Article Cite This: J. Phys. Chem. A 2018, 122, 1960−1966
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Theoretical and Experimental Insights into the Dissociation of 2‑Hydroxyethylhydrazinium Nitrate Clusters Formed via Electrospray Amanda L. Patrick,† Kristen M. Vogelhuber,†,‡ Benjamin D. Prince,† and Christopher J. Annesley*,† †
Space Vehicles Directorate, Air Force Research Laboratory, Kirtland Air Force Base, New Mexico 87117, United States Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts 02467, United States
‡
S Supporting Information *
ABSTRACT: Ionic liquids are used for myriad applications, including as catalysts, solvents, and propellants. Specifically, 2hydroxyethylhydrazinium nitrate (HEHN) has been developed as a chemical propellant for space applications. The gas-phase behavior of HEHN ions and clusters is important in understanding its potential as an electrospray thruster propellant. Here, the unimolecular dissociation pathways of two clusters are experimentally observed, and theoretical modeling of hydrogen bonding and dissociation pathways is used to help rationalize those observations. The cation/deprotonated cation cluster [HEH2 − H]+, which is observed from electrospray ionization, is calculated to be considerably more stable than the complementary cation/protonated anion adduct, [HEH + HNO3]+, which is not observed experimentally. Upon collisional activation, a larger cluster [(HEHN)2HEH]+ undergoes dissociation via loss of nitric acid at lower collision energies, as predicted theoretically. At higher collision energies, additional primary and secondary loss pathways open, including deprotonated cation loss, ion-pair loss, and double-nitric-acid loss. Taken together, these experimental and theoretical results contribute to a foundational understanding of the dissociation of protic ionic liquid clusters in the gas phase.
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INTRODUCTION
Ionic liquids (ILs) have found many application areasfrom catalysis,1 pharmaceuticals,2 and electrochemistry3,4 to propellants for space vehicles. Relevant to this work, ILs have been developed by the United States Air Force Research Laboratory (AFRL) and others with space interests to perform as propellant in either chemical5−9 (as hypergolic bipropellants or catalytic monopropellants) or electrospray10−12 thrusters. The idea of a thruster capable of operating in either mode with a single propellant has also been proposed,13 for which energetic ILs would be prime candidates. 2-Hydroxyethylhydrazinium nitrate (HEHN) (structures of constituent species provided in Figure 1) is a task-specific IL developed by AFRL for use as a catalytic monopropellant.14 HEHN has been analyzed by vacuum electrospray ionization a model experiment for electrospray thruster evaluation.12 Cluster dissociation can lead to a potentially dangerous return current and spacecraft fouling,15 and dissociation in the acceleration region can lead to a reduction in thrust. Thus, dissociation pathways need to be better understood to predict the species expected in a thruster plume and to better predict performance under different modes of thruster operation. Any complete model of thruster performance would need to fully consider the dissociation pathways at play as well as their relative energetics. Studies such as the present one can help provide necessary empirical data. © 2018 American Chemical Society
Figure 1. Structures for constituent ions and neutrals discussed in the text, including (A) 2-hydroxyethylhydrazinium (HEH+), (B) 2hydroxyethylhydrazine (HEH − H), (C) nitric acid (HNO3), and (D) nitrate (NO3−).
