Article pubs.acs.org/JPCB
17
O NMR Investigation of Water Structure and Dynamics
Eric G. Keeler, Vladimir K. Michaelis,† and Robert G. Griffin* Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: The structure and dynamics of the bound water in barium chlorate monohydrate were studied with 17O nuclear magnetic resonance (NMR) spectroscopy in samples that are stationary and spinning at the magic-angle in magnetic fields ranging from 14.1 to 21.1 T. 17O NMR parameters of the water were determined, and the effects of torsional oscillations of the water molecule on the 17O quadrupolar coupling constant (CQ) were delineated with variable temperature MAS NMR. With decreasing temperature and reduction of the librational motion, we observe an increase in the experimentally measured CQ explaining the discrepancy between experiments and predictions from density functional theory. In addition, at low temperatures and in the absence of 1H decoupling, we observe a well-resolved 1H−17O dipole splitting in the spectra, which provides information on the structure of the H2O molecule. The splitting arises because of the homogeneous nature of the coupling between the two 1H−17O dipoles and the 1H−1H dipole.
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spectral resolution. We also note that 17O, much like 15N,22−24 is exquisitely sensitive to secondary chemical environments, particularly hydrogen bonding, making it a powerful probe for structural examinations by NMR. For example, recent investigations from our group and Nour et al. have identified a fingerprint region of 17O for structural water in solids spanning about 40 ppm that is a reflection of the local structure (i.e., O−H bond distance and ∠HOH bond angle).25,26 During our recent studies of H217O spectra,25 it was noted that density functional theory calculations consistently overestimated the quadrupolar coupling constant (CQ) of the bound water. This discrepancy was observed previously during investigations of other systems, particularly the bound water environment in hydroxyapatite.27−30 Here, we show with both experiments and theoretical calculations on the water in barium chlorate monohydrate, Ba(ClO3)2·H217O, that this discrepancy is due to motional averaging that can be diminished at low temperatures. Specifically, we find that the librations present at higher temperatures can be reduced at ∼100 K, and the observed second order quadrupolar coupling increases. In addition, the 1H−17O vector in H2O is oriented approximately at the magic angle with respect to the HOH bisector. Thus, the 2-fold flips executed by H2O molecules at ambient temperatures average the 1H−17O dipole couplings to a few kilohertz. Very interestingly, at low temperatures, these couplings reappear in a homogeneous manner even though they are themselves inhomogeneous and should be averaged by MAS. In
INTRODUCTION Water is the most abundant molecule on the Earth’s surface and is involved in and impacts a diverse variety of chemical, biological, and physical problems.1−3 Accordingly, there have been many NMR studies of H2O and they generally involve 1H spectroscopy, but the resolution in these experiments is often not high for a number of reasons. Two other approaches to studying water involve either 2H or 17O NMR spectroscopy. 2H quadrupole echo techniques are established as excellent probes of molecular dynamics and have been used extensively to characterize the 2-fold hoping rates of 2H2O via 2H lineshapes and anisotropic spin−lattice relaxation.4 In addition, 17O can provide important data on water structure and dynamics, but to date 17O NMR spectroscopy has been challenging because of the low sensitivity associated with the experiments. In particular, 17O is a low natural abundance (0.037%) and low gyromagnetic ratio (γ = −5.774 × 107 MHz T−1) nuclear spin. Furthermore, the resolution in the spectra is compromised by the homogeneous line broadening originating from the residual, second-order quadrupolar interaction (I = 5/2, Q = −2.558 fm2) that is not suppressed by magic-angle spinning (MAS).5 In order to increase the sensitivity of the experiments and address these problems, samples have been isotopically enriched,6−8 population transfer techniques optimized,9,10 and experiments performed at high magnetic fields (≥18.8 T), which improves the Boltzmann polarizaton.11−16 In addition, dynamic nuclear polarization (DNP) has recently provided enhancements in 17O signal intensities of up to 115, offering an additional boost in sensitivity important for studies of challenging systems.17−21 Finally, higher Zeeman fields also narrow the second-order powder patterns, increasing sensitivity and improving the © 2016 American Chemical Society
