IR spectroscopy of α- and β-protonated pyrrole via argon complex photodissociation

Protonated pyrrole cations are produced in a pulsed discharge/supersonic expansion source, mass-selected in a time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy. Vibrational spectra in both the fingerprint and C-H/N-H stretching regions are obtained using the method of tagging with argon. Sharp vibrational structure is compared to IR spectra predicted by theory for the possible α-, β-, and N-protonated structures. The spectral differences among these isomers are much larger than the frequency shifts due to argon attachment at alternative sites. Though α-protonation predominates thermodynamically, the kinetically favored β-protonated species is also observed for the first time (in 3-4 times lower abundance under the conditions employed here). Theoretical investigations attribute the greater stability of α-protonated pyrrole to topological charge stabilization, rather than merely to the greater number of resonance contributors. The far-IR pattern of protonated pyrrole does not match the interstellar UIR bands.