Growth of polyaromatic molecules via ion-molecule reactions: An experimental and theoretical mechanistic study

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C10H7+ with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C16H13+, C16Hn+ (with n=10-12), and C15H10+. From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 A2 (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C16H12+ and C15H10+ ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)x10-11 cm3 s-1 at an average collision energy of about 0.27 eV can be estimated for the process C10H7++C6H6→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H2, CH3). A major signal relevant to the fragmentation of the initial adduct C16H13+ belongs to C15H10+. Since it is not obvious how CH3 loss from C16H13+ can take place to form the C15H10+ radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol-1. From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol-1 with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the ''naphthyl'' part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.