Laboratory of Molecular Spectroscopy & Nano Materials, Pusan National University, Republic of Korea Spectroscopic Identification of New Aromatic Molecular

Laboratory of Molecular Spectroscopy & Nano Materials, Pusan National University, Republic of Korea

Spectroscopic Identification of New Aromatic MolecuSpectroscopic Identification of New Aromatic Molecular Radicals in Corona Discharge: lar Radicals in Corona Discharge:

αα-Methylbenzyl Radical-Methylbenzyl Radical

Chan Ho Park, Gi Woo Lee, Hyeon Geun Ahn, Sang Kuk Lee

Department of Chemistry

Pusan National University

Pusan 609-735, Korea

The 63rd International Symposium on Molecular Spectroscopy, WG11Columbus, Ohio, June 16-20, 2008


Previous worksPrevious works

CH2•

X X

Mono-substituted benzyl radicals

Multi-substituted benzyl radicals

X : F, Cl, CH3, CN

X : F, Cl, CH3

CH2•

(X)n (X)n


0

20

40

60

80

100

17500 18500 19500 20500 21500 22500

Wavenumber (cm-1)

Inte

nsi

tyD1 → D0 (0)

D2 → D0 (0)

CH2•

C2

J. Phys. Chem. A 2007, 111, 6003.

Mesityl RadicalMesityl Radical

CH2•


Tri-substitution: Duryl RadicalTri-substitution: Duryl Radical

CH2•

J. Chem. Phys. 2007, 126, 214308.

0

500

1000

1500

19000 19500 20000 20500 21000

Wavenumber (cm-1)

Inte

nsi

ty 6b


Substitution at Side ChainSubstitution at Side Chain

CH

R

H

Br

-HBr

CBr

RH

Br2

-Br

Intermediate


1. Planar structure with 7 delocalized electrons

2. Many modes with similar vibrational frequencies

Ground state : (1b2)2 (2b2)2 (1a2)2 (3b2)1 12B2

1st Excited state : (1b2)2 (2b2)2 (1a2)1 (3b2)2 12A2

2nd Excited state : (1b2)2 (2b2)1 (1a2)2 (3b2)2 22B2

Benzyl-type RadicalsBenzyl-type Radicals

H

RX

Proto-type Aromatic Molecular Radicals


φ1

φ2

φ3

φ4

φ5

φ6

φ7

b2

b2

a2

b2

a2

b2

b2

Molecular Orbitals of Benzyl Radical Benzyl Radical


D1

D0

D2

2B2

2B2

2A2

B-type (visible region)

A-type

Theoretically, D2 → D0 , D1 → D0 are allowed.

800cm-1

22000cm-1

Energy Levels of Benzyl RadicalEnergy Levels of Benzyl Radical

Collisional Relaxation


CESE Spectra

Carrier gas(He)+

Sample

Vacuum ChamberVacuum Chamber

Supersonic Jet Expansion

Advantage of CESEAdvantage of CESE- Excellent S/N ratioExcellent S/N ratio

- Decreasing Doppler broadeningDecreasing Doppler broadening

- Simplification of spectrumSimplification of spectrum

- Powerful technique for transient speciesPowerful technique for transient species

• Po = 3 atm

• Pv = 10 Torr

• HV = ~1.5 kV

P0

Em

issi

on

Pv

Principle of CESE systemPrinciple of CESE systemCCorona orona EExcited xcited SSupersonic upersonic EExpansionxpansion

(+) (-)e-Corona Discharge


PMT

ResistorBox

High VoltageDC Power

Supply

Pump

ExpansionChamber

QuartzWindow

CollectingLens

Cathode

Ground

Monochromator(Path length = 2.0m)

SpectrometerController

SignalHV

Computer Printer

Oscilloscope

Sample +

Buffer gasOptical Table

(A)(B)(C)

Experimental : CESE system Experimental : CESE system


Overview of CESE systemOverview of CESE system


Discharge in CESE

Jet Emission in CESE systemJet Emission in CESE system


CESE vs. LIF-DF (CESE vs. LIF-DF (pp-fluorobenzyl)-fluorobenzyl)

