Spectroscopic Assessment of Intra-and Intermolecular Hydrogen

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Spectroscopic Assessment of Intra- and Intermolecular Hydrogen Bonding in Ether-Functionalized Imidazolium Ionic Liquids Helen J. Zeng, Mark A. Johnson, Jasodra D. Ramdihal, Rawlric A. Sumner, Chanele Rodriguez, Sharon I Lall-Ramnarine, and James F. Wishart J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b04345 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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The Journal of Physical Chemistry

Spectroscopic Assessment of Intra- and Intermolecular Hydrogen Bonding in Ether-Functionalized Imidazolium Ionic Liquids Helen J. Zeng and Mark A. Johnson,* Sterling Chemistry Laboratory, Yale University, New Haven, CT 06520 Jasodra D. Ramdihal, Rawlric A. Sumner, Chanele Rodriguez and Sharon I. Lall-Ramnarine, Chemistry Department, Queensborough Community College of the City University of New York, Bayside, NY 11364 James F. Wishart,* Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973

ABSTRACT: Functionalization of the imidazolium (Im ) cationic component of ionic liquids (ILs) with ether +

chains affords the possibility of tuning their properties through manipulation of the resulting interion and intramolecular interactions. Herein, we quantify these interactions at the molecular level through analysis of the vibrational spectra displayed by size-selected and cryogenically-cooled ions. These spectra are obtained using the “tagging” approach carried out with photofragmentation tandem mass spectrometry. In the isolated cations, we find that the oxygen atom on the ether chain binds exclusively to the acidic C H position on the Im ring. Upon complexation with BF ‾ to form +

(2)

4

the ternary (Ether-MIm ) (BF ‾) cation, however, the less acidic C H groups also participate as +

2

4

(4,5)

contact points for the ionic assembly, in contrast to the behavior of the closely related (EMIm ) (BF ‾) system. These experimental results support the conclusions derived from earlier X+

2

4

ray scattering and molecular dynamics results on bulk ILs regarding interactions with the ring CH groups and their implications on tuning the viscosities of this class of functionalized ILs.

1 ACS Paragon Plus Environment

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1. INTRODUCTION Room temperature ionic liquids are increasingly useful solvents for many applications due to their low vapor pressures and the possibility of tailoring their properties for specific uses.

1-5

Strategies for the latter include the introduction of functional groups to a common scaffold on either of the ionic constituents. Here, we are concerned with the cations based on imidazole (Im), 6,7

of which the 1-ethyl-3-methylimidazolium (EMIm , 1, see Fig. 1) variation represents one of the +

most widely used IL components.

8-11

Imidazolium cation derivatization typically involves

substituting its two N-bonded hydrocarbon groups to introduce functionalities designed to manipulate directional hydrogen bonding interactions, with notable examples including hydroxylation of the terminal methyl group (2)

7,12-18

and their replacement with ether chains (3-7)

7,19-

. Ether-grafted ILs, for example, tend to have lower viscosities than their alkyl counterparts

7,21-24

21

and thus accelerate transport processes and reactions that are carried out in them.

22,25,26

Understanding

the microscopic origin of the ether ILs viscosities is an active focus of investigations

20,21,27

ranging

from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to molecular 28

dynamics (MD) simulations.

19,21,29-31

The examination of local interactions is important because they

provide the mechanistic basis for the observed differences, such as lower viscosity, of etherfunctionalized imidazolium-based ILs compared to their alkyl analogues.

Figure 1. Eight cations based on the derivatization of the Im ring: (1) 1-ethyl-3-methylimidazolium (EMIm ), +

+

(2) 1-(2-hydroxyethyl)-3-methylimidazolium (HEMIm ), (3) 1-(2-ethoxymethyl)-3-methylimidazolium +

(EOMMIm ), (4) 1-(2-methoxyethoxy)methyl-3-methylimidazolium (MOEOMMIm ), (5) 1-(2-(2+

+

methoxyethoxy)ethyl)-3-methylimidazolium (EOEOMMIm ), (6) 1-(2-(2-ethoxyethoxy)ethyl)-3+

methylimidazolium (EOEOEMIm ), (7) 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium +

(EOEOEOMMIm ) and (8) 1-ethyl-2,3-dimethylimidazolium (EMMIm ) [E = ethyl, O = oxygen atom, M = +

+

methyl and Im = imidazolium] with the numbering scheme for the ring CH groups. +


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The MD simulations indicate that most ether-functionalized imidazolium-based cations can form intramolecular H-bonded structures in which the alkoxy chain curls over or around the Im cationic ring (e.g., EOEOEOMMIm , 7). More specifically, it was proposed that self-blocking +

+

of the C H groups (orange in Fig. 1) by the ether tail decreases the IL viscosity by preventing the (4,5)

intermolecular aggregation of ether chains.

