The dissociation of the naphthalene radical cation has been reinvestigated here by a combination of tandem mass spectrometry and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). Six reactions were explored: (R1) C(10)H(8)(•+) → C(10)H(7)(+) + H (m/z = 127); (R2) C(10)H(8)(•+) → C(8)H(6)(•+) + C(2)H(2) (m/z = 102); (R3) C(10)H(8)(•+) → C(6)H(6)(•+) + C(4)H(2) (m/z = 78); (R4) C(10)H(8)(•+) → C(10)H(6)(•+) + H(2) (m/z = 126); (R5) C(10)H(7)(+) → C(6)H(5)(+) + C(4)H(2) (m/z = 77); (R6) C(10)H(7)(+) → C(10)H(6)(•+) + H (m/z = 126). The E(0) activation energies for the reactions deduced from the present measurements are (in eV) 4.20 ± 0.04 (R1), 4.12 ± 0.05 (R2), 4.27 ± 0.07 (R3), 4.72 ± 0.06 (R4), 3.69 ± 0.26 (R5), and 3.20 ± 0.13 (R6). The corresponding entropies of activation, ΔS(‡)(1000K), derived in the present study are (in J K(-1) mol(-1)) 2 ± 2 (R1), 0 ± 2 (R2), 4 ± 4 (R3), 11 ± 4 (R4), 5 ± 15 (R5), and -19 ± 11 (R6). The derived E(0) value, combined with the previously reported IE of naphthalene (8.1442 eV) results in an enthalpy of formation for the naphthyl cation, Δ(f)H°(0K) = 1148 ± 14 kJ mol(-1)/Δ(f)H°(298K) = 1123 ± 14 kJ mol(-1) (site of dehydrogenation unspecified), slightly lower than the previous estimate by Gotkis and co-workers. The derived E(0) for the second H-loss leads to a Δ(f)H° for ion 7, the cycloprop[a]indene radical cation, of Δ(f)H°(0K) =1457 ± 27 kJ mol(-1)/Δ(f)H°(298K)(C(10)H(6)(+)) = 1432 ± 27 kJ mol(-1). Detailed comparisons are provided with values (experimental and theoretical) available in the literature.

The unimolecular dissociation of the pyrene radical cation, C16H10(+•), has been explored using a combination of computational techniques and experimental approaches, such as multiple photon absorption in the cold ion trap Piège à Ions pour la Recherche et l'Etude de Nouvelles Espèces Astrochimiques (PIRENEA) and imaging photoelectron photoion coincidence spectrometry (iPEPICO). In total, 22 reactions, involving the fragmentation cascade (H, C2H2, and C4H2 loss) from the pyrene radical cation down to the C14(+•) fragment ion, have been studied using PIRENEA. Branching ratios have been measured for reactions from C16H10(+•), C16H8(+•), and C16H5(+). Density functional theory calculations of the fragmentation pathways observed experimentally and postulated theoretically lead to 17 unique structures. One important prediction is the opening of the pyrene ring system starting from the C16H4(+•) radical. In the iPEPICO experiments, only two reactions could be studied, namely, R1 C16H10(+•) → C16H9(+) + H (m/z = 201) and R2 C16H9(+) → C16H8(+•) + H (m/z = 200). The activation energies for these reactions were determined to be 5.4 ± 1.2 and 3.3 ± 1.1 eV, respectively.

