87Rb Nuclear Magnetic Resonance Research Articles

87Rb spin-lattice relaxation times in ferroelectric-paraelectric-incommensurate phases of Rb2CoBr4 using static NMR and MAS NMR

To better elucidate the structural properties of Rb2CoBr4 in paraelectric, incommensurate, and ferroelectric phases, we studied the 87Rb nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times in laboratory frame T1 and in rotating frame T1ρ. The resonance frequency and the chemical shift do not change abruptly near the phase transition temperature of Ti = 333 K and TC = 192 K, whereas T1 and T1ρ display discontinuous changes near Ti and TC. The abrupt changes in the relaxation times near these temperatures seem to be a result of the structural phase transitions. The results are distinctly different from those reported for Rb2CoCl4.

Structural properties of mixed (NH4)2−xRbxZnCl4 (x=0, 1, and 2) crystals studied by 1H and 87Rb nuclear magnetic resonance

The structures of mixed (NH4)2−xRbxZnCl4 (x=0, 1, and 2) single crystals were determined by x-ray diffraction. In addition, the nuclear magnetic resonance (NMR) spectra and spin–lattice relaxation times of the 1H and 87Rb nuclei in the three crystals were determined using NMR spectroscopy. We analyzed the crystallographic structures of these crystals by considering the occupation probabilities (NH4 or Rb) of the two cationic sites. Our NMR results confirm that there are two crystallographically inequivalent NH4 sites, NH4(1) and NH4(2), in (NH4)2ZnCl4, and two crystallographically inequivalent Rb sites, Rb(1) and Rb(2), in Rb2ZnCl4. However, only one NH4 site and two Rb sites were observed in NH4RbZnCl4. The NH4 and Rb occupation rates of each of the two available sites, A1 and A2, were determined from the NMR signals.

NMR Study in the Iron-SelenideRb0.74Fe1.6Se2: Determination of the Superconducting Phase as Iron Vacancy-FreeRb0.3Fe2Se2

77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb(0.3(1))Fe2Se2 as the actual composition of the SC fraction. The resulting estimate of 0.15 electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.

Characteristic features of the molecular dynamics and phase transitions of mixed [(NH4)1−xRbx]3H(SO4)2 (x=0, 0.8, and 1) crystals using 1H and 87Rb NMR

Abstract In this study, we grew [(NH4)1−xRbx]3H(SO4)2 (x = 0, 0.8, and 1) mixed crystals using a slow evaporation method and determined their lattice constants by X-ray diffraction. We investigated the molecular dynamics and phase transition temperatures of the crystals using 1H and 87Rb nuclear magnetic resonance (NMR) relaxation and differential scanning calorimetry. When a fraction of rubidium ions in the pure Rb3H(SO4)2 crystal is partial replaced by ammonium ions, we compared the physical properties of the three types of crystals. A small amount of NH 4 + added to Rb3H(SO4)2 led to a change in the physical properties, bringing about a shift in phase transition temperatures and showed a superprotonic phase transition.

Synthesis, Crystal Structure and Chemical Bonding of the Zintl Phase Rb7NaSi8

AbstractAbstract. The Zintl phase Rb7NaSi8 was synthesized as single‐phase material. Powder and single‐crystal X‐ray diffraction, thermal analysis, chemical analysis, measurement of the magnetic susceptibility, and 23Na, 29Si, and 87Rb nuclear magnetic resonance (NMR) spectroscopy were employed to characterize the material. Rb7NaSi8 crystallizes in the Rb7NaGe8 type of structure forming trigonal pyramidal anions Si44–. Two unique environments of the cations are observed, the linear arrangement [Na(Si4)2]7– with short Na–Si distances of 2.94 Å and the Rb2 atom coordinated by six Si44– anions with long Rb–Si distances of 4.09 Å. The environment of the Rb1 atom is similar to the coordination of the rubidium atoms in Rb4Si4. The chemical bonding was investigated by quantum mechanical calculations of the electron localizability indicator (ELI), the electronic density of states (DOS), and NMR coupling parameters. Good agreement of theoretically calculated and experimentally determined NMR coupling parameters was obtained for both chemical shielding and quadrupole coupling. The anisotropy of chemical shielding (CSA) indicates an anisotropic bonding situation of the silicon atoms. This is confirmed by the observation of a lone‐pair‐like feature and three two‐center Si–Si bonds for each silicon atom using the ELI. The electric field gradients (EFG) of 23Na and 87Rb indicate the anisotropy of the charge distribution of the cations. In particular, the linear arrangement [Na(Si4)2]7– and the Rb1 atom feature anisotropic charge distributions of the cations. This is confirmed by calculating the anisotropy ratio defined as Δnp = n(pz)/(½[n(px)+n(py)]) with n(pi) corresponding to the integrated DOS of the respective state. The long distances of Rb2 to the six coordinating Si44– ions are reflected by the small EFG.