Received: June 7, 2016 Revised: July 13, 2016 Published: July 25, 2016 7851
DOI: 10.1021/acs.jpcb.6b05755 J. Phys. Chem. B 2016, 120, 7851−7858
Article
The Journal of Physical Chemistry B
experiments. Crystals were visually inspected before grinding and then confirmed via powder X-ray diffraction (data not shown). b. Nuclear Magnetic Resonance. 17O experiments at 21.1 T (ω0H/2π = 900 MHz, FBML-MIT), 18.8 T (ω0H/2π = 800 MHz, FBML-MIT) and 14.1 T (ω0H/2π = 600 MHz, Bruker, Billerica, USA) were performed using either a Bruker Avance II or III spectrometer. Experiments at 17.6 T (ω0H/2π = 748 MHz, FBML-MIT) were performed using a home-built spectrometer (courtesy of Dr. David Ruben, FBML-MIT). A recycle delay of 1 s was used for all 17O experiments, unless noted otherwise, and all spectra were referenced to liquid water (18% H217O) via the substitution method.39 All experiments were performed at a temperature of 270 K unless otherwise noted. 17O γB1/2π were between 50 and 100 kHz, as determined from liquid water; stationary experiments were acquired both with and without continuous-wave high-power 1 H-decoupling during acquisition (1H γB1/2π = 50 to 100 kHz). Decoupling under MAS conditions did not affect the second-order line shape of the spectrum at room temperature. One-dimensional MAS 17O experiments were performed using the Hahn echo and RAPT experiments40 with between 512 and 8192 scans with a spinning frequency of 16 and 20 kHz, respectively. Hahn echo stationary experiments were acquired with and without decoupling, using 8192 and 156 000 scans, respectively. The RAPT-enhanced MAS spectra were obtained using frequency-switched Gaussian pulses (γB1/2π = 35 kHz, σ = 2.855 ms, τp = 25.6 ms; 21 pulse pairs) applied with a frequency offset of ±350 kHz. The RAPT profile41 was performed using 1024 scans by stepping the frequency offset from 0 to 1200 kHz in steps of 25 kHz. Variable temperature 17 O MAS experiments were acquired at 600 MHz between 105 and 300 K with between 1024 and 4096 scans and recycle delays of 0.3, 5, and 15 s, respectively, with a spinning frequency of 14 kHz. The experiments at 105 K were acquired both with and without continuous wave 1H decoupling (1H γB1/2π of 100 kHz). Stationary 1H NMR experiments at 5 T (ω0H/2π = 212 MHz) and 16.4 T (ω0H/2π = 699 MHz) were performed using a home-built spectrometer courtesy of Dr. David Ruben, FBML-MIT. Experiments at 300 and 100 K were acquired with 1024 coadded transients and using recycle delays of 2 or 30 s, respectively. c. Quantum Chemical Calculations. Quantum chemical calculations were performed as described previously in Michaelis et al.25 d. Spectral Processing and Simulations. Spectra were processed by RNMR (Dr. D. Ruben, FBML-MIT) or TOPSPIN (Bruker, Billerica, USA) with between 50 and 400 Hz of exponential apodization. Spectral simulations were performed using either the WSolids,42 DMFit,43 or SIMPSON44 software packages. The Euler angles from GIPAW calculations were extracted using the EFGShield software.45 Euler angles for spectral simulations were fixed using the GIPAW determined values.
particular, since the H2O molecule is a three spin system consisting of two 1H−17O couplings and a 1H−1H coupling, the system behaves homogeneously, and a well resolved splitting appears in the second order 17O powder line shape. The splitting and 17O line shape provide direct information on the 1 H−17O distances, 1H−1H distance and the ∠HOH angle. Barium chlorate monohydrate has a monoclinic prismatic unit cell with a space group of I2/c that contains two equivalent bound water molecules per unit cell,31−33 as shown in Figure 1.
Figure 1. Molecular and long-range crystal packing of Ba(ClO3)2·H2O. (a) Molecular unit. (b) 2D long-range crystal structure shown with the b axis normal to the page. Colors represent different elements: barium (black), chlorine (green), chlorate oxygen (red), bound water oxygen (blue), and hydrogen (gray). (c) Interatomic distances and angles that are relevant to the structure of the bound water determined via neutron diffraction,33 where Ow is the water oxygen, and O(2)/O(3) are the nearest chlorate oxygen.
