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The 69 th Meeting of International Symposium on Molecular Spectroscopy , MI01, UIUC,

Eun Hye Yi, Young Wook Yoon, Sang Kuk Lee sklee@pusan.ac.kr Department of Chemistry Pusan National University Pusan 609-735, South Korea. The 69 th Meeting of International Symposium on Molecular Spectroscopy , MI01, UIUC, June 16, 2014.

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The 69 th Meeting of International Symposium on Molecular Spectroscopy , MI01, UIUC,

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  1. EunHye Yi, Young Wook Yoon, Sang Kuk Lee sklee@pusan.ac.kr Department of Chemistry Pusan National University Pusan 609-735, South Korea The 69th Meeting of International Symposium on Molecular Spectroscopy, MI01, UIUC, June 16, 2014 Vibronic Emission Spectroscopy of Benzyl-type Radicals Generated by Corona Discharge A part of this work was published inJ. Phys. Chem. A 117, 2485 (2013).

  2. Motivation • Over the past 20 years, we have tried to identify benzyl-type radicals using a technique of corona excited supersonic jet expansion (CESE) which was well developed in this laboratory. • Substituted benzyl radicals, benzyl-type radicals, have been less studied due to the difficulties associated with analysis of spectra and production of radicals. • Disubstituted benzyl radicals are good candidates to test the red-shift of the electronic transition energy upon chemical substitution. • In this presentation, disubstitutedbenzyl radicals of halo substituents will be discussed.

  3. Characteristics of transient molecules • Molecular radicals, molecular ions, and highly excited molecules. • Very unstable, short lifetime (less than 10-6 sec), and highly reactive in chemical reaction, but determine reaction pathway at the transition state as reaction intermediates. • Cannot exist at ordinary condition. Need a special care for production, preservation, and observation. • Accurate prediction of transition energy is very important for spectroscopic observation.

  4. Bi-substituted benzyl radicals • For homo-substitutions X = CH3, F, Cl • For hetero-substitutions X & Y = CH3 & F, CH3 & Cl F & Cl X X X Y In this talk, the substitutions of F and Cl will be discussed.

  5. Substitution of CH3 and F • 2-fluoro-m-xylyl radical J. Chem. Phys. 136, 024309 (2012) • 3-fluoro-o-xylyl radical Chem. Phys. Lett. 584, 37 (2013) • 4-fluoro-o-xylyl radical J. Phys. Chem. A 117, 2485 (2013) Substitution of CH3and Cl • 2-chloro-m-xylyl radical • Chem. Phys. Lett. 525, 44 (2012)

  6. Substitution of F and Cl • 2-chloro-4-fluorobenzyl radical J. Chem. Phys. 136, 174306 (2012) • 2-fluoro-4-chlorobenzyl radical Bull. Korean Chem. Soc. 34, 3565 (2013) • 2-fluoro-5-chlorobenzyl radical Chem. Phys. Lett. In press (2014) • 2-chloro-5-fluorobenzyl radical Chem. Phys. Lett. In press (2014) Commercially available precursors are very limited.

  7. H H Benzyl radical (C2v) 5b2 4b2 2a2 3b2 1a2 2b2 2nd Excited state: (1b2)2 (2b2)1 (1a2)2 (3b2)222B2 1st Excited state: (1b2)2 (2b2)2 (1a2)1 (3b2)212A2 Ground state: (1b2)2 (2b2)2 (1a2)2 (3b2)1 12B2 1b2

  8. -1 22000cm Energy levels of benzyl radical 2B2 D2 -1 800cm A-type Vibronic relaxation : Transfer of population D1 2A2 B-type (visible region) 2B2 D0 Theoretically, D2 → D0 andD1 →D0 are allowed. Experimentally, D1 →D0 is observable in the visible region.

