Deuteron Nuclear Magnetic Resonance Research Articles

Behavior of mesogenic molecules deposited at the alumina-air interface: a deuteron NMR study.

Thin molecular depositions of 4(')-pentyl-4-cyonobiphenyl (5CB) mesogenic molecules are investigated via quadrupole-perturbed deuteron nuclear magnetic resonance (DNMR) spectroscopy. Uniform and controlled thickness molecular surface depositions are prepared on the inner cylindrical surfaces of Anopore membranes by the solvent-evaporation technique. As a result, 5CB molecules are found in two different configurations: a bulklike one with parallel axial arrangement, and a surface one with planar radial arrangement. If the 5CB surface coverage exceeds c approximately 0.35, only the bulk state is present. In the coverage range between 0.015 and 0.35, the bulklike state and the surface layer coexist, conforming to a typical dewetting scenario. Below c approximately 0.015, only the surface layer is present. The dilution of the surface deposition with decreasing coverage is manifested as an increase in the DNMR doublet frequency splitting. The surface orientational order parameter Q, the surface biaxiality eta, and the diffusion coefficient D(S) are determined from the DNMR spectral patterns obtained at different sample orientations in the external magnetic field. These angular patterns prove that in highly diluted surface depositions the molecules lie flat on the surface. However, they are not frozen and their molecular axes rapidly reorient on the DNMR measurement time scale, typically 10(-4) s, while remaining confined to the surface. Simultaneously, molecules diffuse over the surface with a surface diffusion constant on the order of 10(-11) m(2) s(-1). Such molecular diffusion is responsible for an effective biaxiality on the DNMR time scale. However, an inherent biaxiality cannot be completely ruled out and thus may play a minor role. The surface phase has a two-dimensional (2D) gas character with some (possible) indicators of 2D-liquid properties.

The methyl group as a built-in probe of the glassy dynamics in propylene carbonate

Deuteron nuclear magnetic resonance was employed to study primarily the glassy phase of the fragile glass-former propylene carbonate, selectively deuterated at the methyl group. The analysis of temperature dependent line shapes allowed us to map out the energy barrier distribution below the glass transition temperature. The results were compared to measurements of spin-relaxation times. We determined the relative width of the barrier distribution and the extent of the decoupling between the methyl group rotation time and the structural relaxation time near the glass transition. These findings are discussed in the context of what has been reported for secondary relaxation processes in other glass-formers. Stimulated echo experiments on propylene carbonate demonstrate that, while in the supercooled liquid small angle reorientations dominate, in the glass the methyl group performs 120° (or 60°) jumps.

Rheology and nuclear magnetic resonance measurements under shear of sodium dodecyl sulfate/decanol/water nematics

We report on the rheology and deuteron nuclear magnetic resonance (NMR) measurements performed on the nematic lyotropic system sodium dodecyl sulfate, decanol, and water. For total surfactant and cosurfactant concentrations of about 30% by weight, we prepared nematic calamitic NC and nematic discotic ND solutions. We have studied the flow behavior of the NC and ND phases in terms of the transient shear stress using different rheological histories. In flow-reversal experiments, the transient response exhibits nonperiodic oscillations in the region of small deformations (γ=20), which is followed by a long-lasting relaxation toward the stationary state. In the pseudo-Newtonian regime of the viscosity, we have found that the stress, when normalized to its long term value and plotted versus deformation, is shear-rate independent. 2H NMR measurements at rest and under shear were performed on nematic calamitic and discotic solutions prepared in D2O. At rest, the NMR spectra exhibit well-resolved doublet structures, which indicate an alignment of the director along the axis of the magnetic field. At shear rates large enough (γ̇ > 0.3 s−1), alignment is achieved with respect to the flow velocity. One finds alignment angles of 12 ° and 105 ° for the directors of the calamitic and discotic phase, respectively. Combining the results obtained by rheology and by NMR, we provide strong evidence that the NC and ND nematics of the sodium dodecyl sulfate / decanol system are textured nematics of the flow-aligning type.

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT 1 process and double quantum filtered (DQF) spectral analyses.2H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.

