Photoelectron-Photofragment
Coincidence Spectroscopy of tertButoxide and the Carbanion Isomer
Ben Shen
Continetti Group
University of California, San Diego
June 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
tert-butoxy radical
• Both isomers are combustion products of tert-butanol
• Alkylhydroxy isomer energetics are not well characterized
• Difficult to separate isomers
Alkylhydroxy radical
PPC Experiment
1) Uses anions to analyze transient neutral species
2) Kinematically complete experiment to explore radical
energetics
AB + e-
A + B-
hν + AB- → AB + e-(eKE)
A + B-
hν + AB- → A(KER) + B(KER) + e- (eKE)
Coincidence Spectroscopy
hν + AB- → A(KER) + B(KER) + e- (eKE)
KEtot = eKE + KER
KEmax = hν - D0(AB-) – AEA(B)
Eint = KEmax – (eKE+KER)
Instrumentation
Trap Environment: ~ 20 K
Trap Lifetimes: Many seconds
Beam Energies: 4 – 7 keV
Detectors: Position and time sensitive
Anion Generation
Gas Mixture : Ar (80%)/N2O(20%)
Pulsed Valve
N2O + e- (slow)  N2 + OO- + (CH3)3COH  OH + (CH3)3COOR
O- + (CH3)3COH  OH + (CH3)2COCH2-
Gas Mixture +
tert-butanol vapor
Discharge and Supersonic Expansion
Bubbler
e- beam
Both stable and dissociative channels are observed after photodetachment
Tert-butoxy Stable Channel
Peak
a
b
c
d
e
f
g
Position (eV)
1.908
1.958
2.008
2.058
2.109
2.176
2.208
Simulation (cm-1)
0
410
845
1283
1694
2128
2566
a
b
Non-deuterated
simulation assignment
CCC umbrella
CCC umbrella overtone
C-C stretch
C-C stretch + CCC umbrella
Combination band
C-C overtone
d e
c
537nm
f g
Counts
EA
c
1.0
1.4
1.8
537nm
e
b
C-C stretch
Stable channel spectra (blue)
matches Franck-condon simulated
spectra (red) as well as previous
spectra on tert-butoxy radical1
d
a
Deuterated
CCC umbrella
f
Franck-Condon simulation
performed with PESCAL2 (red)
with MP2/6-311++G(d,p) level of
theory
g
2.2
2.6
electron Binding Energy(eV)
3.0
(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
Calculated
ΔrH (eV)
6.00
2.04
4.74
4.84
2.57
CBS-Q level of theory
Pathways Energetics
1.30
0.73
1.16
hν
hν
CBS-Q level of theory
1.77
388
537nm
nm(3.2
(2.3eV)
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 Lab
US Department of energy
Phase-locking and Bunching
I¯ at 290nm
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
Enol:
mode
Freq(cm-1)
freq(eV
1
201.68
0.025004
2
433.41
0.053734
3
463.48
0.057462
4
516.39
0.064022
5
557.19
0.06908
6
800.37
0.09923
7
918.89
0.113924
8
926.62
0.114882
9
1064.93
0.13203
10
1119.35
0.138777
11
1172.96
0.145424
12
1332.71
0.165229
13
1478.7
0.183329
14
1550.79
0.192267
15
1590.81
0.197229
16
1608.54
0.199427
17
1626.79
0.201689
18
1888.88
0.234183
19
3199.82
0.396714
20
3261.44
0.404353
21
3298.2
0.408911
22
3315.74
0.411085
23
3413.06
0.423151
24
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
Mass 82
Mass 73
15.08
57.80
18.31
63.55
Optimized radical Structure
Transition State
tert-butoxy radical
C4H8OH radical
Calculations carried out with UMP2/6-311++g(d,p)
Minimizing Ion Beam Profile
mm
10
QXDL positions
Minimize profile for
maximum resolution on
conservation of momentum
calculations
0
-10
-10
mm
10
0
mm
10
Ultimately the lens voltage
played the largest factor for
minimizing the beam profile
QXDL positions
0
-10
-10
0
mm
10
Neutral position distribution (x)
Photodetachment Rate (+)
Electrostatic Ion Beam Trap (EIBT)
1.
2.
3.
4.
5.
6.
Entrance mirror
Exit mirror
Bunching electrode
Pickup electrode
Laser-ion interaction point
Coaxial blackbody Radiation
baffles
Beam Lifetimes: Many seconds
Beam Energies: 4 – 7 keV
Ion 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.