21,24,27

Consistent with this hypothesis, the vaporization

enthalpies of (EOMMIm+)(NTf2‾), (EOEOMMIm+)(NTf2‾) and (EOEOEOMMIm+)(NTf2‾) were found to be 10 kJ/mol smaller than those for corresponding alkyl-substituted ILs with the same chain lengths,32 indicating a weakening of the influence of intermolecular interactions, including the network of charges, in favor of intramolecular ones. MD simulations support this conjecture, indicating that the ether chain interacts at the C H positions in spite of the fact that these groups (4,5)

are less acidic than the C H site (blue in Fig. 1) on the Im ring. +

(2)

21

Here we address the intrinsic role of H-bonding at the various CH sites on the Im ring in +

ether-derivatized systems using gas phase cluster ion methods. Using this approach, we establish the structures adopted by isolated ether-functionalized cations of varying chain lengths (3-7). We are specifically concerned with the longest chain EOEOEOMMIm cation that was the only species +

predicted to be able to bind through both of the C H positions by MD simulations in the liquid 21

(4,5)

and in electronic structure calculations of the structure of its ternary cationic complex (2,1), in which two of these cations are attached to the BF ‾ anion. This is accomplished by comparison of 4

the vibrational band patterns of cryogenically cooled, mass selected ions with calculated spectra for various structural candidates obtained at the B3LYP-D3/6-31+G(d) level of theory using the Gaussian 09 package.

15,16,33,34

Of particular interest is the comparison of these structures with those

reported earlier in similar studies of the EMIm and hydroxylated HEMIm (1-(2-hydroxyethyl)-3+

+

methylimidazolium) systems (1 and 2) and their ternary complexes with common anions (e.g. NTf ‾ 2

(bis(trifluoromethylsulfonyl)imide), BF ‾ and halides)

15,33,35,36

4

which did not exhibit contacts at the

C H positions. The (2,1) clusters of the shorter ether chain ILs were not studied here since their (4,5)

chains are not long enough to bind to both of the C H positions (although attachment to the C H (4,5)

(5)

group is a possibility). 2. EXPERIMENTAL DETAILS The experimental methods employed here closely follow our previous studies of the EMIm and HEMIm cations. +

+

15,33,35,36

Pure ionic liquids were diluted to millimolar concentrations in 3 ACS Paragon Plus Environment


Page 4 of 17

acetonitrile and were introduced to the mass spectrometer via electrospray ionization. The resulting ions were then transported through differential pumping regions by RF-only octopole guides and electrostatic ion optics into a three-dimensional quadrupole ion trap (R.M. Jordan) mounted to a closed-cycle helium cryostat held at a temperature of ~10-35 K.37-39 He buffer gas with a trace amount (~10%) of D or N2 was pulsed into the trap to collisionally cool the ions and 2

facilitate the attachment of N2 adducts for messenger spectroscopy.40,41 The ions were stored for ~90 ms before being ejected into the time-of-flight region of the photofragmentation mass spectrometer. A particular m/z target was intersected with the output of a 10 Hz pulsed OPO/OPA tunable infrared light source (Laservision). Resonant absorption of a single photon caused the evaporation of the weakly bound42,43 D or N “tag” and the resulting photofragment 2

2

was separated by a second stage of mass selection. Spectra were generated by monitoring the photofragment yield as a function of continuously scanned photon energy and normalized to fluctuations in laser pulse energy over the scan range. Synthetic protocols for the ILs with the ether-functionalized cations displayed in Fig. 1 are provided in the Supporting Information (SI). 3. RESULTS AND DISCUSSION The spectral response of the C H group of the Im ring was isolated by selective deuteration +

(2)

at that site, which is possible because of the large difference in the pK values of the three ring a

