87Rb NMR Research Articles

High-field QCPMG-MAS NMR of half-integer quadrupolar nuclei with large quadrupole couplings

The quadrupole Carr-Purcell-Meiboom-Gill NMR experiment using magic-angle spinning (QCPMG-MAS) is analysed as a means of determining quadrupolar coupling and anisotropic chemical shielding tensors for half-integer (I > 1/2) quadrupolar nuclei with large quadrupole coupling constants (C Q). This is accomplished by numerical simulations and 87Rb NMR experiments wih Rb2SO4 and Rb2CrO4 using different magnetic fields. It is demonstrated that (i) QCPMG-MAS experiments typically provide a sensitivity gain by more than an order of magnitude relative to quadrupolar-echo MAS experiments, (ii) non-secular second-order terms do not affect the spin evolution appreciably, and (iii) the effect of finite RF pulses needs to be considered when 2ω2 Q/(ω0ωRF) > 0.1, where ωQ = 2πC Q/(4I(2I—1)), ωRF is the RF amplitude, and ω0 the Larmor frequency. Using numerical simulations and iterative fitting the magnitudes and relative orientation of 87Rb quadrupolar coupling and chemical shielding tensors for Rb2SO4 and Rb2CrO4 have been determined.

Temperature dependence of monovalent cation fluxes in isolated rat hearts: a magnetic resonance study

Ion flux studies were performed on Langendorff-perfused rat hearts using 87Rb, 7Li and 23Na NMR at 36, 20 and 10°C, and at constant extracellular pH (7.40). Using 31P NMR, the intracellular pH was estimated and the high energy phosphate content monitored. Compared to 36°C ( k=0.044±0.015 min −1), our measurements showed incomplete Rb + efflux with a dramatically (5-fold) increased rate constant, k, at 20°C, k=0.238±0.080 min −1. 5 μM glibenclamide, a K ATP-channel inhibitor, completely depressed the hypothermia-activated Rb + efflux at this temperature ( k=0.052±0.018 min −1). 7Li NMR efflux studies on KCl-arrested hearts at 20°C also showed an increase (3-fold) in efflux rate constant: k=0.090±0.003 min −1 relative to its value at 36°C. At 10°C, both Rb + and Li + showed efflux rate constants similar to those observed at 36°C, k=0.071±0.016 min −1 and k=0.050±0.005 min −1, respectively, and the washout was complete. 31P NMR at 36, 20 and 10°C indicated cytosolic alkalinization at pH values of 7.05, 7.21 and 7.40, respectively. The ion transport data could be interpreted in terms of a myocyte model allowing for temperature-dependent changes in transport coefficients. The incomplete efflux of Rb + at 20°C may indicate the existence of a mitochondrial Rb +-pool with a very low Rb + permeability for efflux. These findings correlate with previously observed membrane phase transitions in these systems.

Alkali Metal Oxides, Peroxides, and Superoxides: A Multinuclear MAS NMR Study

The many roles of alkali metal−oxygen phases as catalyst components and promoters motivated a survey of the NMR properties of these compounds. Solid state 7Li, 23Na, 39K, 87Rb, and 133Cs magic angle spinning NMR spectra are reported for the following alkali metal oxides, peroxides and superoxides: Li2O, Li2O2, Na2O, Na2O2, NaO2, KO2, Rb2O2, RbO2, Cs2O2, and CsO2. Commercial samples labeled as “Cs2O” proved to be mixtures of Cs2O2 and CsO2. The superoxide (O2-) anion is paramagnetic, and this property leads to highly temperature-dependent chemical shifts for the alkali metal nuclei in all of the superoxides. All four superoxides also show phase transitions between polymorphic forms of cubic and lower symmetry, and these transitions profoundly affect the alkali metal NMR spectra. For CsO2 and RbO2, purely ionic bonding restricts the paramagnetic shift to the through-space dipolar mechanism, and large temperature-dependent shifts in the tetragonal phases vanish upon transition to the cubic phases. 13C in si...

NMR-evidence for absence of floating in structurally incommensurate crystals

The dynamics of the incommensurate (IC) modulation is investigated for several IC systems near the transition to the normal high temperature phase by means of quadrupolar perturbed NMR spectra using first-order effects. For the special case of the IC phase of RB2ZnCl4 also two-dimensional 87Rb NMR exchange spectra are reported. 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 antiferroelectric phase transition in Rb3D(SO4)2 single crystals studied by 87Rb NMR

The antiferroelectric phase transition in Rb3D(SO4)2 single crystals studied by 87Rb NMR

Structure analysis of KxRb1−xX(X = Br, I) mixed crystals by 87 Rb and 39 K NMR and X-ray diffraction methods

