Deuteron Nuclear Magnetic Resonance Research Articles

Phenol, the simplest aromatic monohydroxy alcohol, displays a faint Debye-like process when mixed with a nonassociating liquid.

Solvated in propylene carbonate, viscous phenol is studied using dielectric spectroscopy and shear rheology. In addition, several oxygen-17 and deuteron nuclear magnetic resonance (NMR) techniques are applied to specifically isotope labeled equimolar mixtures. Quantum chemical calculations are used to check the electrical field gradient at phenol's oxygen site. The chosen combination of NMR methods facilitates the selective examination of potentially hydrogen-bond related contributions as well as those dominated by the structural relaxation. Taken together the present results for phenol in equimolar mixtures with the van der Waals liquid propylene carbonate provide evidence for the existence of a very weak Debye-like process that originates from ringlike supramolecular associates.

Deuteron nuclear magnetic resonance and dielectric studies of molecular reorientation and charge transport in succinonitrile-glutaronitrile plastic crystals

Plastic crystals are currently discussed as matrices for highly conducting materials. Among them, mixtures based on succinonitrile (SN) have received particular attention. Long ago, Austen Angell [J. Non-Cryst. Solids 131–133 (1991) 13] has shown that in mixtures with glutaronitrile (GN), the plastic phase of SN can deeply be supercooled. Here, a mixture of 60% SN – featuring deuterated methylene groups – and 40% GN is studied using 2 H nuclear magnetic resonance (NMR), thus allowing selective access to the reorientational dynamics of SN. These dynamics agree with that inferred for partially deuterated SN-GN from dielectric spectroscopy which also reveal that a significant H/D isotope effect is absent. Additionally, in the liquid and slightly below the transition to the plastically crystalline state, mixtures of 60% SN and 40% GN are studied using field-gradient NMR diffusometry as well as rotational viscometry. • Dynamics of plastic crystal from 60%-succinonitrile and 40%-glutaronitrile mixture studied over large temperature range. • Correlation times determined using deuteron NMR, dielectric spectroscopy, proton diffusometry, and viscometry. • Phase transition from liquid to plastic crystal clearly revealed by all these methods. • Time constants of protonated and deuterated mixtures agree, thus no significant H/D isotope effect. • Diffusion in the succinonitrile-glutaronitrile mixture much larger than in pure succinonitrile.

Oxygen NMR of high-density and low-density amorphous ice.

Using oxygen-17 as a nuclear probe, spin relaxometry was applied to study the high-density and low-density states of amorphous ice, covering temperatures below and somewhat above their glass transitions. These findings are put in perspective with results from deuteron nuclear magnetic resonance and with calculations based on dielectrically detected correlation times. This comparison reveals the presence of a wide distribution of correlation times. Furthermore, oxygen-17 central-transition echo spectra were recorded for wide ranges of temperature and pulse spacing. The spectra cannot be described by a single set of quadrupolar parameters, suggesting a distribution of H-O-H opening angles that is broader for high-density than for low-density amorphous ice. Simulations of the pulse separation dependent spin-echo spectra for various scenarios demonstrate that a small-step frequency diffusion process, assigned to the presence of homonuclear oxygen-oxygen interactions, determines the shape evolution of the pulse-separation-dependent spectra.

High-temperature and high-pressure NMR investigations of low viscous fluids confined in mesoporous systems

Abstract In this contribution, the relaxation and diffusional behaviors of low viscous fluids, water and methanol confined into mesoporous silica and controlled size pore glass were investigated. The engineered porous systems are relevant to geologically important subsurface energy materials. The engineered porous proxies were characterized by Brunauer–Emmett–Teller (BET) surface analyzer, nuclear magnetic resonance (NMR) spectroscopy, and electron microscopy (EM) to determine surface area, pore-wall protonation and morphology of these materials, respectively. The confined behavior of the low viscous fluids was studied by varying pore diameter, fluid-to-solid ratio, temperature, and pressure, and then compared to bulk liquid state. Both relaxation and diffusion behaviors for the confined fluids showed increasing deviation from pure bulk fluids as the fluid-to-solid ratio was decreased, and surface-to-volume ratio (S/V) was varied. Variable pressure deuteron NMR relaxation of confined D2O and confined methanol, deuterated at the hydroxyl or methyl positions, were performed to exploit the sensitivity of the deuteron quadrupole moment to molecular rotation. The methanol results demonstrated greater pressure dependence than those for water only in bulk. The deviations from bulk liquid behavior arise from different reasons such as confinement and the interactions between confined fluid and the nano-pore wall. The results of the present report give insight into the behavior of low viscosity fluid in nano-confined geometries under different state conditions.