Substrate-induced order in the isotropic phase of a smectogenic liquid crystal: A deuteron NMR study

Using deuteron nuclear magnetic resonance, we study the pretransitional wetting behavior of the smectogenic and nematogenic liquid crystal 8CB in the isotropic phase confined to 200 nm diameter channels of Anopore membranes. The channels were treated with aliphatic acid to impose homeotropic alignment. Just above the nematic-isotropic transition we observe an enhanced line splitting compared to non-smectogenic homologs such as 5CB; moreover, the line splitting is characterized by a discontinuity of its slope at approximately 5 K above the transition temperature. This deviation from a purely nematic wetting scenario is attributed to the substrate-stabilized smectic order within the wetting film, which appears to be stimulated by the proximity of the smectic-$A$ phase: in 8CB, the smectic-$A$-nematic transition occurs approximately 7 K below the nematic-isotropic transition. We interpret the data in terms of a semimicroscopic Landau--de Gennes type model, which allows us to estimate the thickness of the presmectic film---which consists of 2 bilayers---as well as the parameters of the surface interaction and the effective smectic-nematic coupling.

Solvatochromism in ligand-field absorption bands and solvent-dependent deuteron nuclear magnetic resonance spectra of [CrF 3(tri(2-pyridyl)amine)]: implication of trigonal splitting for the first spin-allowed d–d band and evaluation of π bonding peculiarities

The correlation of the remarkable solvatochromism and solvent-dependent deuteron NMR spectra of fac-[CrF 3(tpa)] (tpa=tri(2-pyridyl)amine) with the acceptor numbers ( A Ns) for solvents demonstrated the sensitivity of the Crligand bond length or the angular overlap model (AOM) parameters to the embedding solvent, and also suggested a significant trigonal splitting in the first 4T 2← 4A 2 ligand-field band. The inverse correlation of the 2H NMR shifts for 4- and 3-deuterons of the pyridyl in the tpa complex with the solvent electron A N gives direct evidence for the π interaction with the pyridyl groups in the tpa ligand that varies from strong acceptor to weak donor behavior depending upon the solvent.

Pressure Effect on Hydrophobic Hydration: Rotational Dynamics of Benzene

Deuteron nuclear magnetic resonance spin−lattice relaxation times T1 have been measured for deuterated benzene molecules (C6D6) in dilute solutions of water (H2O) and methanol at temperatures of 1, 2, 5, 15, and 30 °C and pressures up to 300 MPa. At the higher temperatures of 15 and 30 °C, the rotational correlation time τ2R of C6D6 in the aqueous solution (H2O) increases with pressure, whereas the τ2R of D2O decreases. This difference is interpreted in terms of the compression mechanism involving the packing effect and the orientational dependence of intermolecular interactions. At lower temperatures, the pressure dependence of the τ2R of C6D6 at initial compression is smaller. Especially, at the lowest temperature of 1 °C, the τ2R of C6D6 in the aqueous solution is very weakly dependent on the pressure up to 100 MPa. This implies that a strong hydration shell due to the hydrophobic effect resists compression up to 100 MPa and that the rotational mobility of C6D6 is not affected by the pressure. When the pressure is raised beyond 100 MPa, the pressure dependence of the τ2R at 1 °C is similar to those at the higher temperatures of 15 and 30 °C and the hydration shell is relatively weak. For the methanol solution, in contrast, the τ2R of C6D6 increases monotonically with pressure both at 1 and 30 °C. The activation energies of the τ2R of C6D6 in the aqueous solution and D2O in pure liquid exhibit anomalous reductions as functions of the pressure. The reduction of the activation energy is accounted for by the pressure-induced weakening of hydrogen bonds between water molecules around the solute benzene.

Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T2−1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.

Geometry and tunneling dynamics of CHD2 groups in aspirin: A single-crystal deuteron nuclear magnetic resonance study

CHD 2 groups in aspirin are studied by single-crystal deuteron NMR at temperatures between 7 and 290 K. They perform stochastic reorientations which are governed by a rotational potential possessing three wells, two of which are almost equally deep whereas the third is significantly higher. The three minima of the rotational potential are separated by angles substantially different from 120°. Still, at least in the two lower wells, the geometry of the CHD2 groups is close to that of a symmetric rotor. By selective magnetization transfer experiments, the incoherent tunneling rate between the two lower potential wells is determined. At temperatures below 15 K, Raman processes are the dominating rotor-phonon coupling mechanism, while at higher temperatures, thermally activated processes with activation energies close to the total potential height prevail.

Reorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy

The method of stimulated echoes was used to investigate the reorientational mechanism in the selectively deuterated glass-former glycerol, C3D5(OH)3 about 15 K above its calorimetric glass temperature. The reorientation process is fully isotropic. This enables an accurate determination of the decay constant, T1Q, of the quadrupolar spin order in the regime of ultraslow motion. The knowledge of this time constant has made it possible to reliably determine the rotational correlation function. The experimentally obtained evolution time-dependent correlation functions are compared with those from a simulation procedure involving a distribution of molecular jump angles. It is found that in glycerol small angles in the 2°–3° range dominate. They are accompanied by a small, but significant, fraction of larger jump angles.

A comparative study of dynamics in the nematic and reentrant-nematic phases of 60CB and 60CB/80CB mixture by deuteron nuclear magnetic resonance relaxation

Deuteron longitudinal (T1Z) and quadrupolar (T1Q) spin-lattice relaxation times and quadrupolar splittings were measured over all the stable mesophases in a mixture of perdeuterated 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) at 15.1 and 46 MHz, and compared with those reported previously for a pure 6OCB sample. The 6OCB/8OCB mixture has 28 wt. % of 6OCB and shows a nematic, smectic A and reentrant-nematic (RN) phases. We have carried out data analyses for both samples in order to achieve a consistent physical picture. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings in these samples. A decouple model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a master rate equation. In comparing the NMR results of the pure 6OCB sample and of the 6OCB/8OCB mixture, both the dynamic and static behaviors appear to be similar, and there are no dramatic changes upon entering the RN phase of 6OCB/8OCB, supporting the belief that the effects driving the reentrancy in this mixture are very subtle. The tumbling motion of 6OCB molecules, however, shows quite different behaviors in the two studied samples. Both 6OCB and 8OCB possess a strong terminal dipole and tend to form “loose” dimers. The degree of dimerization can be inferred from the tumbling motion of 6OCB molecules and their internal chain dynamics.

Non-bonded hydrogen in a-Si:H

We report deuteron and proton nuclear magnetic resonance (NMR) measurements on high quality plasma enhanced chemical vapor deposition amorphous silicon films containing from 5 to 15 at.% hydrogen (H and/or D). Our results show that from 2% to 40% of the hydrogen in these samples is not atomically bonded by the Si–H chemical bond. We present conclusive evidence that in highest quality films nearly all of this non-bonded hydrogen is present as isolated hydrogen molecules. These molecules are located in centers of atomic dimensions, perhaps in the analogue of T-sites in crystalline Si. In high quality films these molecules are not the small population of densely packed molecules in the occasional microvoids. The photoelectronic quality of the films (as measured by the ημτ photoresponse product) increases as the fraction of non-bonded hydrogen increases. Our results are consistent with IR spectral measures of hydrogen. Our experiments also suggest that the non-bonded hydrogen is in the vicinity of light induced defects. Finally by comparing successive annealings and other studies it appears that the molecular hydrogen can be identified with the less-clustered proton narrow NMR line.

NMR Investigation of Molecular Motion in the Neat Solid and Plastic-Crystalline Phases of Cyclohexane1

The molecular dynamics of neat solid and plastic phase cyclohexane are investigated using solid state carbon, deuteron, and proton nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on deciphering the dynamical origin of a line width/line shape transformation in the static proton NMR spectrum of plastic phase cyclohexane near 199 K, and it is examined in terms of rotational, translational, and conformational contributions. It is shown that translational diffusion within lattice defect sites is the probable dynamical mode associated with the static proton NMR line shape modulation. The rate of conformational isomerism by ring inversion in the plastic phase is measured for the first time at two temperatures, and the time scale of this process suggests a mechanistic correlation with translational diffusion within single crystals in the plastic phase. Finally, the time scale and symmetry associated with rotational diffusion in solid-phase cyclohexane are found (respectively) to be longer and more complex than previously reported.