CH’s [pK (C H) = 23.8 vs. pK (C H) = 32.97 ]. As described in previous studies from our 44

a

group,

15,33,35,36

45

(2)

a

(4,5)

this modification is necessary to suppress complications arising from strong Fermi

resonance interactions as well as to break up the harmonic coupling between the three CH positions on the Im ring. In the isolated d-EMIm isotopomer (Fig. 2a), the C D fundamental appears as a +

+

(2)

sharp feature (blue) near 2362 cm⁻ , while the C H groups contribute a close doublet (orange) 1

(4,5)

centered at 3170 cm⁻ arising from the asymmetric and symmetric CH stretching normal modes of 1

the two CH oscillators in the C H motif.

15,33,35,36

(4,5)

Fig. S1 compares the calculated spectra of the all-H

isotopologues with those of the selectively deuterated systems at the C position to illustrate this (2)

point. Fig. 2 presents the vibrational predissociation spectra of the seven C D isotopically labeled (2)

Im derivatives displayed in Fig. 1. The region of 2450-3050 cm⁻ (alkyl CH’s) is removed to +

1

highlight the contributions of the three ring modes, and the relative intensities of the peaks of the two remaining regions are directly comparable as indicated in the displays of the complete scans 4 ACS Paragon Plus Environment

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Figure 2. Vibrational predissociation spectra of N -tagged (a) d-EMIm , (b) d-HEMIm , (g) d-EOEOEOMMIm +

2

+

+

cations, and D -tagged (c) d-EOMMIm , (d) d-MOEOMMIm , (e) d-EOEOMMIm , (f) d-EOEOEMIm and (h) d +

2

+

+

+

5

EMMIm cations with the optimized structures of the ether-functionalized cations displayed in Figure 3. The C D +

(2)

stretches are highlighted in blue (doublets are tentatively assigned to isomers I and II), the characteristic unperturbed C H doublets are colored in orange and the C D doublet in pink. Measured transition energies are (4.5)

(4.5)

tabulated in Table S1.

in Fig. S2. For all of the cations studied, the C H features appear at the same positions and with (4,5)

similar intensities as those in bare EMIm . This immediately establishes that these acidic groups + 33,36

do not interact with either the ether or hydroxyl side chains. Interestingly, this is the case even for the longer, ethylene-linked oligoether cation that has the greatest degree of conformational freedom. The C D stretches in the d-HEMIm and d-EOMMIm cations (Figs. 2b and 2c) appear 21

+

+

(2)

in a similar location to that observed for the d-EMIm ion (2371 and 2366, vs. 2362 cm⁻ ), +

1

indicating that their side chains do not interact with the C D position either. This effect has been (2)

reported earlier for HEMIm and is consistent with the spectra (Fig. S3) calculated for the lowest +

energy structure (right of Fig. S3). This is in contrast to the behavior of the ether derivatives with two or three oxygen atoms (Figs. 2d-g), which exhibit significant ~60 cm⁻ redshifts in the C D 1


(2)


Page 6 of 17

stretching fundamentals. However, although only a single redshifted C D peak was observed for (2)

the methylene-linked, d-MOEOMMIm (Fig. 2d), this feature appears as a doublet in the spectra +

of the ethylene-linked ether cations (Figs. 2e-g). The intensities of the two members of the doublet are asymmetric in the cases of d-EOEOEMIm and d-EOEOEOMMIm , suggesting that they arise from two isomers that differ in +

+

the strength of their intramolecular bonds to the C D group. Note that the possibility that the C D (2)

(2)

doublet structure could arise from accidental deuteration at the C H sites was considered. To this (4,5)

end, we forced deuteration in the d -EMMIm isotopologue (EMMIm = 1-ethyl-2,3+

+

5

dimethylimidazolium, 8) to expose the C D doublet with the spectrum displayed in Fig. 2h (see (4,5)

Fig. S4 for more details). These features were not observed in the spectra of the singly deuterated C D isotopologues, although the higher energy C D peak in the d-EOEOEOMMIm spectrum falls (2)

+

(2)

close to the lower energy member of the C D doublet. If only one of the two C H sites was (4,5)

(4,5)

deuterated, however, the doublet would collapse into a single frequency at 3171 cm⁻ (which is 1

observed and displayed in Fig. S5b). Since such a collapse was not observed, we conclude that the doublets near 2300 cm⁻ in the spectra of the single D isotopologues are due to the C D group. 1

