NMR study of fluorine and proton motion in the ionic conductor NH4Sn2F5

The nuclear spin relaxation times of protons and fluorine atoms in the ionic conductor NH₄ Sn₂ F₅ show a particular temperature behaviour: they decrease as the temperature decreases below 200 K and have the same temperature and frequency dependences.Cross relaxation experiments show an important coupling between fluorine atoms and protons. NMR results as well as crystallographic and conductivity data on hydrogenated and deuterated compounds, allow to determine two dominant kinds of motion: at low temperature the reorientation of the ammonium ion is observed and above 250 K the diffusion of the fluorine atoms is responsible for the relaxation. In the high temperature range, the activation energies of the motion deduced from conductivity and NMR measurements are in good agreement.     

Ammonium ion dynamics in NH4I at high pressure

Measurements of spin-lattice relaxation time T ₁ have been performed for polycrystalline ammonium iodide NH₄I as a function of hydrostatic pressure and temperature. Activation parameters were obtained for different models of ammonium ion reorientations. It was found that the dominant mechanism for ammonium reorientational motion in the ordered phases III and IV was the reorientation around C₃ axes. Reorientation activation volume and libration frequency of ammonium ions as a function of pressure were calculated for different phases.     

Calculation of zero-field NMR spectra of the ammonium ion undergoing tunneling motion and fast random reorientation

Zero-field NMR spectra of the ammonium ion in the case of tunneling, tunneling and fast reorientation by 180° jumps around theC ₂ axis, tunneling and fast reorientation by 120° jumps aroundC ₃ axis are calculated.     

6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the ⁷Li NMR chemical shifts. Our research includes ^6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T₁) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter.Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. ⁷Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH₃SO₃–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.     

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH₄SO₄ single crystal were studied at low temperature range of 6 and 280 K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH₄ ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133 K.     

Molecular dynamics of n-dodecylammonium chloride studied by nuclear magnetic resonance

The relation between molecular dynamics and phase properties of the bilayered compound C₁₂H₂₅NH₃C1 is studied by differential scanning calorimetry, proton second moment, and spin-lattice relaxation times. In the low-temperature phase I of the compound methyl and ammonium groups execute a classical threefold reorientation, while the alkylammonium chains are rigid on the nuclear magnetic resonance time scale. In the intermediate-temperature phase δ a trans-gauche isomerization of the alkyl chains is observed. In the high-temperature phase α the reorientation of the whole chains about their long axes, which are parallel to the normal to the ionic layer is evidenced. In the metastable ε phase the dynamics involves classical rotation of methyl and ammonium groups and CH₂ groups motion of the trans-gauche isomerization type.     

铵根离子在水和甲醇溶液中的转动机制

Dynamics of ammonium and ammonia in solutions is closely related to the metabolism of ammoniac compounds, therefore plays an important role in various biological processes. NMR measurements indicated that the reorientation dynamics of NH₄⁺ is faster in its aqueous solution than in methanol, which deviates from the Stokes-Einstein-Debye rule since water has higher viscosity than methanol. To address this intriguing issue, we herein study the reorientation dynamics of ammonium ion in both solutions using numerical simulation and an extended cyclic Markov chain model. An evident decoupling between translation and rotation of methanol is observed in simulation, which results in the deviation of reorientation from the Stokes-Einstein-Debye rule. Slower hydrogen bond (HB) switchings of ammonium with methanol comparing to that with water, due to the steric effect of the methyl group, remarkably retards the jump rotation of ammonium. The observations herein provide useful insights into the dynamic behavior of ammonium in the heterogeneous environments including the protein surface or protein channels.     

Molecular reorientation and phase transition in pyridinium hexafluoroantimonate

The temperature dependence of ¹H and ¹⁹F NMR second moment and spin lattice relaxation times in high (v _L = 60 MHz and low (B ₁ = 2 mT) magnetic fields allow one to determine both ion dynamics in polycrystalline pyridinium hexafluoroantimonate. The solid-solid phase transition discovered at 268 K appears to be connected with symmetrization of energy barriers for pseudohexagonal cation reorientation. The energy difference Δ characterizing the inequivalence of the potential wells can be treated as an order parameter. The effect of coupling of the rotational modes of cations and anions is found at the phase transition.     