Abstract X-ray diffraction profiles indicated that KxRb1−xX(X = Br, I) crystals as prepared by the meltannealing method are a face-centered cubic structure and the method produced a complete solid-solution. The linear relation between the lattice constants and K (or Rb) concentrations of the crystals by X-ray diffraction corresponded fairly well to the result of an eighth unit lattice of NaiK4−i(i = 0–4) as calculated by the energy-gradient optimization method using a semi-empirical MO PM3 program. 87Rb and 39K MAS NMR spectra were broad and the spectral broadening is believed to be attributed to the overlapping peaks of different KiRb14−iI13(i = 0–14) structures in the unit lattice. We emphasize that the high-field shifts of 87Rb and 39K nuclei were observed with increasing the lattice constants of the mixed crystals. The chemical shifts can be explained by considering components which involve second-order quadrupolar coupling tensors.

Double-resonance NMR probes of structural distortions in alkali-metal-fulleride superconductors.

The {sup 87}Rb NMR line shape of the Rb{sub 3}C{sub 60} superconductor contains three distinct peaks: one associated with octahedrally coordinated Rb in the fcc lattice of C{sub 60} molecules and two others, labeled {ital T} and {ital T}{sup {prime}}, both associated with tetrahedrally coordinated Rb. This contrasts with the accepted crystal structure, in which all tetrahedral Rb sites are equivalent. We report multinuclear single and double resonance NMR experiments which probe for effects which could lead to the unexpected splitting, and discuss implications for electronic structure. {copyright} {ital 1996 The American Physical Society.}

Vortex dynamics in Rb3C60 observed by 87Rb and 13C NMR.

The vortex dynamics in Rb{sub 3}C{sub 60} is investigated by {sup 87}Rb and {sup 13}C NMR. It is shown that spin-spin relaxation as well as two-dimensional exchange experiments allow an estimation of the time scale of vortex fluctuations. The effective pinning potential is deduced from the temperature dependence of the NMR parameters. {copyright} {ital 1996 The American Physical Society.}

Proton tunneling and local symmetry of the hydrogen bond in Rb 3H(SO 4) 2

The 87Rb NMR spectrum of polycrystalline Rb 3H(SO 4) 2 indicated that this substance does not possess the center of inversion at the midpoint of the dimeric unit (SO 4)⋯H⋯(SO 4) at and below room temperature in contrast to the result of an X-ray diffraction experiment. The proton spin-lattice relaxation rate showed a hump near 30 K. This phenomenon was attributed to the phonon-assisted tunneling of proton in the asymmetric hydrogen bond in the dimeric unit (SO 4)⋯H⋯(SO 4).

87Rb NMR investigations on binary and ternary fullerene compounds: Rb3C60, K2RbC60 and Rb2CsC60

Results of13C MAS NMR and87Rb NMR measurements on Rb3C60, K2RbC60 and Rb2CsC60 are presented. The87Rb NMR spectra show generally the same characteristics as already reported by other authors. For the ternary compound the degree of site preferential order of the alkali metal ions agrees with the analysis of X-ray absorption data. Furthermore, in the room temperature87Rb NMR spectra of all samples an additional broadened line is detected indicating that at least for the samples prepared from sublimed C60 a considerable amount of the Rb ions is located on sites with symmetry lower than cubic.

NMR studies of alkali fullerides: Rb 1C 60 and Cs 1C 60

We report on 13C and 87Rb NMR measurements on Rb 1C 60 rocksalt, polymerized and quenched phases, as well as on polymerized Cs 1C 60 at room temperature. We present static and high-resolution MAS NMR signatures of these phases in various conditions, and we discuss the different line positions. The study of the temperature dependence of 87Rb NMR on Rb 1C 60 has evidenced the high stability in air of all the Rb 1C 60 phases.

87Rb, 85Rb, and 39K NMR Studies of Alkalides, Electrides, and Related Compounds

Alkali metal anions generally have narrow NMR lines that are only slightly shifted from those calculated for the gaseous anion. When, however, the anions form contact dimers, as in K+(cryptand[2.2.2])K- and Rb+(cryptand[2.2.2])Rb-, or chains, as in Rb+(18-crown-6)Rb-, the NMR peak of the alkali metal anion is significantly broadened and shifted paramagnetically. In order to obtain reliable quadrupole coupling constants, asymmetry parameters, and chemical shifts from polycrystalline samples of alkalides and electrides, spin−echo 87Rb, 85Rb, and 39K static NMR spectra were obtained for 9 alkalides, 3 electrides, and 11 model compounds. A puzzling distortion of the powder line shapes of two alkalides, an electride, and a model salt was traced to orientation-dependent transverse relaxation times (T2) by studies of the single-crystal 87Rb NMR spectra of Rb+(cryptand[2.2.2])Cl-. To analyze the NMR powder patterns, a general computer program was developed and coupled to a nonlinear least-squares fitting program.

