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Electron-Withdrawing Effects in the Photodissociation of CH2ICl To Form CH2Cl Radical, Simultaneously Viewed Through the Carbon K and Chlorine L2,3 X‑ray Edges

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journal contribution
posted on 24.09.2018 by Zheyue Yang, Kirsten Schnorr, Aditi Bhattacherjee, Pierre-Louis Lefebvre, Michael Epshtein, Tian Xue, John F. Stanton, Stephen R. Leone
A fundamental chlorine-containing radical, CH2Cl, is generated by the ultrafast photodissociation of CH2ICl at 266 nm and studied at both the carbon K edge (∼284 eV) and chlorine L2,3 edge (∼200 eV) by femtosecond X-ray transient absorption spectroscopy. The electronic structure of CH2Cl radical is characterized by a prominent new carbon 1s X-ray absorption feature at lower energy, resulting from a transition to the half-filled frontier carbon 2p orbital (singly occupied molecular orbital of the radical; SOMO). Shifts of other core-to-valence absorption features upon photodissociation of CH2ICl to yield ·CH2Cl indicate changes in the energies of core-level transitions of carbon and chlorine to the σ*­(C–Cl) valence orbital. When the C–I bond breaks, loss of the electron-withdrawing iodine atom donates electron density back to carbon and shields the carbon 1s core level, resulting in a ∼0.8 eV red shift of the carbon 1s to σ*­(C–Cl) transition. Meanwhile, the 2p inner shell of the chlorine atom in the radical is less impacted by the iodine atom removal, as demonstrated by the observation of a ∼0.6 eV blue shift of the transitions at the chlorine L2,3 edges, mainly due to the stronger C–Cl bond and the increased energy of the σ*­(C–Cl) orbital. The results suggest that the shift in the carbon 1s orbital is greater than the shift in the σ*­(C–Cl) orbital upon going from the closed-shell molecule to the radical. Ab initio calculations using the equation of motion coupled-cluster theory establish rigorous assignment and positions of the X-ray spectral features in the parent molecule and the location of the SOMO in the CH2Cl radical.