The x-ray absorption spectrum of the tert-butyl radical: An experimental and computational investigation

We report the x-ray absorption spectrum (XAS) of the tert-butyl radical, C4H9. The radical was generated pyrolytically from azo-tert-butane, and the XAS of the pure radical was obtained by subtraction of spectra recorded at different temperatures. The bands in the XAS were assigned by ab initio calculations that are in very good agreement with the experimental data. The lowest energy signal in the XAS is assigned to the C1s electron transition from the central carbon atom to the singly occupied molecular orbital (SOMO), while higher transitions correspond to C1s excitations from terminal carbon atoms. Furthermore, we investigated the fragmentation of the radical following resonant C1s excitation by electron-ion-coincidence spectroscopy. Several fragmentation channels were identified. The C1s excitation of the terminal carbons is associated with a stronger fragmentation tendency compared to the lowest C1s excitation of the central carbon into the SOMO. For this core excited state, we still observe an intact parent ion, C₄H₉⁺, and a comparatively higher tendency to dissociate into CH₃⁺ + C₃H₆⁺.