1 Time-Resolved Raman Studies of the Electron Adducts of Benzoate Anion in Water Deanna O’Donnell University of Notre Dame Radiation Laboratory Department of Chemistry and Biochemistry June 16 th, 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 Benzoic Acid pKa 4.19 Sodium Benzoate

3 Problems with Benzoate Preservative in Food Industry for over 80 years (E211) Lawrence 1 provided mechanism for benzoate conversion to benzene May FDA released report 2 listing 100 soft drinks containing benzene – 10 exceeded the EPA drinking water legal limit May Coca Cola Company released a statement that they will phase out sodium benzoate in their soft drink in the UK 3 1 Lawrence, G. J. Agric. Food Chem., 41, 1993, FDA, "Data on Benzene in Soft Drinks and Other Beverages, " United States Food and Drug Administration. Accessed June 2nd at: 3

4 Radiolysis of Benzoate + ionizing radiation water solvated electron hydroxyl radical hydrogen atom + Electron adducts OH adductsH adduct

5 LINAC – Transient Absorption Benzoate Dianion  445 = 8000 M -1 cm -1 Benzoate Monoanion  435 = 5200 M -1 cm -1 Computer Controlled RF Linac System

6 Time-Resolved Resonance Raman

7 TR-RR Data Collection scattered 440 nm ± 30 nm incident 440 nm Background: H 2 O, sodium benzoate, t-butanol, KOH (pH = 13.2), and degassed with N 2. Signal + Background: H 2 O, sodium benzoate, t-butanol, KOH (pH = 13.2), degassed with N 2, and benzoate dianion

8 TR-RR Data Analysis

9 Radiolysis of Benzoate H 2 O e - aq + OH  + H  DianionMonoanion DianionBenzoate anion

10 Benzoate Dianion TR-RR Spectrum ub3pw91/6-31+g(d,p) 1585 cm cm cm cm cm -1 Wilson 8a ring Wilson 7a Ph-CO 2 - Wilson 19a ring vibration CH bend ring breathing

11 Dianion Structure Structural Conclusions based on Resonance Raman Spectrum 1472cm -1, 7a/Ph-CO 2 - ESR parameters validated by pKa Neta, P.; Fessenden, R.W., J. Phys. Chem., 77, 1973, Hyperfine constants a o H = 4.18 G (2) a m H = 0.83 G (2) a p H = 7.58 G 1600 cm -1 double bond 1107 cm -1 single bond 1472 cm -1

12 Benzoate Monoanion pH 10

13 TR-RR Spectrum Monoanion 1601 cm cm -1 Wilson 8a ring Wilson 7a Ph-CO 2 H Tripathi, G.N.R.; Schuler, R.H.; J. Phys. Chem. 92, 1988,

14 Monoanion Structure 1601 cm cm -1 double bond 1107 cm -1 single bond Hyperfine constants a o H = 4.82 G (2) a m H = 1.33 G (2) a p H = 7.26 G a COOH H = 0.19 G Neta, P.; Fessenden, R.W., J. Phys. Chem., 77, 1973,

15 Is Photoionization occurring? Semiquinone Anion Radical  435 =4769 M -1 cm -1 Monoanion  435 = 5200 M -1 cm -1 47

16 Photon Density Study Note 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 with varying laser power

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 ) Stokes and Anti-stokes Raman Spectrum of CCl 4

22 Signal Enhancement Common method to enhance the Raman scattering is Resonance Raman Occurs when o  em Enhancement is on the order of 10 3 to 10 8  i =  ij E j  i = induced electric dipole  ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation I mn = I o ( 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 B o =0Bo≠0Bo≠0 E Magnetic Field m s = - ½ m s = + ½  E = E +½ - E -½ = g e  B B o  E = h = g  B B o

25 First step: Hydroxyl radical production Cu 2+ + H 2 Asc  Cu + + HAsc  (1) Cu + + O 2  Cu 2+ + O 2  - (2) 2O 2  - + 2H +  O 2 + H 2 O 2 (3) Cu + + H 2 O 2  Cu 2+ + OH - +  OH (4) Second step: Decarboxylation of benzoate  OH + C 6 H 5 CO 2 -   C 6 H 5 (OH)CO 2 -  C 6 H 5 (OH)CO 2 -   C 6 H 5 OH - + CO 2  C 6 H 5 OH - + H 2 O  C 6 H OH - ? Decarboxylation of Benzoate in soft drinks or benzoic acid

26 Pulse Radiolysis Data SystemReacting species pHTransient speciespK a max, nm  max, M -1, cm -1 10mM C 6 H 5 CO 2 H, 1M t-butanol e aq - 3.8[C 6 H 5 C(OH) 2 ]  5.3< > mM C 6 H 5 C0 2 -, 1M t-butanol e aq - 9.1[C 6 H 5 C (OH)O - ]  mM C 6 H 5 CO 2 -, 1M t-butanol e aq [C 6 H 5 CO 2 2- ]  mM C 6 H 5 CO 2 H, 25mM N 2 O  OH3.1[C 6 H 5 (OH)CO 2 H]  mM C 6 H 5 CO 2 -, 25mM N 2 O  OH 9.0 to 13.0 [C 6 H 5 (OH)CO 2 - ]  > mM C 6 H 5 CO 2 H, 1M t-butanol HH1.0[C 6 H 5 (H)CO 2 H]  -350<4200 electron adduct OH adduct H adduct M. Simic and M.Z. Hoffman, J. Phys. Chem., 76, 1398 (1972)