1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.     

Fast hydrogen ion diffusion in solid hydrogen uranyl phosphate (HUP)

NMR relaxation time measurements and pulsed field gradient diffusion measurements have been made for ¹H in hydrogen uranyl phosphate (HUP). The results confirm that for T > 274 K HUP is a fast proton conductor exhibiting self-diffusion coefficients greater than 10^-11m²s^-1.     

Proton diffusion in the room-temperature phase of [(NH4)1−xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation in the rotating frame

Proton diffusion in the room-temperature phase (phase II) of [(NH₄)_1?xRb_x]₃H(SO₄)₂ (0≤x≤1) has been studied by means of ¹H spin-lattice relaxation times in the rotating frame, T_1ρ. The ¹H T_1ρ values were measured at 200.13 MHz in the range of 380–490 K. The ammonium protons and the acidic protons have independent T_1ρ values in the higher temperature range of phase II, suggesting that the spin diffusion between the two species is ineffective. The translational diffusion of the acidic protons is the most dominant mechanism to relax both the ammonium protons and the acidic protons in phase II. The ¹H T_1ρ values in phase II are analyzed theoretically and the motional parameters are obtained. The results of NMR well explain the macroscopic proton conductivity.     

Quantitation of phosphorus metabolites in newborn human brain using internal water as reference standard

A new method for noninvasive, in vivo quantitation of cerebral phosphorus (³¹P) metabolites is described. The technique employs point-resolved spectroscopy (PRESS) to obtain both ³¹P-metabolite and proton (¹H) water spectra: brain water is used as an internal concentration reference. Spin-spin relaxation times (T₂s) of cerebral ³¹P metabolites are much longer than the minimum echo time (TE) usable on a spectrometer equipped with actively shielded gradient coils. With short-TE (≈10 ms) ³¹P PRESS, T₂ relaxation is minimal and phase modulation of the nucleotide triphosphate (NTP) multiplets can be accounted for. ¹H water spectra were acquired using several TEs so that extra- and intracellular water signals could be separated from that due to cerebrospinal fluid. Prior calibration of the ³¹P and ¹H spectrometer channels and an assumed brain-water concentration enabled estimations of metabolite concentrations. Using this method, mean ³¹P metabolite concentrations in the brains of eight normal infants of gestational plus postnatal age 34 to 39 wk were: phosphomonoester (PME) 5.6 (SD 0.9); inorganic phosphate 1.4 (0.4); mobile phosphodiester 2.3 (0.6); phosphocreatine 2.9 (0.3); nucleotide triphosphate 2.8 (0.6); and total mobile phosphate 21.4 (2.8) mmol/kg wet.     

Diffusion of ammonium ions in [(NH4)1−xRbx]3H(SO4)2 studied by 1H spin–lattice relaxation in the rotating frame

Translational diffusion of NH₄⁺ ions in [(NH₄)_1?xRb_x]₃H(SO₄)₂ has been evaluated quantitatively by means of ¹H spin–lattice relaxation times in the rotating frame, T_1ρ. In the high-temperature phase (phase I), the mean residence times of NH₄⁺ ions are three or four orders of magnitude larger than those of the acidic protons. In the room-temperature phase (phase II), they are two or three orders of magnitude larger than those of the acidic protons. The transition from phases II to I causes one order of magnitude enhancement in the diffusion of NH₄⁺ ions. The mean residence time of NH₄⁺ ions increases with increase in the Rb content. The similar trend is also observed in phase II.     

NQR application to the study of hydrogen dynamics in hydrogen-bonded molecular dimers

The temperature dependences of ¹H NMR as well as ³⁵Cl NQR spin-lattice relaxation times T ₁ were investigated in order to study the hydrogen transfer dynamics in carboxylic acid dimers in 3,5-dichloro- and 2,6-dichlorobenzoic acids. The asymmetry energy A/ k _B and the activation energy V/ k _B for the hydrogen transfer were estimated to be 240 K and 900 K, and 840 K and 2500 K, respectively, for these compounds. In spite of a large asymmetric potential the quantum nature of hydrogen transfer is recognized in the slope of the temperature dependence of T ₁ on the low-temperature side of the T ₁ minimum. The NQR T ₁ measurements was revealed to be a good probe for the hydrogen transfer dynamics.     