The role of Na+/K+ ATPase activity during low flow ischemia in preventing myocardial injury: A 31P, 23Na and 87Rb NMR spectroscopic study

An increase in intracellular Na+ during ischaemia has been associated with myocardial injury. In this study, we determined whether inhibition of Na+/K+ ATPase activity contributes to this increase and whether Na+/K+ ATPase activity can be maintained by provision of glucose to perfused rat hearts during low flow, 0.5 ml/min, ischemia. We used 31P NMR spectroscopy to determine changes in myocardial energetics and intracellular and extracellular volumes. 23Na NMR spectroscopy, with DyTTHA3- present as a shift reagent, was used to measure changes in intracellular Na+ and 87Rb NMR spectroscopy was used to estimate Na+/K+ ATPase activity from Rb+ influx rates, Rb+ being an NMR-sensitive congener of K+. In hearts provided with 11 mM glucose throughout ischemia, glycolysis continued and ATP was twofold higher than in hearts without glucose. In the glucose-hearts, Rb+ influx rate was threefold higher, intracellular Na+ was fivefold lower at the end of ischemia and functional recovery during reperfusion was twofold higher. We propose that continuation of glycolysis throughout low flow ischemia allowed maintenance of sufficient Na+/K+ ATPase activity to prevent the increase in intracellular Na+ that would otherwise have led to myocardial injury.

Rb NMR study of the electronic properties of Rb 1C 60

87Rb (and 85Rb) NMR spectra in the orthorhombic phase ( (o) phase) of Rb 1C 60 show up broadening, with respect to the high-T cubic phase and to the quenched phase, which signals change of symmetry of the Rb site. The spin lattice relaxation rate follows a linear T dependence in the (o) phase above 100 K, suggesting that this phase is metallic. The increase of 1/T 1T at low T seems to be correlated with the transition seen by E.S.R.

Relationships Between Cytosolic [ATP], [ATP]/[ADP] and Ionic Fluxes in the Perfused Rat Heart: A 31P, 23Na and 87Rb NMR Study

In order to assess the relationship between cytosolic [ATP] or [ATP]/[ADP] and the intracellular Na + concentration ([Na +] i), we have used the phosphate trap 2-deoxy-D-glucose (DG) to alter the high energy phosphate levels in rat cardiomyocytes. Pyruvate-perfused rat heart were treated with 2 mM DG in the presence of 10 IU/l of insulin for 28 min, followed by perfusion with DG without insulin for 60 min. The DG+insulin treatment resulted in dramatic changes in the 31P NMR spectra: phosphocreatine (PCr) and total ATP decreased (to 15 and 35%, respectively) and deoxyglucose-6-phosphate accumulated, with the little change in either inorganic phosphate or intracellular pH. These changes corresponded to a decrease in cytoplasmic [ATP] (from 7.6 to 1.8 mM). [ATP]/[ADP] (from 494 to 24) and ATP affinity [A(ATP), by 8.9kJ/mol] and an increase in [ADP] (five-fold) and free [Mg 2+] (two-fold). Subsequent perfusion with DG-insulin resulted in slow recovery of PCr. [ATP],[ATP] and A(ADP) such that the "low energy" state lasted an additional 16 min during which ATP remained low and constant. There were no detectable changes in the intracellular Na + content as assessed by shift reagent-aided 23Na NMR at the end of DG+insulin treatment (98 ± 18%, 28-36 min of the protocol). In addition, there was no change in the Rb + influx rate as measured by 87Rb NMR at the beginning of insulin washout which was achieved by replacing 20% of the KCl with RbCl ([K +] = 3.7 mM,[Rb +] = 0.94 mM). During DG+insulin treatment the pressure-rate product (PRP) decreased by half and was restored upon insulin washout to 80% of its initial value both in the presence and in the absence of the shift reagent [5 mM Dy (triethylenetetraminehexaacetate) 3-]. These data imply that unfavorable thermodynamic [low A(ATP)] and kinetic (low [ATP] and [ATP]/[ATP]) conditions induced by DG treatment do not inhibit Na +, K + -ATPase activity. We speculate that during anoxia when changes in [ATP]/[ADP] are comparable to those induced by DG treatment, the observed increase in [Na +] i is not due to inhibition of the Na + pump by reduced [ATP] or [ATP]/[ADP].

Rapid and efficient synthesis of alkali metal-C60 compounds in liquid ammonia

The reaction of stoichiometric amounts of alkali metals with C[sub 60] in liquid ammonia provides a rapid and quantitative route to M[sub x]C[sub 60] superconductors (M[sub x] = K[sub 3]/, Rb[sub 3]/, CsRb[sub 2], RbCs[sub 2], KRbCs). Annealing of the samples for 24-48 h at 375[degrees]C is required to obtain large superconducting fractions. [sup 13]C and [sup 87]Rb NMR line shapes are reported for Rb[sub 3]C[sub 60]. The [sup 13]C line shapes show the degree of rotational motion of the C[sub 60] ions varies considerably from sample to sample and is dependent on the method of preparation and subsequent heat treatment. A correlation between the degree of rotational motion and the superconducting fraction is noted and attributed to the amount of disorder in the sample. [sup 87]Rb NMR shows three peaks indicating that Rb[sub 3]C[sub 60] actually contains three different cation sites, rather than the two suggested by X-ray powder diffraction data. The third site is occupied by approximately 15% of the Rb ions in Rb[sub 3]C[sub 60] and is shown to be a subset of the tetrahedral sites in the cubic close-packed C[sub 60] lattice. Although the exact nature of the distortion required to produce the third site ismore » unknown, it appears to be unrelated to the orientations of the C[sub 60] ions creating the site. 18 refs., 2 figs.« less

87,85Rb NMR spectra and relaxation in Rb 3C 60

85,87Rb NMR spectra and relaxation are reported in the temperature range 4–300 K, for samples of C 60 intercalated with Rb through a direct-reaction technique at the nominal composition Rb 3C 60. The NMR spectrum is characteristic of the second-order quadrupole powder distribution of the central line, indicating a local non-cubic symmetry at the Rb site and a very small Knight shift. The relaxation rate is found to follow a T 2 law above 30 K. This result, together with the ratio of the relaxation for the two Rb isotopes, points towards a quadrupole relaxation mechanism by phonons. Two contributions were found in the nuclear magnetization recovery which have been attributed to the Rb nuclei in the tetrahedral site and in the octahedral site, respectively, with the nucleus in the T-site coupled more strongly to the lattice dynamics. No detectable change in the NMR spectrum and/or relaxation rates was found at the superconducting transition located at T c = 27.5 K.

Structural distortion in Rb3C60 revealed by 87Rb NMR

INTEREST in the alkali-intercalated AxC60 fullerides (where A is an alkali metal and x = 2, 3, 4, 6)1,2 has been greatly enhanced by the discovery of a family of superconducting phases for x = 3 with transition temperatures (Tcs) that are exceeded only by the copper oxide materials3–9. An early structure refinement5 identified K3C60 as a face-centred-cubic (f.c.c.) intercalation phase with two tetra-hedral (T) and one octahedral (O) potassium ion per C60. The other superconducting A3C60 systems are isostructural7. Here we report 87Rb NMR studies of Rb3C60 which show the expected T: O ratio of 2:1 at 440 K. Below 370 K, however, an additional resonance appears; spin-echo double-resonance NMR experiments11,12 show that this arises from a second type of tetrahedral Rb+ site (T′). Thus the bulk of this material has a local symmetry lower than that of the f.c.c. structure derived from X-ray refinements. Models that could account for this include a rotational orientation of C60 that places a C5 ring adjacent to the T′ Rb+ ions. Alternatively, the symmetry lowering might be due to a Jahn-Teller distortion on C3−60, carrier-density modulation or Rb+ clustering. Each of these models could lead to a degree of charge localization and consequent reduction in the density of states and hence in Tc.

Sol-Gel transition of κ-Carrageenan as viewed through NMR

Abstract 23Na, 87Rb, and 133Cs NMR spectra were observed to study an interaction between these cations and κ-carrageenan. It was found that the NMR intensity of gel-forming cations undergoes a remarkable change in the vicinity of the sol-gel transition temperature, while the non-gel-forming cation which coexists together with a gel-forming cation does not show any change. These results suggest that gel-forming cations selectively interact with κ-carrageenan. Motion of water molecules in the gel and the solution of κ-carrageenan was studied by measuring the water proton spin-spin relaxation time. The results indicate three different types of water molecule: a free bulk water molecule, a water molecule strongly bound to κ-carrageenan, and a water molecule weakly bound to the gel network.

Rubidium-87 dynamic-angle spinning NMR spectroscopy of inorganic rubidium salts

Five inorganic rubidium salts, RbCl, RbClO{sub 4}, Rb{sub 2}SO{sub 4}, Rb{sub 2}CrO{sub 4}, and RbNO{sub 3}, were examined using {sup 87}Rb NMR spectroscopy. Significant line narrowing occurs under dynamic-angle spinning (DAS) compared to magic-angle spinning (MAS) or variable-angle spinning (VAS). From DAS spectra acquired at various magnetic field strengths, isotropic chemical shifts and isotropic second-order quadrupolar shifts were obtained. By simulations of single-site line shapes obtained from MAS-detected DAS experiments, complete chemical shift and quadrupolar parameters were determined for each of the three crystallographically distinct rubidium sites in RbNO{sub 3}. 28 refs., 6 figs., 4 tabs.