Effects of Partial Crystallization on the Dynamics of Water in Mesoporous Silica

We use differential scanning calorimetry, broadband dielectric spectroscopy, and deuteron nuclear magnetic resonance to investigate water dynamics in MCM-41 pores with a diameter of d = 2.5 nm. At high pore fillings, partial crystallization at Tm = 221 K leads to a dynamic crossover. The reorientation of all water molecules shows non-Arrhenius temperature dependence above 221 K, while two Arrhenius processes associated with liquid and crystalline water species can be distinguished below this temperature. Thus, the confined liquid water fraction exhibits an apparent fragile-to-strong transition as a consequence of a reduction of the accessible pore volume to a narrow interfacial layer during fractional freezing. The confined crystalline water fraction shows considerable dynamics with water reorientation in the milliseconds regime near 200 K. At low pore fillings, we observe neither partial crystallization nor a dynamic crossover.

Order parameters and time evolution of mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF by DNMR.

Uniaxial order parameters of the nematic and columnar mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF have been determined from deuteron nuclear magnetic resonance, where random confinement of the system by the dispersion of aerosil nanoparticles has been performed to help obtain the angular dependent spectra. The long-time evolution study of the order parameters shows that the system requires tens of hours to stabilize after a deep change in temperature, in contrast with the very fast assembly process of the aggregates. Finally, the degree of order of the water molecules, forced by the uniaxial environment, has been determined.

Communication: Heterogeneous water dynamics on a clathrate hydrate lattice detected by multidimensional oxygen nuclear magnetic resonance

Previous deuteron nuclear magnetic resonance studies revealed conflicting evidence regarding the possible motional heterogeneity of the water dynamics on the hydrate lattice of an ice-like crystal. Using oxygen-17 nuclei as a sensitive quadrupolar probe, the reorientational two-time correlation function displays a clear nonexponentiality. To check whether this dispersive behavior is a consequence of dynamic heterogeneity or rather of an intrinsic nonexponentiality, a multidimensional, four-time magnetic resonance experiment was devised that is generally applicable to strongly quadrupolarly perturbed half-integer nuclei such as oxygen-17. Measurements of an appropriate four-time function demonstrate that it is possible to select a subensemble of slow water molecules. Its mean time scale is compared to theoretical predictions and evidence for significant motional heterogeneity is found.

2H nuclear magnetic resonance study of deuterated water dynamics in perfluorosulfonic acid ionomer Nafion

We have employed deuteron nuclear magnetic resonance (NMR) spectroscopy in order to study the dynamics of the deuterated water (D2O) molecules introduced into a perfluorosulfonic acid ionomer Nafion (NR-211) film. According to the 2H NMR spectral analysis, the deuterated water molecules at low temperatures occupied either relatively rigid or mobile sites up to the temperature TM=240K where all the deuterated water molecules became mobile. The temperature-dependent NMR linewidths sensitively reflected the motional narrowing of the rigid and mobile sites, and the NMR chemical shift reflected significant changes in the hydrogen bonds of the deuterated water. While a slow- to fast-limit motional transition was manifested at TM in the laboratory-frame NMR spin–lattice relaxation, the rotating-frame spin–lattice relaxation indicated no bulk liquid water state down to 200K.

Molecular motions in supercooled and glassy ibuprofen: deuteron magnetic resonance and high-resolution rheology study.

Using deuteron nuclear magnetic resonance, the molecular motions of specifically isotope-labeled ibuprofen were probed at the carboxylic group and at the methin group next to it. Spin relaxometry revealed slight differences between the molecular motions of the two isotopomers that are rationalized with reference to the hydrogen bonding of the COOH moiety. In the glassy state, a small-angle jump process among about four sites, related to the so-called γ-process, was identified using stimulated-echo spectroscopy. Indications for a Debye-like process, previously found to leave a weak signature in the dielectric loss, could not unambiguously be detected in magnetic resonance or shear mechanical experiments carried out for supercooled liquid ibuprofen.