In addition to its practical uses in spacecraft propulsion, HEHN is also more broadly interesting because it is protic thus issues of proton sharing and transfer arise when considering the structure and dissociation pathways of its ion pairs and clusters. Furthermore, the ionic versus molecular nature of protic ILs is of fundamental interest to the IL community.16,17 Anion (e.g., dicyanamide) protonation is also believed to be critical for ignition of hypergolic ionic liquids, Received: December 7, 2017 Revised: January 30, 2018 Published: January 31, 2018 1960
DOI: 10.1021/acs.jpca.7b12072 J. Phys. Chem. A 2018, 122, 1960−1966
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The Journal of Physical Chemistry A
clusters with the constituents in their neutral formmight be viable and produce different results; therefore, the choice to use fully ionic clusters as input for MD is discussed further in the Supporting Information (SI). The force fields were identical to those used in a previous publication by Maginn and coworkers.48 For each species, 200 unique trajectories were equilibrated at 300 K under the canonical (NVT) ensemble, tested for equilibrium for 1 ns under the microcanonical (NVE) ensemble, and then, the configuration on the final step was minimized, and the resulting geometry and potential energies were recorded. The lowest energy structure from the above molecular dynamics (MD) simulations, along with typically 4−7 additional conformers sampled across the energy range produced by MD, for each of the species was submitted to Gaussian 0949 or 1650 for QM energy optimization and frequency calculations with the M06−2x functional51 and 6-311++G** basis set. The lowest energy structure found was then used for thermodynamic analyses. 0 K enthalpies of reaction were calculated using zero-point corrected energies, and 298 K free energies of reaction were calculated from Gaussian output. The visualization software Avogadro52 was used to produce structure graphics and determine bond lengths. Cartesian coordinates for QM-optimized structures are provided in the associated SI. Enthalpy contents at various temperatures were calculated from Gaussian output files using a Perl script available from NIST.53 Experimental Methods. HEHN liquid was obtained from the AFRL Propulsion Directorate at Edwards Air Force Base and was used as received. A solution of HEHN was prepared consisting of ∼0.05% HEHN in 50% water/50% methanol for electrospray ionization. Acid was not necessary for sufficient signal intensity of the desired ions. The addition of acid (e.g., acetic acid) can adversely affect the mass spectrum by inducing the formation of additional cluster species formed by ion exchange of one or more IL anion (e.g., nitrate) for the conjugate base of the acid (e.g., acetate), complicating the fullscan mass spectrum and diluting ion signal from the desired IL cluster channels. All CID mass spectrometry experiments were performed on a custom-built instrument. Relevant to these experiments, ions were formed by electrospray ionization at ambient conditions and introduced into vacuum via a heated metal capillary (90 °C) inlet and skimmer. From there, ions were guided to the first mass selection stage, which was fixed to select the desired precursor ion (or to operate in rf-only mode, if a full-scan mass spectrum was desired). Then, ions were guided to a doubleoctupole collision cell, similar in design to one described previously,54 where ions could be collided with an inert gas at desired kinetic energies. Finally, any resulting fragment ions and the remaining precursor ions were scanned through a second, scanning quadrupole mass filter and detected using standard counting techniques. The kinetic energies of each ion beam as it enters the double octupole was determined by recording and analyzing a retarding potential analysis (RPA) obtained by scanning octupole offsets. Typical RPA results were lab-frame beam energies of ∼16−18 eV and widths of ∼0.9−1.5 eV. These initial kinetic energies were then used to determine the octupole offset required to achieve a desired ion beam kinetic energy through the collision cell for CID measurements. For collision-energy-resolved experiments, krypton gas was leaked into the 4.25 cm collision cell at a pressure of 120−130 μTorr, which placed the experiments primarily in the single-collision
and the influence of the cation on such protonation reactions has recently been investigated.18 Some, primarily aprotic, ILs have been studied previously by collision-induced dissociation (CID). For instance, energyresolved CID of some IL aggregates have been reported,19 a relative cation−anion interaction scale has been developed,20 and the existence of “magic” number cluster sizes for imidazolium-based ILs and H-bonding within those clusters have been explored.21 The CID behavior of a sulfur-containing imidazolium IL was also reported.22 In addition to CID, other allied mass spectrometry approaches, including ion-pair23,24 and ion25−27 spectroscopy, of a few ILs have been reported. Still, this is generally a very nascent field, and complementary experiments on protic ILs are even scarcer. Proton transfer via CID has been mentioned for the salt guanidinium chloride,28 but a detailed investigation of this phenomenon was not reported there. Proton sharing and transfer within ammonium nitrate and related systems have been explored extensively by theoretical and experimental methods.29−33 That salt is of interest both due to its simplicity as a model system and because of its applications in explosive devices. Proton transfer in the ionic liquid 1-ethyl-3methylimidazolium acetate (EMIM Ac) ion pairs has been studied using theoretical approaches and photoelectron spectroscopy.34 The related IL BMIM Ac (where B = butyl) has been studied by electrochemical methods in the liquid phase, where the carbene (deprotonated cation) was detected.35 Notably, imidazolium acetates have important real-world applications as substrates for cellulose pretreatment36 and carbon dioxide capture,37 in which presence of the Nheterocyclic carbene is believed to play a key role in promoting activity. Such carbenes have also been investigated using mass spectrometric detection, where N-heterocyclic carbenes formed via deprotonation of imidazolium ions in solution have been reported,38 as has their reactivity with CO2 by carboxylation in the gas phase.39 Whether a given ionic liquid evaporates via protontransferred neutrals or as ion pairs (or neutral clusters) during distillation has also been discussed extensively in the literature,40−46 and fundamentally understanding proton sharing and dissociation pathways at the molecular/cluster level could play a role in better understanding those phenomena. In the present work, our aim is to study relatively small ionic clusters produced via electrospray ionization of HEHN. Specifically, we are interested in comparing loss of a neutralized cation (e.g., 2-hydroxyethylhydrazine (HEH − H), Figure 1B) or nitric acid (HNO3), either of which would require proton transfer to occur, versus loss of the intact ion pair (which is the typical neutral loss from many IL clusters) as primary dissociation pathways. Additionally, small model systems, consisting of the cation adducted with either a neutralized cation (HEH − H) or a neutralized anion (HNO3), were also investigated. In both cases, thermodynamic calculations are included to help rationalize experimental observations.
