Cyanocyclopentadiene-Annulated Polycyclic Aromatic Radical Anions: Predicted Negative Ion Photoelectron Spectra and Singlet–Triplet Energies of Cyanoindene and Cyanofluorene Radical Anions

Abstract

Isomer-specific negative ion photoelectron spectra (NIPES) of cyanoindene (C 9 H 7 CN) and cyanofluorene (C 14 H 9 N), acquired through the computation of Franck−Condon (FC) factors that utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory (DFT) at the B3LYP/aug-cc-pVQZ and 6-311++G(2d,2p) basis sets, are reported. The adiabatic electron affinity (EA) values of the ground singlet (S 0 ) and the lowest lying triplet (T 1 ) states are used to predict site-specific S 0 −T 1 energies (ΔE ST ). The vibrational spectra of the S 0 and T 1 states are typified by ring distortion and ring C−C stretching vibrational progressions. Among all the S 0 isomers in C 9 H 7 CN, the 2-cyanoindene (2-C 9 H 7 CN) is found to be the most stable at an EA of 0.716 eV, with the least stable isomer being the 1-C 9 H 7 CN at an EA of 0.208 eV. In C 14 H 9 N, the most stable S 0 isomer, 2-cyanofluorene (2-C 14 H 9 N), has an EA of 0.781 eV. The least stable S 0 isomer in C 14 H 9 N is the 9-C 14 H 9 N, with an EA of 0.364 eV. The FC calculations are designed to mimic simulations that would be performed to aid in the analysis of experimental spectra obtained in NIPE spectroscopic techniques. The vibrational spectra, adiabatic EAs, and ΔE ST values reported in this study are intended to act as a guide for future gas-phase ion spectroscopic experiments and astronomical searches, especially with regard to the hitherto largely unexplored C 14 H 9 N isomers.