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

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 1

The 67th International Symposium on Molecular Spectroscopy

June 18-22, 2012

Young Wook Yoon, Chang Soon Huh and Sang Kuk Lee

[email protected]

Department of Chemistry

Pusan National UniversityPusan 609-735, Korea

A part of this work appears in J. Chem. Phys. 136, 174306 (2012).

Spectroscopic identification of benzyl-type radicals generated by corona discharge of precursors of mixed substituents (WI12)


Characteristics of transient molecules

Molecular radicals, molecular ions, and highly excited molecules.

Very short lifetime (less than 10-6 sec), highly reactive in chemical reaction.

Cannot exist at ordinary condition. Need a special care for preservation, especially for benzyl-type radicals of mixed substituents.

Determine reaction pathway at the transition state as reaction intermediates.

Less than 2,000 transient species among more than 18,000,000 molecules in American Chemical Society Database.


Motivation

We have developed a technique of Corona Excited Supersonic Expansion (CESE) which is a laser-free spectroscopic tool for observation of vibronic emission spectra of transient species.

Benzyl-type radicals are excellent candidates for CESE system because they emit visible fluorescence of the D1 → D0 transition.

We want to identify the isomeric benzyl-type radicals using substituent effect on electronic energy by substitution of methyl group, fluorine, chlorine atoms into benzene ring because the effect depends on the nature and position of substituents. This talkTI10 : yesterday


Benzyl radical

The simplest and prototypical aromatic free radical.

Reaction intermediate of aromatic chain reaction.

Seven delocalized π electrons on the molecular plane.

Shows visible emission the D1 → D0 transition.

Substitution into benzene ring reduces the electronic transition energy of delocalized π electrons due to the extended space for delocalized π electrons.

H

HXn


φ1

φ2

φ3

φ4

φ5

φ6

φ7

b2

b2

a2

b2

a2

b2

b2

Molecular orbitals of benzyl radical

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

The position of nodal points depends on the electronic states. At the 1st excited state, it

locates at 1 and 4 position.


D1

D0

D2

2B2

2B2

2A2

B-type (visible region)

A-type

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

800cm-1

22000cm-1

Energy levels of benzyl radical

Transfer of population via vibronic coupling

Experimentally, only D1 → D0 is observable in the visible region.


CESE Spectra

Carrier gas(He)+

Sample

Advantage of CESEAdvantage of CESE- Very simple schemeVery simple scheme

- Reducing Doppler broadening Reducing Doppler broadening

- Improved S/N ratioImproved S/N ratio

- Simplification of spectrumSimplification of spectrum

- Powerful laser-free technique for transient speciesPowerful laser-free technique for transient species

• Po = 3 atm

• Pv = 5 Torr

• HV = ~2.0 kV

• Current = ~3 mA

P0

Vacuum ChamberVacuum Chamber

Supersonic Expansion

Pv

Principle of CESE systemCorona Excited Supersonic Expansion

(+) (-)e-Corona Discharge

High

Press. (P0)

Em

issi

on

anode cathode


Mechanism

X

*

X

X

He*

S0

Sn

− ·H

D0

Collisional relaxationD1

CESE Spectrum

-1

Origin band

Emission

*

X

Radical formation

The CESE spectrum provides vibrational frequencies in the ground state (D0) as well as electronic energy in the D1 → D0 transition of the radicals.


CESE spectrum (p-fluorobenzyl)

LIF- DF spectrum

J. Chem. Phys. 1990, 93, 8488

* He atomic line

* *

* CESE spectrum

Chem. Phys. Lett. 1999, 301 407

CESE spectrum is similar to the LIF-DF spectrum observed while pumping the origin band of the electronic transition.

Thus, the CESE spectrum always shows the origin band of the electronic transition at highest wavenumber with strongest intensity.

Origin band


Characteristics of CESE spectrum of benzyl-type radical

Rotational temperature of 40K due to weak backing pressure of jet expansion and DC corona discharge.

Very cold vibrational temperature through vibrational relaxation, showing the vibronic transition originating from the vibrationless state (v=0) of the D1 state.

Show origin the band of the D1→D0 transition at the highest wavenumber with strongest intensity, from which the electronic transition energy is determined.

The spacing of vibronic bands from the origin band represents the vibrational mode frequencies in the D0 state.