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 dissociation of the anthracene radical cation has been studied using two different methods: imaging photoelectron photoion coincidence spectrometry (iPEPCO) and atmospheric pressure chemical ionization-collision induced dissociation mass spectrometry (APCI-CID). Four reactions were investigated: (R1) C14H10(+•) → C14H9(+) + H, (R2) C14H9(+) → C14H8(+•) + H, (R3) C14H10(+•) → C12H8(+•) + C2H2 and (R4) C14H10(+•) → C10H8(+•) + C4H2. An attempt was made to assign structures to each fragment ion, and although there is still room for debate whether for the C12H8(+•) fragment ion is a cyclobuta[b]naphthalene or a biphenylene cation, our modeling results and calculations appear to suggest the more likely structure is cyclobuta[b]naphthalene. The results from the iPEPICO fitting of the dissociation of ionized anthracene are E0 = 4.28 ± 0.30 eV (R1), 2.71 ± 0.20 eV (R2), and 4.20 ± 0.30 eV (average of reaction R3) whereas the Δ(‡)S values (in J K(-1) mol(-1)) are 12 ± 15 (R1), 0 ± 15 (R2), and either 7 ± 10 (using cyclobuta[b]naphthalene ion fragment in reaction R3) or 22 ± 10 (using the biphenylene ion fragment in reaction R3). Modeling of the APCI-CID breakdown diagrams required an estimate of the postcollision internal energy distribution, which was arbitrarily assumed to correspond to a Boltzmann distribution in this study. One goal of this work was to determine if this assumption yields satisfactory energetics in agreement with the more constrained and theoretically vetted iPEPICO results. In the end, it did, with the APCI-CID results being similar.

Ionized 1,2-dihydronaphthalene (C10H10(+)) and 9,10-dihydrophenanthrene (C14H12(+)) are homologous dihydrogenated polycyclic aromatic hydrocarbons containing adjacent sp(3) carbon sites. Tandem mass spectrometry involving kiloelectronvolt collision induced dissociation was employed to aid in the structural characterization of the products of the main dissociation channels, loss of H (and subsequent H and H2 losses in dihydronaphthalene) and CH3. Evident from both the CID mass spectra and the imaging photoelectron-photoion coincidence (iPEPICO) breakdown curves is the fact that there are two competitive routes to the loss of H. For 1,2-dihydronaphthalene these give activation energies of 2.22 ± 0.10 and 2.44 ± 0.05 eV, whereas only 2.37 ± 0.12 eV was obtained for 9,10-dihydrophenanthrene. The two parallel H-loss chaneels are believed to be the result of isomerization taking place to the methylindene ion and the 9-methylfluorene ion for 1,2-dihydronaphthalene and 9,10-dihydrophenanthren, respectively. Each newly formed isomer dissociates by H loss (one of the two competing H-loss reactions) and, of course, methyl loss. Methyl radical loss has similar kinetics for the two systems, E0 = 2.57 ± 0.12 eV, Δ(‡)S = 18 ± 11 J K(-1) mol(-1) for ionized dihydronaphthalene and E0 = 2.38 ± 0.15 eV, Δ(‡)S = -3 ± 15 J K(-1) mol(-1) for ionized dihydrophenanthrene, but as can be seen, the E0 and Δ(‡)S are slightly lower for the latter. The final bond rupture step in both H and CH3 loss is expected to be accompanied by a positive Δ(‡)S, thus the low energy H loss and CH3 loss originate from the isomer ion in both cases, with the entropy of activation being dominated by the isomerization step. In contrast, sp(3)-H loss from the dihydro-PAHs differ by little in both systems (E0 = 2.44 eV in ionized dihydronaphthalene and 2.37 eV in ionized dihydrophenanthrene and the Δ(‡)S values are 27 and 18 J K(-1) mol(-1), respectively). The presence of a second sp(3) carbon site has decreased the C-H bond dissociation energy relative to protonated naphthalene and protonated phenanthrene, possibly to facilitate the restoration of the unaltered PAH ion. The calculated dihedral angle is -34.3° in C10H10(+•) whereas C14H12(+•) has an angle of -49.6°, indicating that to restore the planar nature of the molecules, which is required for all reaction channels investigated, there is more rearrangement needed for 9,10-dihydrophenanthrene. Energetics and entropic values associated with H and H2 loss from [M - H](+) ions from ionized dihydronaphthalene were determined to be 2.72 eV, 9 ± 17 J K(-1) mol(-1), and 2.85 eV, 9 ± 7 J K(-1) mol(-1), respectively.