The chlorate oxygens form a secondary coordination environment with the bound water via hydrogen bonding. The characteristic dynamics that are known to affect the NMR interactions of the bound water molecule in Ba(ClO3)2·H2O are three librations (sometimes referred to as waving, rocking and twisting) that occur about the plane of the water molecule, the plane bisecting the hydrogens, and along the bisector of the water hydrogens, respectively.34−37 In addition, 2-fold 180° flips about the C2 axis along the HOH bisector transform a Pake pattern observed in 2H quadrupole echo spectra at low temperature to an axially asymmetric powder pattern with η ≈ 1 at room temperature.4,34,36,37 Concurrently, the 2-fold flips average the 1H−17O dipole couplings. Motional averaging of NMR interactions due to librations and higher order motions (e.g., 2-fold flips) are observed experimentally for 13C and 1H chemical shifts.36,38
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EXPERIMENTAL SECTION a. Materials and Synthesis. Two barium chlorate monohydrate, Ba(ClO3)2·H217O, samples were synthesized by recrystallizing 300 and 80 mg of crystalline barium chlorate monohydrate in 400 (30% H217O, Cambridge Isotopes Laboratories, Andover, MA) and 150 uL (90% H217O, SigmaAldrich, St. Louis, MO) 17O enriched water in a sealed 1.5 mL eppendorf tube for 3 days. Upon the onset of nucleation the eppendorf tube was opened to atmosphere over a period of 48 h to allow for partial evaporation. The reaction vial was then sealed until crystals precipitated from the solution (typically 7 to 14 days). The crystals were then removed from any remaining solvent and dried. The transparent and clear in color crystals were ground to a fine powder using an agate mortar and pestle and placed in a zirconia rotor for all NMR
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RESULTS AND DISCUSSION To obtain structural information pertaining to the environment of the quadrupole, chemical shift, and dipole tensors, the spectral line shape of the 17O NMR spectra can be simulated with nine independent variables: quadrupolar coupling constant (CQ), quadrupolar asymmetry parameter (ηQ), isotropic 7852
DOI: 10.1021/acs.jpcb.6b05755 J. Phys. Chem. B 2016, 120, 7851−7858
Article
The Journal of Physical Chemistry B Table 1. Experimental 17O NMR Parameters for the Water in Ba(ClO3)2·H2O at 300, 170, and 105 K experiment
CQ (MHz)
ηQ (±0.02)
δiso (ppm) (±1)
ζδ (ppm) (±5)
ηδ (±0.25)
DO−H (kHz)
DH−H (kHz)
T (K)
B0 (T)
RAPT profile MAS stationary w/ 1H dec. stationary w/o 1H dec. MAS MAS w/ 1H dec MAS w/o 1H dec
6.92 ± 0.17 6.9 ± 0.2 6.9 ± 0.2 6.9 ± 0.2 7.4 ± 0.2 7.55 ± 0.2 7.55 ± 0.2
0.98 0.98 0.98 0.95 0.92 0.92
21 21 21 21 21 21
25 25 25 25 25
0.25 0.25 0.25 0.25 0.25
10 ± 2 (η = 1) 16 ± 1.5
30.5 ± 1
300 ± 5 300 ± 5 300 ± 5 300 ± 5 170 ± 10 105 ± 5 105 ± 5
21.1 21.1; 18.8; 14.1 21.1; 17.6 21.1; 17.6 14.1 14.1 14.1
S3) and 17.6 T (Figure 2b). The chemical shift anisotropy and asymmetry were adjusted to effectively simulate the data at both 17.6 and 21.1 T; despite the moderate error (±20%) associated with the measured CSA, there is a noticeable CSA contribution to the stationary 17O line shape. To more accurately fit the chemical shift tensor components, as has been done in other 17O systems,46,47 either higher magnetic fields or 2D techniques, such as COASTER,48 would need to be performed. The motionally averaged 1H−17O dipolar coupling was determined to be 10 ± 2 kHz based on simulation of the stationary 17O NMR spectra without 1H decoupling (Figures 2c and S3). 