A quantitative experimental study of the core excited electronic states of formamide, formic acid, and formyl fluoride

Abstract

Optical oscillator strength spectra of formamide (HCONH2), formic acid (HCOOH), and formyl fluoride (HCOF) in the region of K-shell excitation have been derived from electron energy loss spectra recorded under electric dipole dominated scattering conditions (>2.7 keV impact energy, small scattering angle). The observed features are assigned to promotions of 1s electrons to π*(C=O), σ*(HCX), σ*(C–X), σ*(C=O), and Rydberg orbitals. Systematic changes in the term values for the 1s→π*(C=O) transitions are related to the π donor strengths of the X substituents of the carbonyl group. Broad weak features, observed only in the carbonyl C1s and O1s spectra around 7 eV above the IP, are assigned to 1s→σ*(C=O) transitions. The positions of these features are in agreement with a previously documented correlation with bond length, as are the positions of features associated with σ*(C–N) in formamide, σ*(C–O) in formic acid, and σ*(C–F) in formyl fluoride. The oscillator strengths of the 1s→π* features in the various K-shell spectra are compared to HAM/3 calculations and are used to estimate the spatial distributions of the π*(C=O) orbital in the (1s−1, π*) core excited states of these three substituted carbonyl species. We discuss the degree to which these derived orbital maps reflect the spatial distributions of π*(C=O) orbitals in the ground state.

Negative- and positive-ion fragmentation of core-excited formic-acid molecules studied with three- and four-ion coincidence spectroscopy

“…1. These resonant features have been studied earlier with near-edge x-ray absorption spectroscopy [20] and inner-shell electron energy loss spectroscopy [21,22].…”

Characterization of refractory organic substances by NEXAFS using a compact X-ray source

“…At first, NEXAFS was used to characterize small, wellknown molecules (Francis and Hitchcock 1992;Ishii and Hitchcock 1987, 1988, Sham et al 1989, Stöhr 1992 and their orientation and bond lengths. Soon, more complex samples from polymer science were investigated using NEXAFS spectroscopy, see for example Kikuma and Tonner (1996); Urquhart and Ade (2002).…”

Soft x-ray spectromicroscopy of environmental and biological samples

“…Often, NEXAFS spectra are fitted using Gaussian curves for the peaks and an arc tangent function to simulate the absorption edge. As seen from NEXAFS spectra of small well-understood molecules [10,46,91,92], shifts of peak positions and absorption edges occur, depending on, e.g., the chemical environment, the substrate or the state of aggregation. Furthermore, the functional groups and other subgroups in such large compositions are regarded as independent units.…”

“…Furthermore, the functional groups and other subgroups in such large compositions are regarded as independent units. Each of these units has its ionization point at another energy, as known from the literature [62,91,92,170,187]. Thus, due to a mix of chemistries leading to chemical shifts of the core level, the arc tangent for large and complex molecules like environmental samples would be very broad, thereby dominating the fit of the spectra and submerging the Gaussian curves, cf.…”

“…This procedure was chosen, because the second derivative is more sensitive to changes in the spectrum than determining zero crossings from the first derivative. Second, advantage was taken of the fact that published NEXAFS-spectra of many chemical compounds from polymer research, biomolecular, and environmental sciences already exist [30,46,50,76,77,91,92,105,123,172,176,178,179]. The spectra were compared to the positions gained from the derivatives and allocated to the expected peaks from transitions to unoccupied orbitals such as σ * C-N , σ * C-S or phenolic π * C=C .…”