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.
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Ishii et al. (1987) studied this question.