Study on the structural properties and relaxation mechanisms in LiRb 1− x(NH 4) xSO 4 ( x=0, 0.5, and 1) mixed crystals by 1H, 7Li, and 87Rb nuclear magnetic resonance

LiRb 1− x (NH 4) x SO 4 ( x=0, 0.5, and 1) mixed crystals were grown using the slow evaporation method, and their lattice constants were determined by X-ray diffraction. The effects of replacing ammonium ions with rubidium on the phase transitions of and dynamics of LiRb 1− x (NH 4) x SO 4 mixed crystals were studied. We investigated the structural properties and phase transition temperatures of the LiRb 1− x (NH 4) x SO 4 mixed crystals by performing 1H, 7Li, and 87Rb NMR relaxation and DSC. When a fraction of rubidium ions in the pure LiRbSO 4 crystal is partially replaced by ammonium ions, we compare the structural properties for LiRbSO 4, LiRb 0.5(NH 4) 0.5SO 4, and LiNH 4SO 4 single crystals. The 1H, 7Li, and 87Rb NMR relaxation mechanisms of LiRb 0.5(NH 4) 0.5SO 4 are different from those of pure LiRbSO 4 and LiNH 4SO 4; the sequence of the structural phase transitions of LiRb 0.5(NH 4) 0.5SO 4 is different, which might be related to the dynamics and orientation of the sulfate tetrahedral groups in the crystal structure.

Relaxation mechanisms of Rb 2Co(SO 4) 2 ⋅ 6H 2O single crystals studied by 1H and 87Rb nuclear magnetic resonance

The spin-lattice relaxation time, T 1 , and the spin–spin relaxation time, T 2 , for 1 H and 87 Rb nuclei in Rb 2Co(SO 4) 2⋅6H 2O crystals were investigated. The phase transitions occur near 370 K ( = T C 1 ) , 402 K ( = T C 2 ) , 413 K ( = T C 3 ) , and 458 K ( = T C 4 ) . The 87 Rb spectrum changes near T C 1 from Rb(1) and Rb(2) central resonance lines to one Rb central resonance line, and this change is due to the structural phase transition. The change in the 1 H T 1 near T C 1 and the changes in T 1 and T 2 for Rb(1) and Rb(2) near T C 1 and T C 2 are also due to the phase transitions, and are related to changes in the symmetry of the octahedra of water molecules and slight rotations of the SO 4 tetrahedra surrounding Rb +. Further, below T C 1 the spin-lattice relaxation rates, T 1 − 1 , for Rb(1) and Rb(2) are proportional to T − 2 , which implies that these relaxations occur mainly via Raman processes.

1H and 87Rb nuclear magnetic resonance study of the order–disorder phase transition of RbHSeO 4 single crystals

The ferroelectric phase transition at T C2 (=370 K) in RbHSeO 4 has been studied by 1H and 87Rb solid-state NMR. Although not large, the spin–lattice relaxation time, T 1, and the spin–spin relaxation time, T 2, of rubidium and of the α- and β-type protons show distinct change near the phase transition. The intensity of the signal due to the α-type protons decreases with increasing temperature, and the intensity of α-type protons is quite weak above 330 K: at a temperature which is about 40 K lower than the phase transition temperature, the ordering of the α-type protons occurs. The α-type protons in the ferroelectric phase lead to a noticeable change in the proton magnetic resonance spectra. Our study of the 1H spectra shows that the ferroelectric phase transition in RbHSeO 4 is of order–disorder type and is due to the ordering of protons in hydrogen bonds.