1H NMR of a stationary sample at 16.4 T was utilized to verify the 1H−1H homonuclear dipolar coupling constant and CSA tensor parameters; our data are in agreement with the detailed findings from Carnevale et al. (Figure S4).49 The 1 H−1H dipolar coupling constant of 29 ± 1 kHz translates into a H−H distance of 1.62 ± 0.02 Å, which is ∼7% larger than the H−H distance determined from neutron diffraction.33 The reduced coupling and larger distance is due to the same librational motions that affect the 17O quadrupole tensor (vide inf ra), as shown in the stationary 1H variable temperature NMR at 5 T (300 and 95 K) in Figure S5, where the 1H−1H dipolar coupling constant increases to 31 ± 1 kHz (1.58 ± 0.02 Å H−H bond distance). We performed gauge-including projector-augmented wave (GIPAW) calculations on the neutron diffraction structure33 with H-optimization and the results are compiled in Table S2. The direction of the EFG tensor components were calculated to be as follows: Vxx along the C2 axis and HOH bisector, Vzz perpendicular to the HOH plane, and Vyy lies in the HOH plane perpendicular to Vxx and Vzz as illustrated in Figure 3a. Similarly the nuclear shielding tensor elements are oriented along the axes of molecular symmetry. In particular, the σxx component is perpendicular to the HOH plane approximately coaxial with Vzz and σyy lies along the HOH bisector coaxial with Vxx. These orientations are similar to those found for
chemical shift (δiso), chemical shift anisotropy (ζδ), chemical shift asymmetry parameter (ηδ), the Euler angles (α, β, γ) that relate the EFG and chemical shift anisotropy (CSA) tensors, and the 1H−17O dipolar coupling constant, D. To assess these variables, a series of careful experiments were performed to deconvolute these interactions, and Table 1 describes the experiments from which these variables were determined. Spectral simulations, using WSolids42 or DMFIT43 software packages, of the 17O MAS spectrum (Figure 2a), shown at 21.1
Figure 2. MAS and stationary 17O NMR spectra of Ba(ClO3)2·H217O. (a) MAS spectrum (solid, black) and simulation (dashed, black) at 21.1 T (ω0H/2π = 900 MHz), (b) stationary 17O NMR spectrum (solid, red) and simulation (dashed, red) with continuous-wave 1H decoupling (γB1/2π = 100 kHz) at 17.6 T (ω0H/2π = 748 MHz), and (c) stationary 17O NMR spectrum (solid, green) and simulation (dashed, green) without 1H decoupling spectra at 17.6 T (ω0H/2π = 748 MHz). NMR interactions from GIPAW calculations are included in each spectral simulation displayed with water molecule in the inset. NMR parameters used in the spectral simulations are given in Table 1.
T (Figure S1 for other fields), yielded a quadrupolar coupling constant and asymmetry parameter of the 17O nucleus that were determined to be 6.9 ± 0.2 MHz and 0.98 ± 0.02, respectively. Using the rotor-assisted population transfer (RAPT) method,40 the 17O central transition of Ba(ClO3)2· H217O was enhanced by a factor of 1.88 at a frequency offset of 350 kHz (Figure S2, inset); the CQ was verified, utilizing the RAPT profile, to be 6.92 ± 0.17 MHz (Figure S2). The RAPT profile relates the quadrupolar coupling constant to the edge frequency (νedge) of the RAPT profile by the equation: νedge = 3CQ/(2I(2I − 1)), where CQ is the quadrupolar coupling constant, and I is the spin of the nucleus.41 The chemical shift parameters of the 17O nucleus were determined to be δiso = 21 ± 1 ppm, ζδ = 25 ± 5 ppm, and ηδ = 0.25 ± 0.25, based on spectral fitting of the stationary 17O spectrum at 21.1 (Figure
Figure 3. EFG and NSA tensor components taken from GIPAW calculations in CASTEP on the structure of the bound water in Ba(ClO3)2·H2O. (b,c) Visualization of the motional averaging of the Vzz component by the two librational modes of the bound water about the (b) Vyy (waving) and (c) Vxx (twisting) axes. 7853
DOI: 10.1021/acs.jpcb.6b05755 J. Phys. Chem. B 2016, 120, 7851−7858
