UC Berkeley

Radical organic ions can be stabilized by complexation with neutral organics via interactions that can resemble chemical bonds, but with much diminished bond energies. Those interactions are a key factor in cluster growth and polymerization reactions in ionizing environments such as regions of the interstellar medium and solar nebulae. Such radical cation complexes between naphthalene (Naph) and pyridine (Pyr) are characterized using mass-selected ion mobility experiments. The measured enthalpy of binding of the Naph^+•(Pyr) heterodimer (20.9 kcal/mol) exceeds that of the Naph^+•(Naph) homodimer (17.8 kcal/mol). The addition of 1-3 more pyridine molecules to the Naph^+•(Pyr) heterodimer gives 10-11 kcal/mol increments in binding enthalpy. A rich array of Naph^+•(Pyr) isomers are characterized by electronic structure calculations. The calculated Boltzmann distribution at 400 K yields an enthalpy of binding in reasonable agreement with experiment. The global minimum is a distonic cation formed by Pyr attack on Naph^+• at the α-carbon, changing its hybridization from sp² to distorted sp³. The measured collision cross section in helium for the Naph^+•(Pyr) heterodimer of 84.9 ± 2.5 Ų at 302 K agrees well with calculated angle-averaged cross sections (83.9-85.1 Ų at 302 K) of the lowest energy distonic structures. A remarkable 16 kcal/mol increase in the binding energy between Naph^+•(Pyr) and Bz^+•(Pyr) (Bz is benzene) is understood by energy decomposition analysis. A similar increase in binding from Naph^+•(NH₃) to Naph^+•(Pyr) (as well as between Bz^+•(NH₃) and Bz^+•(Pyr)) is likewise rationalized.