LIF- DF spectrumJ. Chem. Phys. 1990, 93, 8488

* He atomic line

* *

* CESE spectrumChem. Phys. Lett. 1999, 301 407


CESE vs. LIF-DF (CESE vs. LIF-DF (pp-chlorobenzyl)-chlorobenzyl)


Mechanism of formationMechanism of formation

X

*

X

*

X

X

e-

S0

Sn

− ·H

D0

Collisional relaxationD1

CESE Spectrum

-1

Origin band

Emission


Substitution at Side ChainSubstitution at Side Chain

Precursor : Toluene

6b

1

6a

Origin

Do(C-H) = 415 kJ/mol

-H

Toluene Benzyl radical


Precursor: Ethylbenzene

weak intensity: α-methylbenzyl radical(?)

Major product

α-Methylbenzyl Radical Radical

Do(C-C) = 344 kJ/molDo(C-H) = 415 kJ/mol

strong transition: benzyl radical


Precursor : Isopropylbenzene

Major Product

origin of α-methylbenzyl radical

α-Methylbenzyl Radical Radical

Do(C-C) = 344 kJ/molDo(C-H) = 415 kJ/mol


a) Benzyl radical from toluene

b) α-Methylbenzyl radical from ethylbenzene

The region of C2

observed

Not observed

c) α-Methylbenzyl radical from isopropylbenzene

origin

6a6b

1

Comparison of SpectraComparison of Spectra


Assignment of α-Methylbenzyl RadicalAssignment of α-Methylbenzyl Radical

Precursor : EthylbenzeneCalculation : B3LYP/cc-pvdz


pp-Chloro-α-Methylbenzyl Radical -Chloro-α-Methylbenzyl Radical

H2C

Cl

CH3

CH2

Cl

C

Cl

CH3H

- CH3- H


Spectrum from Spectrum from pp-Chloro-Ethylbenzene-Chloro-Ethylbenzene


pp-Chlorobenzyl Radical-Chlorobenzyl Radical


From precursor

p-chlorobenzyl radical


Assignment of Assignment of pp-Chloro-α-Methylbenzyl-Chloro-α-Methylbenzyl

position From origin mode calculation Reference

21140 0 origin -- --

20782 358 6b 359 376

20354 786 1 797 815

Reference : p-chloro-fluorobenzene

Calculation : B3LYP/cc-pvdz


H2C

F

CH3

CH2

F

C

F

CH3H

- CH3- H

pp-Fluoro-α-Methylbenzyl Radical -Fluoro-α-Methylbenzyl Radical


Spectrum from Spectrum from pp-Fluoro-Ethylbenzene-Fluoro-Ethylbenzene



Spectrum from precursor

p-fluorobenzyl radical

?


21040 21140 21240 21340

21680 21780 21880 21980

α-methylbenzylbadical

p-chloro-α-methylbenzyl radical

p-fluoro-α-methylbenzyl radical

Origin band


Assignment of Assignment of pp-Fluoro-α-Methylbenzyl-Fluoro-α-Methylbenzyl

position From origin mode calculation Reference

20854 0 origin -- --

20476 378 6b 379 375

20062 792 1 807 800

Reference : 1,4-difluorobenzene

Calculation : B3LYP/cc-pvdz


SummarySummary

1.Developed CESE system for spectroscopy of molecular radicals

2.Observed many substituted benzyl radicals in the gas phase.

3.Detected new spectroscopic evidence of α-methylbenzyl radicals for the first time.


AcknowledgmentsAcknowledgments

1.Basic Research Program, Korea Research Foundation

2. Special Basic Research Program, Korea Science and Engineering Foundation

Funding for Basic Sciences


Thank you for your attention

Documents

Laboratory of Molecular Spectroscopy & Nano Materials, Pusan National University, Republic of Korea Spectroscopic Identification of New Aromatic Molecular