NMR Line Shifts Research Articles

Structural, thermodynamic, and local probe investigations of the honeycomb material Ag3LiMn2O6

Here we present the structural and magnetic properties of the new honeycomb material ${\mathrm{Ag}}_{3}{\mathrm{LiMn}}_{2}{\mathrm{O}}_{6}$. The system $\mathrm{Ag}[{\mathrm{Li}}_{1/3}{\mathrm{Mn}}_{2/3}]{\mathrm{O}}_{2}$ belongs to a quaternary $3R$-delafossite family and crystallizes in a monoclinic symmetry with space group $C\phantom{\rule{0.16em}{0ex}}2/m$, and the magnetic ${\mathrm{Mn}}^{4+}\phantom{\rule{0.28em}{0ex}}(S=3/2)$ ions form a honeycomb network in the $ab$ plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature ${\ensuremath{\theta}}_{CW}\ensuremath{\sim}\ensuremath{-}51\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48 K, and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. An asymmetric Bragg peak (characteristic of two-dimensional order), seen in neutron diffraction, gains intensity even above the ordering temperature, thus showing the existence of short-range spin correlations. Our electron spin resonance (ESR) experiments corroborate the bulk magnetic data. Additionally, the ESR line broadening upon approaching the ordering temperature ${T}_{\mathrm{N}}$ could be described in terms of a Berezinskii-Kosterlitz-Thouless scenario with ${T}_{\mathrm{KT}}=40(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The $^{7}\mathrm{Li}$ NMR line shift probed as a function of temperature tracks the static susceptibility (${K}_{\mathrm{iso}}$) of magnetically coupled ${\mathrm{Mn}}^{4+}$ ions. The $^{7}\mathrm{Li}$ spin-lattice relaxation rate ($1/{T}_{1}$) exhibits a sharp decrease below about 50 K. A critical divergence is absent at the ordering temperature perhaps because of the filtering out of the antiferromagnetic fluctuations at the Li site, i.e., at the centers of the hexagons in the honeycomb network. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system ${\mathrm{Ag}}_{3}{\mathrm{LiMn}}_{2}{\mathrm{O}}_{6}$. Our ab initio electronic structure calculations suggest that in the $ab$ plane, the nearest-neighbor (NN) exchange interaction is strong and AFM, while the next-NN and the third-NN exchange interactions are FM and AFM, respectively. The interplanar exchange interaction is found to be relatively small. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.

14N Nuclear Magnetic Resonance and Relaxation in the Paramagnetic Region of Uranium Mononitride

The spin susceptibility of a polycrystalline sample of uranium mononitride UN is studied by measuring the 14N NMR line shift, spin–lattice relaxation rates of the nuclear spin, and static magnetic susceptibility in the temperature region of 1.5TN < T < 7TN A joint analysis of the results obtained has revealed the temperature dependence of the characteristic energy of spin fluctuations of the uranium 5f electrons: Γnmr(T) ∝ T0.54(4) close to the dependence Γ(T) ∝ T0.5 characteristic of concentrated Kondo systems above the coherent state formation temperature.

Noncovalent Hydrogen Isotope Effects in Paramagnetic Molecules

Zero-point energies (ZPE) of intermolecular, non-bonded vibrations and isotope effects, induced by noncovalent interactions, are computed for paramagnetic molecules. They appear to be not significant for complexation of HO2 and oxygen with C–H bonds and results to isotope effect, which deviates from unit by 5–10%. However, ZPE and isotope effects in complexes of HO2 and nitroxyl radicals with water are larger and reach 50–70%. The largest effect, about 12, is found for complexation of hydrogen atom with water. Complexation of nitroxyl and peroxy radicals by hydrogen bonds is accompanied by transfer of spin density of unpaired electron from radical to the ligand molecules and induces high field paramagnetic shifts of the ligand NMR lines. It evidences that the spin transfer via intermolecular bonds occurs by mechanism of spin polarization.