High Pressure Study of Dynamics in Tetraphenyltin

Measurements of proton NMR relaxation times as well as inelastic and elastic neutron scattering of tetraphenyltin (C 6 H 5 ) 4 Sn as a function of temperature and pressure were performed in the work. It allowed to determine activation volume j V * for a small amplitude reorientation/oscillation of the phenyl rings. The values of activation volume is very small and the ratio of j V */ V M ( V M is molar volume) is less than 1%.     

NMR Study of the Molecular Dynamics of D-Amphetamine Sulfate Salt Powder

The temperature dependence of the proton spin–lattice relaxation times was measured for the D-amphetamine sulfate salt in the temperature range from 80 to 466 K. Two minima of T ₁ were observed and explained in terms of reorientation of the CH₃ and NH₃ groups. The T ₁ minimum attributed to the motion of methyl groups was analyzed assuming two dynamically inequivalent types of these groups in the unit cell of the studied salt. The activation parameters for the C ₃ reorientation of the methyl and ammonium groups were determined. A structural phase transition was evidenced at about 335 K in the polycrystalline sample of the D-amphetamine sulfate salt. Authors' address: Joanna Kaszyńska, Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, Poznań 60-179, Poland     

NMR relaxometry study of gelatin based low-calorie soft candies

ABSTRACT

Soft candies are popular confectionery products. The most significant concern on the consumption of these products is the high amount of sugar and thus the high calories. The use of low-calorie sweeteners is a desirable trend in confectionery research. In this study, gelatin-based soft candies were formulated by using different sweeteners and their characterisation was performed using high and low field nuclear magnetic resonance (NMR) relaxation experiments. To complement the information obtained by NMR experiments, moisture content, water activity, texture analysis and differential scanning calorimeter experiments were also conducted. T₁ and T₂ relaxation times were measured at both low and high fields and also temperature-dependent measurements were conducted at the high field system. Candies were formulated by substitution of sucrose with maltitol, isomalt and stevia at 30%, 50% and 70% ratios. Significant difference was observed on relaxation times. T₁ values were best described by a mono-exponential model, whereas for T₂ relaxation times a bi-exponential model gave better results at both fields.     

NMR study of the motion of oxygens in some oxysalts

The temperature dependence of the proton spin-lattice relaxation times T₁ and T_1? were measured in some partially deuterated ammonium compounds; ammonium perchlorate and ammonium dichromate. The extremely large minimum values of T_1? (2 ～ 3 sec) were found to be independent of the concentration of deuterons. These minima of T_1? were attributed to the random modulation of the dipolar interaction between the protons and ¹⁷O (0.037%) of low abundance. The activation energy E_a of the reorientation of ClO₄ and CrO₃ were determined to be 6.2 and 10.7 kcal mol^-1, respectively.     

Dielectric and anelastic relaxation of crystals containing point defects. II

In part I (1965, Adv. Phys., 14, 101), a theory was developed which treated the thermodynamics of dielectric and anelastic relaxation due to point defects in crystals from the viewpoint of the point symmetry of the defect as well as of the crystal. In the present paper this theory is extended to treat the kinetics of relaxation. Equations are derived which express the relaxation times of electrically and stress active modes of relaxation in terms of the rates of reorientation between one particular defect orientation and all of the other equivalent configurations. Explicit expressions are then given for these relaxation times for commonly occurring crystal and defect symmetries. The reorientation frequencies which appear in these expressions may be converted into the appropriate atom or ion jump rates; this final step can generally be carried out merely by inspection of the crystal model. The possibility that two or more relaxations due to a given point defect may be widely separated on a frequency or temperature scale (a situation which is called a ‘frozen-free split’), and the anomalies connected with such behaviour, are discussed. Finally, various examples which have been studied in the literature, of relaxations due to point defects, are reviewed in the framework of the present theory.     

Muon spin relaxation as a probe of molecular dynamics of organometallic compounds

LF‐Muon Spin Relaxation data are reported for the organometallic compounds Pb(C₆H₅)₄, (C₆H₆)Cr(CO)₃ and (C₅H₅)₂Ru. In each case the change in relaxation rate with temperature shows a peak analogous to the T_1 minimum in NMR. The activation parameters were calculated, and the mechanism of muon spin relaxation in the case of (C₆H₆)Cr(CO)₃ is shown to be the reorientation motion of the benzene ring. This revised version was published online in August 2006 with corrections to the Cover Date.     