Ferroelastic to paraelastic phase transition of K3H(SeO4)2 and Rb3H(SeO4)2 single crystals studied by nuclear magnetic resonance and external stress

AbstractThe temperature dependences of the nuclear magnetic resonance (NMR) and the stress–strain hysteresis curves have been measured for K3H(SeO4)2 and Rb3H(SeO4)2 crystals grown using the slow evaporation method. By analysis of the 39K and 87Rb NMR results, it has been confirmed that the phase below TC is ferroelastic with twin domains and that the phase above TC is paraelastic with a single domain. The ferroelastic domain switching that occurs in the two single crystals due to external stress was also studied. The critical stress values for converting the twin‐domain state to the single‐domain state were obtained. When external stresses of 0.6 and 0.4 MPa were applied to the K3H(SeO4)2 and Rb3H(SeO4)2 crystals, respectively, transitions from twin‐domain to single‐domain states were observed. The ferroelastic domain switching resulting from an external stress may be associated with the atomic rearrangements of the SeO4 tetrahedra in K3H(SeO4)2 and Rb3H(SeO4)2 crystals; the ferroelastic domain switching is explained by the restriction of the rotation of the SeO4 tetrahedra as a result of the breaking of all hydrogen bonds by the external stress. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional tetramer-cuprate [formula omitted]:phase transitions and AFM order as seen by 87Rb NMR

We report the 87Rb nuclear magnetic resonance (NMR) results in a recently synthesized Na 5 RbCu 4 ( AsO 4 ) Cl 2 . This complex novel two-dimensional (2D) cuprate is an unique magnetic material, which contains layers of coupled Cu 4 O 4 tetramers. In zero applied magnetic field, it orders antiferromagnetically via a second-order low-entropy phase transition at T N = 15 ( 1 ) K . We characterize the ordered state by 87Rb NMR, and suggest for it a non-collinear rather than collinear arrangement of spins. We discuss the properties of Rb nuclear site and point out the new structural phase transition(s) around 74 and 110 K.

NMR Studies on Crystals with Structurally Incommensurately Modulated Phases

The dynamics of the incommensurate (IC) modulation is investigated for several one-dimensionally incommensurately modulated crystals near the transition to the normal high temperature phase at the temperature T i . Using nuclei with nuclear quadrupole moments as local probe, nuclear magnetic resonance (NMR) line shape and spin-lattice relaxation have shown to be very sensitive to study the order parameter dynamics and slow elementary excitations in crystals with IC phases. All results can be described consistently in terms of a static modulation in the IC phase without any indication for “floating” or large scale fluctuations of the modulation wave. The critical exponents derived from the NMR line shape and the relaxation times T1 very nicely fit to the universality class of the 3d-XY model. Recently, the validity of this conclusion was also confirmed for the particular IC system of bis(4-chlorophenyl)sulphone ((ClC6H4)22SO2). For Rb2ZnBr4 and Rb2ZnCl4 crystals the characteristic frequency of the critical dynamics of the order parameter (OP) slows down to a frequency range which has been estimated to lie between 2 … 10 MHz. In this particular case one can derive the characteristic frequency of the critical dynamics of the OP above T i and of the phason below T i from the Larmor frequency dependence of 87Rb NMR relaxation times T1. The relatively broad temperature range where the critical dynamics was observed may be explained.

87Rb NMR study of the magnetic structure of the quasi-two-dimensional antiferromagnet RbFe(MoO4)2 on a triangular lattice

87Rb nuclear magnetic resonance was experimentally studied in a quasi-two-dimensional Heisenberg antiferromagnet RbFe(MoO4)2. Dipole fields at the 87Rb nuclei were found over a wide range of temperatures and static magnetic fields. Magnetic structures in the ordered phase were determined at various magnetic fields.

X-ray diffraction, opticalbirefringence, and 87Rb nuclear magnetic resonance spectroscopyof the paraelectric and antiferroelectric phases ofRb3DxH1-x(SO4)2

The antiferroelectric (AFE) phase transition ofRb3DxH1-x(SO4)2 was studied using x-raydiffraction, optical birefringence, and nuclear magnetic resonance.The orientation dependence of the resonance lines deduced from thequadrupole-perturbed 87Rb nuclear magnetic resonance ofRb3D(SO4)2 single crystals indicates slightdeviations from the monoclinic symmetry in the paraelectric and theAFE phases. The dynamical critical exponents as deduced frommeasurements of the spin-lattice relaxation times depend on thedeuteron concentration. Additionally, we have carried out x-raysingle-crystal diffraction as well as optical birefringencemeasurements and find clear evidence for a structural phasetransition in Rb3D(SO4)2. The low-temperature spacegroup of this compound is A2.