Dynamical shift of NMR lines in nanostructured Ga–In–Sn melt

NMR spectrum and recovery of longitudinal nuclear magnetization after inverting pulses were measured for isotopes 69Ga and 71Ga in a liquid eutectic alloy Ga–In–Sn introduced in porous glasses having pores with the sizes of 25 and 7 nm. The measurements were conducted in the fields of 9.4, 11.7, and 17.6 T and compared with those for the bulk melt sample. Differences between the shape and position of the NMR lines in various fields and for different gallium isotopes were revealed in the melt introduced into the porous glass with the pores size of 7 nm. Data obtained for this sample were interpreted based on the theoretical model of dynamic quadrupolar shift, and correlation time of atomic motion was found within this model. Characteristics of the atomic motion calculated based on the dynamic shift model and from the spin relaxation data were shown to agree with each other.

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2 T

The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy’s salt nitroxide radicals was measured in a magnetic field of 9.2 T (corresponding to 260 GHz microwave (MW) and 392 MHz NMR frequencies) in the temperature range of 15–65 °C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of −38 (signal integral) or −81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor.

Influence of size effects on the Knight shift of NMR lines in the gallium-indium alloy

A study is reported of the Knight shift of 71Ga, 69Ga, and 115In NMR lines in a liquid gallium-indium alloy with a composition of 90 at % Ga and 10 at % In, introduced into porous glasses with pore sizes of 5 and 200 nm, relative to the corresponding shifts in the bulk alloy. The measurements have been performed at room temperature. The study has revealed a size-dependent decrease in the Knight shift. A sample with a 5-nm pore size has demonstrated a noticeable difference in the magnitudes of the Knight shift of both gallium isotopes measured in magnetic fields of 9.4 and 17.6 T, which implies a dependence of the electronic susceptibility of the melt on the magnetic field under the nanoconfinement conditions.

The Intercalation of N,N,N′,N′-Tetramethyl-ethane-1,2-diamine (tmeda) into C6Li and C12Li

Liquid N,N,N',N'-tetramethyl-ethane-1,2-diamine (tmeda) was intercalated into preprepared C(6)Li or C(12)Li. X-ray diffraction from the ternary compound indicates an identity period of 11.5 Å. The (13)C NMR line shifts show that tmeda molecules form a screen between the graphene planes and the lithium ions. Small-angle X-ray scattering showed that no higher structural ordering was present but revealed a progressive roughening of the surfaces with successive intercalation of lithium and amine into the graphite galleries.

Ground state and intrinsic susceptibility of the kagome antiferromagnet vesignieite as seen by51V NMR

The intrinsic magnetic susceptibility and local magnetization of the near-kagome quantum magnet vesignieite, Cu${}_{3}$BaV${}_{2}$O${}_{8}$(OH)${}_{2}$, are presented as measured using ${}^{51}$V NMR. The NMR line shift gives an accurate measurement of the intrinsic susceptibility of the kagome sites which closely resembles that of the quantum spin liquid herbertsmithite [A. Olariu et al., Phys. Rev. Lett. 100, 087202 (2008)]. It is therefore surprising that, at ${T}_{C}\ensuremath{\simeq}9$ K, a transition to a heterogeneous ground state is observed. A gradual wipeout of half the NMR intensity indicates a slowly fluctuating spin liquid component, and a detailed analysis of the linewidth reveals the onset of static magnetism at the remaining half of the sites. It is proposed that this transition and unusual ground state originate from a nearby quantum critical point induced by the Dzyaloshinskii-Moriya interaction.

ChemInform Abstract: NMR Study of the Influence of Adsorbed Sulfur on Hydrogen Chemisorption on Supported Ruthenium Catalysts.

