Unimolecular reaction energies for polycyclic aromatic hydrocarbon ions

Abstract

Imaging photoelectron photoion coincidence spectroscopy was employed to explore the unimolecular dissociation of the ionized polycyclic aromatic hydrocarbons (PAHs) acenaphthylene, fluorene, cyclopenta[d,e,f]phenanthrene, pyrene, perylene, fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, coronene and corannulene. The primary reaction is always hydrogen atom loss, with the smaller species also exhibiting loss of CH to varying extents. Combined with previous work on smaller PAH ions, trends in the reaction energies (E) for loss of H from sp-C and sp-C centres, along with hydrocarbon molecule loss were found as a function of the number of carbon atoms in the ionized PAHs ranging in size from naphthalene to coronene. In the case of molecules which possessed at least one sp-C centre, the activation energy for the loss of an H atom from this site was 2.34 eV, with the exception of cyclopenta[d,e,f]phenanthrene (CPP) ions, for which the E was 3.44 ± 0.86 eV due to steric constraints. The hydrogen loss from PAH cations and from their H-loss fragments exhibits two trends, depending on the number of unpaired electrons. For the loss of the first hydrogen atom, the energy is consistently ca. 4.40 eV, while the threshold to lose the second hydrogen atom is much lower at ca. 3.16 eV. The only exception was for the dibenzo[a,l]pyrene cation, which has a unique structure due to steric constraints, resulting in a low H loss reaction energy of 2.85 eV. If CH is lost directly from the precursor cation, the energy required for this dissociation is 4.16 eV. No other fragmentation channels were observed over a large enough sample set for trends to be extrapolated, though data on CH and CH loss obtained in previous studies is included for completeness. The dissociation reactions were also studied by collision induced dissociation after ionization by atmospheric pressure chemical ionization. When modeled with a simple temperature-based theory for the post-collision internal energy distribution, there was reasonable agreement between the two sets of data.

Formation and photochemistry of covalently bonded large functional PAH clusters

“…We stress here that the derived dissociation energy (Fig. 7B) for all the dehydrogenation and functional group loss steps is lower than the dehydrogenation energy barrier of pure PAHs (Chen et al 2015;Croiset et al 2016;Castellanos et al 2018;West et al 2018).…”

“…With increasing laser energy (e.g., 1.3 mJ, blue line), the peaks due to multiple fragmentation steps become more prominent and some graphene products (e.g., C + 60 , C + 62 and C + 64 , blue line) become accessible. The photochemical behavior in these two spectra suggests that after direct dehydrogenation or loss of its C 2 H 3 unit and then with further dehydrogenation, DC/9-vinylanthracene A57, page 4 of 11 cluster cations convert into aromatic species West et al 2018).…”

“…After that, the newly formed large PAHs will transformed into C + 62 upon irradiation following full dehydrogenation, then with further C 2 losing chemical pathways. The photochemical behavior of DC/9-methylanthracene cluster cations are similar to fluorene cluster cations and large PAH cations (Zhen et al 2015;Castellanos et al 2018;West et al 2018;Zhang et al 2019). We do not observe C + 63 in the mass spectra in Fig.…”

“…The dissociation energy is 1.7 eV for H loss from the aliphatic carbon, which is the dominant dissociation channel for P 1 . In contrast, H loss from the pure aromatic carbon rings located in anthracene molecules group takes around 5.5 eV (Chen et al 2015;West et al 2018). As shown in Fig.…”

Laboratory Photochemistry of Covalently Bonded Fluorene Clusters: Observation of an Interesting PAH Bowl-forming Mechanism

“…The increased density in the fluorene plume allows cation cluster-formation as discussed in . In our experiments, the low energy barrier for H-loss from fluorene (2.3 eV, West et al (2018)) leads to efficient radical formation upon electron gun irradiation and promotes covalently bonded cluster formation. Once formed, cation species are transported into a quadrupole ion trap and trapped.…”