title: IR spectroscopy of α- and β-protonated pyrrole via argon complex photodissociation.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Mosley
authors.forename: A M
authors.initials: AM
authors.lastname: Ricks
authors.forename: P v R
authors.initials: Pv
authors.lastname: Schleyer
authors.forename: J I
authors.initials: JI
authors.lastname: Wu
authors.forename: M A
authors.initials: MA
authors.lastname: Duncan
mesh_terms: D001128:Argon / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D055668:Photochemical Processes; D017785:Photons; D011522:Protons; D011758:Pyrroles / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D011522:Protons; D011758:Pyrroles; D001128:Argon