Reactivity and Structures of Hydrogenated Carbon Cluster Ions C n H x + ( n = 8,20;x = 4–12) Derived from Polycyclic Aromatic Hydrocarbons toward Benzene: Ion/Molecule Reactions as a Probe for Ion Structures

Ion/molecule reactions of two series of isomeric graphitic carbon/hydrogen cluster ions C _n H _x ⁺, n = 18,20; x = 4–12, in the gas phase with benzene and benzene-d₆ have been studied by using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The cluster ions C _n H _x ⁺ have been generated by consecutively removing H atoms from the four ionized polycyclic aromatic hydrocarbon (PAH) C₁₈H₁₂ isomers triphenylene 1, chrysene 2, 1,2-benzanthracene 3 and 2,3-benzanthracene 4, and from the two C₂₀H₁₂ isomers perylene 5 and benzo[e]pyrene 6. For the removal of H atoms, the method of sustained off-resonance irradiation collision-induced dissociation (SORI-CID) in FT-ICR has been applied. Consistent with our previous studies of ions C₁₈H _x ⁺, the ions C₂₀H _x ⁺ differ in their reactions from the ions formed by laser evaporation of graphite as reported by Bowers and coworkers and retain very likely the graphitic carbon skeletons of the original PAHs. The ion/molecule reaction products, the reaction channels and their branching ratios, the bimolecular rate constants k_bi and the SORI-CID fragments of the product ions provide valuable information about the reactivity and structure of the cluster ions C _n H _x ⁺. The dependence of the bimolecular rate constants k_bi on the structures of the isomeric parent PAHs indicates that the ions C _n H _x ⁺ “remember” the skeleton of their precursor PAH. Two kinds of reactions are observed, an addition of a benzene molecule probably by a strained triple aryne bond as in the carbon cluster ion C _n H _x ⁺ (aryne reactivity) and an addition of a benzene molecule followed by H elimination which can be attributed to a carbenium-like reactivity (C⁺ reactivity). Then there are unreactive components of the isomeric cluster ions C _n H _x ⁺, x = 4–9, which represent the non-reactivity. For the whole series of the cluster ions C₁₈H _x ⁺ and C₂₀H _x ⁺ the relative importance of these reactivities and the corresponding rate constants k_bi with benzene alternates with the odd- and even numbers of H atoms indicating their structural features and the preferable formation of aryne bonds especially in the cluster ions C _n H _x ⁺ containing a small number of hydrogen atoms. SORI-CID studies of the product ions [C _n H _x •C₆X₆]⁺ (X = H or D) or [C _n H _x •(C₆H₆)₂]⁺ support the operation of a Diels–Alder mechanism in the addition process of both reactive channels, in which a triple aryne bond behaves as a dienophile toward benzene. The ion/molecule reactions with benzene can be considered as a useful probe of the structures of the studied cluster ions C _n H _x ⁺.