Abstract NMR of hydrogen chemisorbed on silica-supported ruthenium catalysts was used to study the dissociation of H 2 S and its effect on subsequent hydrogen adsorption. A mixture of H 2 S/He was used to sulfide the reduced catalyst. The hydrogen coverages resulting from the dissociation of H 2 S as measured by NMR yielded estimates of sulfur coverages in good agreement with the results of X-ray fluorescence analysis of sulfur on the samples. The H 2 S dissociation reaction was found to be enhanced on higher dispersion catalysts suggesting an important role for defect-like sites. At sulfur coverages greater than 0.25 sulfur atoms per surface ruthenium atom, no hydrogen from the dissociation of H 2 S could be detected remaining on the surface. On the presulfided catalysts dosed to saturation with additional hydrogen, the NMR spectra revealed the presence of two species of hydrogen: a mobile species and a less mobile species. At sulfur coverages greater than 0.5 no detectable hydrogen could be adsorbed on the surface when exposed to 30 Torr of H 2 . The ratio of weakly bound hydrogen to the strongly bound hydrogen at saturation coverage decreased with increasing sulfur coverage. The small change in the magnitude of the NMR lineshift of hydrogen-on-metal with sulfur coverage suggested that the adsorbed sulfur does not induce a strong electronic effect on the chemisorption of hydrogen.

O17NMR evidence for vanishing of magnetic polarons in the paramagnetic phase of ceramicCaMnO3

An $^{17}\text{O}$ NMR study of the distribution of the spin and charge densities of a lightly electron-doped ${\text{CaMnO}}_{3\ensuremath{-}x}$ $(x&lt;0.01,\text{ }{T}_{N}=123\text{ }\text{K})$ ceramic in the paramagnetic phase up to $T=670\text{ }\text{K}$ is reported. The isotropic and anisotropic components of the NMR line shift probe selectively the local spin susceptibility of the itinerant $({e}_{g})$ and the localized $({t}_{2g})$ electrons of the Mn neighbors whereas the nuclear quadrupole parameters look at the distribution of the charge density along the Mn-O bond. When approaching ${T}_{N}$, the spin density of the doped electrons becomes inhomogeneously distributed: a separation into slow carriers, forming magnetic polarons, and fast carriers develops few tens of degrees above ${T}_{N}$. The energy barrier corresponding to the change in carriers mobility was estimated to $\ensuremath{\sim}1100\text{ }\text{K}$. The spin and valence charge densities at the oxygen ions were also measured. From the comparison of these data, the ground state of ${\text{CaMnO}}_{3}$ appears as a charge-transfer insulator state.

Parallel NMR Based on Solution Magnetic-Susceptibility Differences. Application to Isotopic Effects on Self-Diffusion

Different susceptibility NMR line shifts can be induced in distinct liquid solutions by dissolving different concentrations of complexes of paramagnetic lanthanide Ln3+ ions. We show how these solutions, put in capillaries, can be simultaneously studied with a standard high-resolution spectrometer. After theoretical justification the method is illustrated by an investigation of the effects of H/D substitution on self-diffusion in heavy water. Non-Stokesian effects are observed.

2H and 47, 49Ti nuclear magnetic resonance in the γ phase of titanium deuterides TiD x

Titanium deuterides TiD1.92, TiD1.98, and TiD2.0 have been studied by 2H and 47, 49Ti NMR in a magnetic field of 7.04 T and a temperature range of 120–500 K. At all temperatures and compositions, the 2H NMR line is a singlet described by the Gaussian function. The contribution of demagnetizing fields to the 2H NMR shift is ∼50 ppm. The titanium NMR spectra for all compositions comprise two signals due to the 47Ti and 49Ti isotopes. The shift between these signals depends on the deuterium content and temperature. The 47, 49Ti NMR line shape, width, and shifts have been considered in the framework of second-order quadrupole effects for a tetragonal lattice distortion and random distribution of vacancies. The Knight shifts σ(2H) and K(47, 49Ti) are a function of temperature with a clearly pronounced singularity at ∼300 K. The contact, orbital, and polarization contributions to the Knight shifts have been estimated from analysis of the temperature dependences of σ(2H) and K(47, 49Ti).