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The Journal of Physical Chemistry B −13CH2− groups in single crystal studies of glycine50 where the central component of the tensor lies in the HCH plane bisects the HCH angle. This orientation is different from that observed for carbonyl and carboxyl groups, where the most shielded element of the tensor is perpendicular to the COC/OCO plane, and the middle element lies in the plane perpendicular to the CC bond.51−55 It is important to note that the CQ is proportional to the Vzz component of the EFG tensor, which is perpendicular to the HOH plane. The calculations overestimate the experimental room temperature quadrupolar coupling constant, CQ, by ∼35% and underestimated the asymmetry parameter by ∼25%. Other parameters determined via GIPAW are isotropic nuclear shielding (σiso) = 254.99 ppm (δiso = 20.7 ppm, reference shielding (σref) = 275.7 ppm),25 ζδ = 20 ppm, and ηδ = 0.23. The Euler angles determined using GIPAW are α = 270°, β = 74° and γ = 0°. GIPAW as implemented within the CASTEP software calculates the chemical system without accounting for any molecular motions, effectively treating the molecule at 0 K; therefore, any motional averaging that is observed in the experimental NMR data at conventional temperatures would not be observed. GIPAW has been shown to be in good agreement with other 17O chemical systems, including inorganic oxides,56−58 carboxylic (C17O17OH) oxygens of amino acids,27,59 and small organic molecules.13,60,61 However, the molecular motions of these chemical systems are not known to have an effect on the measurable 17O NMR parameters. Both previous studies of 17O in bound water observed a similar overestimation in CQ and underestimation in ηQ in a series of monohydrates, where it was proposed this discrepancy was likely caused by dynamics;25,26 below we demonstrate experimental evidence of the effect of molecular dynamics on measurable quadrupole parameters with variabletemperature NMR spectra. The quadrupolar coupling constant is indicative of the asymmetry of the electronic environment of the nucleus, and with departures from cubic symmetry, the magnitude of CQ will increase. Therefore, the EFG tensor is sensitive to not just local symmetry but also to the secondary bonding environment of molecules, i.e., hydrogen bonding.7,47,62−64 The quadrupolar coupling constant and asymmetry parameter determined in this study are in good agreement with previous NMR studies of different forms of bound water molecules.18,25,26,65,66 However, nuclear quadrupole resonance (NQR) studies of some inorganic hydrates have determined the CQ of bound water to be significantly larger than 7 MHz, including Ba(ClO3)2· H2O.67,68 This is not the case for all hydrates that have been studied via NQR, demonstrating the sensitivity of the EFG tensor to many different factors and highlighting the complexity of the discrepancy.69,70 The differences in temperature between the NMR (300 K, CQ = 6.92 MHz) and NQR (77 K, CQ = 7.61 MHz) experiments could explain the discrepancies in measured CQ due to changes in dynamics. A detailed understanding of the EFG tensors is required to ascertain which dynamic processes could impact the Vzz component. The axes of the librational modes of the water in Ba(ClO3)2· H2O are coaxial with the principal axes of the EFG tensor. Thus, the two librational modes about the Vyy and Vxx axes could motionally average CQ with the reduction in the Vzz component proportional to the amplitude of the libration. The frequency of the librational modes in the bound water of Ba(ClO3)2·H2O is much larger than the quadrupolar coupling constant (∼1013 vs 107 Hz at 120 K)34 and results in a
quasistatic line shape. The dynamics will not cause distortions in the 17O NMR line shape that occur when the frequencies associated with the dynamic processes have the same order as the relevant NMR interaction. This situation occurs in 2H quadrupole echo spectroscopy and leads to the well-known echo distortions.4,36,37,71 As shown in Figure 3b,c the oscillations about the Vyy and Vxx axes will attenuate the magnitude of the Vzz component. To experimentally investigate the effect of the water dynamics on the measured CQ, we recorded spectra at three temperatures (300, 170, and 105 K) and observed the effective CQ increased from 6.92 to 7.55 MHz with