Molecular mobility in poly(ethylene-2,6-naphthalene dicarboxylate) as determined by means of deuteron NMR

Selectively deuterated poly(ethylene-2,6d-naphthalene dicarboxylate) (PEN) was synthesized, obtaining a material in which either the ethylene groups (PEN-Ed1 or the naphthalene ring (PEN-Nd6) was deuterated. By means of deuteron nuclear magnetic resonance (2H-NMR), the molecular mobility of the deuterated groups was investigated as a function of temperature. The spectra were measured by the solid echo method, applying different waiting times tw The longitudinal relaxation curves were obtained by determination of the maximum intensity of the echo as a function of tw It was found that in the amorphous regions of PEN, the molecular mobility of the ethylene groups is higher than in the amorphous regions of PET. In PEN crystals in the β-modification form, trans-gauche jumps of the ethylene groups with almost equal occupation of the trans and the gauche conformation take place already at 90°C. This is not the case in either PEN crystals in the a-modification form or in PET crystals. The rotational motion of the naphthalene ring, which according to dynamic mechanical analysis at 20 Hz starts at 60°C, could not be observed in the NMR spectra. This is explained by the high frequency applied in the NMR experiment.

Rotational diffusion of colloid spheres in concentrated suspensions studied by deuteron NMR

We present a study of the application of deuteron-nuclear magnetic resonance spectroscopy (NMR) to the investigation of the rotational diffusion of spherical colloidal particles. We performed NMR pulse experiments on colloidal suspensions of polystyrene latex spheres in water-glycerol mixtures in a wide range of particle volume fractions \ensuremath{\varphi} from the dilute suspension up to \ensuremath{\varphi}=0.504. We have analyzed the stimulated echo NMR signal in the time domain. The full shape of the orientational correlation function deviates from an exponential behavior in the whole \ensuremath{\varphi} range examined. We evaluate the rotational diffusion coefficient and calculate its \ensuremath{\varphi} dependence up to the ${\mathrm{\ensuremath{\varphi}}}^{2}$ term in view of the theory proposed recently [V. Degiorgio, R. Piazza, and R. B. Jones, Phys. Rev. E 52, 2707 (1995)], which considers the effect of two- and three-body hydrodynamic interactions upon particle reorientation. We find considerable slowing down of sphere reorientation for \ensuremath{\varphi}\ensuremath{\geqslant}0.2. The agreement between experimental results and theoretical considerations is satisfactory.

Pressure- and temperature-variable viscosity dependencies of rotational correlation times for solitary water molecules in organic solvents

Deuteron nuclear magnetic resonance spin-lattice relaxation times T1 have been measured for solitary water molecules (D2O) at low concentrations in apolar and polar organic solvents at 30 °C at high pressures; D2O (30 mM) in C6H6 up to 90 MPa, D2O (60 mM) in CHCl3 up to 300 MPa, D2O (100 mM) in CH3CN up to 300 MPa. The rotational correlation times τ2R for D2O in the organic solvents increase with increasing pressure. The pressure effect on τ2R for D2O in solution is considerably larger than that on τ2R and η (viscosity) for the neat solvent. We have tested the two forms of modified Stokes–Einstein–Debye law; the linear and nonlinear forms are τ2R=τ02R+S(η/T) and τ2R=B(η/T)α, respectively. The rotational correlation times are linearly related to solvent viscosity divided by temperature (η/T) with a large positive intercept (τ02R≳0). It is shown that the linear form is practically better, and that the nonlinear form constrained at η/T=0 is invalid. The temperature-variable slope (ST) and the pressure-variable one (Sp) are markedly different, the ratios of Sp to ST being 0.2–0.3. The extended-diffusion models based on isolated binary collisions cannot be used to explain the observed pressure effect because of the neglect of the attractive solute–solvent interactions.

