A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface

Abstract

The fragmentation mechanisms of the naphthalene molecular ion to [M-C 4 H 2 ] +•, [M-C 2 H 2 ] +•, [M-H 2 ] +•, and [M-H • ] + were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G(d) level of theory and were subsequently used to calculate the microcanonical rate constants, k(E)'s, for all the steps by the Rice-Ramsperger-Kassel-Marcus formalism. The pre-equilibrium and steady state approximations were applied on different regions of the potential energy profiles to obtain the fragmentation k(E)'s and calculate the relative abundances of the ions as a function of energy. These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0-18.8 eV that was previously reported by our group. Prior to dissociation, the molecular ion rapidly equilibrates with a set of isomers that includes the Z- and E-phenylvinylacetylene (PVA) radical cations. The naphthalene ion is the predominant isomer below 10 eV internal energy, with the other isomers remaining at steady state concentrations. Later on, new steady-state intermediates are formed, such as the azulene and 1-phenyl-butatriene radical cations. The naphthalene ion does not eject an H atom directly but eliminates an H 2 molecule in a two-step fragmentation. H • loss occurs instead from the 1-phenyl-butatriene ion. The PVA ions initiate the ejection of diacetylene (C 4 H 2) to yield the benzene radical cation. Acetylene elimination yields the pentalene cation at low energies (where it can account for 45.9%-100.0% of the rate constant of this channel), in a three-step mechanism starting from the azulene ion. However, above 7.6 eV, the major [M-C 2 H 2 ] +• structure is the phenylacetylene cation.