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THEORETICAL AND EXPERIMENTAL METHODS Theoretical Methods. Classical all-atom force fields were used as input to LAMMPS47 to generate reasonable starting structures for the follow-on quantum mechanics (QM) investigation. All MD input structures consisted of clusters in which the constituents were in their ionic form. For this protic system, it is possible that proton-transferred clustersor 1961
DOI: 10.1021/acs.jpca.7b12072 J. Phys. Chem. A 2018, 122, 1960−1966
Article
The Journal of Physical Chemistry A
Notably, in the HEHN “ion pair”, the proton transfers from HEH to the nitrate, effectively forming (HEH − H) and nitric acid, while still maintaining a hydrogen-bonding interaction. Specifically, the NH bond lengthens to 1.426 Å, and the distance from the proton to the oxygen of the nitrate is only 1.103 Å. Calculations on ammonium-dinitrimide-55 and triazolium-based56 ionic liquids have previously suggested that isolated ion pairs may undergo proton transfer to form stable associated neutral pairs. Furthermore, it was suggested that, while this proton transfer may not predominate in the bulk, it might have profound influence on the decomposition mechanism of energetic protic IL systems. The idea of proton transfer specifically in HEHN was previously introduced to help rationalize the observed vacuum electrospray ionization (ESI) mass spectrum,12 and it was studied theoretically by that group, where they also found that proton transfer occurs in the isolated ion pair. In the larger, positively charged cluster, [HEH(HEHN)2]+, the extent of proton sharing/transfer is less marked, but it is certainly present and contributes to the stability of the cluster and, likely, to the dissociation pathways. Representative examples of H-bonding are labeled with bond distances in Figure 2. An OH bond is slightly lengthened to ∼0.971 Å, with only a distance of ∼1.947 Å between the OH proton and the nearest oxygen of the nitrate. Likewise, one of the NH bonds of the internal nitrogen is lengthened to 1.041 Å in the cluster, with a distance of 1.776 Å separating the labeled NH proton from the nearest nitrate oxygen. This indicates that protontransfer-mediated dissociation pathways must be considered for these systems. As an aside on hydrogen bonding within this system, there is substantial cation−cation binding, facilitated by the various H-bond donor and acceptor groups on each cation. Specifically, for the donor−acceptor pair labeled in Figure 2, the NH bond distance is increased to 1.035 Å, and the distance to the nearest hydroxyl oxygen of another cation is only 1.937 Å. Additional views of this cluster, as well as coordinates, are provided in the SI. Stability and Dissociation of Small Clusters. If proton transfer readily occurs between HEH and the nitrate, as is suggested by the theory presented in the previous section, it is conceivable that species consisting of the neutralized cation (i.e., (HEH − H)) or neutralized anion (i.e., HNO3) associated with the cation, HEH, could form directly from positive-mode ESI. Previous reports of vacuum electrospray mass spectrometry of the neat IL indicated the presence of the former, but not the latter.12 This is not surprising when comparing the calculated thermodynamics for each of these species (Table 1). Considerably less energy would be required to dissociate the
regime. Five averages of five scans each were recorded for each energy point with the gas on. Five averages of single-scan backgrounds (gas diverted toward the main chamber, rather than the collision cell) were also recorded for each energy point. The breakdown curve is plotted as yield as a function of collision energy. In this case, yield is defined by eq 1 yield =
Iproduct − Ibackground Itotal
×
1 p
(1)
where Iproduct is the integral intensity of the product ion of interest with the collision gas present in the cell, Ibackground is the background intensity of the product ion of interest (when the gas flow is diverted away from the collision cell and toward the main chamber), Itotal is the summed integral intensity of all ions, and p is the pressure in mTorr in the collision cell, which is included to correct for any fluctuations throughout the time frame of the experiment. These yields are plotted as a function of center-of-mass collision energy (CECoM), which is determined using eq 2 CECoM =
mn (CE Lab) mn + m i
(2)
where mn is the mass of the neutral (in amu), mi is the mass of the precursor ion, and CELab is the lab-frame collision energy.
