Internal Energy Dependence of the Pyrrole Dimer Cation Structures Formed in a Supersonic Plasma Expansion:...

The structures of the pyrrole dimer cation (Py₂⁺) formed in an electron-ionization-driven supersonic plasma expansion of Py seeded in Ar or N₂ are probed as a function of its internal energy by infrared photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. The IRPD spectra recorded in the CH and NH stretch ranges are analyzed by dispersion-corrected density functional theory (DFT) calculations at the B3LYP-D3/aug-cc-pVTZ level. The spectra of the cold Ar/N₂-tagged Py₂⁺ clusters, Py₂⁺L (n = 1-5 for Ar, n = 1 for N₂), indicate the exclusive formation of the most stable antiparallel π-stacked Py₂⁺ structure under cold conditions, which is stabilized by charge-resonance interaction. The bare Py₂⁺ dimers produced in the ion source have higher internal energy, and the observation of additional transitions in their IRPD spectra suggests a minor population of less stable hydrogen-bonded isomers composed of heterocyclic Py/Py⁺ structures formed after intramolecular H atom transfer and ring opening. These intermolecular isomers differ from the chemically bonded structures proposed earlier in the analysis of IRPD spectra of Py₂⁺ generated by VUV ionization of neutral Py clusters.