1H NMR study of proton dynamics in (NH4)3H(SO4)2

Proton dynamics in (NH₄)₃H(SO₄)₂ has been studied by means of ¹H solid-state NMR. The ¹H magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT) and at Larmor frequency of 400.13 MHz. ¹H static NMR spectra were measured at 200.13 MHz in the range of 135–490 K. ¹H spin-lattice relaxation times, T₁, were measured at 200.13 and 19.65 MHz in the ranges of 135–490 and 153–456 K, respectively. The ¹H chemical shift for the acidic proton (14.7 ppm) indicates strong hydrogen bonds. In phase III, NH₄⁺ reorientation takes place; one type of NH₄⁺ ions reorients with an activation energy (E_a) of 14 kJ mol^− 1 and the inverse of a frequency factor (τ₀) of 0.85 × 10^− 14 s. In phase II, a very fast local and anisotropic motion of the acidic protons takes place. NH₄⁺ ions start to diffuse translationally, and no proton exchange is observed between NH₄⁺ ions and the acidic protons. In phase I, both NH₄⁺ ions and the acidic protons diffuse translationally. The acidic protons diffuse with parameters of E_a = 27 kJ mol^− 1 and τ₀ = 4.2 × 10^− 13 s. The translational diffusion of the acidic protons is responsible for the macroscopic proton conductivity, as the NH₄⁺ translational diffusion is slow and proton exchange between NH₄⁺ ions and the acidic protons is negligible.     

Electrical relaxation in proton conductor composites based on (NH4)H2PO4/TiO2

We report on electrical relaxation measurements of (1 − x)NH₄H₂PO₄-xTiO₂ (x = 0.1) composites by admittance spectroscopy, in the 40-Hz–5-MHz frequency range and at temperatures between 303 and 563 K. Simultaneous thermal and electrical measurements on the composites identify a stable crystalline phase between 373 and 463 K. The real part of the conductivity, σ’, shows a power-law frequency dependence below 523 K, which is well described by Jonscher’s expression , where σ ₀ is the dc conductivity, ω _p /2π = f _p is a characteristic relaxation frequency, and n is a fractional exponent between 0 and 1. Both σ ₀ and f _p are thermally activated with nearly the same activation energy in the II region, indicating that the dispersive conductivity originates from the migration of protons. However, activation energies decrease from 0.55 to 0.35 eV and n increases toward 1.0, as the concentration of TiO₂ nanoparticles increases, thus, enhancing cooperative correlation among moving ions. The highest dc conductivity is obtained for the composite x = 0.05 concentration, with values above room temperature about three orders of magnitude higher than that of crystalline NH₄H₂PO₄ (ADP), reaching values on the order of 0.1 (Ω cm)^− 1 above 543 K.     

Imaging of the (1)H NMR second moment with (13)C chemical-shift resolution

A method of (13)C chemical-shift-resolved (1)H second moment imaging is proposed for molecular mobility imaging of heterogeneous materials. For evaluating the (1)H second moment, the method relies on the curve fitting procedure using spin-echo shapes indirectly: The information of (1)H echo shapes is transferred to the (13)C signal amplitude through (1)H-(13)C cross polarization and then the curve fitting is made using the (13)C signal amplitude. The (13)C signal is detected under (1)H dipolar decoupling and magic angle spinning, resulting in the incorporation of (13)C chemical-shift resolution. Imaging information is included in the (13)C signal by application of phase-encoding gradients. The second moment images obtained can reflect the molecular mobility at every molecular site separated by (13)C chemical shifts, yielding detailed information on the molecular mobility. The method is demonstrated by spatially 1D experiments performed on a model sample. Copyright 2000 Academic Press.     

Interface magnetization reversal and anisotropy in Fe/AlGaAs(001)

The reversal process of the Fe interface layer magnetization in Fe/AlGaAs heterostructures is measured directly using magnetization-induced second-harmonic generation, and is compared with the reversal of the bulk magnetization as obtained from magneto-optic Kerr effect. The switching characteristics are distinctly different due to interface-derived anisotropy--single step switching occurs at the interface layer, while two-jump switching occurs in the bulk Fe for the magnetic field orientations employed. The angle between the interface and bulk magnetization may be as large as 40-85 degrees. Such interface switching will dominate the behavior of nanoscale structures.     

Anomalous spin dephasing in (110) GaAs quantum wells: anisotropy and intersubband effects

A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on a (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along [110]. The spin lifetimes of both spin orientations decrease monotonically above temperatures of 80 and 120 K, respectively. The decrease is very surprising for spins along the [110] direction and cannot be explained by the usual Dyakonov-Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of electrons between different quantum well subbands.     

Low temperature differential magnetization of Ni(ClO4)26NH3 and Ni(BF4)26NH3

The differential magnetization of Ni(ClO₄)₂6NH₃ and Ni(BF₄)₂6NH₃ was measured as a function of temperature (20 to 0.3 K) and magnetic field (up to 40kOe). An antiferromagnetic transition was found at T_N = 0.45 K for the Ni(ClO₄)₂6NH₃ and T_N = 0.43 K for the Ni(BF₄)₂6NH₃, and a portion of the magnetic phase diagram was determined. The interpretation of the data in terms of a uniaxial model yielded (D/k) ～ 0.2 K for both salts.     