  9. Pinhole-type glass nozzle Rev. Sci. Instrum57, 2274 (1986). Schematics of glass nozzle designed for corona discharge and supersonic jet expansion Useful for OH radical, but not suitable for hydrocarbons

  10. Modification of glass nozzle • Original Engelking glass nozzle • Rev. Sci. Instrum. 57, 2274 (1986) • Made by grinding one end of glass tube • Flat bottom surface : Large deposition • Short path length : deflected beam • Useful for carbon-free precursor • Modified Engelking glass nozzle • Chem. Phys. Lett.358, 110 (2002) • Made a hole through one end of glass tube • Round bottom surface : Small deposition • Long path length : straight beam • Useful for hydrocarbon precursor

  11. 1.2cm Emission in CESE system Demonstration with Helium Discharge in CESE Electrode inside tube The bright emission disappears with injection of precursor because of energy transfer from He* to precursor.

  12. Radical formation * −·H * Sn Emission He* D0 CESE Spectrum Origin band X -1 X X X Mechanism D2 D1 Vibronic relaxation S0 Precursor • The CESE spectrum provides directly • Electronic energy of the D1 → D0 transition. • Vibrational mode frequencies in the D0 state.

  13. Production of benzyl-type radicals Three benzyl-type radicals are possible from precursors by corona discharge. 356 kJ 423 kJ e- + + 519 kJ 397 kJ major minor no production bond dissociation energy

  14. Radicals from 2-chloro-5-fluorotoluene Vdis=1.6 kV Vdis=1.5 kV

  15. Spectrum from 2-chloro-4-fluorotoluene

  16. Spectrum from 2-fluoro-4-chlorotoluene

  17. Spectrum from 2-chloro-5-fluorotoluene

  18. Spectrum from 2-fluoro-5-chlorotoluene

  19. Electronic energy and red-shift of origin bands of isomeric chlorofluorobenzyl radicals in D1→D0 transition • Features • 2-X-4-Y shows smaller shift than 2-X-5-Y. • 2-Cl-4(5)-F shows larger shift than 2-F-4(5)-Cl. aSpacing from the origin band of benzyl radical at 22002cm-1. bData based on the shift of mono-substitution.

  20. 5b1 ψ6 z 4b1 ψ5 ψ4 2a2 y 3b1 ψ3 1a2 ψ2 ψ2p 2b1 Shift of energy levels with substitution into benzene ring. ψ1 1b1 Shift of energy levels with substitution Most of substituted benzenes show red shift of electronic transition energy compared to benzene.

  21. Red/Blue shift of electronic transition energy Substitution Unsubstitution Upper state Lower state Red-shift Blue-shift

  22. Interpretation in terms of Hϋckel MO theory The 2-chloro-4-fluoro- and 2-fluoro-4-chlorobenzyl radicals havethe shifts similar to the 2-chloro- and 2-fluorobenzyl radicals, respectively, suggesting negligible contribution from 4-position. • Nodal point, zero amplitude of orbital, cannot extend π electron conjugation to substituents, giving a negligible contribution to red-shift. (new discovery) Nodal point D1 (A2) D0 (B2)

  23. The larger red-shifts of 2-fluoro-5-chloro and 2-chloro-5-fluorobenzyl radicals are attributed to orientation of substituents. • Anti-parallel alignments change the 2-dimensional molecular plane to be elongated, reducing translational energy of πelectrons and giving large shift. Anti-parallel alignments (large red-shift) Parallel alignments (small red-shift)

  24. Summary • Modified glass nozzle was highly effective for production of benzyl-type radicals which strongly depends on the discharging conditions. • Several hetero halo disubstituted benzyl radicals were successfully identified from analysis of the vibronic emission spectra. • The red-shift of electronic transition energies of benzyl-type radicals was well explained in terms of 1) orientation of substituents and 2) nodal points in Hϋckel molecular orbital theory.

  25. Acknowledgments Financial Support for Basic Sciences (2013-2016) and (2014-2019) from National Research Foundation of Korea

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