(2)

These empirical conclusions regarding structure are consistent with the calculated lowestenergy structures, presented in Fig. 3, which all feature cyclic intramolecular H-bonds to the C H (2)

group with a bifurcated attachment motif involving two or three oxygen atoms on the ether chain. Differences in the local interactions of the chain with the C D group among these conformers can (2)

account for the C D doublet structure observed for the longer, ethylene-linked cations. For (2)

example, the two isomers recovered for the EOEOEOMMIm cation (I and II in Figs. 3g and 3h) +

differ in their H-bond bond angles (e.g. C -H···O1/O2) and dihedral angles. The dihedral angle of (2)

C -N-C-C for Isomer I is calculated to be -14° whereas the angle for Isomer II is -60°. The (2)

calculated redshifts of the C D stretches for the two isomers (2304 and 2317 cm⁻ , in Figs. S6m 1

(2)

and S6n) reflect this structural variation, thus accounting for the doublet observed experimentally (Fig. 2g). Note that enantiomers also exist for all of these structures in Fig. 3, but only one of the two enantiomers is presented for simplicity, as they result in the same predicted vibrational contributions (i.e. identical H-bond distances). Rotatable pdfs for all of the ether-functionalized cationic structures mentioned in the text are provided in Fig. S7, with XYZ coordinates in Table S2.


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For the shorter, two-oxygen ether side chain derivatives [MOEOMMIm (Fig. 3d), +

EOEOMMIm (Figs. 3b and 3c) and EOEOEMIm (Figs. 3e and 3f)], H-bonding from the C H site +

+

(2)

to the terminal oxygen (O2) atom is generally stronger than to the inner O1. An exception occurs in Isomer II of EOEOEMIm , whereby the lengths of the H-bonds to both of the oxygen atoms are +

Figure 3. Calculated minimum energy structures [B3LYP-D3/6-31+G(d) level of theory] of the etherfunctionalized Im ring cations with the one oxygen ether chain (a) EOMMIm , two oxygen ether chains: +

+

(d) MOEOMMIm , (b, c) EOEOMMIm , (e, f) EOEOEMIm , and the longest, three oxygen ether chain +

+

+

(g-j) EOEOEOMMIm cation, separated into four classes of isomers. Isomer I denotes the cations with the +

strongest intramolecular H-bonding interactions at the C H site whereas Isomer II has a weaker interaction. (2)

Isomer III is the proposed structure whereby the ether chain bends over the ring without direct interactions with the Im ring CH sites and Isomer IV is a higher-energy, straight-chain variant. +



Page 8 of 17

similar. The main difference for the longer ether chain EOEOEOMMIm system (compared to the +

shorter chain ether cations) is that C D attachment could occur to the outer two oxygen (O2 and (2)

O3) atoms of the ether tail, as depicted in the most favorable Isomer I in Fig. 3g. Isomer II (Fig. 3h), on the other hand, exhibits stronger H-bonding to the inner O1 and O2 atoms, which is more similar to the conformations adopted by shorter ether chain species. For all of the structures with intramolecular H-bonding occurring at the C H position, the attachment to the O2 atom is always (2)

calculated to be the most favorable out of the available oxygen atoms, as evidenced by the shorter H-bond distances. As such, this arrangement is likely the motif that accounts for the largest redshifts of the C D band (Fig. S6). We note that in an MD theoretical study, Shimizu et al. (2)

concluded that the terminal oxygen (O2) is most likely to interact with the ring proton in the methylene-linked oligoether cation. This is consistent with our calculations of the isolated cations, 19

despite the fact that O2 is not the terminal oxygen on the longer EOEOEOMMIm cation. +

For completeness, we also explored the possibility that the ether side chains might curl back to interact with the imidazolium ring in a ‘scorpion’ tail fashion that does not directly interact with any of the ring CH groups. That motif has been identified, for example, through X-ray and MD calculations.

