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.

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH 3 OC(O)H, were elucidated to be CH 3 OH + CO, 2 CH 2 O and CH 4 + CO 2 as in part distinct from the dissociation of the radical cation (CH 3 OH +• + CO and CH 2 OH + + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH 2 O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.

1-Methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modeling to determine a 0 K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K mol for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the Δ H (1-methylenepyrene cation) of 945 ± 31 kJ mol.

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O=C(H)-O-C(H 2 )-CH 3 and trans (staggered) configuration of the O=C(H)-O-C(H 2 )-CH 3 dihedral angles. We observed the following ethyl formate pyrolysis products: CH 3 CH 2 OH, CH 3 CHO, C 2 H 6 , C 2 H 4 , HC(O) OH, CH 2 O, CO 2 , and CO, with HC(O)OH and C 2 H 4 pyrolyzing further, forming CO + H 2 O and C 2 H 2 + H 2 . The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH 3 CH 2 O + CHO and CH 3 CH 2 + HC(O)O, were studied computationally at the M06-2X-GD3/ aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.

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