1. 1 A Study of Hydroxycyclohexadienyl Radical Absorption Using Time-Resolved Resonance Raman Spectroscopy Deanna O’Donnell University of Notre Dame Radiation Laboratory Department of Chemistry and Biochemistry 64 th OSU International Symposium on Molecular Spectroscopy June 25 th, 2009

3. 3 Question: What is the chemical and physical nature of the transient isomers that contribute to the single broad transient absorption in the visible region? C 6 H 5 SO 3 - C 6 H 5 NO 2 C 6 H 5 CN C 6 H 5 CO 2 - C 6 H 5 COCH 3 C 6 H 5 CH 2 CO 2 - C 6 H 5 CH 2 OH C 6 H 5 OCH 3 p-NO 2 C 6 H 4 CO 2 - Figure 1 – Model structure of hydroxycyclohexadienyl radical 1. Neta, P.; Dorfman, L. M., Advances in Chemistry Series 1968, 81, 222-230. Research Objective Figure 2 - Absorption bands of various hydroxycyclohexadienyl radicals in aqueous solution containing N 2 O at neutral pH 1 2.5 2.6 0.55

4. 4 Why Resonance Raman? Aim: To identify species contributing to absorption at a given wavelength In Resonance Raman, the intensity of a given vibrational mode is enhanced when the molecule is in resonance with the excitation wavelength ( o ) EeEe E m, m n me Intensity of mn Intensity of o Probe frequency Raman scattering freq. polarizability tensor Frank-Condon factor Damping factor Freq. of excited state

5. 5 Model System Benzoate anion Choose X such that the formation of OH-adduct is favored over oxidation by ● OH

6. 6 ESR of Benzoate OH-adduct Eiben, K.; Fessenden, R.W.; J. Phys. Chem. 1971, 75, 1186-1201. Figure 1 - High-field side of the spectrum obtained under high resolution with an N 2 O-saturated, 10 -2 M benzoate solution. Determined 1:2:1 ratio of formation forortho:meta:para adducts per position on the ring Did not detect ipso adduct

7. 7 Research Strategy Questions to Answer: What species contribute to the broad transient absorption? How does each species contribute? 1.Transient Absorption Spectroscopy Identify transient absorption of adducts of interest Ensure proper chemistry employed 2.Time-Resolved Resonance Raman (TR-RR) Studies Structural information 3.DFT Calculations Turn qualitative analysis into a quantitative analysis

8. 8 Transient Absorption - LINAC Computer Controlled RF Linac System pulse width = 2ns to 1  s absorbed dose yields ~10 -6 M observe transients 10 -9 to 10 -3 s after pulse

9. 9 OH-adduct Transient Absorption Absorption of Benzoate OH-adduct(s) 10  s after electron pulse at pH 9.80 Solution contained 1mM sodium benzoate, bubbled with N 2 O, KOH

10. 10 Time-Resolved Resonance Raman produce ~10 -4 M transient species

11. 11 RR Benzoate OH-adduct(s) Solution contains 1mM sodium benzoate, bubbled with N 2 O, KOH (pH~10) Signal acquired 1  s after electron pulse at 341nm, signal averaged: 250 minutes accumulation

12. 12 The structure and electron distribution of the carboxylate group does not change upon OH addition Therefore, the vibrational modes specific to that this functional group will not shift significantly (e.g. C=O symm stretch ~1400cm -1 ) 2 What do we already know about isomers’ structure? 1.Simic, M; Hoffman, M.Z.; J. Phys. Chem., 1972, 76, 1398-1404 2.Green, J.H.S.; et al.; Spectrochimica Acta, 1961, 17, 486-502

13. 13 Tripathi, G.N.R., Schuler, R.H.; J. Phys. Chem. 1988, 92, 5129-5133 http://home.arcor.de/rothw/gauss/varsanyi/molekuele/Bz/ Vibrational Coupling 8a ~1600cm -1 ± 50cm -1 benzene:1595cm -1 9a ~1160cm -1 ± 30cm -1 benzene:1178cm -1 where X = -OCH 3, -Br, -Cl, -CH 3, -F

14. 14 Spectroscopic Assignment Frequency cm -1 Assignment 1606Parent (8a) 15668a 1548 shoulder 1533 shoulder 149919a 1404 shoulder 1368C=O symm. st. 1308-1290 1198 shoulder 11807a 11689a 1040 9601 or 18a 8071 or 18a 602 525 3236a Benzoate OH-adduct RR spectrum at pH 10 and 341nm

15. 15 B3LYP/6-311G(d,p) Benzoate p-OH adduct DFT Calculations AssignmentExperimental Frequency cm -1 Calculated Frequency (no H 2 O) Calculated Frequency (2 H 2 O) 8a15661597.11604.5 19a14991433.61447.6 C=O136813211363.5 7a1180 or 10401094.31115.7 9a11681185.11195.4 18a807 or 960924.8948.5 1960 or 807805.6811.8 6a323332.5362.9

16. 16 DFT Calculations para OH-adduct of Benzoate B3LYP/6-311G(d,p) 1.501 Å 1.460 Å 1.360 Å 1.422 Å 1.522 Å 1.257 Å 1.360 Å 1.084 Å 1.422 Å 1.089 Å 1.109 Å 1.086 Å 1.786 Å 1.258 Å 1.790 Å 0.985 Å 0.965 Å 1.868 Å 0.965 Å 0.985 Å 1.084 Å 0.963 Å 1.497 Å

17. 17 ESR studies predict concentration of meta OH-adduct is twice that of ortho and para OH-adducts If molar extinction coefficient (  ) is equivalent in isomers, absorption at max should correspond to meta OH-adduct TR-RR spectra at = 341nm due to para isomer contribution This indicates  is not equivalent in all isomers and  para >  ortho,  meta A note on 

18. 18 Probed absorption at 341nm – RR spectrum indicates para OH-adduct dominate species – presence of 9a at 1167cm -1 and strong 8a at 1566cm -1  is not equivalent in all isomers and  para >  ortho,  meta Unassigned modes likely other OH-adduct isomers Future Work Probe spectrum off-resonance (360nm) to minimize resonance with other isomers Adjust instrumentation to probe other features in absorption spectrum To Sum It All Up

19. 19 Acknowledgments Dr. G.N.R. Tripathi – research advisor Dr. Ian Carmichael – academic advisor Dr. Irek Janik – assisted with Raman instrumentation Dr. Nicole Brinkmann – DFT calculations Prof. Wolfgang Roth – provided vibrational mode animations This work was sponsored by the Department of Energy

21. 21 Terephthalate o-OH adduct Solution contains 1mM disodium terephthalate, bubbled with N 2 O,KOH (pH~10) Signal acquired 1  s after electron pulse at 341nm, signal averaged: 215 minutes accumulation

22. 22 Raman Spectrum of Solid Hydroxybenzoic acid Isomers: Looking for trends in symmetry