Study of the diffusion of hydrogen in LaNi5+xH6 compounds by 1H NMR relaxation

The diffusion of hydrogen in LaNi_5+xH₆ (x=?0.2, 0.0, 0.2) has been investigated by NMR from 150 to 300 K. High-temperature data of the spin-spin relaxation time T₂ and the rotating frame spin lattice relaxation time T₁? are independent of stoichiometry but the data of the spin lattice relaxation time T₁ and low-temperature T₁? data are not, and they do not fit Torrey's relaxation model.     

Magic-angle-spinning NMR on solid biological systems. Analysis Of the origin of the spectral linewidths

Magic-angle-spinning (MAS) high-power ¹H-decoupled ¹³C and ³¹P NMR has been applied to solid biological materials to obtain information about the mechanisms that determine the spectral linewidths. The line broadening in MAS ³¹P NMR spectra of solid tobacco mosaic virus (TMV) has been investigated by selective saturation and T₂ measurements. About 90 Hz stems from homogeneous effects, whereas the inhomogeneous contribution is approximately 100 Hz. The inhomogeneous line broadening is assigned to macroscopic inhomogeneities in the sample and not to variations in the nucleotide bases along the RNA strand in TMV. It is concluded that sample preparation is of vital importance for obtaining well-resolved spectra. Under optimal preparation techniques the isotropic values of the chemical shift of the different ³¹P sites have been determined to obtain information about the secondary structure of the viral RNA. The chemical shift anisotropy has been determined from the relative intensities of the spinning side bands in the spectra. The chemical shift information is used to make a tentative assignment of the resonance in terms of the three structurally distinguishable phosphate groups in TMV. The origin of the linewidths in MAS NMR has been examined further by ¹³C NMR of approximately 10% ¹³C-enriched coat protein of cowpea chlorotic mottle virus, using selective excitation and saturation techniques, as well as measurements of the relaxation times T_1γ and T₂. The CO resonance in the spectrum is composed of an inhomogeneous and homogeneous part with a total linewidth of 700 Hz. The homogeneous linewidth, contributing with 200 Hz, is found to arise from slow molecular motions in the solid on a millisecond timescale.     

Diffusion and NMR spin lattice relaxation of 1H in α′ TaHx and NbHx

The concentration and temperature dependence of the self diffusion coefficient, D, of H in Group V transition metals Nb and Ta has been measured for the α^' phase. The nuclear magnetic resonance spin lattice relaxation time, T₁, was measured in Ta only. A pulsed field gradient, NMR spin echo technique was utilized to measure D. In both systems, the activation energy increases with hydrogen concentration while the pre-exponential factor is not strongly concentration dependent. The diffusion results are compared with published values of the macroscopic diffusion coefficient, $\text{D}{}^1$, obtained from Gorsky effect measurements. Values of the thermodynamic factor [$\text{(}\text{ρ}\text{kT}\text{)(}\text{?μ}\text{(?ρ)}$] are found for selected ρ and T, where μ is the chemical potential and ρ is the density of hydrogen atoms. These values agree with known determinations of the same factor obtained from the Gorsky effect relaxation strengths, but the agreement with results from solubility measurements is less satisfactory. NMR relaxation is partitioned into conduction electron (T^?1_1e) and dipolar (T^?1_1d) relaxation rates. The observed x dependence of ($\text{D}\text{T}{}_{\mathrm{1d}}$) is inconsistent with random occupancy of tetrahedral sites, and it is suggested that a repulsive interaction exists between H atoms on nearest neighbor sites.     

NMR study of hydrogen diffusion in uranium hydride

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T₂, the activation energy for hydrogen diffusion in β-UH₃ is determined to be $\text{E}{}_{\mathrm{a}}\text{= (19.25 ± 0.4)}\text{kcal}\text{mole}$ and the preexponential factor to be A₀ ≈ 5 × 10¹⁴ Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T₁ data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E_a is the sum of the energies of vacancy formation and barrier height, and that A₀ contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E_b ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v₀ determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about $\text{S}\text{k}{}_{\mathrm{B}}\text{≈3}$. Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH₃, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH₃ is found to be approximately 10⁶ Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.     

