionization results in a number of prominent fragmentations producing two singly-ionized fragments with kinetic energies of up to a few eV. A number of fragmentations produce ions with four or more carbon atoms, which can only be formed by the breaking of at least three C-C bonds.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Formation and infrared identification of protonated fluoranthene isomers 3-, 9-, and 10-C16H11+ in solid para-H2

Polycyclic aromatic hydrocarbons (PAH) and their derivatives are prospective carriers of unidentified infrared (UIR) emission features observed in interstellar media. Fluoranthene (C16H10) is a simple planar PAH with five- and six-membered rings; it can be considered as a fragment of C60, which, along with its cationic counterpart, has been identified in interstellar media. Protonated fluoranthene, C16H11+, was generated upon electron bombardment during deposition at 3.2 K of p-H2 containing fluoranthene in a small proportion. The intensities of infrared features of C16H11+ decreased after maintaining the matrix in darkness because of its neutralization with trapped electrons. According to the correlations in intensities upon neutralization and secondary photolysis, observed lines were classified into three groups which are assigned to isomers 3-C16H11+, 9-C16H11+, and 10-C16H11+. Experimental vibrational wavenumbers and relative IR intensities of the features agree with corresponding calculated values predicted for these three isomers of C16H11+ with the B3PW91/6-311++G(2d,2p) method. 3-C16H11+ and 9-C16H11+ are predicted to have the lowest energy (within 5 kJ mol-1), whereas 10- and 1-C16H11+ are lying above the global minimum 3-C16H11+ by ∼20 kJ mol-1. However, definitive identification of 1-C16H11+ could not be made as only the most intense line is tentatively assigned. Although the observed spectra of these isomers match unsatisfactorily with the UIR bands, they will facilitate the potential terrestrial and extraterrestrial identification of these species.

Capturing the Elusive Water Trimer from the Stepwise Growth of Water on the Surface of the Polycyclic Aromatic Hydrocarbon Acenaphthene

Polycyclic aromatic hydrocarbons (PAHs) are key players in reaction chemistry. While it is postulated that they serve as a basis for ice grains, there has been no direct detection of PAHs in astronomical environments. We aim to investigate the hydration of PAHs to set a foundation for the future exploration of potential ice formation pathways. We report results from chirped pulse Fourier transform microwave spectroscopy and quantum-chemical calculations for the PAH acenaphthene and acenaphthene complexed with up to four water molecules. The acenaphthene-(H₂O)₃ complex is of particular interest as the elusive cyclic water trimer was observed. It appears in a slightly distorted configuration when compared with the pure water trimer. This is explained by hydrogen-bond net cooperativity effects. Binding energies for the complexes are presented. Our results provide insight into the onset of complex aggregation that could be occurring in extraterrestrial environments as part of ice grain formation.