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RESULTS AND DISCUSSION Hydrogen Bonding within HEHN Species. To better understand the role of proton sharing and transfer within HEHN, we first examined hydrogen bonding in relevant systems, namely the ion pair, HEHN, and the cluster, [(HEHN)2HEH]+. These calculated structures, along with relevant bond length annotations, are provided in Figure 2. The isolated cation is also shown.
Table 1. Theoretical Stability of Each Considered Small Cluster reaction
ΔH0K (kJ/mol)
ΔG298K (kJ/mol)
[(HEH)2 − H]+ → HEH+ + (HEH − H) [HEH + HNO3]+ → HEH+ + HNO3
132 55
84 11
nitric acid adduct than to dissociate the hydroxyethylhydrazine adduct. Thinking in the direction of cluster formation, rather than dissociation, these thermodynamics also suggest that the [(HEH)2 − H]+ cluster would be more likely to form than [HEH + HNO3]+ in the first place.
Figure 2. Structures of the isolated cation, HEH (top), ion pair, HEHN (middle), and [(HEHN)2HEH]+ cluster (bottom), along with some annotations of bond distances pertinent to hydrogen bonding and proton sharing. 1962
DOI: 10.1021/acs.jpca.7b12072 J. Phys. Chem. A 2018, 122, 1960−1966
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The Journal of Physical Chemistry A We observed an ion at a nominal m/z of 153, which is tentatively assigned as [HEH2 − H]+. The CID mass spectrum of this ion was measured, and the results are presented in Figure 3. Given the fact that it dissociates via loss of neutralized cation (e.g., loss of 76 amu), assignment as [HEH2 − H]+ is further supported.
further, sequential pathways (E−K). Table 2 provides the theoretical 0 K enthalpy and 298 K free energy of reaction for each of these pathways. Table 2. Enthalpies of Considered Unimolecular Dissociation Reactions pathwaya
reaction
A
[(HEHN)2HEH]+ → (HEH − H) + [(HEHN)(HNO3)HEH]+ [(HEHN)2HEH]+ → (HNO3) + [(HEHN)(HEH)2 − H]+ [(HEHN)2HEH]+ → (HEHN) + [(HEHN)HEH]+ [(HEHN)2HEH]+ → 2(HEHN) + [HEH]+ [(HEHN)(HNO3)HEH]+ → (HNO3) + [(HEHN)HEH]+ [(HEHN)(HEH)2 − H]+ → (HNO3) + [(HEH)3 − 2H]+ [(HEHN)(HEH)2 − H]+ → (HEH − H) + [(HEHN)HEH]+ [(HEH)3 − 2H]+ → (HEH − H) + [(HEH)2 − H]+ [(HEHN)HEH]+ → (HNO3) + [(HEH)2 − H]+ [(HEHN)HEH]+ → (HEHN) + [HEH]+ [(HEH)2 − H]+ → (HEH − H) + [HEH]+
B C D E F G H
Figure 3. CID mass spectrum of the [HEH2 − H] cation, along with the structures for the theoretical dissociation pathway used to calculate relative adduct stability and dissociation. +
I J K
The enthalpy content of [HEH + HNO3]+ was calculated to be ∼31.5 kJ/mol at 298.15 K and ∼60.6 kJ/mol at 450 K. The value at 450 K exceeds the enthalpy of reaction (55 kJ/mol). Thus, the fact that this species is not observed in this experiment suggests that our source may produce relatively “hot” ions, making it impossible to form stable [HEH + HNO3]+ ions. On the other hand, the enthalpy content of [(HEH)2 − H]+ was calculated to be ∼38.0 kJ/mol at 298.15 K and ∼75.5 kJ/mol at 450 K, both of which are less than the 132 kJ/mol required to break up the cluster, consistent with the fact that this species is experimentally observed. Theoretical Dissociation of [(HEHN)2HEH] + . The [(HEHN)2HEH]+ cluster provides a system large enough to provide multiple potential dissociation pathways to be explored, yet small enough for QM calculations to remain tractable. Typically, for aprotic IL clusters, the only active unimolecular dissociation channels are loss of one or more ion pairs. However, given the propensity for proton transfer in the HEHN system, several additional pathways should be considered. Figure 4 summarizes both the hypothetical primary dissociation channels (A−D, highlighted with blue arrows) and
a
ΔH0K (kJ/mol)
ΔG298K (kJ/mol)
189
119
154
90
180
102
182
122
69
15
84
44
105
43
97
39
77
40
130
92
132
84
Pathways are illustrated in Figure 4.
