1

Time-Resolved Raman Studies of the Electron Adducts of Benzoate

Anion in Water

Deanna O’DonnellUniversity of Notre Dame

Radiation Laboratory Department of Chemistry and Biochemistry

June 16th, 2008

2

Introduction• Research Goals:

study kinetics and structure of short-lived transient species

• Model system: Benzoate• Simple acid• 6 radiolytic products derived from benzoate and

benzoic acid• OH adduct lifetime may be long enough to observe experimentally

O

O

Na+

OH

O

Benzoic Acid pKa 4.19 Sodium Benzoate

3

Problems with Benzoate• Preservative in Food Industry for

over 80 years (E211)

• 1993 - Lawrence1 provided mechanism for benzoate conversion to benzene

• May 2006 - FDA released report2 listing 100 soft drinks containing benzene – 10 exceeded the EPA drinking water legal limit

• May 2008 - Coca Cola Company released a statement that they will phase out sodium benzoate in their soft drink in the UK31 Lawrence, G. J. Agric. Food Chem., 41, 1993, 693.

2 FDA, 2006. "Data on Benzene in Soft Drinks and Other Beverages, " United States Food and Drug Administration. Accessed June 2nd at:http://www.cfsan.fda.gov/~dms/benzdata.html 3 http://www.BeverageDaily.com

4

Radiolysis of Benzoate

+ionizing radiation

water solvated electron

hydroxyl radical

hydrogen atom

+

O

O

OH

O

OH

OHH+ H

+. .pKa 12

.pKa 5

Electron adducts

OH adductsH adduct

5

LINAC – Transient Absorption

InjectedElectron Beam(130 KV)

e- e -

RF acceleratedelectron beam

8MeV

lightsource

l

Monochromatorand

Photomultiplier Tube

Data acquisition, computer control

and analysis

Computer Controlled RF Linac system

Block Diagram Model TB-8/16-15

Benzoate Dianion 445 = 8000 M-1 cm-1

Benzoate Monoanion 435 = 5200 M-1 cm-1

Computer Controlled

RF Linac System250 300 350 400 450 500

0.00

0.01

0.02

0.03

0.04

0.05

2mM Benzoic Acid, 1M t-Butanol, pH 13.2, N

2 2mM Benzoic Acid,

1M t-Butanol, pH 9, N2

Opt

ical

Den

sity

Wavelength nm

6

Time-Resolved Resonance Raman

DyeLaser

XeClExcimer

Accelerator

Laser

DM

DM

M

M

M

G

Lf

L1

2L

D

RC

A

A

Legend

L2L1

Lf

PD

tOscilloscope pulse monitor

pulse monitorpulse monitor

ElectronLaserGate

pulsepulsepulse

CCD gateLaser triggerPulse and

delaygenerator

Gatepulsemonitor

CCD gatedpulse

ComputerWinSpec 3.0

2 MeV

Laser pulsemonitor

directionof flow

C

Van de Graaff

RC - Raman CellM - MirrorG - Grating

C - Controller

DM - Dielectric Mirror

- Focusing Lens- Collecting Lens- Collimating Lens

PD - Fast photodiaodeA - Aperture

D - PI-Max CCD

Van de Graaff

7

TR-RR Data Collectionlscattered

440 nm ± 30 nm

lincident

440 nm

Background:H2O, sodium benzoate, t-butanol,KOH (pH = 13.2), and degassed with N2.

1100 1200 1300 1400 1500 1600 1700

1.0

1.5

2.0

2.5

3.0

3.5

Ram

an S

igna

l x 1

06 a

rb. u

nits

Raman Shift cm-1

Signal + Background:H2O, sodium benzoate, t-butanol,KOH (pH = 13.2), degassed with N2,and benzoate dianion

8

TR-RR Data Analysis

1100 1200 1300 1400 1500 1600 1700

(signal + background) - 0.68*background

(signal + background) - 0.62*background

(signal + background) - 0.58*background

Raman Shift cm-1

9

Radiolysis of BenzoateH2O e-

aq + OH + H

Dianion Monoanion

DianionBenzoate anion

10

Benzoate Dianion TR-RR Spectrum

ub3pw91/6-31+g(d,p)

1585 cm-1

1472 cm-1

1385 cm-1

1083 cm-1

987 cm-1

Wilson 8a ring

Wilson 7aPh-CO2

-

Wilson 19aring vibration

CH bend

ring breathing

11

Dianion StructureStructural Conclusions based on

• Resonance Raman Spectrum• 1472cm-1, 7a/Ph-CO2

-

• ESR parameters • validated by pKa

Neta, P.; Fessenden, R.W., J. Phys. Chem., 77, 1973, 620-625

Hyperfine constantsao

H = 4.18 G (2)am

H = 0.83 G (2)ap

H = 7.58 G

OO

1600 cm-1

double bond

O

OPh

1107 cm-1

single bond

O

OPh

OO OO

..1472 cm-1

12

Benzoate Monoanion

pH 10

O

O OH

O

O

OH

H

OH-

OH-

OH

pKa 12

dianion

+

.

monoanion .

monoanion

1

2

+

+

.2

13

TR-RR Spectrum Monoanion

1601 cm-1

1545 cm-1

Wilson 8a ring

Wilson 7a Ph-CO2H

Tripathi, G.N.R.; Schuler, R.H.; J. Phys. Chem. 92, 1988, 5129-5133

14

Monoanion Structure

OHO OHO O OH

.. . 1601 cm-1

1600 cm-1

double bond

OH

OPh

1107 cm-1

single bond

OH

OPh

Hyperfine constantsao

H = 4.82 G (2) am

H = 1.33 G (2)ap

H = 7.26 GaCOOH

H= 0.19 G

Neta, P.; Fessenden, R.W., J. Phys. Chem., 77, 1973, 620-625

15

Is Photoionization occurring?

