The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, The 70 th Meeting of International Symposium on Molecular Spectroscopy, UIUC, June 22-26, 2015 Young Wook Yoon, Sang Kuk Lee Department of Chemistry Pusan National University Pusan , Korea A part of this work was published in J. Chem. Phys. 136, (2012) and Chem. Phys. Lett. 612, 134 (2014). Vibronic Spectroscopy of Hetero Dihalo-Benzyl Radicals Generated by Corona Discharge : Jet-Cooled Chlorofluorobenzyl Radicals

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, 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 has been well developed in this laboratory.  Chloro, fluoro, methyl-substituted benzyl radicals show substituent effect on electronic transition energy which can explain π electronic interaction between benzene ring and substituents.  Last year, we presented substituent effect of dimethylbenzyl radicals.  In this presentation, hetero dihalo-benzyl radicals will be discussed.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Substituent Effect of Methyl Substituents Benzyl (21345) 3- (21485) 4- (21700) Wavenumber (cm -1 ) 2,5- (20558) 2,6- (20616) 3,5- (20842) 2,3- (21164) 2,4- (21306) 3,4- (21594) 2,3,5- (19536) 2,3,6- (19796) 2,4,6- (20800) 2,4,5- (20832) 3,4,5- (20836) 2,3,4- (20844) Dimethyl- Trimethyl- Methyl-

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Redshift of Electronic Transition Energy with Substitution b a Shift of energy levels with substitution into benzene ring. ψ6ψ6 ψ5ψ5 ψ3ψ3 ψ2ψ2 ψ1ψ1 ψ4ψ4 5b 1 4b 1 3b 1 1a 2 2b 1 1b 1 2a 2 ψ 2p Benzene

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Dihalo-substituted Benzyl Radicals  For homo dihalo-benzyl radicals X = F, Cl  For hetero dihalo-benzyl radicals X & Y = F & Cl In this talk, the substitutions of F and Cl will be discussed. XXYX

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Hetero Dihalo-Substitution of F and Cl 2-chloro-4-fluorobenzyl radical J. Chem. Phys. 136 (2012) fluoro-4-chlorobenzyl radical Bull. Korean Chem. Soc. 34 (2013) fluoro-5-chlorobenzyl radical Chem. Phys. Lett (2014) 81 2-chloro-5-fluorobenzyl radical Chem. Phys. Lett. 608 (2014) 6 2-chloro-6-fluorobenzyl radical Chem. Phys. Lett. submitted Commercially available precursors are very limited.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, 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 a b Delocalized π electrons may extend to substituents.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, D1D1 D0D0 D2D2 2B22B2 2B22B2 2A22A2 B-type (visible region) A-type Theoretically, D 2 → D 0 and D 1 → D 0 are possible. Experimentally, D 1 → D 0 is observable in the visible region. 800cm 22000cm Energy Levels of Benzyl Radical Vibronic coupling transfers population to D 1 state. Symmetry

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Rev. Sci. Instrum 57, 2274 (1986). Useful for OH radical, but not suitable for hydrocarbons due to soot deposition. Schematics of glass nozzle designed for corona discharge and supersonic jet expansion Pinhole-type Glass Nozzle

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Original Engelking glass nozzle Rev. Sci. Instrum. 57, 2274 (1986)  Made by grinding one end of glass tube  Flat bottom surface : More 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 : Less deposition  Long path length : straight beam  Useful for hydrocarbon precursor Modification of Glass Nozzle

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, 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.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, 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,Y * He* SnSn − ·H− ·H D0D0 D1D1 CESE Spectrum Origin band Emission * X,Y Radical formation Precursor D2D2 Vibronic relaxation S0S0

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Production of Benzyl-type Radicals Three benzyl-type radicals are possible from chlorofluorotoluene precursors by corona discharge. ++ e-e- majorminorno production 519 kJ 397 kJ 423 kJ 356 kJ Bond dissociation energy Products depend on discharge strength.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, V dis =1.6 kV V dis =1.5 kV Radicals from 2-chloro-5-fluorotoluene C-Cl dissociation increases with discharge strength.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Spectrum from 2-chloro-4-fluorotoluene

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23,

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Spectrum from 2-fluoro-4-chlorotoluene

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Spectrum from 2-chloro-6-fluorotoluene

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Spectrum from 2-fluoro-5-chlorotoluene

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Transition energy (cm -1 ) Shift a (cm -1 ) = 1040 a With respect to original benzyl radical at cm -1

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Transition energy (cm -1 ) Shift a (cm -1 ) a With respect to original benzyl radical at cm -1 X

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Transition energy (cm -1 ) Shift (cm -1 ) a With respect to original benzyl radical at cm -1 X

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Transition energy (cm -1 ) Shift a (cm -1 ) = 1173 a With respect to original benzyl radical at cm -1

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Transition energy (cm -1 ) Shift a (cm -1 ) = 886 a With respect to original benzyl radical at cm -1

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, The 2-chloro-4-fluoro- and 2-fluoro-4-chlorobenzyl radicals show the similar shifts to the 2-chloro- and 2-fluorobenzyl radicals, respectively.  Delocalized π electronic system cannot be extended to substituents at node, giving a negligible contribution to red-shift. Nodal points D 1 (A 2 ) D 0 (B 2 )

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 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 π electrons and giving larger shift. Anti-parallel alignments (large substituent effect) Parallel alignments (small substituent effect)

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Summary 1.The modified glass nozzle was effective for production of vibronically excited but jet cooled substituted benzyl radicals from stable precursors. 2.Formation of hetero dihalo-substituted benzyl radicals was clearly confirmed by analyzing the vibronic emission spectra observed. 3.The substituent effect of hetero dihalo-substituents was well explained in terms of 1) orientation of substituents and 2) nodes in H ϋ ckel molecular orbital theory.

The 70 th International Symposium on Molecular Spectroscopy, TH07, June 23, Acknowledgments Financial Support for Basic Sciences ( ) and ( ) from National Research Foundation of Korea