The Energetics for Hydrogen Addition to Naphthalene Cations

Abstract

Energetics of H-addition reactions to the naphthalene cation are determined using computational chemistry techniques. While most of the reactions have little to no barrier, the shape of the potential energy surfaces favors the formation of the product with the added hydrogen atoms being on adjacent carbons. Thus, while many products will be formed, one expects that the product in which all the hydrogen atoms are added to the same or adjacent rings will be favored over a random distribution of added hydrogen atoms. This has implications on the species that will be formed when polycyclic aromatic hydrocarbons (PAHs) are photolyzed in ices in the interstellar medium (ISM ). Barrierless reactions are consistent with the view that PAHs with extra hydrogens contribute to the 3.4 m band.

Formation and stability of hydrogenated PAHs in the gas phase

“…For protonated quinoline, the binding energy of a hydrogen atom to H n QuiH + is ∼2 eV higher for n even than for odd n. The odd-even pattern is only seen weakly in the experimental mass spectrum of protonated quinoline, although H 4 QuiH + has the highest abundancy after QuiH + . The trend of hydrogenation site of the investigated molecules agrees with the calculations of Ricca et al (2007).…”

“…Our results are supported by calculations (Ricca et al 2007;Rasmussen et al 2011). Rasmussen et al (2011) show that chemical binding of hydrogen to neutral pyrene (a small PAH consisting of four fused aromatic rings) is exothermic by 1.6 eV, and the barrier for further hydrogenation is negligible (0.06 eV).…”

“…Rasmussen et al (2011) show that chemical binding of hydrogen to neutral pyrene (a small PAH consisting of four fused aromatic rings) is exothermic by 1.6 eV, and the barrier for further hydrogenation is negligible (0.06 eV). Ricca et al (2007) find that single hydrogenation of the napthalene cation has no barrier, while double hydrogenation has a slightly higher barrier. In our study, the reaction barriers for hydrogen addition were not calculated, but the measurements show that the barrier can be passed both in solution and in gas-phase, because hydrogenated PAHs are observed from both ion sources, indicating low reaction barriers.…”

Growth kinetics and formation mechanisms of complex organics by sequential reactions of acetylene with ionized aromatics

“…The measured rate coefficient of 4.2 (AE2.5) Â 10 À11 cm 3 s À1 ) indicates a reaction efficiency (defined here as the ratio of the measured rate coefficient to the Langevin capture rate coefficient taken as 1.5 Â 10 À9 cm 3 s À1 ) of nearly 2.8% at 304 K. The higher reactivity of the benzonitrile cation towards acetylene is in contrast to the very low reaction efficiency (10 À5 ) observed for the addition of acetylene on the benzene cation [38]. This confirms that the barrier to the cation ring growth can be reduced or overcome by introducing appropriate substituent groups in the cation ring [44][45][46]. Thus, nitrogen-containing complex organics can be produced by the sequential reactions of acetylene with the benzonitrile cation at room temperature and relatively high pressure conditions (0.1 Torr).…”

“…Through our previous studies we discovered novel reactions of the benzene cation with acetylene molecules over a wide temperature range from 120 to 650 K, demonstrating that benzene ions autocatalyze the conversion of associated acetylene molecules into benzene at low temperatures (120 K), and form naphthalene-type covalent products at high temperatures (650 K) [38]. The barrier to the covalent addition of acetylene to the benzene cation can be eliminated by removing a hydrogen atom from the benzene ring [44][45][46]. Recently, we provided the first direct evidence for the formation of covalently bonded C 8 H 7 + (2-phenyl ethenylium ion) and C 10 H 9 + (protonated naphthalene)…”