Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 1 The 69 th Meeting of International Symposium on Molecular Spectroscopy, MI01, UIUC, June 16, 2014 Eun Hye Yi, Young Wook Yoon, Sang Kuk Lee Department of Chemistry Pusan National University Pusan , South Korea A part of this work was published in J. Phys. Chem. A 117, 2485 (2013). Vibronic Emission Spectroscopy of Benzyl-type Radicals Generated by Corona Discharge

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 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, disubstituted benzyl radicals of halo substituents will be discussed.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 3 Characteristics of transient molecules  Molecular radicals, molecular ions, and highly excited molecules.  Very unstable, short lifetime (less than 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.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 4 Bi-substituted benzyl radicals  For homo-substitutions X = CH 3, F, Cl  For hetero-substitutions X & Y = CH 3 & F, CH 3 & Cl F & Cl In this talk, the substitutions of F and Cl will be discussed. XXYX

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 5 Substitution of CH 3 and F 2-fluoro-m-xylyl radical J. Chem. Phys. 136, (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 CH 3 and Cl 2-chloro-m-xylyl radical Chem. Phys. Lett. 525, 44 (2012)

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 6 Substitution of F and Cl 2-chloro-4-fluorobenzyl radical J. Chem. Phys. 136, (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.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 7 Benzyl radical (C 2v ) H H 2 nd Excited state: (1b 2 ) 2 (2b 2 ) 1 (1a 2 ) 2 (3b 2 ) B 2 1 st Excited state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 1 (3b 2 ) A 2 Ground state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 2 (3b 2 ) B 2 2b 2 3b 2 4b 2 5b 2 1a 2 2a 2 1b 2

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 8 D1D1 D0D0 D2D2 2B22B2 2B22B2 2A22A2 B-type (visible region) A-type Theoretically, D 2 → D 0 and D 1 → D 0 are allowed. 800cm 22000cm Energy levels of benzyl radical Vibronic relaxation : Transfer of population Experimentally, D 1 → D 0 is observable in the visible region.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 9 Rev. Sci. Instrum 57, 2274 (1986). Useful for OH radical, but not suitable for hydrocarbons Schematics of glass nozzle designed for corona discharge and supersonic jet expansion Pinhole-type glass nozzle

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 10 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 Modification of glass nozzle

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 11 Discharge in CESE Emission in CESE system Demonstration with Helium 1.2cm Electrode inside tube The bright emission disappears with injection of precursor because of energy transfer from He* to precursor.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 12 Mechanism The CESE spectrum provides directly 1)Electronic energy of the D 1 → D 0 transition. 2)Vibrational mode frequencies in the D 0 state. X * X X He* SnSn − ·H− ·H D0D0 D1D1 CESE Spectrum Origin band Emission * X Radical formation Precursor D2D2 Vibronic relaxation S0S0

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 13 Production of benzyl-type radicals Three benzyl-type radicals are possible from precursors by corona discharge. ++ e-e- majorminorno production 519 kJ 397 kJ 423 kJ 356 kJ bond dissociation energy

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 14 V dis =1.6 kV V dis =1.5 kV Radicals from 2-chloro-5-fluorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 15 Spectrum from 2-chloro-4-fluorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 16

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 17 Spectrum from 2-fluoro-4-chlorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 18

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 19 Spectrum from 2-chloro-5-fluorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 20

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 21 Spectrum from 2-fluoro-5-chlorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 22

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 23 Electronic energy and red-shift of origin bands of isomeric chlorofluorobenzyl radicals in D 1 →D 0 transition MoleculesObs.Red-shift a Calc. Empirical b Diff benzyl chloro-4-fluoro chloro-5-fluoro fluoro-4-chloro fluoro-5-chloro chloro chloro fluoro fluoro a Spacing from the origin band of benzyl radical at 22002cm -1. b Data based on the shift of mono-substitution. 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.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 24 Shift of energy levels with substitution ψ6ψ6 ψ5ψ5 ψ3ψ3 ψ2ψ2 ψ1ψ1 ψ4ψ4 5b 1 4b 1 3b 1 1a 2 2b 1 1b 1 2a 2 ψ 2p Shift of energy levels with substitution into benzene ring. y z Most of substituted benzenes show red shift of electronic transition energy compared to benzene.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 25 Red/Blue shift of electronic transition energy Red-shift Blue-shift Upper state Lower state UnsubstitutionSubstitution

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 26 The 2-chloro-4-fluoro- and 2-fluoro-4-chlorobenzyl radicals have the 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 D 1 (A 2 ) D 0 (B 2 ) Interpretation in terms of Hϋckel MO theory

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 27 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 π electrons and giving large shift. Anti-parallel alignments (large red-shift) Parallel alignments (small red-shift)

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 28 Summary 1.Modified glass nozzle was highly effective for production of benzyl-type radicals which strongly depends on the discharging conditions. 2.Several hetero halo disubstituted benzyl radicals were successfully identified from analysis of the vibronic emission spectra. 3.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 H ϋ ckel molecular orbital theory.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 29 Acknowledgments Financial Support for Basic Sciences ( ) and ( ) from National Research Foundation of Korea