Photoionization of pentalene and derivatives

The vertical, valence ionization energies for pentalene (C8H6) and some of its derivatives have been calculated as free molecules in the gas phase using high-level ab initio coupled-cluster method: EOM-DLPNO-CCSD. Their valence electronic structures depend on the number, type and position of substituents. We have used the calculated vertical, valence ionization energies to simulate UV photoelectron spectra. HOMO-LUMO gaps and the influence of substituents on ionization energies and electronic structure were also studied. We discuss the antiaromaticity and bis-annulation effects in pentalenes.