Internal Energy Dependence of the Pyrrole Dimer Cation Structures Formed in a Supersonic Plasma Expansion: Charge-Resonance and Hydrogen-Bonded Isomers.

The structures of the pyrrole dimer cation (Py) formed in an electron-ionization-driven supersonic plasma expansion of Py seeded in Ar or N 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/N-tagged Py clusters, PyL ( = 1-5 for Ar, = 1 for N), indicate the exclusive formation of the most stable antiparallel π-stacked Py structure under cold conditions, which is stabilized by charge-resonance interaction. The bare Py 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 Py generated by VUV ionization of neutral Py clusters.