In examination of Figure 4 and Table 2, of the four potential primary dissociation channels (Pathways A−D), nitric acid loss (Pathway B) is the most energetically probable. Pathways C, D, and A then follow. Due to the higher enthalpy of reaction, deprotonated cation loss (Pathway A), is expected to be small; furthermore, any product ion arising from Pathway A might be expected to be readily depleted, given that Pathway E is predicted to be facile. Again, we can consider the enthalpy content of the precursor ion at two example temperatures. At 298.15 K, the enthalpy content is ∼79.5 kJ/mol, and at 450 K it is ∼156.8 kJ/mol. Thus, again, if ions are assumed to be relatively “hot” from the source, nitric acid loss (∼154 kJ/mol enthalpy of reaction) would be expected, even with minimal activation. From the most energetically favorable primary dissociation pathway, nitric acid loss, there are two secondary dissociation pathways to evaluate. These two channels are denoted Pathways F and G. Again, loss of the neutralized cation (HEH − H) or loss of the neutralized anion (HNO3) should be considered. When comparing the ΔG of these two pathways, they are expected to be competitive. From either secondary product, sequential fragmentation to the (HEH2 − H)+ fragment and then on to the bare cation is expected to occur at the highest collision energies. Experimental Dissociation of [(HEHN)2HEH]+. The experimentally measured CID mass spectrum of the [(HEHN)2HEH]+ cluster is presented in Figure 5. This spectrum is considerably more diverse than those typically seen for aprotic IL clusters, wherein dissociation occurs solely by loss of one or more ion pairs. In this experiment, losses of nitric acid, neutralized cation, and ion pair, as well as double nitric acid, double ion pair, and ion pair combined with nitric acid loss are all observed, giving rise to product ions assigned
Figure 4. Potential dissociation pathways of the [(HEHN)2HEH]+ cluster. Here, HEHN represents the ion pair, HEH the cation, and (HEH − H) the neutralized (deprotonated) cation. Each reaction is labeled with a letter, which corresponds to its nomenclature in Table 2. Blue arrows indicate potential primary dissociation pathways, and black arrows indicate potential sequential dissociation pathways. 1963
DOI: 10.1021/acs.jpca.7b12072 J. Phys. Chem. A 2018, 122, 1960−1966
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secondary Pathways F and Gwhich would be observed as loss of 2HNO3 or HEHN loss, respectively, in Figure 6HEHN loss is much more prevalent, despite it being slightly less energetically favorable. This can be rationalized by the fact that there are several additional pathways, including direct HEHN loss and sequential dissociation through the neutralized cation primary pathway, contributing to the signal observed in the (− HEHN) channel. To examine solely the competition between HNO3 and neutralized cation loss from the [(HEHN)(HNO3)HEH]+ primary product ion, energy-resolved multistage mass spectrometry (MS3) experiments would be necessary, but such experiments cannot be implemented on our current instrumental apparatus.