decreasing sample temperature, demonstrating a clear effect of the librational motions on the measurable CQ in Ba(ClO3)2·H2O. The asymmetry parameter was also affected by the motional averaging, decreasing from 0.98 to 0.92. The reduction in amplitude of the librational modes caused by the decrease in temperature leads to an increase in the influence of the strong hydrogen bonding network on the EFG tensor; which in turn causes an increase in the observable CQ via a decrease in the symmetry around the 17O nucleus (Figure 4). At temperatures where the 2-fold flipping motion is in the rigid lattice limit, below ∼150 K according to 2H NMR,4 the 17 O MAS experiment required 1H decoupling to recover the
Figure 4. (a) 17O MAS NMR spectrum (solid, red) at 300 ± 5 K; spectral simulation (dashed, red): CQ = 6.9 ± 0.1 MHz, ηQ = 0.98 ± 0.05, δiso = 21 ± 1 ppm. (b) 17O MAS NMR spectrum (solid, green) at 170 ± 10 K, spectral simulation (dashed, green): CQ = 7.4 ± 0.1 MHz, ηQ = 0.95 ± 0.05, δiso = 21 ± 1 ppm. (c) 17O MAS NMR spectrum with 100 kHz continuous-wave 1H decoupling (solid, blue) at 105 ± 5 K, spectral simulation (dashed, blue): CQ = 7.55 ± 0.10 MHz, ηQ = 0.92 ± 0.05, δiso = 21 ± 1 ppm. Spectra were acquired with ωR/2π = 14 kHz, spinning sidebands are noted by asterisks. The gray vertical dashed line indicates the left edge of all VT MAS spectra, while the colored vertical dashed lines indicate the right edge of each spectrum. 7854
DOI: 10.1021/acs.jpcb.6b05755 J. Phys. Chem. B 2016, 120, 7851−7858
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The Journal of Physical Chemistry B
(DHH > DOH) is also required to produce such a splitting, as such an effect is not seen with the analogous simulation of Ba(ClO3)2·D217O (Figure S6). The low-temperature 17O MAS NMR experiments indicate that the primary cause of the discrepancy between the experimental and GIPAW calculated results is due to motional averaging. To support this suggestion, nine different crystal structures of Ba(ClO3)2·H2O were generated using the minimum, average, and maximum O−H bond distance and ∠HOH bond angle for hydrates according to the prescriptions of Ferraris and co-workers73,74 to investigate the effect of structure on the calculated EFG parameters. Three additional structures were generated with a fixed H−H distance (1.62 Å) using the NMR determined 1H−1H dipole coupling. The resulting structural details along with the CQ and ηQ from the GIPAW calculations are given in Table S1, and the results are summarized in Figures S7−S9. The calculated results indicate that the CQ and ηQ are sensitive to the symmetry of both the local environment (i.e., the 17O EFG has a dependence on O− H bond distance and ∠HOH bond angle) and the hydrogenbonding network with an increase in the symmetry associated with a reduction of the CQ.25,70 It is apparent that experimental NMR data at temperatures that inhibit motional averaging of the EFG tensor due to librations could be rich in structural details about the H-bonding network in systems with bound water. However, such a temperature was unable to be reached in the current study. Structural studies of chemical systems with molecular motions via NMR will be an exciting avenue with the advancements and availability of MAS NMR at cryogenic temperatures.75−78
room temperature MAS line shape, as shown in Figure 5. The 2-fold flip averages the 1H−17O dipolar tensor to an η ≈ 1 line
Figure 5. (a) 17O MAS NMR spectrum (solid, blue) at 105 ± 5 K without 1H decoupling; spectral simulation (dashed, blue). (b) 17O MAS NMR spectrum (solid, black) at 105 ± 5 K with 100 kHz continuous-wave 1H decoupling; spectral simulation (dashed, black). Spectra were acquired with a ωR/2π = 14 kHz, spinning sidebands are noted by asterisks (*). NMR interactions that are included in each spectral simulation displayed with a water molecule in the inset. NMR parameters used in the spectral simulations are given in Table 1.
