1. Jet-Cooled Chlorofluorobenzyl Radicals: Spectroscopy and Mechanisma
Young Wook Yoon, Sang Kuk Lee
Department of Chemistry
Pusan National University
Pusan , Korea
aSupported by the National Research Foundation (NRF) of Korea

2. Motivation
In our laboratory, we have developed a technique of corona excited supersonic expansion (CESE) for production of vibronically excited but jet-cooled benzyl-type radicals by corona discharge.
Substituted benzyl radicals, so called benzyl-type radicals, have been less studied due to the difficulties associated with purchasing precursors and producing radicals.
We studied substituent effect on electronic transition energies by observing vibronic emission spectra of various benzyl-type radicals.
Chlorofluorobenzyl radicals are very recent subject for identification of substituent effect.

3. Characteristics of Transient Molecules
Molecular radicals, molecular ions, and highly excited molecules belong to this class.
Determine reaction pathway at the transition state as reaction intermediates.
Very unstable, short lifetimes (less than 10-6 sec), and highly reactive in chemical reaction.
Cannot exist at ordinary condition. Need a special care for production, preservation, and observation.

4. Benzyl Radical A prototype of aromatic molecular radicals
Seven delocalized π electronic system
Strong visible emission of the D1 → D0 transition
Well studied by Fukushima and Obi H
C2V point group
2nd Excited state: (1b2)2 (2b2)1 (1a2)2 (3b2)2 22B2
1st Excited state: (1b2)2 (2b2)2 (1a2)1 (3b2)2 12A2
Ground state: (1b2)2 (2b2)2 (1a2)2 (3b2)1 12B2

5. B-type (visible region)
Energy Levels of Benzyl Radical
2B2 D2 -1
Vibronic relaxation: Transfer of population
800cm
A-type D1
2A2
22000cm -1
B-type (visible region) D0
2B2
Theoretically, D2 → D0 and D1 → D0 are allowed.
Experimentally, only D1 → D0 is observable in the visible region.

6. Homo-Substituted Benzyl-type Radicals studied
Fairly strong visible emission.
Isomers studied up to n=3.
The substituent effect of –CH3 on electronic transition energy has been identified.
Fairly strong visible emission.
Isomers studied up to n=2.
Difficult to get precursor molecules of n>2.
Weak visible emission.
Isomers studied up to n=2.
Difficult to get precursor molecules of n≥2.
Difficult to produce radicals from precursors.

7. Hetero-Substituted Benzyl-type Radicals
Fairly strong visible emission.
Easy to produce from precursors.
Recent subjects.
Very weak visible emission.
Difficult to get precursors.
C-Cl dissociation is possible.
Very recent subject.(Subject of this presentation)
Commercially available precursors are limited.
C-Cl dissociation is also possible.

8. Experimental : CESE system
Resistor
Box
High Voltage
DC Power
Supply
Pump
Expansion
Chamber
Quartz
Window
Collecting
Lens
Cathode
Ground
Monochromator
(Path length = 2.0m)
Spectrometer
Controller
Signal HV
Computer
Printer
Oscilloscope
Sample +
Buffer gas
Optical Table
(A)
(B)
(C)
Nozzle
Very simple scheme

9. Pinhole-type Glass Nozzle
Rev. Sci. Instrum 57, 2274 (1986).
Large soot deposits of carbon near hole, destabilizing the discharge
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 glass nozzle
Rev. Sci. Instrum. 57, 2274 (1986)
Made by grinding one end of glass tube
Flat bottom surface : Large soot deposit
Short path length : Deflecting beam
Useful for carbon-free precursors
Modified glass nozzle
Chem. Phys. Lett. 358, 110 (2002)
Made a hole through one end of glass tube
Round bottom surface : Reducing soot deposit
Long path length : Straight beam
Useful for hydrocarbon precursors

11. Principle of CESE System Corona Excited Supersonic Expansion
Carrier gas(He) +
Sample
Vacuum Chamber Pv
Po = 3 atm
Pv = 5 Torr
HV = ~2.0 kV
Current = ~3 mA
Supersonic Expansion
(+)
(-) e-
Corona Discharge P0
High
Press. (P0)
anode
cathode
Emission
Very simple scheme
Reducing Doppler broadening up to 30K
Improved S/N ratio
Simplification of spectrum
Laser-free technique for transient species
CESE Spectra

12. Visible Emission in CESE System
Demonstration with Helium
Discharge in CESE
1.2cm
Anode inside glass tube
The bright visible He emission disappears with injection of precursor because of energy transfer from He* to precursor.

