Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose C₂H₂?

Abstract

The reaction mechanisms for the loss of C2H2 from the ions of anthracene, phenanthrene, tetracene, and pyrene were calculated at the B3-LYP/6-311++G­(2d,p) level of theory and compared to that previously published for ionized naphthalene. A common pathway emerged involving the isomerization of the molecular ions to azulene-containing analogues, followed by the contraction of the seven-member ring into a five- and four-member fused ring system, leading to the cleavage of C2H2. The key transition state was found to be for this last process, and its relative energy was consistent going from naphthalene to tetracene. That for pyrene, though, was significantly higher due to the inability of the azulene moiety to achieve a stable conformation because of the presence of the three fused rings. Thus, C2H2 loss is discriminated against in pericondensed PAHs. For catacondensed PAHs, C2H2 loss also drops in relative abundance as the PAH gets larger due to the increase in the number of available hydrogen atoms, increasing the rate constant for H atom loss over that for C2H2 loss as PAH size increases. The unimolecular reactions of four cyano-substituted polycyclic aromatic hydrocarbon (PAH) ions were also probed as a function of collision energy by collision-induced dissociation tandem mass spectrometry. As the size of the ring system increases, HCN loss decreases in importance relative to other processes (H and C2H2 loss). 9-Cyanophenanthrene ions were chosen for further exploration by theory and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The calculated reaction pathway and energetics for C2H2 loss were consistent with those found above. The calculations suggest that larger PAHs of interest in the interstellar environment will behave independently of a CN substituent.

Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

“…With regard to the loss of a C 2 H 2 unit, pathways similar to those described by Trinquier et al (2017b) and West et al (2019) for Pyr + were verified. Like Pyr + , 1EtyPyr + features direct routes for the loss of C 2 H 2 from an inner ring or from an outer one (respectively dissociation types 1 and 2 in Trinquier et al 2017b).…”

IR photofragmentation of the phenyl cation: spectroscopy and fragmentation pathways

“…34,35 Furthermore, many studies on the larger PAH species have shown a trend for smaller and irregular PAHs to be more likely to lose [2C,2H] than H or H 2 upon photofragmentation. [36][37][38][39][40][41][42] It would thus be of great interest to find out if phenylium follows this trend.…”

Structural investigation of doubly-dehydrogenated pyrene cations

“…However, for pericondensed PAHs, including pyrene, loss of C 2 H 2 is inhibited because a key transition state is too high in energy. 10 Indeed, experiments reveal that for PAHs in the astrophysically-relevant size range (50-100 carbon atoms), fragmentation is dominated by hydrogen-loss until mostif not all -hydrogen atoms are lost. 8,10,11 For completeness, we mention that in regions with high atomic hydrogen density and low UV fields, interstellar PAHs can be superhydrogenated (i.e.…”

“…10 Indeed, experiments reveal that for PAHs in the astrophysically-relevant size range (50-100 carbon atoms), fragmentation is dominated by hydrogen-loss until mostif not all -hydrogen atoms are lost. 8,10,11 For completeness, we mention that in regions with high atomic hydrogen density and low UV fields, interstellar PAHs can be superhydrogenated (i.e. some or all of the edge carbon atoms have acquired an additional hydrogen atom).…”