Frustrated magnetism in the quantum Kagome Herbertsmithite ZnCu3(OH)6Cl2 antiferromagnet

The frustration of antiferromagnetic interactions on the kagome lattice of corner sharing triangles, associated to the enhancement of quantum fluctuations for S=1/2 spins, was acknowledged long ago as a keypoint to stabilize novel ground states of magnetic matter. Only very recently, the model compound Herbersmithite, ZnCu3(OH)6Cl2, a structurally perfect kagome antiferromagnet could be synthesized and allows close comparison to theories. Muon spin resonance investigation has demonstrated the absence of any spin freezing at least down to 50 mK, an energy scale 4000 times smaller than the main antiferromagnetic interaction (J ≊ 180 K). The kagome plane susceptibility could be accurately measured from the 17O NMR lineshift. It goes through a broad maximum at T ≊ J/2 at variance with the diverging SQUID susceptibility which can be consistently interpreted as being dominated by a paramagnetic-like defect contribution arising from Zn/Cu intersite mixing. At lower temperatures (T ≊ J/100), the kagome susceptibility is found to saturate to a non zero value in agreement with the absence of a gap in relaxation measurements.

Anisotropic Transferred Hyperfine Interaction in a Heavy-Fermion Compound: Paramagnetic NMR Line Shift in a CeCu6 Single Crystal

We report on single crystal nuclear magnetic resonance spectroscopy for 63Cu at the Cu(5) sites in the heavy-fermion paramagnet CeCu6 for constant external magnetic field strength (7 T) applied along the “hardest” magnetic direction b and varied temperature (5 − 100 K). The transferred field coupling constant α b (5) = + 3.40(31) kOe/μ B is derived from the van Vleck like paramagnetic line shift. These results establish the predominance of anisotropic spin and orbital polarization acting on the Cu sites neighbouring the Ce 4f local moment. Because magnetically induced electric quadrupolar interaction at this Cu site is not unusually enhanced, we conclude that the exchange polarization mechanism at the Ce site is the source of the observed anisotropy. Impact of the substitution of Cu(2) by Au on the Ce3+ single-ion ground state properties and magnetic characteristics of CeCu6-x Au x is discussed.

Protonation equilibria of [formula omitted] complexes possessing electron donating sites on the diimine ligand

The protonation equilibria of [Ru(LL)(CN) 4] 2− complexes, where LL = 4,4′-dicarboxy-2,2′-bipyridine (dcb) and 2,3-bis(2-pyridyl)-pyrazine (bppz), have been investigated by spectrophotometric titration and 13C NMR spectroscopy in the acidic region. Two protonation steps with p K a values of 2.0 and 0.5 and the absorption spectra of protonated species are distinguished for [Ru(bppz)(CN) 4] 2− in water. The 13C NMR spectra unambiguously prove the first protonation step occurring on the free pyridyl nitrogen of bppz. The second protonation step occurring on the cyanide ligand under pH = 2 results in a blue shift of the MLCT absorption maximum. The weakly emitting excited state of [Ru(bppz)(CN) 4] 2− is presumably more acidic than the ground state and a complete quenching of luminescence is observed with a diffusion controlled quenching rate of 4 ± 0.5 × 10 10 s −1. Four separate protonation steps with p K a values of 3.2, 2.2, 1.1, −0.2 and the absorption spectra of the different protonated complex of [Ru(dcb)(CN) 4] 4− have also been determined. The protonation of carboxyl groups gives rise to an upfield shift of 13C NMR lines (6.1 and 4.8 ppm) of the C4 and COO − carbon atoms. The chemical shift of cyanide ligands are also affected by the protonation of carboxyl group and 1.5 ppm upfield shift was measured between pH = 4 and 2. The emission intensity and apparent lifetimes of [Ru(dcb)(CN) 4] 4− decrease to ca. 10% of the original values at pH = 2, while the position of emission maxima shifts from 666 to 715 nm. The change of emission intensity versus pH suggests that the monoprotonated form of [Ru(dcbH)(CN) 4] 3− is a weaker emitter than [Ru(dcbH 2)(CN) 4] 2−.