Deuteron magnetic resonance and dielectric studies of guest reorientation and water dynamics in six clathrate hydrates containing ring-type guests

Using deuteron nuclear magnetic resonance and high-resolution dielectric spectroscopy the guest dynamics of tetrahydropyran, cyclopentane, trimethylene oxide, 1,3-dioxolane, and 1,4-dioxane clathrate hydrates is studied. By investigating lattice-deuterated as well as guest-deuterated crystals, the anisotropic guest reorientation is scrutinized and compared with previous results for tetrahydrofuran clathrate hydrate. The reorientational energy barriers depend linearly on the size of the guest molecules except for the clathrate hydrate of cyclopentane, a molecule which exhibits a dipole moment of the order of 0.01D. The ether oxygens of the other guests can induce Bjerrum L defects on the hydrate lattice. Their concentration is examined for ammonia-doped tetrahydrofuran clathrate hydrate. Covering a wide range of NH3 concentrations x, a minimal overall Bjerrum defect concentration is observed that leads to a maximum mobility on the hydrate lattice at x≈0.03%. To examine guest-induced Bjerrum L defects further, the translational motion on the undoped hydrate lattices is studied using stimulated-echo spectroscopy: The proton dynamics of CP clathrate hydrate is virtually identical to that of hexagonal ice and the water motion of THP clathrate hydrate closely matches that of the tetrahydrofuran compound.

High Pressure NMR Investigation of 3M Fuel Cell Ionomers

Nuclear Magnetic Resonance (NMR) measurements were obtained for polymer electrolyte membranes (PEM) provided by 3M Corp. through the courtesy of Dr. Steve Hamrock. These membranes have potential for use within PEM Fuel Cells (PEMFCs) and can operate at somewhat lower relative humidity than NAFION. NMR spin-lattice relaxation (T1) and static gradient self-diffusion measurements were obtained as a function of applied hydrostatic pressure for three different equivalent weight (EW) membranes at several values of water content. T1 measurements allow us to study the rotational molecular motion of water within the membranes, whereas diffusion directly probes translational motion (1). By measuring both T1 and self-diffusion, possible relationships between rotational and translational motion within the membranes can be observed and effects due to water content or EW can be found.The membranes studied had EWs of 700, 825, and 1000, and each was prepared with both 10% and 20% water content (wt. %). Measurements were conducted at several temperatures. For T1 measurements, we used deuteron NMR in membranes equilibrated with D2O because of the high sensitivity of nuclear quadrupole relaxation to rotational motions (2). In order to measure diffusion (using proton NMR on samples equilibrated with H2O) in samples contained within a high pressure Cu/Be cell, it was necessary to use a spin-echo pulse sequence in the static gradient in the fringe field of a 7.3 T superconducting magnet. Figure 1 displays isothermal variable pressure T1 results obtained for the 700 EW membrane with 20 wt. % D2O. It can be seen that T1 values increase with temperature and decrease with pressure.The results for different membranes also suggest that values for T1 increase with higher membrane EW and with higher D2O content. It is found that there is an increase in activation volume, Δv, with lower water content and lower temperatures, which was calculated using Δv = -kT[lnT1/P]T [1] Table 1 displays the activation volume of each membrane at various temperatures with 20 wt. % D2O.Measurements of water self-diffusion as a function of pressure for each sample at different water content are underway. Diffusion activation volumes will thus be obtained and compared with those for deuteron spin-lattice relaxation. Such comparison will shed light on the relationship between water translational and rotational motion inside the ionomer, and hopefully lead to insight into the proton transport mechanism.REFERENCES1. Jayakody, JRP, Stallworth, PE, Mananga, ES, Farrington-Zapata, J, and Greenbaum, SG. J. Phys. Chem. B 108, 4260 (2004).2. Fontanella, JJ, Wintersgill, MC, Chen, RS, Wu, Y, and Greenbaum, SG. Electrochimica Acta 40, 2321 (1995).This work was supported by the Office of Naval Research.

Dynamics in Glass Forming Sulfuric and Nitric Acid Hydrates

Deuteron nuclear magnetic resonance (NMR) and dielectric spectroscopy are utilized to investigate the dynamics in sulfuric acid hydrates as well as in nitric acid hydrates for various degrees of hydration. Near the glass transition temperature the electrical response is up to four decades faster than the calorimetric one, a feature found also for several other inorganic ionic liquids. The acid hydrates display pronounced super-Arrhenius behavior with fragility indices of the order of 100. The relaxation strength of the acid hydrates increases with increasing temperature, an observation that was rationalized with reference to the degree of molecular dissociation. Spin relaxometry and stimulated-echo spectroscopy revealed an overall isotropic reorientation process featuring a jump angle of about 30°. Finally, the implications of the present results for the understanding of the glass transition of pure ultraviscous water are discussed.