Water Absorption of the Polymer Electrolyte Systems Pb ( CF 3 SO 3 ) 2 PEO n and Zn ( CF 3 SO 3 ) 2 PEO n

The water absorption behavior has been studied for poly(ethylene oxide) films (PEO) and for films of with and in the range of 9 to 24. Quantitative measurements of the kinetics of water absorption have been made. Proton nuclear magnetic resonance (NMR) spectroscopy has been used to characterize the effects of water absorption on the polymer properties, such as degree of crystallinity and polymer chain mobility. Impedance spectroscopy measurements have also been performed on humid samples. Water absorption is found to increase the mobility of the polymer chain as well as the ionic conductivity of the electrolyte, and a possible mechanism for these effects is proposed. Water absorption also causes a decrease of the amount of crystalline phase present in the sample. In addition, deuteron NMR has been used to study the mobility of the absorbed water, demonstrating the absence of liquid water surface layers on the polymer, and that the conductivity increase observed upon hydration is a property of the hydrated polymer electrolyte itself.

Deuteron nuclear magnetic resonance study of high-pressure effects on the molecular dynamics in amorphous polyethylene

The effect of high pressure up to 2500 bar on the chain motion in amorphous chain-deuterated polyethylene is studied in a wide range of temperature (203–393 K) using the deuteron nuclear magnetic resonance ( 2H n.m.r.) quadrupole echo and spin alignment echo technique. Quadrupole echo spectra simulated on the basis of Flory's rotational isomeric state model appear to reproduce the experimental spectra quite well. The comparison yields information on the time-scale of the motion. The temperature dependence of the mean correlation time shows an Arrhenius behaviour in the temperature range from 223 to 383 K. The activation energy at ambient pressure is 11.3 kJ mol −1, which is slightly lower than the value of the barrier between the trans and gauche conformations (13.2 kJ mol −1). The effect of increasing the pressure by 1 kbar corresponds roughly to lowering the temperature by 15 K.

Molecular dynamics of new thermoplastic elastomers based on hydrogen bonding complexes: a deuteron nuclear magnetic resonance investigation

Abstract4‐(4‐Carboxyphenyl)‐3,5‐dioxo‐1,2,4‐triazolidin‐1‐yl moieties resulting from 4‐(3,5‐dioxo‐1,2,4‐triazolin‐4‐yl)benzoic acid (U4A) randomly attached to 1,4‐polybutadiene form supramolecular aggregates which act as effective junction zones in new thermoplastic elastomers. Deuteron nuclear magnetic resonance (2H NMR) spectroscopy was used to investigate the molecular dynamics of the polymer backbone and the junction zones in selectively labelled samples. The spectra show motional heterogeneity along the polymer chain as well as in the supramolecular domains. Polybutadiene segments which are adjacent to these polar clusters are restricted in their mobility, even at temperatures 100 K above the glass transition temperature Tg. The immobilized regions are estimated to involve about two monomeric units. The functional groups in the supramolecular structures are rigid or exhibit 180° ‐phenyl flips dependent on their location in the middle or in the edges, respectively. Polar units which are not enclosed in the clusters and dispersed in the polymer matrix undergo fast isotropic motions.

Deuteron NMR study of dynamics and of coexistence of paraelectric and ferroelectric phases in Rb0.90(ND4)0.10D2AsO4.

The deuteron glass ${\mathrm{Rb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(${\mathrm{ND}}_{4}$${)}_{\mathit{x}}$${\mathrm{D}}_{2}$${\mathrm{AsO}}_{4}$ (DRADA) is a mixed crystal of ${\mathrm{RbD}}_{2}$${\mathrm{AsO}}_{4}$ (DRDA) and ${\mathrm{ND}}_{4}$${\mathrm{D}}_{2}$${\mathrm{AsO}}_{4}$ (DADA). Deuteron nuclear magnetic resonance has been performed on the acid and ammonium deuterons. The crystal studied has an ammonium concentration (x=0.10) that puts it in the coexistence region of the phase diagram. Line-shape measurements of the ammonium deuterons show the coexistence of the ferroelectric (FE) and paraelectric (PE) phases as the temperature is lowered below the ferroelectric-phase-transition temperature ${\mathit{T}}_{\mathit{c}}$. The acid deuteron line shape on the other hand is found to broaden as the temperature is reduced but is unaffected by the ferroelectric transition. Spin-lattice-relaxation measurements have been performed and the activation energies for the relaxation processes have been computed. The relaxation-rate anomaly for acid deuterons in the ferroelectric-transition range indicates a short correlation length for the FE phase in the coexistence region of the phase diagram.