19,29-31

Such isolated cationic structures were indeed recovered as higher energy local

minima and denoted Isomer III in Fig. 3i. This survey also identified locally stable open structures with an all-trans ether chain (Isomer IV, Fig. 3j). The predicted spectra of both isomer classes III and IV (Figs. S3f and S3g) do not feature a redshifted C D stretch, however, and are thus not (2)

present under our experimental conditions. The analysis of isolated cations indicates that the ether derivatives behave differently from the alkyl and hydroxyl compounds in their ability to generate cyclic structures through the formation of intramolecular H-bonds to the C H group. This raises the question, however, of (2)

whether such interactions are in play when multiple ions (including counterions) are present in the macroscopic ILs.

19,21,29

MD simulations of the condensed phase suggest that the anions prefer to bind

to the ether-functionalized imidazolium-based cations through the C H group. If that is the case, 21

(2)

the ether tail could be engaged in less favorable interactions with the cations, such as with the C H (4,5)

contact points (as discussed in the Introduction section). We therefore extended this study to explore the behaviors of the cationic ternary complexes comprised of two cations bound to a single anion, hereafter denoted a (2,1) cluster composition, in order to assess the competing interactions in the ether-derivatized ILs. 8 ACS Paragon Plus Environment

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Our focus on the (2,1) complexes has the advantage that we previously characterized the interactions that drive the structures adopted when two EMIm and HEMIm cations are bound to +

+

the NTf ‾ and BF ‾ anions as well as to the halides (X‾). For the smaller BF ‾ and X‾ anions, the 2

33

4

4

cations are completely separated from each other by the anion in a sandwich motif in which both C H groups are directly docked to the anion.

46-49

(2)

Furthermore, using a two-laser technique, we

succeeded in identifying two different isomers for the (d-DEMIm ) (NTf ‾) complex, whereby +

2

2

d-DEMIm is an isotopologue of HEMIm with deuterium atom substitutions at both the C H and +

+

(2)

OH positions. With the addition of the hydroxyl group, the cations were able to display 15

intermolecular H-bonding through the OH groups between the cations, which then attached to the basic nitrogen site of the NTf ‾ anion. This interaction is termed as cation-cation (c-c),

12,14-16,50

2

which

Figure 4. Vibrational predissociation spectra of N -tagged (a) (d-EMIm ) (NTf ‾), (b) (d-DEMIm ) (NTf ‾), (c) (d+

2

EOEOEOMMIm+) (BF ‾) 2

4

2

2

+

2

2

compared to the inverted, calculated spectra of optimized structures of

(d-EOEOEOMMIm ) (BF ‾) [B3LYP-D3/6-31+G(d), scaled to 0.954] for Isomers (d) A and (e) B shown in +

2

4

Figure 5. The extent of the redshift of the C H asymmetric and symmetric doublet is indicated by the red arrow (4,5)

above (c). The region of 2450-3050

cm⁻1

(alkyl CH’s) is removed to highlight the contributions of the three ring

modes. The relative intensities of the peaks of the two remaining regions are directly comparable as indicated in the displays of the complete scans in Figure S8. Measured transition energies are tabulated in Table S1.



Page 10 of 17

are evident by extremely redshifted OH bands in the IR spectrum. The other type of isomer is comprised of a cation that is bound primarily through the C H site leaving the OH group free, (2)

whereas the other cation binds with the OH group. Importantly, neither the EMIm nor the HEMIm +

+

cations bind to the C H positions, even for larger cationic clusters (Figs. 4a and 4b), as is (4,5)

evidenced by the persistence of the simple C H doublet in the C D derivatives. (4,5)

(2)

The vibrational spectrum of the (d-EOEOEOMMIm ) (BF ‾) ion is presented in Fig. 4c, and +

2

4

interestingly displays a broad, redshifted peak (FWHM: 89 cm⁻ which is centered at 3144 cm⁻ ), 1

1

demonstrating that binding to the C H groups is significant. The introduction of the additional (4,5)

ions (an identical cation and an anion) to the EOEOEOMMIm cation provides a bigger system +

with additional possibilities for electrostatic and H-bonding interaction sites involving the C H (4,5)

groups. The broadness of the C H peak could have occurred due to the splitting of the C H (4,5)

(4,5)

doublet as the two cations are asymmetrically bound to the anion, presence of isomers causing an overlap of the C H contributions, or Fermi interactions of the fundamentals with the overtones of (4,5)

bending modes involving these groups. The C D doublet obtained for this larger assembly is (2)

relatively blueshifted (Fig. 4c) in comparison with that obtained for a single cation (Fig. 2g). This observation can be explained by the lowest-energy calculated configuration, whereby one of the

Figure 5. Lowest-energy calculated isomers (a) A and (b) B of the ether-functionalized ionic liquid ternary ion complexes with two EOEOEOMMIm cations and one BF ‾ anion, with their calculated spectra presented +

4

in Figures 4(d) and 4(e) respectively.


