1. Air Force Research Laboratory
Spectroscopic Measurements for Next-Generation Space Technologies
25 June 2015
Justin W. Young Ryan S. Booth Jaime A. Stearns Space Vehicles Directorate Air Force Research Lab Kirtland AFB, NM

2. Ionic Liquids: lots of variables

3. How Gas Phase Spectroscopy Improves Air Force Technology
Draw
Synthesize
Test in lab
Redesign
Test in silico
Calculate
Measure
Revise theory
Develop/transition
Status Quo
(slow)
Ideal
(fast)
Benchmark
(this task)
h = 21.0 cP
h = 74.0 cP
Model verification; understanding and mitigating viscosity; understanding clustering; understanding/improving reactivity vs

4. Hydrogen Bonding and Viscosity
[emmim][Tf2N]
h = 74.0 cP
[emim][Tf2N]
h = 21.0 cP
even though emim doesn’t have in-plane conformations in the gas phase they’re still more favorable than in emmi
Maginn, PCCP, 16, 13489, 2014

5. Supersonic Jet Time-of-Flight
helium
sample holder
(225°C)
pulsed valve UV IR
TOF
UV spectra M M* M+
IR Spectra M
v=1 M* M+
UV only
IR+UV
Tdecomp=455˚C
4x10-4
Experimental setup. Standard supersonic jet source and time of flight detection. Detect UV absorption by fragmenting IL ion pair into component ions.
Ngo, H. L., et al. Thermochim. Acta 2000.
Zaitsau, D. H., et al. J. Phys. Chem. A 2006.

6. [emim][Tf2N] IR Spectroscopy
MP2/aug-cc-pVTZ
Jerry Boatz, AFRL
Family I
0 kJ/mol
Family II
3 kJ/mol
Family III
7 kJ/mol
In-plane
21 kJ/mol
Overview of spectrum – CH assignments. Four families of conformations, differing in C2-H interaction. Calculations show that experiment best matches Family II. The peak is broad and unresolved – three possible reasons why
Highlight that we worked pretty hard understanding this.
Causes of C2-H complexity
Hydrogen bonding
Fermi resonances
Multiple conformations
Agrees generally with He nanodroplet work
(Douberly and coworkers, JPCA 2013, 117, 9047)
Cooper, R.; Zolot, A. M.; Boatz, J. A.; Sporleder, D. P.; Stearns, J. A., JPCA, 2013, 117 (47),

7. [emmim][Tf2N] IR Spectroscopy
alkyl C-H
Likely fully “stacked” structure
No detailed assignments yet
C2-H
ring C-H
M06-2X/ G**
(scaled 0.95)

8. Hydrogen Bonding and Viscosity* *
*[emim][Tf2N] Family II structure
Molecular Dynamics simulations: 25 snapshots of 400 runs at 300K
Additional step minimized in each of 400 runs
Different conformational distributions for [emim] [Tf2N] and [emmim][Tf2N]
Suggests [emim] more easily crosses through C2 barrier than [emmim]
Consistent with previous work of Maginn, Izgorodina, Kiefer, etc.
Force fields courtesy of Ed Maginn and Ben Prince – JPCA 2014, 119, 352.
thank Ed and Ben for force fields

9. Hydrogen Bonding and Viscosity
[emmim][Tf2N]
h = 74.0 cP
[emim][Tf2N]
h = 21.0 cP
even though emim doesn’t have in-plane conformations in the gas phase they’re still more favorable than in emmi
Maginn, PCCP, 16, 13489, 2014

10. UV Temperature Dependence
Difference between cold and warm hints at major rearrangements

11. IR Temperature Dependence
ditto

12. [emim] vs [emmim]: A Suggestion
Both ILs have minimum energy “stacked” structures
[emim] starts to tilt off “stacked” position as beam warms
less interaction between anion and pi-cloud changes UV spectrum
IR shows stronger H-bonding interaction even if conformers are higher energy
[emmim] is locked into “stacked” structure even when beam warms
no drastic change in UV spectrum
perhaps evidence of ring C-H interactions when cold

13. Conclusions and Future Directions
Using UV and IR spectroscopy to probe ionic liquid ion pair structure
Related viscosity change upon methylation to mobility of anion around cation
Extend IR to pinpoint structure
Spectroscopy of clusters

14. Acknowledgements Dr. Ryan Booth (NRC) Dr. Justin Young (BC)
Dr. Chris Annesley
(BC)
Alumni
Dr. Russell Cooper
Dr. Alex Zolot
Collaborators
Dr. Jerry Boatz (AFRL)
Dr. Steven Chambreau (AFRL)
Prof. Joe Subotnik (Penn)
Acknowledgements
AFOSR Molecular Dynamics
National Research Council
Boston College Institute for Scientific Research
Space Scholars Program
DOD High Performance Computing Modernization Program
Dr. Jaime Stearns

15. [emim][Tf2N] UV Spectroscopy
• Anion
- Cation
• Laser Power
x140
1.12 ± 0.07
1.92 ± 0.13
0.88 ± 0.09
J. Phys. Chem. A 2013, 117, 12419
These are the mass spectra and UV spectra. The fact that the cation and anion differ in signal level (a lot) and spectrum (a little) hint at something more interesting going on.
[emim][Tf2N] UV Spectroscopy

16. [emmim][Tf2N] UV Spectroscopy: an aside on photophysics
1.04 ± 0.03
1.00 ± 0.03
anion
cation
Similar structure but different photophysics:
C2-H directing excited state dynamics?
despite similar structure, surprisingly different UV spectra and photophysics
compare warm/cold UV here
[emmim][Tf2N] UV Spectroscopy: an aside on photophysics

17. Hypergolic Reactivity
+9.5 kcal/mol
(B3LYP/6-31++G**)
Is the cation required to make the first steps downhill?
+12.4 kcal/mol (B3LYP/6-31++G**)
…same thing at other end …
+11.4 kcal/mol (B3LYP/6-31++G**)
DNB
Russo, et al. JPCA 2013, 117, 9216
Chambreau et al JPCA 2008, 112, 7816
Hypergolic Reactivity

18. Micro-Hypergolic-Reactor
QMS
DCA-
EMIM+
ESI He
HNO3
detector ?
Micro-Hypergolic-Reactor

19. Micro-Hypergolic-Reactor
[emim]2[dca]3 + HNO3
Low HNO3 concentration
High HNO3 concentration
Micro-Hypergolic-Reactor