1. Vibronic Emission Spectroscopy of Benzyl-type Radicals Generated from Chloro-Substituted o-Xylenes in Corona Dischargea
Young Wook Yoon and Sang Kuk Lee
Department of Chemistry
Pusan National University
Busan 46241, Korea
aSupported by the National Research Foundation(NRF) of Korea.

2. Chemistry in corona discharge
What is the reaction intermediate?
A + B → [A-B]☨ → C + D
How to detect and identify the intermediate?

3. Seven delocalized π electrons on the ring plane
What is benzyl radical?
A prototype of aromatic molecular radicals
Reaction intermediate in aromatic reactions
Seven delocalized π electronic system
Fairly strong visible emission of the D1 → D0 transition H
Seven delocalized π electrons on the ring plane
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

4. Motivation We have identified many benzyl-type radicals of
Up to the present, the hetero disubstituted benzyl radicals of CH3 and Cl have been much less studied due to the difficulty associated with weak emission intensity and dissociation of C-Cl bond.
X=F, Cl, CH3, CN
X, Y=F, Cl, CH3
Home-type
Hetero-type

5. Pinhole-type glass nozzle for generation of transient molecules
Rev. Sci. Instrum 57, 2274 (1986).
Large soot deposits near hole, destabilizing the discharge system
Schematics of glass nozzle for corona discharge and supersonic jet expansion
Useful for OH radical,
but not suitable for hydrocarbons

6. Modification of glass nozzle for use of heavy aromatic compounds
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

7. Glass nozzle Nozzle holder Nozzle holder Anode Anode holder
1.27 cm
< 0.3 mm
Soot deposition

8. Visible emission in CESE system Demonstration with He Discharge
Discharge in CESE
Glass nozzle
The bright visible He emission disappears with injection of precursor because of energy transfer from He* to precursor.

9. Characteristics of CESE system Corona Excitation and Supersonic Jet 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
Laser-free technique for transient species
Simple scheme and easy to operate
Very small electric current
Reduce sample temperature up to 30K
Strong emission intensity
CESE Spectra

10. Spectrum from corona discharge of 3-chloro-o-xylene*
Precursor
2M3Cl
2M6Cl
o-Xylyl
Already reported

11. Spectrum from corona discharge of 4-chloro-o-xylene
Precursor
2M5Cl
2M4Cl

12. Assignment of the origin band of isomeric chloro-substiututed-o-xylyl radical
We estimated the electronic transition energy of the di- substituted benzyl-type radicals by using the additivity rule, summing up the contribution from each substituent which has a limitation up to di-substitution.
With the confirmation of the origin band, other vibronic bands were assigned by comparing with the ab initio calculation.
We identified the radical species generated by assigning the vibronic bands and the molecular structure of precursor.

13. Red-shift of electronic transition energy with substitution
Substituent
Position
Origin banda
Red-shifta,b
X=None
22002
X=CH3 2-
21345
657 3-
21485
517 4-
21700
302
X=Cl
21040
962
21194
808
21645
357
aIn unit of cm-1 bRed-shift from benzyl radical of 22002cm-1
The size of red-shift depends on the position and type of the substituents on benzene ring.

14. 2-Methyl-3-Chlorobenzyl Radical
657
808
Additivity rule : =1465 cm-1
Prediction : =20537 cm-1
Observation : cm-1
2-Methyl-6-Chlorobenzyl Radical
657
Additivity rule : =1619 cm-1
Prediction : =20383 cm-1
Observation : cm-1
962
The 2M3Cl and 2M6Cl benzyl radicals show excellent agreements with the observation.

15. Assignment of origin bands of benzyl-type radicals observed from 3-chloro-o-xylene*
Already reported

16. 2-Methyl-4-Chlorobenzyl Radical
Additivity rule : =1014 cm-1
Prediction : =20988 cm-1
Observation : cm-1
657 ●
● Nodes at D1 state X
357
2-Methyl-5-Chlorobenzyl Radical
657
Additivity rule : =1465 cm-1
Prediction : =20537 cm-1
Observation : cm-1
Anti-parallel orientation of two substituents ● ●
808
The 2M4Cl and 2M5Cl benzyl radicals show a large discrepancy from the observations.

17. Assignment of origin bands of benzyl-type radicals observed from 4-chloro-o-xylene

18. Shows excellent agreement with the ab initio calculation.
Determination of vibrational structures of 2M3Cl and 2M6Cl benzyl radicals
Shows excellent agreement with the ab initio calculation.

19. Shows excellent agreement with the ab initio calculation.
Determination of vibrational structures of 2M4Cl and 2M5Cl benzyl radicals
Shows excellent agreement with the ab initio calculation.

20. Assignments of Isomeric Radicals
Question: How to explain the distribution of the electronic transition energies of isomeric chloro-substituted methylbenzyl radicals?

21. Electronic energy of substituted benzyl radicals
We assume that the rotational period of electron determines the electronic transition energy.
for elliptic orbit
a : major axis
b : minor axis
Elliptic orbit has a longer circumference than circular orbit, showing lower electronic transition energy.

22. Shape of molecular orbitals at the D1 state Nodes are located at 1- and 4-positions●
(Benzyl)
(2M4Cl)
(2M3Cl)
(2M6Cl)
(2M5Cl) ● ●
MO shape ● ● ● ● ● ● ● ● ● X
Medium size
Elliptic
Large size
Circular
Near Elliptic
Small size
21376
20270
20418
20680
22002
Obs. (cm -1)
Red-shift (cm-1)
626
1322
1584
1732

23. Tetrafluorobenzyl Pentafluorobenzyl● X
21910(92)
21857(145)
Although they have different numbers of substituents, they show a similar LUMO shape due to the modal position, confirming that LUMO determines the electronic transition energy.

24. Orbit shape of disubstituted benzyl radicals
for elliptic
Molecules
Energy (cm-1)
Red-shift (cm-1)
Circular/Elliptic
a/b
Benzyl
22002
1.000
2M3Cl
20680
1322
1.124
2M6Cl
20418
1584
1.150
2M5Cl
20270
1732
1.165
It is possible to determine the shape of the elliptic orbit from the red-shift of the electronic transition energies.

25. Summary
We have observed the vibronic emission spectra from corona discharge of chloro-substituted o-xylenes.
We have identified the benzyl-type radicals from the assignments of the origin and vibronic bands.
More interestingly, we discovered the red-shift of electronic transition energy with substitution is related to the shape and size of LUMO for the first time.
We confirmed the CESE system is useful for emission spectroscopy of benzyl-type radicals.

26. * Scheme of CESE system D2 D1 Sn D0 S0 -1X *
He* Sn
− ·H D0 D1
CESE Spectrum
-1
Origin band
Emission
Radical Production
Precursor D2
Vibronic relaxation S0
The CESE spectrum provides directly
Origin band : Electronic energy of the D1 → D0 transition.
Vibronic structure : Vibrational mode in the D0 state.

27. Characteristics of CESE spectrum
LIF- DF spectrum
J. Chem. Phys. 1990, 93, 8488
No bands exist to blue region of the origin band
Origin band
p-fluorobenzyl radical
* He atomic line *
CESE spectrum
Chem. Phys. Lett. 1999,
Origin band
Vibrational structures in the D0 state

28. Displacement reaction of Cl by H
2M6Cl
2Mbenzyl
The production of o-xylyl radical can be explained in terms of bond dissociation energy.