Broadband dynamics in neat 4-methyl-3-heptanol and in mixtures with 2-ethyl-1-hexanol

The relatively small dielectric Debye-like process of the monohydroxy alcohol 4-methyl-3-heptanol (4M3H) was found to depend slightly on the intramolecular conformation. Proton and deuteron nuclear magnetic resonance demonstrate that the hydroxyl dynamics and the overall molecular dynamics take place on similar time scales in contrast to the situation for the structural isomer 2-ethyl-1-hexanol (2E1H) [S. Schildmann et al., J. Chem. Phys. 135, 174511 (2011)]. This indicates a very weak decoupling of Debye-like and structural relaxation which was further probed using volume expansivity experiments. Shear viscosity as well as diffusometry measurements were performed and the data were analyzed in terms of the Debye-Stokes-Einstein equations. In mixtures of 4M3H with 2E1H the Debye-like process becomes much stronger and for 2E1H mole fraction of more than 25% the behavior of this alcohol is rapidly approached. This finding is interpreted to indicate that the ring-like supramolecular structures in 4M3H become energetically unfavorable when adding 2E1H, an alcohol that tends to form chain-like molecular aggregates. The concentration dependence of the Kirkwood factor in these mixtures displays a high degree of similarity with experimental results on monohydroxy alcohols in which the pressure or the location of the OH group within the molecular structure is varied.

Small-angle water reorientations in KOH doped hexagonal ice and clathrate hydrates

Using deuteron nuclear magnetic resonance and dielectric spectroscopy KOH doped tetrahydrofuran clathrate hydrates and KOH doped hexagonal ice are studied at temperatures above 60 and 72 K, respectively. Below these temperatures proton order is established on the lattice formed by the water molecules. In the clathrate hydrate a new type of small-angle motion is discovered using deuteron spin-spin relaxation, line-shape analysis, and stimulated-echo experiments. Based on the latter results a model is developed for the local proton motion that could successfully be tested using random-walk simulations. It is argued that the newly identified small-angle motion, obviously absent in undoped samples, is an important feature of the mechanism which accompanies the establishment of proton order not only in doped clathrate hydrates but also in doped hexagonal ice. Specific motions of OH(-) defects are demonstrated to explain the experimentally observed behavior. The relative importance of localized versus delocalized OH(-) defect motions is discussed.

Translational and rotational mobility of methanol-d4 molecules in NaX and NaY zeolite cages: A deuteron NMR investigation

Nuclear magnetic resonance (NMR) provides means to investigate molecular dynamics at every state of matter. Features characteristic for the gas phase, liquid-like layers and immobilized methanol-d4 molecules in NaX and NaY zeolites were observed in the temperature range from 300K down to 20K. The NMR spectra at low temperature are consistent with the model in which molecules are bonded at two positions: horizontal (methanol oxygen bonded to sodium cation) and vertical (hydrogen bonding of hydroxyl deuteron to zeolite framework oxygen). Narrow lines were observed at high temperature indicating an isotropic reorientation of a fraction of molecules. Deuteron spin–lattice relaxation gives evidence for the formation of trimers, based on observation of different relaxation rates for methyl and hydroxyl deuterons undergoing isotropic reorientation. Internal rotation of methyl groups and fixed positions of hydrogen bonded hydroxyl deuterons in methyl trimers provide relaxation rates observed experimentally. A change in the slope of the temperature dependence of both relaxation rates indicates a transition from the relaxation dominated by translational motion to prevailing contribution of reorientation. Trimers undergoing isotropic reorientation disintegrate and separate molecules become localized on adsorption centers at 166.7K and 153.8K for NaX and NaY, respectively, as indicated by extreme broadening of deuteron NMR spectra. Molecules at vertical position remain localized up to high temperatures. That indicates the dominating role of the hydrogen bonding. Mobility of single molecules was observed for lower loading (86molecules/uc) in NaX. A direct transition from translation to localization was observed at 190K.

Photoisomerization-Controlled Phase Segregation in a Submicron Confined Azonematic Liquid Crystal

Deuteron nuclear magnetic resonance is used to study the phase segregation behavior of photoisomerizable liquid crystal diheptylazobenzene (7AB) confined into cylindrical pores of Anopore membranes. It is demonstrated that the concentration of both components in a binary trans-7AB and cis-7AB mixture can be controlled dynamically using UV-illumination stimulated trans-to-cis photoisomerization and thermally induced cis-to-trans backisomerization. The so far elusive temperature-concentration phase diagram of such system is determined by comparative analysis of the behavior in bulk, thin-planar, and Anopore-confining geometry.