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CONCLUSIONS AND FUTURE DIRECTIONS This study provides the first experimental insights, beyond fullscan mass analysis,12 into the behavior of HEHN clusters generated by ESI. Specifically, we have performed CID measurements of two clusters: [HEH 2 − H] + and [(HEHN)2HEH]+. Theoretical thermodynamic analysis of various dissociation pathways were determined and used to help explain experimental observations. Key insights from this study include the following • There is a high degree of H-bonding between the cation and anion, with the isolated neutral ion pair effectively undergoing proton transfer from the cation to the anion. H-bonding is extensive in the larger, charged cluster, but the proton remains predominantly on HEH in this case. • The small cluster [HEH2 − H]+ is observed directly from electrospray; whereas the complementary cluster [HEH(HNO3)]+ is not. This is in line with the much lower calculated stability of the latter species and in agreement with previously reported experimental results. • The CID mass spectrum of [(HEHN)2HEH]+ is much more complex than those of aprotic IL clusters. • The [(HEHN)2HEH]+ cluster loses HNO3 at low energies. A second loss of HNO3 can also occur from the resulting ion and is observed in the CID spectrum. • Ion-pair loss is observed upon dissociation at higher energies. This species is likely to originate from more than one pathway, including a direct pathway and sequential pathways from primary ions resulting from loss of HNO3 and loss of (HEH − H), in either order. The work presented herein provides a basis upon which to build a fundamental understanding of this system of importance to the space community and of other protic ILs. Future work will focus on investigating other systems (larger clusters, different ILs) by both CID and complementary techniques, including ion/neutral reactions and ion spectroscopy.
Figure 5. CID-MS of the [(HEHN)2HEH]+ cluster with tentative peak assignments labeled. A lab-frame collision energy of ∼10 eV and a collision cell gas pressure of ∼400 μTorr were used to obtain this spectrum.
here as [(HEHN)(HEH)2 − H]+, [(HEHN)(HNO3)HEH]+, [(HEHN)HEH]+ [(HEH)3 − 2H]+, [HEH]+, and [HEH2− H]+, respectively. The prevalence of double-nitric-acid loss is especially interestingthough not energetically unreasonabledue to it straying quite far afield from the ion-pair-loss picture familiar for aprotic IL clusters. To further probe this system, collision-energy-resolved experiments were performed. The results are provided in Figure 6. At the lowest collision energies, nitric acid loss is
Figure 6. Dissociation of [(HEHN)2HEH]+ as a function of collision energy.
observed. This pathway is then successively followed by HEHN loss, HEHN + HNO3 loss, and 2HEHN loss at progressively higher collision energies. Based on this and the general shapes of the curves, sequential dissociation through Pathways B, G, I, and K appears to be prevalent. Minor pathways include loss of two nitric acid neutralslikely also as a sequential mechanism (of Pathway B followed by Pathway F)and loss of deprotonated cation (HEH − H). Loss of two nitric acid molecules peaks at a similar position as HEHN loss, indicating that these two pathways are competitive as secondary loss channels. Deprotonated cation loss is always of low intensity. Now we can consider the experimental results in the context of the theoretical predictions. At low collision energies, the predominant primary dissociation pathway is indeed nitric acid loss (i.e., Pathway B), as predicted by theory. When comparing
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b12072. Comparison of structures obtained from MD simulations with ionic versus neutral constituents of “ion pairs”; additional views of the [(HEHN)2HEH]+ cluster; coordinates for the structures used to obtain the structures and thermodynamics presented in the main text (PDF) 1964
DOI: 10.1021/acs.jpca.7b12072 J. Phys. Chem. A 2018, 122, 1960−1966
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The Journal of Physical Chemistry A
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AUTHOR INFORMATION
Corresponding Author
*Phone: 505-846-1042; E-mail:
[email protected]. ORCID
Amanda L. Patrick: 0000-0003-4525-0970 Kristen M. Vogelhuber: 0000-0002-8439-4013 Christopher J. Annesley: 0000-0002-5877-803X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by the US Air Force Office of Scientific Research (AFOSR) under task numbers 15RVCOR171 and 16RVCOR275. A grant of computing time from the Department of Defense (DoD) High Performance Computing Modernization Program at AFRL DSRC is also acknowledged. This work was completed while A.L.P. was supported by the National Research Council Research Associateship Program (NRC RAP) as a postdoctoral associate at AFRL. The authors thank Adam Brand for providing the HEHN sample used in this work.
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