What do we expect from the dissociation of ionized nitro-substituted polycyclic aromatic hydrocarbons in the interstellar medium?

Abstract

The unimolecular dissociations of four nitro-substituted polycyclic aromatic hydrocarbon (PAH) ions were explored using tandem mass spectrometry, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and computational chemistry. Ionized 1-nitronaphthalene and 9-nitroanthracene both exhibit the loss of C2O2 (likely sequential loss of two CO molecules), NO and CO2 as primary fragmentation channels, while for the larger 3-nitrofluoranthene and 1-nitropyrene ions CO2 loss is replaced by NO2 loss. The reaction pathways for 1-nitropyrene ions were calculated at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory. NO loss is preceded by a nitro-to-nitrite isomerization requiring 2.09 eV, while NO2 loss was found to form the triplet state [pyrene – H]+ ion (requiring 2.36 eV), consistent with the behavior of this class of PAH ions. iPEPICO-derived energies for these two reactions are in excellent agreement with theory, 2.05 ± 0.09 eV and 2.38 ± 0.12 eV, respectively. The iPEPICO-derived reaction energy for C2O2 loss is 2.76 ± 0.08 eV, but we were unable to calculate this reaction pathway. The most energetically-favorable products of sequential CO loss lie 3.2 eV and 2.05 eV, respectively, lower in energy than the precursor ion, indicating that what was measured was an effective barrier height for this reaction.