H2O/OH ratio determination in hydrous aluminosilicate glasses by static proton NMR and the effect of chemical shift anisotropy

Static ¹H NMR spectra of hydrous NaAlSi₃O₈ glasses have been acquired at low temperature (140 K) in order to quantitatively determine OH and H₂O concentrations. Since both components overlap in the spectra, an unambiguous determination of the line shapes is required. The structurally bonded hydroxyl groups are well described by a Gaussian line and the water molecules exhibit a Pake doublet-like line shape due to the strong proton–proton dipolar interaction. However, at proton resonance frequencies used in this study (360 MHz), the Pake doublet has an asymmetric line shape due to chemical shift anisotropy (CSA), which is significant and must be included in any simulation in order to reproduce the experimental line shape successfully. The simulations for rigid water molecules dissolved in our hydrous aluminosilicate glasses result in a CSA of 30±5 ppm and a dipolar interaction constant of 63.8±2.5 kHz (i.e., dipolar coupling constant (DCC) of 42.6±1.7 kHz), corresponding to a proton–proton distance of r_ij=154±2 pm. In contrast to earlier work, water speciation obtained from the simulations of our ¹H NMR spectra are in excellent agreement with those obtained from infrared (IR) spectroscopy.     

1H relaxation study in (NH4)2SbF5

¹H spin-lattice relaxation rate (T ₁ ⁻¹ ) has been measured using inversion recovery technique in polycrystalline (NH₄)₂SbF₅ system in the temperature range 140–400 K. From the plot of log (M ₀−M) againstτ, we have estimated two differentT ₁ corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT ₁ in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.     

The tautomerization of H2NCH2C(OH)NH to H2NCH2CONH2 in the interstellar medium

The tautomerization of H2NCH2C(OH)NH to H2NCH2CONH2 is an important step by way of Strecker synthesis for the production of glycine in the interstellar medium (ISM) with respect to the origin of amino acids on the early Earth.Our work indicates two mechanisms for the tautomerization to occur.k CVT/SCT (rate constant) is 45.6s-1 at 50 K,obtained with the small curvature tunneling (SCT) approximation and canonical variational transition state theory (CVT),to support one mechanism assisted by quantum tunneling...     

Experimental observation of the 1/3 magnetization plateau in the diamond-chain compound Cu3(CO3)2(OH)2

The magnetic susceptibility, high field magnetization, and specific heat measurements of Cu3(CO3)2(OH)2, which is a model substance for the frustrating diamond spin chain model, have been performed using single crystals. Two broad peaks are observed at around 20 and 5 K in both magnetic susceptibility and specific heat results. The magnetization curve has a clear plateau at one third of the saturation magnetization. The experimental results are examined in terms of theoretical expectations based on exact diagonalization and density matrix renormalization group methods. An origin of magnetic anisotropy is also discussed.     

Dzyaloshinsky-Moriya anisotropy in the spin-1/2 kagome compound ZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.     

Separation of anisotropy and exchange broadening using (15)N CSA-(15)N-(1)H dipole-dipole relaxation cross-correlation experiments

Based on the measurement of cross-correlation rates between (15)N CSA and (15)N-(1)H dipole-dipole relaxation we propose a procedure for separating exchange contributions to transverse relaxation rates (R(2) = 1/T(2)) from effects caused by anisotropic rotational diffusion of the protein molecule. This approach determines the influence of anisotropy and chemical exchange processes independently and therefore circumvents difficulties associated with the currently standard use of T(1)/T(2) ratios to determine the rotational diffusion tensor. We find from computer simulations that, in the presence of even small amounts of internal flexibility, fitting T(1)/T(2) ratios tends to underestimate the anisotropy of overall tumbling. An additional problem exists when the N-H bond vector directions are not distributed homogeneously over the surface of a unit sphere, such as in helix bundles or beta-sheets. Such a case was found in segment 4 of the gelation factor (ABP 120), an F-actin cross-linking protein, in which the diffusion tensor cannot be calculated from T(1)/T(2) ratios. The (15)N CSA tensor of the residues for this beta-sheet protein was found to vary even within secondary structure elements. The use of a common value for the whole protein molecule therefore might be an oversimplification. Using our approach it is immediately apparent that no exchange broadening exists for segment 4 although strongly reduced T(2) relaxation times for several residues could be mistaken as indications for exchange processes.     

Optical studies of the ferroelastic phase transitions in Rb3H(SeO4)2, (NH4)3H(SO4)2 and (NH4)3H(SeO4)2

Optical observations of growth twins and ferroelastic domains and measurements of the rotation of the optical indicatrix were carried out for Rb₃H(SeO₄)₂ and (NH₄)₃H(SO₄)₂ using an optical microscope. Taking into account the symmetry reduction from the rhombohedral (Rm) to the monoclinic phase (B2/a) the occurrence of domains and growth twins can be well described. The orientations of oblique ferroelastic walls are well determined by the spontaneous strains ^s e ₁₁ and ^s e ₂₃ at room temperature.     