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

14. Characteristics of CESE Spectrum
LIF- DF spectrum ( pumping)
J. Chem. Phys. 1990, 93, 8488
* He atomic line *
CESE spectrum
Chem. Phys. Lett. 1999,
The CESE spectrum is similar to LIF-DF spectrum observed with excitation of origin band.

15. Production of Benzyl-type Radicals
Two precursors can be used for benzyl-type radicals.
Benzylic C-Cl bond (285 kJ/mol) dissociation
Methyl C-H bond (355 kJ/mol) dissociation
Removal of Cl
Removal of F

16. Spectra Observed from Two Different Precursors
Difference
R-CH2Cl
R-CH3
R-CH2Cl generates only one benzyl-type radicals, but R-CH3 produces two radicals.

17. Suggested Mechanism for Benzyl-type Radicals
519J
397J

18. Chlorofluorobenzyl Radicals Studied
We observed only 3 isomers among 6 possible isomers due to the limitation of available precursors
2,4-disubstitutions
2,5-disubstitutions
2,6-disubstitution

19. 2,4-disubstitutions JCP 136, 174306 (2012)
Precursor: 2-chloro-4-fluorotoluene.
Observed 2-chloro-4-fluorobenzyl and 4-fluorobenzyl radicals.
BKCS 34, 3565 (2013)
Precursor: 2-fluoro-4-chlorotoluene.
Observed 2-fluoro-4-chlorobenzyl and 2-fluorobenzyl radicals.

20. 2,5-disubstitutions CPL 612, 134 (2014)
Precursor: 2-chloro-5-fluorotoluene
Observed 2-chloro-5-fluorobenzyl and 3-fluorobenzyl radicals.
CPL 608, 6 (2014)
Precursor: 2-fluoro-5-chlorotoluene
Observed 2-fluoro-5-chlorobenzyl and 2-fluorobenzyl radicals.

21. 2,6-disubstitutions CPL 637, 148 (2015)
Precursor: 2-chloro-4-fluorotoluene
Observed 2-chloro-4-fluorobenzyl and 4-fluorobenzyl radicals.

22. Spectroscopic Data (cm-1) Determined

23. Substitution at the 4-position
The substituent at the 4-position has a negligible contribution to the substituent effect on electronic transition energy because nodal point, zero amplitude of π electronic orbitals is located at the 4-position in the D1 state.
Thus, 2-chloro-4-fluorobenzyl and 2-fluoro-4-chlorobenzyl radicals show the similar effects to the 2-chlorobenzyl and 2- fluorobenzyl radicals, respectively.
Node at 4-position
at D1 state
D1 (A2)
D0 (B2)
No extension of delocalization of π electrons to the substituent at 4-position

24. Orientation effect
The 2,5-isomers, two substituents of anti-parallel alignment, have quite large substituent effect on electronic transition energy because the plane available for delocalized π electrons changes from circular to elliptical motion of π electrons.
Anti-parallel alignments :
Large substituent effect
Parallel alignments :
Small substituent effect

25. For a linear translational motion,
me : Mass of electron
For a circular motion,
R: Radius of circular motion
Elliptic motion has larger transition energy than a circular one, giving a larger red-shift from benzyl radical.

26. Summary
We successfully observed the vibronic emission spectra of chlorofluorobenzyl radicals using a technique of CESE.
We determined the electronic transition energies and vibrational mode frequencies of the newly observed chlorofluorobenzyl radicals.
The red-shift of the electronic transition energy of benzyl-type radicals was discussed using the model of substituents orientation and node position in Hϋckel molecular orbital theory.

27. Acknowledgments Financial Support for Basic Sciences from
The National Research Foundation of Korea

28. Thank you for your attention