Photoelectron-PhotofragmentCoincidence Spectroscopy of tert-

Butoxide and the Carbanion Isomer

Ben ShenContinetti Group

University of California, San DiegoJune 18, 2012

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

hν + AB- → A(KER) + B(KER) + e- (eKE)

A + B-

hν + AB- → AB + e-(eKE)

AB + e-

A + B-

PPC Experiment1) Uses anions to analyze transient neutral species2) Kinematically complete experiment to explore radical

energetics

Coincidence Spectroscopy

KEtot = eKE + KER

KEmax = hν - D0(AB-) – AEA(B)

Eint = KEmax – (eKE+KER)

hν + AB- → A(KER) + B(KER) + e- (eKE)

Instrumentation

Trap Environment: ~ 20 KTrap Lifetimes: Many seconds

Beam Energies: 4 – 7 keVDetectors: Position and time sensitive

Anion Generation

Gas Mixture : Ar (80%)/N2O(20%)

N2O + e- (slow) N2 + O-

O- + (CH3)3COH OH + (CH3)3CO-

OR

O- + (CH3)3COH OH + (CH3)2COCH2-

Pulsed Valve

Bubbler

e- beam

Gas Mixture + tert-butanol vapor

Discharge and Supersonic Expansion

Both stable and dissociative channels are observed after photodetachment

Tert-butoxy Stable ChannelPeak Position (eV) Simulation (cm-1) simulation assignment

a 1.908 0b 1.958 410 CCC umbrellac 2.008 845 CCC umbrella overtoned 2.058 1283 C-C stretche 2.109 1694 C-C stretch + CCC umbrellaf 2.176 2128 Combination bandg 2.208 2566 C-C overtone

Stable channel spectra (blue) matches Franck-condon simulated spectra (red) as well as previous spectra on tert-butoxy radical1

Franck-Condon simulation performed with PESCAL2 (red) with MP2/6-311++G(d,p) level of theory

1.0 1.4 1.8 2.2 2.6 3.0

electron Binding Energy(eV)

Cou

nts

a b

c

d e

f g

abc

de

fg

537nm

537nmDeuterated

Non-deuterated

EA

CCC umbrella C-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

Dissociative Channels CalculatedΔrH (eV)

6.00

4.74

4.84

2.04

2.57

CBS-Q level of theory

Pathways Energetics

CBS-Q level of theory

hν

hν

0.731.16

1.30 1.77 388 nm (3.2 eV)537 nm (2.3 eV)

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

Acknowledgements

Current and past members of the Continetti LabUS Department of energy

Phase-locking and BunchingPhase 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

Enol:

mode Freq(cm-1) freq(eV1 201.68 0.0250042 433.41 0.0537343 463.48 0.0574624 516.39 0.0640225 557.19 0.069086 800.37 0.099237 918.89 0.1139248 926.62 0.1148829 1064.93 0.13203

10 1119.35 0.13877711 1172.96 0.14542412 1332.71 0.16522913 1478.7 0.18332914 1550.79 0.19226715 1590.81 0.19722916 1608.54 0.19942717 1626.79 0.20168918 1888.88 0.23418319 3199.82 0.39671420 3261.44 0.40435321 3298.2 0.40891122 3315.74 0.41108523 3413.06 0.42315124 4108.51 0.509373

CH3

1 257.98 0.031984

2 1526.1 0.189206

3 1526.1 0.189206

4 3267.63 0.405121

5 3450.53 0.427797

6 3450.53 0.427797

Dissociative Channel

15.08 57.8018.31 63.55

Mass 73 Mass 82

Transition StateOptimized radical Structure

Calculations carried out with UMP2/6-311++g(d,p)

tert-butoxy radical

C4H8OH radical

Minimizing Ion Beam Profile

-10 0 10mm

QXDL positions

-10

0

10

mm

-10 0 10mm

QXDL positions

-10

0

10

mm 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 (+)

Electrostatic Ion Beam Trap (EIBT)

1. Entrance mirror2. Exit mirror3. Bunching electrode4. Pickup electrode5. Laser-ion interaction point6. Coaxial blackbody Radiation

baffles

Beam Lifetimes: Many secondsBeam Energies: 4 – 7 keVIon Frequency: 50 – 250 kHz

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.

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.