VUV photoprocessing of oxygen-containing polycyclic aromatic hydrocarbons: iPEPICO study of the unimolecular dissociation of ionized benzofuran

Abstract

Oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) are potential contributors to the 11.3 m band in interstellar observations. To further explore their role in the interstellar medium, we have investigated their fate after photoprocessing by VUV radiation; in particular, we studied the dissociative photoionization of the simplest OPAH, benzofuran, with imaging photoelectron photoion coincidence spectroscopy, iPEPICO. Ionized benzofuran dissociates by loss of CO, followed by a sequential H atom loss. The parallel HCO-loss channel, leading to the same bicyclic C7H5+ fragment ion, is not competitive at low excess energies above the ionization threshold. However, the collision-induced dissociation tandem mass spectrometry results suggest that CO and HCO may be formed in parallel at higher energies. An RRKM fragmentation model reproduced the iPEPICO data well assuming the initial 1,2-H shift transition state to be rate determining to CO loss. The breakdown diagram and the measured dissociation rates agreed well at the CBS-QB3-calculated activation energy of 2.99 eV, which could be relaxed to 3.25 eV, and only a slight adjustment of the ab initio activation entropy. The model barrier to sequential H-loss is larger than the computed H-loss threshold and the breakdown diagram rises less steeply than predicted, which indicates suprastatistical kinetic energy release after the tight H-transfer transition state of the first step. HCO cleavage is possible after a ring-opening transition state, which is looser than and isoenergetic with the CO-loss transition state. However, a subsequent ring formation transition state at 3.85 eV is moderately tight, which suppresses HCO loss at low excess energies.

Gas-phase electronic action absorption spectra of protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs)

“…This is followed by further fragmentation into one of the three additional channels via either loss of an additional CO, COH n , or C 2 H 4±n resulting in the (257 ± 2) amu channel or breaking apart resulting in two fragments that are similar in size, for example the smaller two-carbon ring aromatics naphthalene (128 amu) and naphtaldehyde, ethylnaphthalene, or dimethylnaphthalene (all three species have a mass of 156 amu). These results are consistent with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy experiments investigating the dissociative photoionization of the OPAH benzofuran (Zinck et al 2022) and of hydroxy-substituted PAH cations (Lesniak et al 2019) that revealed their primary dissociation channel to be CO-and HCO-loss.…”

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde

“…On the other hand, one of the CH 2 hydrogens in the five-membered ring in [ 46 ] may transfer to the OH group over the transition state [ 48 ] ‡ at a barrier of 1.97 eV, which leads to water loss and the m / z 118 fragment ion benzofuran [ 49 ]. As evidenced by their similar ionization energy but lower DPI onset, ,− oxygen-containing polycyclic aromatic hydrocarbon cations are less stable than the hydrocarbon analogues, which would be indene in this case. Therefore and despite the large kinetic energy release upon water loss on account of the 2.06 eV reverse barrier, [ 49 ] is a likely intermediate on the way to the m / z 90 fragment ion by consecutive CO loss at higher photon energies in ortho -anisaldehyde DPI.…”

“…The transition state [54] ‡ energy is calculated to be lower than that of the intermediate state [53], which shows a slight disagreement between the density functional theory geometry optimization and the wave function theory energies in the G4 composite method. 49 At the same time, it also indicates a flat reaction energy curve, which may act as a bottleneck and, together with quantum tunneling across the [48] ‡ barrier with a 1373 cm −1 critical frequency, may explain the dominance of the m/z 118 fragment ion in the BD (Figure 3c), while the m/ z 94 channel stays relatively minor.…”

“…As evidenced by their similar ionization energy but lower DPI onset, 35,46−48 oxygen-containing polycyclic aromatic hydrocarbon cations are less stable than the hydrocarbon analogues, which would be indene in this case. Therefore and despite the large kinetic energy release upon water loss on account of the 2.06 eV reverse barrier, [49] is a likely intermediate on the way to the m/z 90 fragment ion by consecutive CO loss 48 at higher photon energies in ortho-anisaldehyde DPI. Alternatively, [46] can also lose ketene, CH 2 CO, and form m/z 94 [56] via H transfer, ring opening, and continued H migration along [46]…”

“…On the one hand, direct C−OH bond breaking in [46] leads to m/ z 119 [47] over a barrier of 1.78 eV relative to [46]. On the other hand, one of the CH 2 hydrogens in the five-membered ring in [46] may transfer to the OH group over the transition state [48] ‡ at a barrier of 1.97 eV, which leads to water loss and the m/z 118 fragment ion benzofuran [49]. As evidenced by their similar ionization energy but lower DPI onset, 35,46−48 oxygen-containing polycyclic aromatic hydrocarbon cations are less stable than the hydrocarbon analogues, which would be indene in this case.…”