Secondary Water Relaxation in a Water/Dimethyl Sulfoxide Mixture Revealed by Deuteron Nuclear Magnetic Resonance and Dielectric Spectroscopy

We exploit the potential of a combined dielectric spectroscopy (DS) and deuteron nuclear magnetic resonance ((2)H NMR) approach to investigate the molecular dynamics in a supercooled 2:1 molar mixture of deuterated water (D(2)O) and dimethyl sulfoxide (DMSO). While DS probes the rotational motion of both components, application of (2)H NMR allows us to single out the dynamical behavior of the water molecules. Combining the results of both methods, we can follow the slowdown of the α-process of the mixture over more than 10 orders of magnitude in time, revealing that the Vogel-Fulcher-Tammann (VFT) equation describes well its temperature dependence down to the glass transition temperature, T(g) = 146 K. While the (2)H NMR data do not provide evidence for a secondary relaxation process in the weakly supercooled regime, they indicate that, in the deeply supercooled regime, T(g) ≤ T ≤ 160 K, the water molecules do show a secondary dynamical process, which is faster and exhibits a weaker temperature dependence than the α-process of the mixture. Consistently, the shape of the dielectric spectra changes in this temperature range. (2)H NMR rotational correlation functions reveal that this faster secondary water process destroys essentially all orientational correlation. In addition, these data show that the water reorientation process is characterized by a mean elementary jump angle smaller than 13°. Possible origins of the faster secondary water process in the deeply supercooled mixture are discussed.

Intramolecular Ligand Dynamics in d15-(PPh3)-Capped Gold Nanoparticles Investigated by 2H NMR

The knowledge of intramolecular dynamics, geo-metric confinement, and structure of surface-bound ligands are important factors for understanding surface chemistry, catalysis, and design of catalytically optimized nanoparticles. In this report, intramolecular dynamics of perdeuterated triphenylphosphine ligands, d15-(PPh3), bound to the surface of gold nanoparticles (AuNPs) are characterized by pulsed deuteron nuclear magnetic resonance (2H NMR) in the solid state. The 2H quadrupolar echo Fourier transform NMR line shapes indicate fast 180° flip dynamics (π flip) of phenyl rings in surface-bound d15-(PPh3) ligands about the P−C bond at the rate of k ≥ 7 × 107 s−1. In contrast, the d15-(PPh3)AuCl complex, a model compound that is also present in this nanoparticle system, does not exhibit measurable π flips on the time scale sampled by the experiment (∼10−5 s). Its phenyl rings appear rigid from ambient temperature up to 386 K. The experimental 2H NMR line shape results are consistent with a theoretical mode...

Comparative Dynamics of Leucine Methyl Groups in FMOC-Leucine and in a Protein Hydrophobic Core Probed by Solid-State Deuteron Nuclear Magnetic Resonance over 7−324 K Temperature Range

Quantitative dynamics of methyl groups in 9-fluorenylmethyloxycarbonyl-leucine (FMOC-leu) have been analyzed and compared with earlier studies of methyl dynamics in chicken villin headpiece subdomain protein (HP36) labeled at L69, a key hydrophobic core position. A combination of deuteron solid-state nuclear magnetic resonance experiments over the temperature range of 7-324 K and computational modeling indicated that while the two compounds show the same modes of motions, there are marked differences in the best-fit parameters of these motions. One of the main results is that the crossover observed in the dynamics of the methyl groups in the HP36 sample at 170 K is absent in FMOC-leu. A second crossover at around 95-88 K is present in both samples. The differences in the behavior of the two compounds suggest that some of the features of methyl dynamics reflect the complexity of the protein hydrophobic core and are not determined solely by local interactions.

Deuteron nuclear magnetic resonance and dielectric study of host and guest dynamics in KOH-doped tetrahydrofuran clathrate hydrate

The host as well as the guest dynamics in ion doped clathrate hydrates were studied via several deuteron nuclear magnetic resonance techniques and using broadband dielectric spectroscopy in conjunction with the application of large electrical fields. At a given temperature evidence for up to three relaxation processes was found for samples with mole fractions larger than ${10}^{\ensuremath{-}4}\text{ }\text{KOH}$. The two slower processes, unraveled via an electrical cleaning procedure, are similar to those detected on undoped samples in which they proceed on slightly longer time scales. The fastest process exhibits a weak temperature dependence except close to the transition into the low-temperature phase. Here, an incomplete proton order is established on the hydrate lattice and a residual orientational motion of the guest molecules could be detected at low temperatures. These results demonstrate the large degree of coupling between host and guest motions.