7Li and 87Rb nuclear magnetic resonance in aLiRbSO4 single crystal with the growth twin domain

The 7Li and 87Rb NMR in a LiRbSO4 single crystal with the growthtwin domain grown by the slow evaporation method were investigated byemploying a Bruker FT NMR spectrometer. Instead of the simple one set ofsignals caused by quadrupole interaction, three sets of NMR signals wereobtained. From these results, all parameters were determined, and all lead tothe same values for the quadrupole coupling constant and asymmetry parameter.The 7Li NMR parameter in the LiRbSO4 crystal was axially symmetric,whereas the 87Rb NMR parameter was non-axially symmetric; the lithium ionis surrounded by oxygen atoms located on a regular tetrahedron, and theresultant coordinating rubidium ion is surrounded by slightly distorted oxygenatoms located at nine regular tetrahedra. The obtained results can beexplained by the existence of three kinds of growth twin domain, rotatedwith respect to each other by 120° around the c-axis.

Mechanisms of intracellular Mg2+ regulation affected by amiloride and ouabain in the guinea-pig taenia caeci.

1. The effects of amiloride and ouabain on the regulation of the intracellular, free Mg2+ concentration ([Mg2+]i) were investigated in the taenia isolated from the guinea-pig caecum, using nuclear magnetic resonance (NMR) techniques. 2. [Mg2+]i were mainly estimated from the separation of the alpha- and beta-ATP peaks observed in 31P NMR spectra. In normal (physiological) and nominally Ca(2+)-free solutions, [Mg2+]i was approximately 0.3-0.4 mM. Application of either amiloride or ouabain in Ca(2+)-free solutions significantly increased [Mg2+]i, with only a small change in ATP content. Washout of the drugs reversed the changes in [Mg2+]i. 3. Changes in pHi were estimated from: (1) the chemical shift of phosphoethanolamine, and (2) solving two relational equations of pHi and [Mg2+]i obtained from the beta- and gamma-ATP peaks. Both estimations revealed some intracellular alkalosis during application of these two drugs. After correction for pHi, a significant increase in [Mg2+]i was still obtained 150 min after application of either drug. 4. In the presence of amiloride, simultaneous removal of extracellular Mg2+ and Ca2+ significantly depleted intracellular Mg2+. This result suggests the presence of an amiloride-insensitive (or less sensitive) pathway which passively transports Mg2+ across the plasma membrane. 5. The intracellular Rb+ concentration was monitored as an index of Na(+)-K+ pump activity, using 87Rb NMR. In Ca(2+)-free solutions containing 5 mM Rb+, the intracellular Rb+ concentration was hardly changed by amiloride, but was depleted by additional applications of ouabain. Wash-out of ouabain restored the intracellular Rb+ in the presence of amiloride. 6. These results are consistent with the presence of Na(+)-Mg2+ exchange as an effective Mg(2+)-extruding mechanism in smooth muscle. Although many other factors may cause changes in [Mg2+]i, it seems likely that amiloride directly inhibits the Na(+)-Mg2+ exchanger, whilst ouabain does so indirectly through reduction of the Na+ gradient across the plasma membrane.

