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

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.