Benzyl-type radicals of mixed substituents

H

HX Y

Two different substituents, X and Y, X and Y = -CH3, F, and Cl

There are many possible combinations of X and Y. 2-chloro-4-fluorobenzyl 2-methyl-3-fluorobenzyl 2-chloro-5-fluorobenzyl 2-methyl-6-fluorobenzyl 2-fluoro-4-chlorobenzyl 2-methyl-4-fluorobenzyl 2-fluoro-5-chlorobenzyl 2-methyl-5-fluorobenzyl

2-fluoro-3-methylbenzyl (WI11, previous talk) 2-chloro-3-methylbenzyl (WI11, previous talk)

Yesterday, we presented benzyl-type radicals of X,Y = -CH3. (TI10)


Precursors generating one product

F

Cl

CH2•

F

Cl

CH2•

F

Cl

F

Cl

Cl

F

CH2•

Cl

F

CH2•

Cl

F

Cl

F

2-chloro-4-fluorobenzyl 2-chloro-5-fluorobenzyl

2-fluoro-4-chlorobenzyl 2-fluoro-5-chlorobenzyl

There is only one methyl group.


F

•H2C

F

Cl

•H2C

Cl

Precursors generating one products

2-fluoro-3-methylbenzyl (WI11)

2-chloro-3-methylbenzyl (WI11)

Precursors produce only one product because both methyl groups are equivalent.

There are two equivalent methyl groups.


F

CH2•

F

CH2•

F

+

F

CH2•

F

CH2•

F

+

Precursors generating two isomers

2-methyl-3-fluorobenzyl 2-methyl-6-fluorobenzyl

2-methyl-4-fluorobenzyl 2-methyl-5-fluorobenzyl

Precursors produce 2 isomers because two methyl groups are different.

There are two non-equivalent methyl groups.


Spectrum of 2-chloro-5-fluorobenzyl radical

Origin

Typical vibronic emission spectrum

of benzyl-type radicals


Elimination of Cl atom at higher voltage

Production of 3-fluorobenzyl radical by elimination of Cl atom at higher discharging voltage


Spectrum from 2-fluoro-4-chlorotoluene

0

50

100

150

200

20000 20500 21000 21500 22000

Wavenumber (cm-1)

Inte

nsit

y

0(1)

0(2)

*

*

1(2)1(1)

*6b(1)

7a(1)

8a(1)

14(1)

12(1)

C2

7a(2)

CH2•F

Cl

CH2•F

[2][1]

Produce two different products by dissociation

of C-Cl bond.


Origin

F

CH2•

F

CH2•

F

+

Spectrum from 2-methyl-3-fluorotolueneProduce two isomers


Spectrum from 2-methyl-4-fluorotoluene

F

CH2•

F

CH2•

F

+


Assignment procedure

The assignments of vibronic bands belonging to benzyl-type radical were obtained by comparing with those of precursor as well as those of ab initio calculation.

The vibrational mode frequencies obtained by DFT/6-311G* are accurate within ±3% of the observation.

The prediction of electronic energy obtained by TDDFT is accurate within ±20% of the observation.

However, the assignments of isomeric benzyl-type radicals generated from the same precursor is obtained by the shift of the origin band based on the mono-substituted benzyl radicals.


ModeThis work

(D0)

Ab initioB3LYP/6-311G*

(D0)

2-Chloro-4-Fluorotoluene

(S0)

Symmetry(Cs)

Origin 21014

9b 416 416 418 a'

6a 902 911 906 a'

6b 468 472 484 a'

1 680 687 689 a'

TABLE III. Vibrational frequencies of 2-chloro-4-fluorobenzyl radicala

a Precursor produces only one kind of benzyl-type radical.


Substituent effect of mono-substitutiona

a Represents the red-shift of the origin bands from parental benzyl radicals at 22,002 cm-1.

Substituent o - m - p -

CH3 657 517 302

F 78 311 475

Cl 962 808 357

CN 1824 1678 1256

Substitution into benzene ring shifts the origin band to red region of the D1→D0 transition of the radicals. The shift depends on the

kind and position of substituents.


Molecules Origin band Shift

Benzyl 22002 0

2-Methylbenzyl 21345 657

2,4-Dimethylbenzyl 21306 696

2-Fluorobenzyl 21924 78

2,4-Difluorobenzyl 21846 156

2-Chlorobenzyl 21040 962

2-Chloro-4-fluorobenzyl 21014 988

2,4-Dichlorobenzyl 20980 1022

TABLE. Origin bands in the D1 → D0 transition of benzyl-type radicals

Substituent o- m- p-

F 78 311 475

Cl 962 808 357

CH3 657 517 302


Table 1. The origin bands (cm-1) of difluorobenzyl radicals in D1→D0 transition Substituent o- m- p-

F 78 311 475

Cl 962 808 357

CH3 657 517 302

a Without substituent at 4-position


The origin bands (cm-1) of dichlorobenzyl radicals in D1→D0 transition

Molecules Obs. Shift

Calc.