(1)H-(13)C INEPT MAS NMR correlation experiments with (1)H-(1)H mediated magnetization exchange to probe organization in lipid biomembranes

Two-dimensional (1)H-(13)C INEPT MAS NMR experiments utilizing a (1)H-(1)H magnetization exchange mixing period are presented for characterization of lipid systems. The introduction of the exchange period allows for structural information to be obtained via (1)H-(1)H dipolar couplings but with (13)C chemical shift resolution. It is shown that utilizing a RFDR recoupling sequence with short mixing times in place of the more standard NOE cross-relaxation for magnetization exchange during the mixing period allowed for the identification and separation of close (1)H-(1)H dipolar contacts versus longer-range inter-molecular (1)H-(1)H dipolar cross-relaxation. These 2D INEPT experiments were used to address both intra- and inter-molecular contacts in lipid and lipid/cholesterol mixtures.     

Transition of magnetic anisotropy in Ni/GaAs (0 0 1) observed by magnetization and ferromagnetic resonance

Polycrystalline thin Ni films deposited onto GaAs (0 0 1) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film ⩽12 nm. This becomes in-plane in the films having thicknesses ⩾15 nm. The films are deposited onto the n-type GaAs (0 0 1) substrate by the usual thermal evaporation method and also by the electron beam evaporation in ultra high vacuum onto a GaAs epilayer in the standard molecular beam epitaxy system. The magnetization and ferromagnetic resonance (FMR) are observed in the temperature range from 4.2 to 300 K. For the discussion of the microscopic origin of the anomalous properties in magnetization and FMR experiments, the experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization data. This calculated anisotropy is able to explain the temperature and angle dependency of the FMR spectra of the Ni films. Hence the magnetization and FMR spectra are in agreement with the type of the anisotropy and its temperature dependency. In addition to these, the temperature dependence of the in-plane magnetic anisotropy is able to explain the previously reported anomalous effect of reducing the squareness at low temperatures in Ni/GaAs.     

Relaxation-selective magnetization preparation based on T1 and T2

A magnetization-preparation scheme is described that combines the spin-echo and inversion-recovery (SEIR) to select spins based on both T1 and T2 characteristics. The inclusion of T2 weighting allows for greater relative suppression of some tissues with respect to others, depending on their respective relaxation times, than does inversion-recovery alone. Formulae describing the observed magnetization following SEIR and double-SEIR (DSEIR) are presented with the corresponding formulae for inversion-recovery (IR) and double-IR (DIR). The formulae are validated with experimental studies on MnCl2 solutions and compared numerically for a variety of possible applications. Results indicate that DSEIR may yield 2x or more signal than DIR in some potential applications.     

Optical properties of the proton conductor (NH2(CH3)2)2CoCl4

Based on the data from optical-spectral studies of (NH₂(CH₃)₂)₂CoCl₄ crystals, the existence of high-temperature phase transitions at 419, 380, 352, and 313 K is confirmed. It was shown that with the exception of the first transition, they are related to a considerable extent to rearrangement of the network of hydrogen bonds and are not accompanied by, considerable deformation of the lattice. In this connection the character of the electron-phonon interaction remains unchanged in the entire temperature range of the measurements. The (NH₂(CH₃)₂)₂CoCl₄ structure at room temperature is close to that of the ordered low-temperature phases of such isomorphous crystals as β-K₂SO₄. I. Franko State University, L’vov, Ukraine, 8, Kirill and Mefodii St., UA-290005, L’vov. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 412–418, May–June, 1998.     

Elastic properties of monoclinic telluric acid ammonium phosphate,Te(OH)6·2NH4H2PO4·(NH4)2HPO4, near the paraelectric-ferroelectric transition

The elastic constants of Te(OH)₆·2NH₄H₂PO₄·(NH₄)₂HPO₄, TAAP, point symmetrym, have been measured by ultrasonic resonance methods passing through the paraelectric-ferroelectric transition at ca. 320 K. In the range between 273 and 340 K the elasticity tensor exhibits only a slight anisotropy. No discontinuity of the elastic constants is observed. However, some temperature derivatives of the elastic constants show slight anomalies within the range 310 to 325 K. The strongest effect occurs with the longitudinal elastic resistancec ₂₂. The thermal expansion which varies only slightly between 263 and 340 K, is highly anisotropic in contrast to the thermoelastic behaviour. A strong negative thermal expansion is observed in a direction within the mirror plane, ca. 45° apart from the direction of spontaneous polarization.This effect is not accompanied by a corresponding thermoelastic anomaly. The interactions connected with the transition are of the totally symmetric type. Like many other properties the elastic and thermoelastic behaviour of TAAP resembles that of triglycine sulphate (TGS). Larger differences between TAAP and TGS are found in the pressure dependence of various properties. For example the pressure dependence of the transition temperatureT is negative for TAAP (–3.8 K/kbar) and positive for TGS (3.9 K/kbar).Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday