Nuclear magnetic resonance study of Li and H diffusion in the high-temperature solid phase of LiBH4

In order to study the atomic jump motions in the high-temperature solid phase of LiBH₄, we have measured the ¹H and ¹¹B nuclear magnetic resonance (NMR) spectra and the ¹H, ⁷Li and ¹¹B spin–lattice relaxation rates in this compound over the resonance frequency range of 14–34.4 MHz. In the temperature range 384–500 K, all the spin–lattice relaxation data are satisfactorily described in terms of a thermally activated jump motion of Li ions with the pre-exponential factor τ₀=1.1×10⁻¹⁵ s and the activation energy E_a=0.56 eV. The observed frequency dependences of the spin–lattice relaxation rates in this temperature range exclude a presence of any distributions of the Li jump rate or any other jump processes on the frequency scale of 10⁷–10¹⁰ s⁻¹. The strong narrowing of the ¹H and ¹¹B NMR lines above 440 K is consistent with the onset of diffusive motion of the BH₄ tetrahedra.     

Magnetic properties of RY2Ni9 compounds and their hydrides (R=La, Ce)

The magnetic properties of the RY₂Ni₉ ternary alloys (R=La, Ce) and their hydrides have been investigated. Both LaY₂Ni₉ and CeY₂Ni₉ are ferromagnets with T_C=15 and 92 K, respectively. Upon H absorption LaY₂Ni₉H₁₂ becomes a Pauli paramagnet, whereas CeY₂Ni₉H₈ remains ferromagnetic with an additional magnetic contribution of trivalent Ce atoms belonging to the MgZn₂ units occupied by hydrogen atoms.     

Structural properties of two deuterides LaY2Ni9D12.8 and CeY2Ni9D7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy

The crystal structure of two deuterides RY₂Ni₉D_x (R=La; x=12.8 and R=Ce; x=7.7) have been investigated by means of neutron powder diffraction and X-ray absorption spectroscopy. The structures are best described in the space group . The deuterium location has been determined for both compounds. The nature and the occupancy factors of the different D sites are presented. Comparisons are made between the La-based deuteride and the Ce-one in relation with the crystal structure of the intermetallic compounds. Differences in site occupancies within the RM₂ and RM₅ building units of the PuNi₃-type structure are discussed and heterogeneous mixed valence state is reported for the cerium compound.     

Nuclear magnetic resonance of paramagnetic metalloporphyrins

The present article reviews and discusses proton magnetic resonance studies on metalloporphyrins which provide good models for heme proteins in their physical and chemical properties. Emphasis is given on the discussion of the₁Hnmr work done in our laboratory.     

The anti-perovskite type hydride InPd3H0.89

Hydrogenation of tetragonal InPd₃ in the ZrAl₃ type structure (four-fold ccp superstructure) yields a hydride with a cubic AuCu₃ type structure (one-fold ccp superstructure). Deuterium can be located by neutron powder diffraction in octahedral voids surrounded exclusively by palladium, [Pd₆], which are 88.5(6)% occupied in a statistical manner. The resulting deuteride InPd₃D_0.89 thus crystallizes in a cubic anti-perovskite type structure (space group Pm3¯m (no. 221), a=402.25(1) pm at 299(2) K). The Pd-D distance of 201.13(1) pm is typical for interstitial hydrides with palladium. Inelastic neutron scattering on the hydride InPd₃H_0.89, which shows a spectrum similar to that of binary palladium hydride, confirms the cubic site symmetry of hydrogen in [Pd₆] interstices. This is also confirmed by the absence of any quadrupole splitting in the ²D-NMR signal of the deuteride. ¹H NMR spectra of InPd₃H_0.89 do not show any motional narrowing. Values found for the H jump rate τ⁻¹ in InPd₃H_0.89 remain below 10⁶ s⁻¹ in the studied temperature range 28-360 K, indicating a small hydrogen mobility in InPd₃H_0.8 as compared with binary palladium hydride, PdH_≤1. This can be attributed to the large spatial separation of the [Pd₆] sites.     