TDDFT Diff Empirical Difff

2,4-dichloro 20980 1022 24674 2629 1319(962)a -297(60)a

2,5-dichloro 19984 2018 25764 3918 1770 248

2,6-dichloro 20153 1849 23532 2484 1924 75

3,4-dichloro 21098 904 24980 3206 1165(808)a -261(96)a


F 78 311 475

Cl 962 808 357

CH3 657 517 302



The origin bands (cmThe origin bands (cm-1-1) of the benzyl-type radicals in D) of the benzyl-type radicals in D11→D→D

00 transition transition

Molecules Obs. ShiftCalc.

TDDFT Diff Empirical Diff2-chloro-4-fluoro 21014 988 24780 2778 1437(962)a 26

2-chloro-5-fluoro 20256 174622678

　 676 　 1273 473

2-fluoro-4-chloro 21708 294 25729 4021 435(78)a 2162-fluoro-5-chloro 20762 1240 23327 2565 886 354

2-methyl-3-fluoro 20486 151623795

　1793

　 968 548

2-methyl-6-fluoro 21174 828 24284 3110 735 93

2-methyl-4-fluoro 21480 52225331

　 3329 1132(657)a -135

2-methyl-5-fluoro 20505 1497 23455 1453 968 529

3-methyl-2-fluoro 21376 62624334

　 2332 595 31

3-methyl-2-chloro 20688 131423777

　3089

　 1479 -165


F 78 311 475

Cl 962 808 357

CH3 657 517 302



Shift of electronic energy

Conjugated organic molecules Transition (λ, nm)

H-C=C-H 170

H-C=C-C=C-H 220

H-C=C-C=C-C=C-H 260

For 1-D delocalization

For 2-D delocalization

Benzene 38461 (cm-1)

Toluene 37474

o-Chlorotoluene 36863

m-Chlorotoluene 36602

p-Chlorotoluene 36281

Extension of space for delocalized π electrons reduces electronic transition energy. Substitution into benzene ring

increases the delocalizing space.


Nodal points in Hückel MO

Φ2

Φ3

Φ1

Φ4

Φ5

Φ6

Φ7b2

a2

a2

b2

b2

b2

b2

D0 (b2)

D2 (b2)

D1 (a2)Nodal point at 1 and 4-positions

No nodal point

The position of nodal points changes with electronic states. At D1 state, it is located at 1 and 4 positions.


Possible benzyl-type radicals from precursor

F

Cl

CH2•

F

CH2•

F

Cl

CH2•

Cl

+ +

[3][2][1]

[1] Product of cleavage of methyl C-H bond and C-Cl dissociation

[2] Product of cleavage of methyl C-H bond

[3] Product of cleavage of methyl C-H bond and C-F dissociation


Spectrum observed from corona discharge of 2-chloro-4-fluorotoluene

0

150

300

450

20000 20500 21000 21500 22000

Wavenumber (cm-1)

Inte

nsit

y

6b(2) **

0(1)

0(2)

6a(1)

1(1)6a(2)

7a(1)

1(2) 9b(2)

CH2•

F

Cl

CH2•

F

Spectrum shows only two products can be obtained from the corona discharge of 2-chloro-4-fluorotoluene


Proposed dissociation process

C-Cl Cleavage

C-H CleavageCH3

F

Cl

CH2•

F

CH2•

F

Cl

[1]

[2]

CH2

F

Cl

H

CH2

F

Cl

H CH2

F

H

•

519kJ/mol

398kJ/mol

356kJ/mol

We could not observe the product of cleavage of C-F bond.


Summary

We successfully observed the vibronic emission spectra of 2-chloro-4fluorobenzyl radical using a technique of CESE.

We explain the smaller red-shift of the origin bands of benzyl-type radicals with substituent at 4-position using Hückel MO theory for the first time. The smaller shift is observed from other multi-substituted benzyl radicals.

This explanation provides direct evidence of nodal points at a given electronic state.

We found the dissociation products agree with the bond dissociation energy.


Acknowledgments

National Research Foundation of Korea (2011-2014)

Funding for Basic Sciences

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