Orbital physics in sulfur spinels: ordered, liquid and glassy ground states

Measurements of magnetization M(T, H), heat capacity C(T), NMR lineshift K(T) and linewidth Δ(T), neutron scattering S(Q, ω, T) and broadband dielectric spectroscopy ϵ(ω, T) provide experimental evidence of the different orbital ground states in the cubic sulfur spinels under investigation. In all compounds, the tetrahedrally coordinated Jahn–Teller ions Fe2+ are characterized by a degeneracy of the orbital degrees of freedom. Particularly, we found a long-range orbital ordering in polycrystalline (PC) FeCr2S4, and a glassy freezing of the orbital degrees of freedom in FeCr2S4 (single crystals) (SCs). In contrast, FeSc2S4 belongs to the rare class of spin–orbital liquids, where quantum fluctuations accompanying the glassy freezing of the orbitals suppress long-range magnetic order.

Evolution of NaNO2 in porous matrices

NMR and dielectric studies of NaNO2 loaded into an SBA-15 mesoporous matrix are reported. The spin-lattice relaxation rate and the 23Na NMR line shift, as well as the permittivity, were measured within a broad temperature interval including the ferroelectric phase transition in NaNO2. The phase transition temperature of sodium nitrite in as-prepared samples was shown to differ substantially from that characteristic of a bulk crystal. The permittivity grows strongly in the vicinity of the phase transition. Heating a sample causes the properties of NaNO2 embedded in pores to gradually approach those of bulk crystals.

Formula omitted] and [formula omitted] NMR study in copper hydride

We have obtained the 2D and 63Cu NMR spectra of hexagonal copper hydride and deuteride CuH(D)x with x ∼ 0.8 in the temperature range 11–292K. Single, broad, symmetrical and without resolved structure absorption lines were observed for both 2D and 63Cu resonances in the investigated temperature range. The resonances are broadened far beyond the rigid-lattice dipolar width due to quadrupolar interactions. The linewidths are temperature independent and imply a lack of proton (deuteron) self-diffusion processes. No measurable 2D NMR line shift is observed in the whole temperature range investigated. Temperature independent 63Cu chemical shifts (measured with respect to 63Cu signal in CuCl) occur at ∼780ppm, largely reduced from 2350ppm Knight shift value in pure metallic copper.

Transferred hyperfine fields in the heavy-fermion compound CeCu6−Au

Abstract We present temperature-dependent studies of 63 Cu - and 65 Cu -NMR line shifts for CeCu 6−x Au x (x=0,0.1,1) powder samples. More than 20 main lines and quadrupole satellites are resolved. The paramagnetic line-shifts are both to higher and lower frequencies proving the different sign of the coupling constants of the transferred hyperfine fields.

Metal-insulator transition of donor-acceptor-type organic charge-transfer complex(BETS)2(Br2TCNQ):Site-selective NMR measurements

The roles of donor layers and acceptor stacks in the electronic properties of the charge transfer complex, $(\mathrm{BETS}{)}_{2}({\mathrm{Br}}_{2}\mathrm{TCNQ})$ were investigated by means of site-selective NMR measurements. Line broadening of ${}^{77}\mathrm{Se}$ NMR indicating a magnetic ordering on the BETS site was observed at the metal-insulator transition temperature 12 K. This transition is caused by the spin-density-wave formation on the BETS layers. ${}^{13}\mathrm{C}$ NMR on the cyano carbon in the ${\mathrm{Br}}_{2}\mathrm{TCNQ}$ molecule confirms that a spin singlet formation starts to develop on the one dimensional ${\mathrm{Br}}_{2}\mathrm{TCNQ}$ columns below 170 K and its three dimensional ordering appears below 80 K. From the analysis of the NMR line shape and shift, magnetic susceptibility was decomposed into the contributions of the BETS and ${\mathrm{Br}}_{2}\mathrm{TCNQ}$ sites.