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Characterization of Substituted Radicals by Multi-Edge Femtosecond X-Ray Transient Absorption Spectroscopy


Femtosecond x-ray transient absorption spectroscopy is used in conjunction with selective bond breaking to characterize the electronic structures of substituted radicals. By utilizing the A band photodissociation of iodides, a series of carbon-centered radicals is produced and their frontier molecular orbitals are probed via core-level transitions by x-ray photons produced through high harmonic generation, whose limit has been extended to 390 eV with the apparatus detailed in the second chapter.

Two types of substituted radicals are studied in this work to demonstrate substitution effects: halogenated radicals and alkyl substituted radicals. The third chapter focuses on the halogenated radicals, in particular ·CH2Cl and ·CH2Br, which are produced under ultraviolet radiation of CH2ICl and CH2IBr, respectively. The ·CH2Cl radical is probed via the carbon K edge and the chlorine L2,3 edges to reveal the electron withdrawing effect impacting different atoms in the photodissociation of CH2ICl. Core-level transitions in ·CH2Br are studied in comparison to ·CH3 and ·CH2Cl to understand possible stabilization of the frontier molecular orbital by π bonding and the strength of such stabilization. The fourth chapter turns to the alkyl substituted radicals, where the methyl, ethyl, isopropyl and tert-butyl radical are produced from methyl, ethyl, isopropyl and tert-butyl iodide, respectively. While the characteristic carbon 1s→σ*(C-I) transition takes place in the parent iodides at similar energies, the transition from carbon 1s to the frontier singly occupied molecular orbital differs drastically in the radicals. This can be explained by either a significant change in the carbon 1s orbital of the radicals because of methyl substitution or complications due to vibrational excitation. The results point to new directions for core-level theoretical investigations.

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