Influence of hydrogen on the magnetic and electrical properties of GdI2Hx (0?x?1)

We have studied the effect of hydrogen insertion in GdI₂ on its magnetic and electrical properties. The ferromagnetic layered metal GdI₂ reversibly absorbs hydrogen with the formation of GdI₂H_x which exhibits a range of homogeneity 0?x?1. These phases crystallize with the hexagonal MoS₂-type heavy atom arrangement. Variation of the hydrogen content is accompanied by a monotonic change in the lattice parameters, and a decreases from 4.074(1) to 4.023(1) Å whereas c increases from 15.050(5) to 15.394(5) Å for x=0-0.97. The molar volume passes through a maximum at x≈0.35. Magnetization and resistance measurements reveal a substantial change in the physical properties of GdI₂H_x as a function of the hydrogen content, particularly as x approaches a critical value of ∼1/3. The Curie temperature rapidly decreases with increasing hydrogen content, and the long-range magnetic ordering is destroyed for x>0.33. The phases GdI₂H_x with 0.42?x?0.69 exhibit characteristics of a spin-glass below the magnetic freezing temperatures T_f=24 and 3 K for x=0.42 and 0.69, respectively. The hydride halides GdI₂H_x (x>0.19) show thermally activated conduction, accompanied by a steep increase of the activation energy at x≈0.33. The anomalies observed can be understood by assuming an ordering of the hydrogen atoms for x=1/3 within each layer.     

Nuclear magnetic resonance spectroscopic study on ionic liquids of 1-alkyl-3-methylimidazolium salts

Chemical shifts of ¹H and ¹³C NMR of series of methylimidazolium salts (MIM⁺, X=Br⁻, BF₄⁻ and PF₆⁻) function on the length of alkyl groups on the ring, type of solvents and the concentration. The bromides series demonstrate more chemical shift variation on H2 upon the change of solvents and concentration. Unexpected H-D exchange reactions were also observed in the MIM⁺Br⁻ by using CD₃OD and D₂O. The exchange rates strongly depend on the length of the alkyl group, which could cause more steric factor to reduce the interaction between deuterium atom from solvent and C2 of the ring.     

Analysis of glycans in glycoproteins by diffusion-ordered nuclear magnetic resonance spectroscopy

Enzymatically cleaved glycans from sub-milligram quantities of erythropoietin (EPO) and ovalbumin have been analyzed, without further purification, by two-dimensional diffusion-ordered nuclear magnetic resonance spectroscopy. At NMR sample concentrations below 50 μmol L⁻¹ the major components of the oligosaccharide fractions could be distinguished by their anomeric proton chemical shift and their size-dependent diffusion coefficients. Figure ¹H NMR diffusion decay curves of anomeric protons in the EPO glycan fraction     

Nuclear magnetic resonance metabonomic profiling using tO2PLS

Blood plasma collected from adult fish (black bream, Sparidae) exposed to a dose of 5 mg kg⁻¹ 17β-estradiol underwent metabonomic profiling using nuclear magnetic resonance (NMR). An extension of the orthogonal 2 projection to latent structure (O2PLS) analysis, tO2PLS, was proposed and utilized to classify changes between the control and experimental metabolic profiles. As a bidirectional modeling tool, O2PLS examines the (variable) commonality between two different data blocks, and extracts the joint correlations as well as the unique variations present within each data block. tO2PLS is a proposed matrix transposition of O2PLS to allow for commonality between experiments (spectral profiles) to be observed, rather than between sample variables. tO2PLS analysis highlighted two potential biomarkers, trimethylamine-N-oxide (TMAO) and choline, that distinguish between control and 17β-estradiol exposed fish. This study presents an alternative way of examining spectroscopic (metabolite) data, providing a method for the visual assessment of similarities and differences between control and experimental spectral features in large data sets.     

Diffusion of hydrogen isotopes and their mutual perturbation in Ti0.33V0.67HxDy (x+y≈0.9) studied by H and H NMR

Ti_0.33V_0.67H_xD_y (x+y≈0.9) alloys have been studied by means of X-ray powder diffraction (XRD), differential scanning calorimetry (DSC) and ¹H and ²H NMR. The crystal structures are body-centered-cubic (bcc) dominantly, being mixed with a face-centered-cubic (fcc) phase. A phase transition similar to that from the δ_D phase to the α_D phase in the V-D system is observed in all the samples except for the protide. H and D are considered to occupy tetrahedral sites. The temperature and frequency dependence of spin-lattice relaxation times T₁ of ¹H and ²H has been analyzed by Bloembergen-Purcell-Pound equations with a distribution of correlation times, and parameters of hydrogen diffusion are estimated. The mean activation energy for D diffusion (E_D) is higher than that for H diffusion (E_H). E_H is constant while E_D increases slightly with the [D]/[H] ratio. The distribution of the correlation times increases as the [D]/[H] ratio decreases.     

