Electronic excitation of gaseous pyrrole and pyrazole by inner-shell electron energy loss spectroscopy

Abstract

International audienceThe spectroscopy of the unoccupied molecular orbitals of gaseous pyrrole and pyrazole has been studied by electron impact under electric dipole scattering conditions (2 KeV, small angle) and the inner-shell electron energy loss method. The core-excitation spectra have been recorded at the C1s and N1s edges with 0.2 eV resolution, resolution allowing to observe separately the transitions from the carbon atoms with different chemical environment. Ab initio calculations, using the equivalent core model, were performed to help in the interpretation of the spectral features. The spectra are dominated by transitions to the π* and σ* molecular orbitals. The calculations confirm that the 3b1(π*) orbital is the lowest-energy unoccupied one in pyrrole. In several cases, the intensity of the Rydberg excitations is enhanced by an important valence σ* character of the Rydberg orbitals

Auto-oligomerization and hydration of pyrrole revealed by x-ray absorption spectroscopy

“…14,17 Five major features have been identified in the spectra (labeled 1-5). Another feature has been reported previously in ISEELS studies at approximately 300 eV; however, the evidence for this feature, given the signal to noise ratio, is not compelling.…”

“…14,17 The relative alignment of C1 and C2 is obtained by total energy differences. 32 The first feature was originally believed to comprise two transitions, however, higher resolution spectra 17 DFT leads to spectra which appear contracted with respect to experiment.…”

“…14,17 The relative alignment of C1 and C2 is obtained by total energy differences. 32 The first feature was originally believed to comprise two transitions, however, higher resolution spectra 17 DFT leads to spectra which appear contracted with respect to experiment. 29 The third segment of Feature 1, visible as a shoulder at approximately 287.1 eV, has previously been assigned to a combination of a π * /σ * orbital transition from carbon 1; 17 however, our DFT calculations did not reproduce this feature with adequate intensity.…”

“…13 Previous study of pyrrole has shown that its solid form has very similar NEXAFS spectra to the inner shell electron energy loss spectroscopy (ISEELS) measurements of pyrrole in the gas phase. [14][15][16][17] This implies that in the solid, the orbitals of pyrrole do not exhibit significant hybridization.…”

Effects of vibrational motion on core-level spectra of prototype organic molecules

“…HCH LCAO calculations produce a spurious peak after the first main peak and the second peak is too intense compared to experiment. Previous experiment has identified a shape resonance [34], labeled 3 here, which has some oscillator strength above the decay that occurs well above the ionization potential; some features are apparent there, but nothing definitive emerges from either calculation.…”