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two cations has its C D group H-bonding to the O2 position on the ether chain of the second cation (2)

(see Isomer A in Fig. 5a). The length of this intercationic bond (2.52 Å) is indicative of a weaker H-bonding than the intramolecular interaction of the single cation (2.16 Å) to the O2 site of its own ether chain (Fig. 3g). Therefore the higher intensity, bound C D peak is assigned to the first (2)

cation, which is redshifted by 23 cm⁻ from the C D fundamental, a smaller extent compared to 1

(2)

that observed (2317 cm⁻ ) in the bare cation. The smaller C D shoulder can then be assigned to the 1

(2)

free C D of the second cation. Calculations of other isomers of this ternary cationic cluster have (2)

also been conducted, such as another representative low-energy isomer, Isomer B, presented in Fig. 5b, which has the two cations bound to the anion electrostatically in a quasisymmetric, outof-plane sandwiched motif. However, this structure only gave rise to a single C D frequency (Fig. (2)

4e), and hence the lowest-energy structure Isomer A still seems most plausible as its calculated harmonic spectrum (Fig. 4d) reproduces the asymmetric C D doublet that was observed (2)

experimentally. Moreover, given the length of the ether tail, such intercationic interactions are highly possible

51,52

since they can help to stabilize the overall structure (by at least 1.0 kcal/mol

compared to Isomer B). These calculated isomers with the BF ‾ anion are also consistent with the 4

behavior noted in previous study of the ternary cluster (EMIm ) (BF ‾), whereby the C H site does +

2

4

(2)

not form the traditional H-bonding to the anion and interactions between the imidazolium ring and anion are mostly electrostatic in nature.

36

4. CONCLUSION In summary, we present vibrational spectroscopic evidence that confirms interactions at the C H sites of the imidazolium ring that arise from contacts with the ether chain in ether(4,5)

functionalized imidazolium-based ILs. These interactions are not observed in the isolated cations, but occur in the ternary clusters comprised of two cations and one anion (BF ‾). These data are 4

consistent with results obtained from earlier X-ray scattering and MD results of the bulk ILs, which proposed associations at the C H sites. By systematically studying the isolated ether cations with 21

(4,5)

varying chain lengths, the structural parameters required to form intramolecular interactions and multiple conformational isomers were also identified. Moving forward, it may be useful to study these systems with smaller halide anions to induce strong directional H-bonding through the C H (2)

sites, or look into the methylated-C derivative of the ether-functionalized cation, which may 35

(2)

induce interactions via the C H sites even for a single cation. (4,5)



ASSOCIATED CONTENT Supporting Information Supporting experimental and computational data, rotatable structures (PDF), and the full citation for reference 34. This information is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M.A.J.). *E-mail: [email protected] (J.F.W.). ORCID Helen J. Zeng: 0000-0002-1850-4923 Mark A. Johnson: 0000-0002-1492-6993 Rawlric A. Sumner: 0000-0002-6627-902X James F. Wishart: 0000-0002-0488-7636 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS M.A.J. thanks the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under grant DE-FG02-06ER15800 for support of this work. The cryogenic ion spectrometer critical to these measurements was provided by the Air Force Office of Scientific Research (AFOSR) under grants FA9550-17-1-0267 (DURIP) and FA9550-18-1-0213. Work at BNL was supported by the DOE Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under contract DE-SC0012704. Work and funding for J.D.R and R.A.S at QCC was supported by the Transfer CUNY Research Scholars Program (CRSP) at Queens College and the PSC-CUNY Traditional A Award 61066-0049. Summer student internships for J.D.R., R.A.S. and C.R. at BNL were supported by the BNL Office of Educational Programs and the DOE Office of Workforce Development for Teachers and Scientists. The authors thank Dr. Gopal Subramaniam for helpful discussions.


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TABLE OF CONTENTS (TOC) GRAPHIC:


Spectroscopic Assessment of Intra-and Intermolecular Hydrogen

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