The C 1s and N 1s near edge x-ray absorption fine structure spectra of five azabenzenes in the gas phase

Abstract

Near edge x-ray absorption fine structure spectra have been measured and interpreted by means of density functional theory for five different azabenzenes (pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine) in the gas phase. The experimental and theoretical spectra at the N 1s and C 1s edges show a strong resonance assigned to the transition of the 1s electron in the respective N or C atoms to the lowest unoccupied molecular orbital with pi(*) symmetry. As opposed to the N 1s edge, at the C 1s edge this resonance is split due to the different environments of the core hole atom in the molecule. The shift in atomic core-level energy due to a specific chemical environment is explained with the higher electronegativity of the N atom compared to the C atom. The remaining resonances below the ionization potential (IP) are assigned to sigma or pi [corrected] orbitals with mixed valence/Rydberg [corrected] character. Upon N addition, a reduction of intensity is observed in the Rydberg region at both edges as compared to the intensity in the continuum. Above the IP one or more resonances are seen and ascribed here to transitions to sigma(*) orbitals. Calculating the experimental and theoretical Delta(pi) term values at both edges, we observe that they are almost the same within +/-1 eV as expected for isoelectronic bonded pairs. The term values of the pi(*) and sigma(*) resonances are discussed in terms of the total Z number of the atoms participating in the bond.

Vibrationally resolved NEXAFS at C and N K-edges of pyridine, 2-fluoropyridine and 2,6-difluoropyridine: A combined experimental and theoretical assessment

“…A very weak transition is calculated at 402.1 eV and assigned to an antibonding σ * (C-H) orbital with a strong admixture of Rydberg character, in agreement with previous calculations. 14,15 It corresponds to a weak peak observed in the experiment just before the second well-defined experimental peak (around 402.6 eV on the energy scale of Fig. 2).…”

“…For pyridine in the gas phase, also previous calculations on the vibrational fine structure have been reported. It is worth noting the NEXAFS has been also employed to study not only free pyridine in the gas phase 14,15 but also when adsorbed over the silicon (110) surface, both experimentally 16 and theoretically. 17…”

Resonant Auger spectroscopy at the carbon and nitrogen K-edges of pyrimidine

“…11.22 ± 0.20 11.5 (π 2 ) 11.40 (π 2 ) 11.34 (π ) 11.48 (π 2 ) 11.40 (π 2 ) 11.30 (π 2 ) 11.364 (π ) 11.20 (π ) 7a 1 (n N+ ,σ ) N 1,3 (57.6%) C 4,6 (13.8%) C 5 (13.4%) 11.66 ± 0.30 11.2 (n N+ ,σ ) 11.20 (n N+ ,σ ) 11.48 (n) 12.88 (n N+ ,σ ) 10.96 (n N+ ,σ ) 11.31 (n N+ ,σ ) 11.471 (n) 10.93 (n) (see Table II). Experimentally, the first two peaks have an energy spacing of about 1 eV and their reversed ordering has been confirmed by β-parameter measurements, 14,16 which associate the outermost band (with a measured β = 0.61 at 45 eV photon energy) to a hole in the σ -type orbital, n N , and the second one (with a measured β = 1.42 at the same photon energy) to a hole in the π -type orbital, π 3 .…”

“…Among organic polyatomic molecules, pyrimidine ( Figure 1) is of particular relevance because its six member ring structure, made up of two N and four C atoms, is the base structure for three nucleic acids: uracil, cytosine, and thymine. Thus on the one hand pyrimidine represents an ideal case to study site-and state-selective electron decay because (i) it has three non-equivalent carbon sites, which produce features well separated in energy both in the NEXAFS 11,12 and XPS C(1s) 13 spectra; (ii) the interpretation of the present results can benefit from a comprehensive spectroscopic characterisation provided by the combined experimental and theoretical studies J. Chem. Phys.…”

“…Phys. 136, 154308 (2012) of the valence shell ionization, [14][15][16][17][18] inner-shell excitation, 11,12 and ionization 13 and Auger decay 19 ; (iii) a few "similar" aromatic molecules, such as benzene, furan, and pyridine can be used for comparative studies. On the other hand, the RAE spectroscopy provides a test of consistency of the assignments made in the previously mentioned works and allows the experimental confirmation of some theoretical predictions.…”