Semiquinone Anion Radical435=4769 M-1cm-1

unitsarbxunitsarbx

IntensityRamanMonoanionIntensityRamaneSemiquinon

.105.1.107

5

6

min5sec300.750*.1

3*5.7sec1 oraccum

accumgates

gatestimetotal

Monoanion435 = 5200 M-1cm-1

O

O

47

1800 1600 1400 1200 1000

0

1x106

2x106

3x106

4x106

5x106

6x106

7x106

116514

36

1621

Ram

an S

igna

l arb

. uni

ts

Raman Shift cm-1

16

Photon Density StudyNote the increase in the 1605cm-1 peak with respect to the water peak as laser power changes

If no effect due to the laser

this ratio should not change withvarying laser power

IntensityRamanOHfactorscalingIntensityRamanTransient

2*

1800 1700 1600 1500 1400 1300 1200

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

for I

nter

nal S

tand

ard

1605

cm-1

2mM Sodium Benzoate, 50mM t-butanol, pH 10, N

2

Inte

nsity

Raman Shift cm-1

2 mJ 4 mJ 6 mJ

0.0 5.0x105 1.0x106 1.5x106 2.0x106

0.2

0.4

0.6

0.8

1.0

Tran

sien

t Ram

an In

tens

itysc

alin

g fa

ctor

* H

2O R

aman

Inte

nsity

Raman Intensity of H2O arb. units

Jan-18-2008 (monoanion) Jan-22-2008 (dianion) Jan-25-2008 (monoanion)

17

Summary• Benzoate electron adducts

– Benzoate Dianion Radical– Benzoate Monoanion Radical

• Photoionization• Future Work includes:

– OH adduct– product analysis– para-substituted benzoate

18

Acknowledgments• Dr. G.N.R. Tripathi• Dr. Ian Carmichael• Dr. Irek Janik

• This work was sponsored by the Department of Energy

19

20

Extra Slides

21

More Raman Basics• Raman shifts can be expressed as o ± m

Stokes and Anti-stokes produce same spectrum, differing in intensity. Intensity is governed by the Maxwell-Boltzmann Distribution law.

• Raman shifts are measured in wavenumbers (cm-1)

600 400 200 0 -200 -400 -600

460 31

221

7

- 217

- 312

- 460

Rayleigh Scattering

Anti-Stokes Scattering

Stokes Scattering

Raman Shift cm-1

Stokes and Anti-stokes Raman Spectrum of CCl4

22

Signal Enhancement

• Common method to enhance the Raman scattering is• Resonance Raman

Resonance Raman• Occurs when o em• Enhancement is on the order of

103 to 108

280 300 320 340 360 380 400 420 440 460 480 500 520

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

2mM Benzoic Acid, 1M t-BuOHpH 13.2, N

2

e- adduct

440 nm

320 nm

Abso

rptio

n

Wavelength nmi = ij Ej

i = induced electric dipole ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation

Imn = Io(o-mn)4|(ij)mn|2

(ij)mn (em-o)-1

23

Instrument Setup

Laser Light

440nm

SIDE VIEW

mirror

Focusing Lens

Raman cell

slit

PI gated CCD

Raman scattering

Excimer laser

Dye laser

Direction of flow

24

Electron Spin Resonance• Same principles as NMR• Measures resonance of electron rather than nuclei• Species must be paramagnetic

Hyperfine Constant• analogous to J-coupling in NMR• when e interact with N, allow additional energy states• spacing between lines is hyperfine constant

Bo=0 Bo≠0

E

Magnetic Field

ms= - ½

ms= + ½

E = E +½ - E -½ = geBBo E = h = gBBo

25

First step: Hydroxyl radical production Cu2+ + H2Asc Cu+ + HAsc (1)

Cu+ + O2 Cu2+ + O2 - (2)

2O2 - + 2H+ O2 + H2O2 (3)

Cu+ + H2O2 Cu2+ + OH- + OH (4)

Second step: Decarboxylation of benzoate OH + C6H5CO2

- C6H5(OH)CO2-

C6H5(OH)CO2- C6H5OH- + CO2

C6H5OH- + H2O C6H6 + 2 OH-?

Decarboxylation of Benzoate

in soft drinks

or benzoic acid

26

Pulse Radiolysis Data System Reacting

speciespH Transient species pKa lmax,

nmmax,

M-1, cm-1

10mM C6H5CO2H,

1M t-butanol

eaq- 3.8 [C6H5C(OH)2] 5.3 <290

420>16000

1600

2mM C6H5C02-,

1M t-butanol

eaq- 9.1 [C6H5C (OH)O-] 12.0 310

435250005200

2mM C6H5CO2-,

1M t-butanol

eaq- 13.2 [C6H5CO2

2-] - 322445

270008000

1mM C6H5CO2H,

25mM N2O

OH 3.1 [C6H5(OH)CO2H] 4.4 350 3800

1mM C6H5CO2-,

25mM N2O

OH 9.0 to 13.0

[C6H5(OH)CO2- ] >14 330 3800

2mM C6H5CO2H,

1M t-butanol

H 1.0 [C6H5(H)CO2H] - 350 <4200

electron adduct

OH adduct

H adduct

M. Simic and M.Z. Hoffman, J. Phys. Chem., 76, 1398 (1972)