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 2 + ) formed in an electron-ionization-driven supersonic plasma expansion of Py seeded in Ar or N 2 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 2 -tagged Py 2 + clusters, Py 2 + L n ( n = 1-5 for Ar, n = 1 for N 2 ), indicate the exclusive formation of the most stable antiparallel π-stacked Py 2 + structure under cold conditions, which is stabilized by charge-resonance interaction. The bare Py 2 + 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 2 + generated by VUV ionization of neutral Py n clusters.