1. 1 Integrity  Service  Excellence 19 June 2014 Jaime A. Stearns, Russell Cooper, a David Sporleder, a Alexander Zolot b Space Vehicles Directorate Air Force Research Lab Jerry Boatz Aerospace Systems Directorate Air Force Research Lab Ion Pair Structure and Photodissociation Dynamics of Ionic Liquid [emim][Tf 2 N] Air Force Research Laboratory a National Research Council Associate b Boston College Institute for Scientific Research

2. 2 Air Force Research Lab “leading the discovery, development, and integration of warfighting technologies for our air, space, and cyberspace forces” ~10,000 government employees and contractors ~$2 billion budget 8 technical directorates + AFOSR Space Vehicles (1000 people, $378 million) Aerospace Systems (1800 people) Boston College National Research Council Collaborations with academia Space Scholar Program

3. 3 Chemistry in Space: Emerging Satellite Applications Chambreau, et al, Energy & Fuels, 2008, 22, 2871 Ionic Liquid Hypergolic Fuels Electrospray Thrusters Chiu et al, IJMS 265, 146, 2007 Reversible Electrochemical Mirrors Low-Threshold Plasmas Thruster Plume Modelling vs Electrolyte Ag + + 0 bulk silver Ag + Ag+Ag+ Mirror Substrate Ag + h 121 nm NH(A)  NH(X) + 310 nm NH 3 HNO 3

4. 4 Ionic Liquid Propellants Salts that melt below 100˚C Room temperature ILs melt below 20 ˚C Molecular size/asymmetry prevents crystallization Low vapor pressure = “green” Highly tailorable saltionic liquid hydrazine alternative reaction too slow ILs too viscous Hypergolic Propulsion Electrospray Propulsion Hall thruster alternative single ions vs droplets

5. 5 [emim][Tf 2 N] Typical IL tested in EP systems High decomposition temp: T onset =455˚C [1] Vapor pressure: 4x10 -4 torr @201˚C [2] Interesting computational question [3] [1] Ngo, H. L., et al. Thermochim. Acta 2000, 357-358, 97-102. [2] Zaitsau, D. H., et al. J. Phys. Chem. A 2006, 110, 7303-7306. [3] Boatz, J, personal communication DFT minimum (BE=315 kJ/mol) MP2 minimum (BE=355 kJ/mol)

6. 6 Supersonic Jet Time-of-Flight - + h h - + helium carrier gas sample holder (225ºC) pulsed valve UV IR TOF UV spectra M M* M+M+

7. 7 UV Dissociation Anion - Cation Laser Power x140 Little fragmentation of ions Broad UV absorption Much smaller anion signal Slightly different UV spectra More than simple dissociation!

8. 8 Dissociation Mechanism 1.12 ± 0.07 1.92 ± 0.13 0.88 ± 0.09 Similar quantities of cation and anion at long wavelengths Similar product results in VUV dissociation of [emim] + [Tf 2 N] - Laser Power Dependences

9. 9 IR-UV Gain Spectroscopy IR spectra using ion gain M v=1 M* M+M+ UV only IR+UV Does not measure absorbance directly At a set UV wavelength we monitor ion signal as IR frequency varies Measure CH stretch frequencies of cation Set UV laser for IR experiments ~500 cm -1

10. 10 IR Spectroscopy Cause of C2-H complexity debated Hydrogen Bonding Fermi Resonances Multiple Conformations Lassègues et al, JPCA 2009, 113, 6419-6421 Douberly, and coworkers, JPCA 2013, 117, 9047 Family I 0 kJ/mol Family II 3 kJ/mol Family III 7 kJ/mol In-plane 21 kJ/mol

11. 11 Backing Pressure Dependence Main change in C2-H Small changes in C4/C5-H Multiple conformers with slightly different C2-H stretch frequencies Cooper, R.; Zolot, A. M.; Boatz, J. A.; Sporleder, D. P.; Stearns, J. A. “IR and UV Spectroscopy of Vapor-Phase Jet-Cooled Ionic Liquid [emim] + [Tf2N] − : Ion Pair Structure and Photodissociation Dynamics.“ J. Phys. Chem. A 2013, 117, 12419-12428.

12. 12 Conclusions [emim][Tf 2 N] has two photodissociation mechanisms IR spectra point to stacked structure B3LYP incorrectly predicts in-plane structure Significant interaction between C2-H and anion O Complexity arises from multiple conformers  = 21.0 cP  = 74.0 cP Current work: linking structure with viscosity

13. 13 Future Work: Electrospray

14. 14 Acknowledgements Dr. Russell Cooper Dr. Alex Zolot Dr. Jerry Boatz Dr. Ryan Booth Dr. David Sporleder AFOSR National Research Council Boston College Institute for Scientific Research Space Scholars Program