Radicals are important short-lived species whose valence orbitals are characterized by the singly occupied molecular orbital (SOMO), which plays a large role in stability and chemical reactivity. A table top pump-probe x-ray apparatus is used to investigate the electronic structure of radicals by core level spectroscopy. Employing the broad A band photodissociation of iodinated species, radicals are generated in a consistent fashion with sub 80-fs 266 nm light to break the C-I bond to form CH3 from CH3I, CH2Cl from CH2ICl, CH2Br from CH2IBr, allyl radical from allyl iodide, and tert-butyl radical from tert-butyl iodide. A soft x-ray beam, resulting from high harmonic generation up to 300 eV is used to probe the radicals from the carbon 1s core level, yielding SOMO absorption spectra as shown in Fig. 1. By comparing the SOMO peak positions of methyl and other substituted radicals, effects of the substituents are explored. In the case of CH2Cl and CH2Br, halogenation results in a shift of the C 1s core level due to the electron withdrawing effect of halogens, in agreement with C K edge photoelectron studies. Additionally, in the case of CH2Br, the effect of π bonding between Br 4p and C 2p is inferred. Resonance stabilization strongly affects allyl radical, where the singly occupied C 2p orbital constitutes a conjugated system with the double bond, explaining the 1 eV difference in the SOMO between CH3 and allyl radical. Tert-butyl radical whose SOMO is 1.5 eV higher than that of CH3 is interpreted as a combined effect of hyperconjugation and vibrational excitation from photodissociation. Electron withdrawing effects, π bonding, resonance stabilization and hyperconjugation can each be quantified, providing insights about frontier orbital reactivity.

Radical characterization by ultrafast core-level spectroscopy

256th National Meeting (2018)

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