Crystal chemistry and thermodynamic properties of anisotropic Ce2Ni7H4.7 hydride

A new intermetallic deuteride Ce₂Ni₇D_4.7 with an anomalous volume expansion has been studied. Its structure was solved on the basis of in situ neutron diffraction data. Expansion proceeds along the c-axis and within the CeNi₂ slabs only. All D atoms are located inside these slabs and on the border between CeNi₂ and CeNi₅. Ordering of D atoms in the bulk of CeNi₂ is accompanied by substantial deformation of these slabs thus lowering the hexagonal symmetry to orthorhombic [space group Pmcn (No. 62); a=4.9251(3) Å, b=8.4933(4) Å, c=29.773(1) Å]. Inside the CeNi₂ layer the hydrogen sublattice is completely ordered; all D-D distances exceed 2.0 Å. Local coordination of Ni by D inside the CeNi₂ blocks is of “open”, saddle-like type. Hydrogen ordering is mainly determined by Ce-H and H-H interactions. The pressure-composition-temperature measurements yielded the following thermodynamic parameters of the formation of the hydride: ΔH=−22.4 kJ/mol_H, ΔS=−59.9 J/(K mol_H).     

Deuterium diffusion in vanadium deuterides (VDx; 0.4?x?0.6) studied by H NMR

Sites and dynamics of deuterium in VD_0.50 as well as in VD_0.40 and VD_0.57 have been studied by means of X-ray diffraction, differential scanning calorimetry and ²H NMR. VD_0.40 consists of the body-centered-cubic (bcc) α_D and body-centered-tetragonal (bct) β_D phases at room temperature and only the octahedral (O) sites are occupied in the β_D phase. On the other hand, in VD_0.50, consisting of the bctβ_D phase at room temperature, the O site is dominantly occupied with a small occupancy of the tetrahedral (T) sites in the β_D phase. VD_0.57 is composed of the bccα′_D (a high-deuterium-concentration phase of the α_D phase) and bctβ_D phases at room temperature, and the occupancy of the T sites is observed. The mean activation energy (E_D) of the deuterium diffusion is much larger for the O sites than for the T sites, and the diffusion is also much slower in the O sites. The mean E_D values for the O sites are almost the same in the β_D phase of VD_0.40 and VD_0.50, although the distribution of the mean residence time is much larger in VD_0.50 than in VD_0.40.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

Nuclear magnetic resonance relaxation studies of polyacrylamide solution

 The cohesive interaction among polymer chains in a polyacrylamide (PAAm)–D₂O solution has been studied by NMR relaxation. The NMR relaxation times of PAAm in the good solvent D₂O were measured at different temperatures. The results show that the solution system has a high local viscosity and that its relaxation characteristic is soft-solid-like. The temperature dependence of the relaxation behavior of the solution is obviously different from that of ordinary polymer solutions. The difference lies in the relaxation behavior of the methylene protons in the main chain of PAAm, as shown by analyzing the relaxation process with single exponential and biexponential decays. As the temperature increases, the solvation is weakened, leading polymer chains to form curling coils, thus hindering the movement of the methylene protons among the main chains. It can be expected from the existence of 80% fast-relaxing protons that there are a zhigh number of entanglements among the polymer chains in PAAm solution. The information about entanglements among the polymer chains can be deduced from the biexponential dependence of the spin–spin relaxation on the concentration of the polymer solutions. Received: 14 April 1999/Accepted in revised form: 12 October 1999     

Crystal structure and hydrogenation properties of pseudo-binary Mg6Pd0.5Ni0.5 complex metallic alloy

The crystal structure of the Ni-substituted Mg_6.10(2)Pd_0.52(2)Ni_0.41(2) complex metallic alloy has been determined by X-ray and neutron powder diffraction. The reaction of this compound at 573 K towards deuterium absorption for pressures up to 23 bar has also been studied. The crystal structure of Mg_6.10(2)Pd_0.52(2)Ni_0.41(2) compound was determined in the light of Samson's [Acta Crystallogr. B 28 (1972) 936) and Makongo's (Philos. Mag. 86 (2006) 427] models for the binary Mg₆Pd compound. It crystallizes in space group with lattice parameter 20.13331(7) Å. The refined unit-cell composition is Mg₃₄₂₍₁₎Pd₂₉₍₁₎Ni₂₃₍₁₎ with Z=56. Nickel by palladium substitution is not fully random. Nickel atoms preferentially locate on Pd sites with low coordination number due to steric effects. Deuterium uptake is 9.6 D/f.u. under the given conditions of pressure and temperature. Upon absorption, the intermetallic compound disproportionates into MgD₂, Mg₅Pd₂ and Mg₂NiD₄ phases. The Mg₂NiD₄ phase is observed to crystallize in the orthorhombic LT2 modification for which an averaged crystal structure in the Pcc2 space group is proposed.     