Electronic properties and phase transitions of alkali fullerides: Investigations by nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) measurements on alkali fullerides A x C 60 ( A = K, Rb, Cs; x = 0, 1, 3, 4, 6) are reviewed, with an emphasis on the interpretation of data on the superconducting A 3C 60 compounds. It is argued that 13C NMR spectra and spin-lattice relaxation rates in the A 3C 60 compounds are most strongly affected by electron-nucleus hyperfine couplings of the dipolar, rather than Fermi contact, type and that antiferromagnetic electron spin fluctuations, structural disorder, or a combination of these two factors may influence the measurements substantially. Interpretation of the NMR data in terms of electronic densities of states is therefore not straightforward. 13C, 87Rb and 133Cs NMR measurements on A 1C 60 compounds provide information about the phase transitions and electronic properties of these materials. All A 1C 60 compounds have structural phase transitions in the 270–420 K range. The high temperature phases are paramagnetic insulators. Below their respective phase transitions, Rb 1C 60 and Cs 1C 60 are single-phase compounds with greatly reduced spin susceptibilities, while K 1C 60 is a phase-separated mixture of K 3C 60, and K δC 60 with δ ⪡ 1. 13C spin-lattice relaxation measurements on Rb 4C 60 show non-metallic behavior, but the relaxation rates are anomalously large. Relaxation measurements on A 6C 60 compounds are consistent with electronic band structure calculations, which show them to be diamagnetic insulators.

Evidence for increased in vivo sodium-potassium pump activity and potassium efflux in skeletal muscle of spontaneously hypertensive rats

We have used 87Rb nuclear magnetic resonance spectroscopy (NMR) to study in vivo rubidium kinetics in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls, using rubidium as a marker for potassium. We gave 15 male, 13-week-old SHR, mean +/- s.d. blood pressure 180 +/- 10 mmHg, and 15 age-matched normotensive controls, mean blood pressure 120 +/- 9 mmHg, a daily dose of RbCl (2 mmol/kg intraperitoneally). We made repeated NMR measurements of skeletal muscle rubidium concentrations until steady state was reached. We then withdrew rubidium and made further measurements of rubidium concentrations, at intervals, for up to 1 week after the last injection. We also measured plasma and erythrocyte rubidium concentrations by flame atomic absorption spectroscopy at similar intervals after the withdrawal of rubidium. Rubidium concentrations rose at a faster rate in SHR skeletal muscle, but the steady-state muscle rubidium concentration was the same (45 mmol/l) in both SHR and WKY rats. There was also a threefold increase in the rate of rubidium efflux from both muscle and erythrocytes in SHR. These results are consistent with a marked increase in Na+,K(+)-ATPase activity and an increase in the rate of rubidium efflux in vivo in SHR. The increased rate of rubidium efflux in SHR could represent increased K+ efflux via calcium-activated K+ channels and/or result as part of cell volume regulation secondary to increased Na(+)-H+ antiporter activity.

A non-invasive method of measuring concentrations of rubidium in rat skeletal muscle in vivo by 87Rb nuclear magnetic resonance spectroscopy: implications for the measurement of cation transport activity in vivo

1. We have used n.m.r. spectroscopy to measure rubidium concentrations in the skeletal muscle of live intact rats. Using a 1.9 T superconducting magnet and an ear-phone coil tuned to both protons (1H) and rubidium (87Rb), it was possible to make measurements of both tissue rubidium content and water content, and from these measurements to obtain the rubidium concentration. 2. The n.m.r. estimate of rubidium concentration in muscle in vivo was found to be a constant 31% (SEM 4%) of that estimated by flame atomic absorption spectroscopy in an extract of excised muscle. This is close to the predicted theoretical n.m.r. visibility of 33%. The visibility was constant for muscle rubidium concentrations ranging between 10 and 34 mmol/l. 3. Rubidium concentration measurement by this method is unaffected by variations in sample geometry, sample volume, tissue conductivity, coil tuning and amplifier gain. 4. By using this method to measure changes in tissue rubidium concentration with time in the same animal, it should now be possible to assess the activity of ion transport systems, such as sodium- and potassium-activated adenosine triphosphatase in vivo, by measuring the rates of change of tissue rubidium concentrations during the administration of rubidium salts. 5. This method could also be used to measure the absolute concentration of any n.m.r.-visible nucleus and could be applied to man.

Measurement of cromakalim-induced 87Rb flux in intact cells by NMR

87Rb nuclear magnetic resonance signals from intracellular and extracellular Rb + can be distinguished by the use of shift reagents, thus permitting the simultaneous measurement of intracellular and extracellular Rb concentrations. When smooth muscle cells are suspended in a medium containing Rb + in place of K +, there is a slow influx of Rb +, which is greatly increased by the addition of cromakalim to the medium. The concentration of cromakalim required to stimulate influx is similar to that needed to stimulate 86Rb efflux from isolated tissue.