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

J Phys Chem A. 2012 Oct 4;116(39):9689-95. doi: 10.1021/jp307631n. Epub 2012 Sep 20.

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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Argon / chemistry*
  • Models, Molecular
  • Molecular Conformation
  • Photochemical Processes*
  • Photons
  • Protons*
  • Pyrroles / chemistry*
  • Spectrophotometry, Infrared

Substances

  • Protons
  • Pyrroles
  • Argon