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7-H(D)2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)₂Ni₇ alloys. Substitution of La in La₂Ni₇ by Mg to form La_1.5Mg_0.5Ni₇ preserves the initial Ce₂Ni₇ type of the hexagonal P6₃/mmc structure and leads to contraction of the unit cell. The system La_1.5Mg_0.5Ni₇-H₂ (D₂) was studied using in situ synchrotron X-ray and neutron powder diffraction in H₂/D₂ gas and pressure-composition-temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi₄ slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La₂Ni₇, the unit cell of La_1.5Mg_0.5Ni₇D_9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δa/a=7.37%) and along [001] (Δc/c=9.67%). In contrast with La₂Ni₇D_6.5 where only LaNi₂ layers absorb hydrogen atoms, in La_1.5Mg_0.5Ni₇D_9.1 both LaNi₅ and LaMgNi₄ layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg)₂Ni₂, (La,Mg)Ni₃, Ni₄, tetragonal pyramidal La₂Ni₃ and trigonal bipyramidal (La,Mg)₃Ni₂ interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD₆ and a 16-vertex polyhedron LaD₁₆ around La. The interatomic distances are in the following ranges: La-D (2.28-2.71), Mg-D (2.02-2.08), Ni-D (1.48-1.86 Å). All D-D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the ΔH and ΔS of hydrogenation/decomposition; ΔH_H=−15.7±0.9 kJ (mol_H)⁻¹ and ΔS_H=−46.0±3.7 J (K mol_H)⁻¹ for H₂ absorption, and ΔH_H=16.8±0.4 kJ (mol_H)⁻¹ and ΔS_H=48.1±1.5 J (K mol_H)⁻¹ for H₂ desorption.     

Characterisation of a synthesised fluorescent ligand (4-acridinol-1-sulphonic acid) using nuclear magnetic resonance spectroscopy

The synthesis and complete characterisation of the fluorescent ligand, 4-acridinol-1-sulphonic acid (the acridine analogue of 8-quinolinol-5-sulfonic acid) is described. Using a judicious array of nuclear magnetic resonance spectroscopy experiments, the structural elucidation and full assignment of all proton and carbon chemical shifts were afforded. The 4-acridinol-1-sulphonic acid was found to behave in a similar manner to 8-quinolinol-5-sulphonic acid, forming fluorescent complexes with magnesium(II) and zinc(II). The uncorrected emission maxima for the metal-acridinol complexes were found to be at around 620 nm compared to 505 nm for the respective quinolinol complexes. Unfortunately, preliminary spectrofluorimetric analytical figures of merit revealed that the detection limits of the new acridinol metal complexes were one and a half orders of magnitude poorer than those attained with the corresponding quinolinol ligand. However, in contrast to 8-quinolinol-5-sulphonic acid, the 4-acridinol-1-sulphonic acid ligand showed considerable selectivity for magnesium(II) and zinc(II) over aluminium(III).     

Nuclear magnetic resonance approaches to the rationalization of chromatographic enantiorecognition processes

NMR spectroscopy represents a valuable tool for obtaining information about structure and dynamics at a molecular level on the diastereoisomeric complexes formed by enantiomeric substrates and chromatographic chiral selectors or modifiers. Some examples collected from the literature show the potentialities of solution NMR spectroscopy in the rationalization of chromatographic enantiorecognition processes and the different NMR approaches needed according to the chiral selector features.     

Computer-aided spectral assignment in nuclear magnetic resonance spectroscopy

The proton-carbon correlation spectra, HMBC (heteronuclear multiple bond correlation) and HMQC (heteronuclear multiple quantum correlation), respectively, provide direct and remote connectivity information with high sensitivity. Their combination enables carbon-carbon proximity relationships to be deduced, which are formally identical to those produced by a fictitious INADEQUATE-2D experiment, where correlations would be established exclusively between atoms linked by one or two bonds. The CASA program uses these relationships, as well as DEPT spectra and elementary chemical-shift considerations to assign the ¹³C spectrum of a compound if its structure is known or assumed. If the structure conflicts with the experimental data, no assignment is produced. The CASA program serves as an aid to either spectral assignment or structural elucidation.     

Doping studies of the magnetic cobaltocuprate CoSr2Y2−xCexCu2O9±δ

A structural, magnetic and electronic study of the cobaltocuprate CoSr₂Y_2−xCe_xCu₂O_9±δ (x=0.5-0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO₄ tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5-0.8; T_M increases with x. A change in the dimensionality of the magnetic order occurs at x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co³⁺ is partially oxidised to Co⁴⁺ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.