Can OPAH ions be a source of CO and HCO in the interstellar medium? Lessons learned from the unimolecular dissociation of dibenzofuran and dibenz[b,f]oxepin radical cations

Abstract

Unlike polycyclic aromatic hydrocarbons (PAHs), which are recognized to be key players in interstellar and astrochemistry, less is known about the astrochemical relevance of oxygen-containing PAHs (OPAHs). Small O-containing molecules such as CO and HCO are ubiquitous in the interstellar medium and understanding how OPAHs may be a source for these critical small molecules is important. To this end, we have studied the unimolecular reactions of two ionized OPAHs, dibenzofuran (1^+•) and dibenz[b,f]oxepin (2^+•) with tandem mass spectrometry (collision-energy resolved dissociation) and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). Collision-induced dissociation (CID) results show the competition between the loss of carbon monoxide (CO) and loss of 29 Da (either the formyl radical (HCO) or sequential H loss), with the latter being the dominant reaction. Rice–Ramsperger–Kassel–Marcus (RRKM) modeling of the iPEPICO data, on the other hand, is consistent with the loss of CO from the parent ion at the dissociative ionization onset, and, in the case of 2^+•, sequential H-atom loss from this product. There is significant difference between the two structurally similar systems. In 1^+•, dissociation requires around 4 eV of ion internal energy, while only 2.5 eV internal energy is required for 2^+• to fragment. Calculations at the CAM-B3LYP/6–311++G(d,p) level of theory were used to examine the reaction pathways. For CO loss in 1^+•, the reaction is initiated by a ring expansion followed by contraction of the central ring forming an ion–molecule complex between protonated cyclopenta[3,4]cyclobuta[1,2]benzene and CO. HCO loss is preceded by H migration to a bridging carbon vicinal to the oxygen atom and subsequent ring re-organization to form a low energy cyclopenta[c][1]benzopyran cation. This channel is higher enough in energy to preclude its participation near threshold, but not at higher internal energies reached in the CID experiment, which could therefore involve both sequential H loss and HCO loss. In 2^+•, the reaction starts with an opening of the central O-containing ring, lowering the energy demand relative to 1^+•.