Deuteron NMR spectra of NH3D+ Ions

A theory of deuteron nuclear magnetic resonance (NMR) spectra is outlined for NH₃D⁺ ions. Dipolar interactions among all five spins in the ion contribute to the shape of the deuteron spectra. Dimensions of the ammonium ion can be derived from the structure of the single-crystal spectra. The analysis of the spectra supplies ways to distinguish between cases of rigid, tunnelling or reorienting protons. Single-crystal spectra of partially deuterated ammonium perchlorate measured at 5 K provide examples of the applicability of the theory. Distances between nuclei are derived. Evidence is provided for tunnelling and reorientation of protons in NH₃D⁺ ions. Deuteron spectra indicate that NH₃D⁺ ions exhibit a diverse mobility. There exist ions with immobile deuterons which populate one position in the crystal unit cell (isotopic ordering), while the deuterons in the remaining ions reorient about C₃ axes parallel to the N-D bond orientation of the rigid ones. The contribution of the latter increases with increasing temperature.     

Li and K NMR relaxation study of a LiKSO4 single crystal

The ⁷Li and ³⁹K NMR relaxations in a LiKSO₄ single crystal grown by the slow evaporation method were investigated by employing a pulse NMR spectrometer. From the experimental data, the quadrupole coupling constant and asymmetry parameter were determined at the temperatures of 180 and 300 K. The relaxation processes of ⁷Li and ³⁹K were studied for the LiKSO₄ crystal, and the relaxation times for the ⁷Li and ³⁹K nuclei exhibit remarkable changes near T_c2 (=190 K). The activation energies for ⁷Li and ³⁹K were determined in phases I and III. The large change in the activation energy at 190 K indicates that the Li and K ions are significantly affected during this transition. The correlation time of the ⁷Li calculated from the spin-lattice relaxation time and quadrupole parameters was larger than that of the ³⁹K calculated using the same method. The reason for this is that the Li ion undergoes molecular motion as in the LiO₄ groups.     

Polarization and energy barriers in ferroelectric pyridinium tetrafluoroborate

Measurements of the ¹H and ¹⁹F nuclear magnetic resonance (NMR) second moments were performed for a polycrystalline sample of (PyH)BF₄, whereas the shape of the ²H NMR line was analysed for a polycrystalline sample of (d₅PyH)BF₄. Asymmetry parameter δ has been calculated for four models of pyridinium cation reorientation among inequivalent potential energy minima, using the experimental value of the ¹H NMR second moment as well as the ²H NMR line width. From knowledge of the potential shape and the population of its minima, the temperature dependence of polarization for all the models has been found. From the comparison of the experimentally determined polarization with the calculated polarization, the most appropriate model of the pyridinium cation reorientation has been chosen.     

1H and 13C n.m.r. Conformational Analysis of Adrenergic Drugs. Part 2. N-Isopropyl-N- {3-[4(4-Methoxybenzoylamino)Phenoxy]-2-Hydroxypropyl} Ammonium Chloride,a New β1-Blocking Agent

Abstract

Dynamic and structural features of N-Isopropyl-N- {3-[4(4-Methoxybenzoylamino)Phenoxy]-2-Hydroxypropyl} Ammonium Chloride in [²H₆]DMSO were investigated by measuring ¹³C and ¹H spin-lattice relaxation rates and ¹³C- {¹H} and ¹H- {¹H} n.O.e. Correlation times for main and internal reorientational motions were interpreted in terms of internal rotation around the two planal axes. Selective and double-selective ¹H spin-lattice relaxation rates were measured, wherefrom relevant proton-proton intramolecular distances were calculated. It was shown that the β₁? blocking agent assumes a preferred conformation where extensive intramolecular H-bonding prevents segmental motion along the quaternary ammonium sidechain.     

NMR studies of hydrogen diffusion in hydrogen uranyl phosphate tetrahydrate (HUP)

¹H NMR spin-lattice relaxation times, T₁ (Zeeman) and T_1ϱ (rotating frame) and spin-spin relaxation times, T₂, and ³¹P NMR solid-echoes are reported for phase I and II of hydrogen uranyl phosphate tetrahydrate (HUP) at temperatures in the range 200–323 K. The spectral density functions extracted from the measured relaxation times for phases I and II are consistent with a 2D diffusion mechanism for hydrogen motion. ³¹P second moments determined from the solid-echoes show that all the hydrogens diffuse rapidly in phase I, and that the hydrogen-bond site nearest to the phosphate oxygen is not occupied in phase II. The mechanism for diffusion in phase II is discussed.