1. Photoelectron-Photofragment Coincidence Spectroscopy of tert- Butoxide and the Carbanion Isomer Ben Shen Continetti Group University of California, San Diego June 18, 2012

2. tert-Butoxy and Its Alkylhydroxy Radical Isomer Butanols are considered a biofuel candidate as well as other oxygenated molecules Model molecule for combustion of tertiary alkoxy radicals Both isomers are combustion products of tert-butanol Alkylhydroxy isomer energetics are not well characterized Difficult to separate isomers tert-butoxy radical Alkylhydroxy radical

3. hν + AB - → A(KER) + B(KER) + e - (eKE) A + B - hν + AB - → AB + e - (eKE) AB + e - A + B - PPC Experiment 1)Uses anions to analyze transient neutral species 2)Kinematically complete experiment to explore radical energetics

4. Coincidence Spectroscopy KE tot = eKE + KER KE max = hν - D 0 (AB - ) – AEA(B) E int = KE max – (eKE+KER) hν + AB - → A(KER) + B(KER) + e - (eKE)

5. Instrumentation Trap Environment: ~ 20 K Trap Lifetimes: Many seconds Beam Energies: 4 – 7 keV Detectors: Position and time sensitive

6. Anion Generation Gas Mixture : Ar (80%)/N 2 O(20%) N 2 O + e - (slow)  N 2 + O - O - + (CH 3 ) 3 COH  OH + (CH 3 ) 3 CO - OR O - + (CH 3 ) 3 COH  OH + (CH 3 ) 2 COCH 2 - Pulsed Valve Bubbler e - beam Gas Mixture + tert-butanol vapor Discharge and Supersonic Expansion Both stable and dissociative channels are observed after photodetachment

7. Tert-butoxy Stable Channel PeakPosition (eV)Simulation (cm-1)simulation assignment a1.9080 b1.958410CCC umbrella c2.008845CCC umbrella overtone d2.0581283C-C stretch e2.1091694C-C stretch + CCC umbrella f2.1762128Combination band g2.2082566C-C overtone Stable channel spectra (blue) matches Franck-condon simulated spectra (red) as well as previous spectra on tert-butoxy radical 1 Franck-Condon simulation performed with PESCAL 2 (red) with MP2/6-311++G(d,p) level of theory EA CCC umbrellaC-C stretch (1) Ramond, T. M., Davico, G. E., Schwartz, R. L., & Lineberger, W. C. J. Chem Phys, 2000 112(3), 1158. (2) K.M. Ervin, T.M. Ramond, G.E. Davico, R.L. Schwartz, S.M. Casey, W.C. Lineberger, J. Phys. Chem. A. 2001, 105, 10822

8. Dissociative Channels Calculated Δ r H (eV) 6.00 4.74 4.84 2.04 2.57 CBS-Q level of theory

9. Pathways Energetics CBS-Q level of theory hνhν hνhν 0.73 1.16 1.301.77 388 nm (3.2 eV)537 nm (2.3 eV)

10. Conclusions PPC allows for disentanglement of tert-butoxide from its isomer through coincidence detection The major dissociative channel for alkylhydroxy radical is consistent with theoretical calculations for the decomposition to methyl radical and propen-2-ol Experimental energetic values provides a point of reference for theoretical modeling of tert-butanol combustion

11. Acknowledgements Current and past members of the Continetti Lab US Department of energy

12. Phase-locking and Bunching Phase locking the ion oscillation with the laser pulse allows for faster data acquisition Resolution of Conservation of momentum calculations are dependent on the least amount of perturbation of the ion KE I¯ at 290nm

13. Enol: modeFreq(cm-1)freq(eV 1201.680.025004 2433.410.053734 3463.480.057462 4516.390.064022 5557.190.06908 6800.370.09923 7918.890.113924 8926.620.114882 91064.930.13203 101119.350.138777 111172.960.145424 121332.710.165229 131478.70.183329 141550.790.192267 151590.810.197229 161608.540.199427 171626.790.201689 181888.880.234183 193199.820.396714 203261.440.404353 213298.20.408911 223315.740.411085 233413.060.423151 244108.510.509373 CH3 1257.980.031984 21526.10.189206 31526.10.189206 43267.630.405121 53450.530.427797 63450.530.427797

14. Dissociative Channel 15.0857.80 18.3163.55 Mass 73 Mass 82

15. Transition State Optimized radical Structure Calculations carried out with UMP2/6-311++g(d,p) tert-butoxy radical C 4 H 8 OH radical

16. Minimizing Ion Beam Profile Minimize profile for maximum resolution on conservation of momentum calculations Ultimately the lens voltage played the largest factor for minimizing the beam profile Neutral position distribution (x) Photodetachment Rate (+)

18. Electrostatic Ion Beam Trap (EIBT) 1.Entrance mirror 2.Exit mirror 3.Bunching electrode 4.Pickup electrode 5.Laser-ion interaction point 6.Coaxial blackbody Radiation baffles Beam Lifetimes: Many seconds Beam Energies: 4 – 7 keV Ion Frequency: 50 – 250 kHz

19. Tert-butanol bond strengths (1) Lefkowitz, J. K.; Heyne, J. S.; Won, S. H.; Dooley, S.; Kim, H. H.; Haas, F. M.; Jahangirian, S.; Dryer, F. L.; Ju, Y. Combustion and Flame. 2011.

20. Total Energy The total kinetic energy plot has some bands in it, but are unevenly spaced The separation range around 0.05 eV, but difficult to assign unambiguously due to the number of vibrational modes in enol and methyl radical.