Reactivity and structures of hydrogenated carbon cluster ions CnHx+ (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 CnHx+, n = 18,20; x = 4-12, in the gas phase with benzene and benzene-d(6), have been studied by using Fourier transform ion cyclotron re sonance (FT-ICR) mass spectrometry. The cluster ions CnHx+ have been genera ted by consecutively removing H atoms from the four ionized polycyclic arom atic hydrocarbon (PAH) C18H12 isomers triphenylene 1, chrysene 2, 1,2-benza nthracene 3 and 2,3-benzanthracene 4, and from the two C20H12 isomers peryl ene 5 and benzo[e]pyrene 6. For the removal of H atoms, the method of susta ined 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 C20Hx+ differ in their reactions from the ions formed by laser evaporation of graphite as reported by Bowers and coworkers and retain ver y likely the graphitic carbon skeletons of the original PAHs. The ion/molec ule reaction products, the reaction channels and their branching ratios, th e bimolecular rate constants k(bi) and the SORI-CID fragments of the produc t ions provide valuable information about the reactivity and structure of t he cluster ions CnHx+. The dependence of the bimolecular rate constants k(b i) on the structures of the isomeric parent PAHs indicates that the ions Cn Hx+ "remember" the skeleton of their precursor PAH. Two kinds of reactions are observed, an addition of a benzene molecule probably by a strained trip le aryne bond as in the carbon cluster ion CnHx+ (aryne reactivity) and an addition of a benzene molecule followed by H elimination which can be attri buted to a carbenium-like reactivity (C+ reactivity). Then there are unreac tive components of the isomeric cluster ions CnHx+, x = 4-9, which represen t the non-reactivity. For the whole series of the cluster ions C18Hx+ and C 20Hx+ the relative importance of these reactivities and the corresponding r ate constants k(bi) with benzene alternates with the odd- and even numbers of H atoms indicating their structural features and the preferable formatio n of aryne bonds especially in the cluster ions CnHx+ containing a small nu mber of hydrogen atoms. SORI-CID studies of the product ions [CnHx.C6X6](+) (X = H or D) or [CnHx. (C6H6)(2)](+) support the operation of a Diels-Alde r mechanism in the addition process of both reactive channels, in which a t riple aryne bond behaves as a dienophile toward benzene. The ion/molecule r eactions with benzene can be considered as a useful probe of the structures of the studied cluster ions CnHx+.