NMR in well logging and hydrocarbon exploration

Nuclear magnetic resonance (NMR) has become a versatile tool for the evaluation of underground hydrocarbon reservoirs. Formation attributes such as rock porosity and rock pore size distributions, as well as the relative concentrations of water, oil and gas, can be inferred from subsurface NMR. The hydrogen NMR signal encodes porosity as amplitude, pore sizes as relaxation times and fluid properties as a mixture of relaxation and diffusion rates. The paper describes the basic operating principles for NMR on cable (wireline), NMR on a drill string (logging-while-drilling) and NMR for downhole fluid sampling. The geometry of the borehole requires a magnet that projects its field into the surrounding rock, implying a grossly inhomogeneous field distribution. Experience shows that even under these circumstances, saturation-recovery and Carr-Purcell-Meiboom-Gill sequences can work well and yield meaningfulT ₁ andT ₂ information.     

In vivo relaxation time measurements on a murine tumor model—Prolongation of T1 after photodynamic therapy

RIF tumors implanted on mice feet were investigated for changes in relaxation times (T₁ and T₂) after photodynamic therapy (PDT). Photodynamic therapy was performed using Photofrin II as the photosensitizer and laser light at 630 nm. A home-built proton solenoid coil in the balanced configuration was used to accommodate the tumors, and the relaxation times were measured before, immediately after, and up to several hours after therapy. Several control experiments were performed using the untreated tumors, tumors treated with Photofrin II alone, or tumors treated with laser light alone. Significant increases in T₁s of water protons were observed after PDT treatment. In all experiments, ³¹P spectra were recorded before and after the therapy to study the tumor status and to confirm the onset of PDT. These studies show significant prolongation of T₁s after the PDT treatment. The spin-spin relaxation measurements, on the other hand, did not show such prolongation in T₂ values after PDT treatment.     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

NMR studies of magnetic properties in Heusler-type Mn 3 Si

In order to microscopically investigate the magnetic properties of both paramagnetic and antiferromagnetic phases in Mn₃Si (T _N?=?23 K), the ⁵⁵Mn NMR has been carried out at temperatures between 2.2 K and 300 K. The temperature dependences of the spectrum, Knight shift (or resonance frequency shift) and spin-lattice relaxation time T ₁ of ⁵⁵Mn NMR have been measured. In the paramagnetic phase, only one resonance spectrum can be obtained. The observed spectrum is identified to be a signal corresponding to the Mn(II) site. In the antiferromagnetic phase, two different spectra corresponding to the Mn(I) and Mn(II) sites are found at the resonance frequencies of 145 and 6 MHz, respectively, by the zero field NMR at 4.2 K. From these results, the internal magnetic fields on the ⁵⁵Mn(I) and ⁵⁵Mn(II) nuclei are found to be 13.6 and 0.6 T, respectively. According to the NMR results, the helical structure in incommensurate Mn spin states is better explained compared with the transverse sinusoidal structure.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

A 1H NMR study of the effects of Ni additions on the behavior of hydrogen in β-vanadium hydride

The behavior of hydrogen in the hydrides of vanadium metal containing small amounts of nickel was studied by ¹H NMR spectroscopy. FT spectra and spin-spin and spin-lattice relaxation times for VH_0.77, V_0.95Ni_0.05H_0.73, and V_0.90Ni_0.10H_0.65 were measured at temperatures between 77 and 400 K. On the addition of nickel the number of hydrogen atoms on O₂₂ sites decreases and the superstructure of hydrogen is altered. Different effects of nickel on hydrogen diffusion are observed above and below about 200 K, and, therefore, the mechanism of hydrogen diffusion is assumed to change at this temperature.     

Hydrogen diffusion in β-phase titanium iron hydride

NMR pulse techniques have been used to measure proton relaxation times in high purity β-TiFeH_1.03 between 94K and 413K. Recent crystal structure parameters from neutron diffraction studies of orthorhombic β-TiFeD_x have been explicitly included in the analysis of the T₂ relaxation times to obtain more accurate hydrogen diffusion parameters than were possible from previous NMR studies of β-TiFeH_x. The room temperature (300K) hydrogen diffusion constant in β-TiFeH_x is verified to be about 1.2 ± 0.8 × 10^?12cm²/s, which is 10^?4–10^?5times smaller than in many other metal hydrides such as PdH_x or LaNi₅H_x.     

Proton NMR relaxation study of the CsHSO4 solid acid system

At 141 °C the solid acid CsHSO₄ is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin-spin relaxation time T₂ measurements reported here for CsHSO₄ also reveal substantial increases (factors of 20-30) in the vicinity of the transition temperature. In the temperature range just below the transition (70-136 °C), T₂ increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T₂ results. NMR spin-lattice relaxation T₁ measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T₁'s decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T₁ exhibits a minimum in which the correlation time is found to be ∼2 ns.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.