Polyaromatic Radical Anions: Utilization of a Harmonic Model in Simulating Radical Vibrational Structure and Gas-Phase Acidity Determination

Abstract

Through their diverse molecular structure, charge and protonation states, polyaromatic hydrocarbons (PAHs) play a central role in the field of Biochemistry, combustion chemistry, and Astrochemistry. In this poster, a harmonic model was utilized in the Franck–Condon (FC) analysis of the vibrational structure of negative radical anions of PAHs such as naphthalene and anthracene, following the ultra violet (UV) photodetachment of the initially prepared deprotonated anionic species. The two PAHs are utilized as a prototype for the vibronic analysis of the ground-state and lowest lying excited states of similar gas-phase isomers that contain a rigid, ring structure. The spectra are interpreted based on the comparison with quantum-mechanical data obtained from ab initio calculations as well as the Franck-Condon (FC) calculations. The geometric and frequency of the optimized structures of the anion and the neutral radical are calculated using the GAUSSIAN 09 software package. The FC factors of the anion PES are simulated using the PESCAL program. PESCAL obtains the FC factors using molecular geometry, normal mode vectors, and normal mode harmonic vibrational frequencies of the anion and neutral states. These FC simulations are based on a harmonic oscillator approximation model that utilizes the Duschinsky rotation between the normal mode vectors of the anion and neutral radical species. The calculated adiabatic electron affinity is utilized in the negative ion thermochemical cycle to determine gas-phase acidity values of neutral PAH molecules.