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CONCLUSIONS In determining the 17O quadrupolar and chemical shift tensor parameters of Ba(ClO3)2·H2O via multiple magnetic field NMR experiments, we observed a systematic discrepancy in the 17O CQ in comparing experimental NMR, GIPAW, and NQR data. Utilizing variable temperature experiments, the quasistatic observed CQ was shown to increase with decreasing temperature. This increase is due to the partial quenching of large amplitude torsional oscillations of the bound water molecule, which perturb the medium-range hydrogen-bonding network. NMR experiments performed at temperatures low enough to sufficiently slow the librational motions of bound water would be necessary to measure the static CQ and be used in conjunction with DFT calculations to determine finer structural constraints. In addition, at 105 K we observe 1H−17O dipole couplings in the 17O MAS line shape. These are due to the cessation of the 180° 2-fold flips of the H2O molecules and the coupling of three spins of HOH to form a spin system that behaves homogeneously. Thus, the 1H−17O couplings are not averaged by MAS at the spinning frequency utilized, allowing the 1H−17O dipole couplings to be determined via spectral simulation and the H−O distance to be measured. As the application of cryogenic temperatures75−78 (e.g., the use of dynamic nuclear polarization) becomes more widespread, the effect of dynamics and other temperature effects18,19,79 on the measured NMR parameters will undoubtedly reveal additional interesting new effects.
shape at 300 K. However, at low temperatures this motion is absent and yields a rigid lattice 1H−17O coupling. The dipolar interactions in the bound water give rise to a new contribution to the MAS line shape that manifests as a splitting that is observed without 1H decoupling (Figure 5b, solid), because the magnitude of the dipolar interactions is larger than the spinning frequency (∼16 kHz vs 14 kHz). While the 1H−17O dipole tensor interactions are inhomogeneous, the presence of the 1 H−1H dipole tensor, which is a homogeneous interaction, causes the three-spin interaction to act wholly homogeneously, as predicted by Maricq and Waugh,72 giving rise to the observed splitting. The presence of the residual dipolar splitting due to the interaction of the 1H−17O and 1H−1H dipole coupling tensors portends the possibility to determine structural information about the water of hydration. Simulation of the 17O MAS NMR experiment without 1H decoupling (Figure 5b, dashed), using the SIMPSON software package,44 yielded an 1H−17O dipole coupling of 16 ± 1.5 kHz; which translates into an O−H bond distance of 1.00 ± 0.03 Å in the rigid lattice limit. However, due to the presence of librations, the 1H−17O dipole tensor continues to be motionally averaged at 105 K, causing the observed H−O distance to be larger than what is observed from neutron diffraction (0.958 Å).33 The simulation required the proper orientation of all three dipole tensors within the molecule, as shown in the inset of Figure 5b, to reproduce the residual dipolar splitting. The orientation of the three interactions gives rise to the particular splitting that is observed in the 17O MAS spectrum; however, different orientations give rise to different effects on the MAS line shape. The large magnitude of the 1H−1H dipole coupling
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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.jpcb.6b05755. 7855
DOI: 10.1021/acs.jpcb.6b05755 J. Phys. Chem. B 2016, 120, 7851−7858
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The Journal of Physical Chemistry B
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Terminology and experimental details as well as additional tables (S1−S2) and figures (S1−S9). (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; Phone: 617-253-5597. Present Address
† Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Institutes of Health (NIH) through Grant Numbers EB-003151 and EB-002026. V.K.M. is grateful to the Natural Sciences and Engineering Research Council of Canada and the Government of Canada for a Banting Postdoctoral Fellowship. Variable temperature NMR experiments were made accessible by Drs. M.A. Caporini and S. Pawsey (Bruker BioSpin, Billerica, MA). The authors thank Dr. V. Terskikh at the National Ultrahigh-field NMR Facility for Solids, NRC, Ottawa, Canada for scientific discussion and access to the CASTEP software. The authors thank Prof. T. Vosegaard (University of Aarhus, Aarhus, Denmark) and Dr. S. Jain (MIT) for their assistance with the SIMPSON simulation of the variable temperature 17O spectra.
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