Temperature Dependence of <Superscript>1</Superscript>H and <Superscript>17</Superscript>O NMR Shifts of Water: Entropy Effect

Density functional theory together with statistical thermodynamics based on the equilibrium constants method and concept of orientational entropy were applied to reproduce the temperature dependences of ¹H and ¹⁷O nuclear magnetic resonance (NMR) chemical shifts in liquid water. Despite a rather simple theoretical model, a satisfactory agreement between calculated NMR quantities and corresponding experimental data was found. By using only a single adjustable parameter of arbitrary directionality, we succeeded to imitate the first-order temperature effect for both (¹H and ¹⁷O) NMR signals in the neat water. ¹H and ¹⁷O magnetic shielding tensors of water molecules in various water clusters have been calculated using the density functional theory. The full geometry optimization was performed using Becke's three-parameter hybrid method and the Lee–Yang–Parr correlation functional (B3LYP) combined with 6-311++G^** basis set. Magnetic shielding tensors have been calculated using the modified hybrid functional of Perdew, Burke and Ernzerhof, and the gauge-including atomic orbital approach was applied to ensure gauge invariance of the results. Solvent effects were taken into account by the polarized continuum model. Authors' address: Vytautas Balevicius, Faculty of Physics, Vilnius University, Sauletekio 9, Vilnius 10222, Lithuania     

<Superscript>1</Superscript>H and<Superscript>13</Superscript>C chemical shift assignments and stereochemistry of matrine and oxymatrine

Matrine and oxymatrine were extracted fromSophora flavescens, and their¹H and¹³C nuclear magnetic resonances (NMR) were unambiguously assigned by a combination of different two-dimensional 2-D¹H-¹³C and¹H-¹H correlation experiments of HMQC, HMQC-TOCSY and MAXY. The technique of using those experiments to make the assignment of the heavily overlapped spectrum is demonstrated. The coupling constants of matrine were measured by 2-DJ-resolved spectrum and 1-D spectra extracted from the slices of 2-D MAXY spectrum. The stereochemistry of the titled compounds was established from the NMR spectroscopy.     

<Superscript>29</Superscript>Si, <Superscript>27</Superscript>Al, <Superscript>1</Superscript>H and <Superscript>23</Superscript>Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers

²⁹Si, ²⁷Al, ¹H and ²³Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The ²⁹Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast ¹H MAS and ²⁹Si T _SiH/T _1ρ relaxation measurements demonstrated that occluded pore H₂O mobility within the disordered cavities is slow in comparison with H₂O mobility characteristics observed within the ordered channel structures of zeolites. The ²⁷Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the ²⁹Si MAS data, the corresponding ²⁷Al MAS line shapes are relatively narrow, suggesting that the AlO₄ tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding ²³Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved ²³Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed. Authors' address: John V. Hanna, NMR Facility, Institute of Materials and Engineering Science, Lucas Heights Research Laboratories, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia     

Firing-Induced Microstructural Properties of Quasi-Diamagnetic Carbonate-Based Porous Ceramics: a <Superscript>1</Superscript>H NMR Relaxation Correlation Study

This study deals with the application of two-dimensional proton nuclear magnetic resonance relaxometry (2D ¹H NMR-R) to the characterization of porous ceramics nearly free of magnetic compounds. Different microstructural properties were obtained by firing a diamagnetic mixture of kaolin, calcium, and magnesium carbonate over a wide range of maximum temperatures (600–1100 °C) and firing times at the maximum temperature (soaking times) (0–10 h). The 2D ¹H NMR-R method relies on the correlated measurement of ¹H longitudinal (T ₁) and transverse (T ₂) relaxation times of pore-filling water by which the properties of the interconnected pore space may be investigated. In the absence of significant magnetic susceptibility effect due to para- and ferro-magnetic compounds, the 2D ¹H NMR-R maps allow studying the conjoint effects on pore size distribution and inter-pore coupling due to the variations in both time and temperature of firing. The NMR experiments were performed with a low-field ¹H NMR sensor, which allows non-destructive and in situ analysis. For ceramic specimens fired at 600 and 700 °C, the fraction of smallest pores increases with firing time at the expenses of those with intermediate size. The pore shrinkage occurring at this stage, and likely associated with the transformation of kaolinite in metakaolinite, is affected in a similar way by soaking time and firing temperature, in line with the concept of equivalent firing temperature. At temperatures from 800 to 1100 °C, the structural modifications involving interconnectivity and average pore size are driven primarily by firing temperature and, secondarily, by soaking time. The 2D ¹H NMR-R results are confirmed by more traditional, but destructive, mineralogical, and structural analyses like X-ray powder diffraction, helium pycnometry, mercury intrusion porosimetry, and nitrogen adsorption/desorption method.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

<Superscript>1</Superscript>H and <Superscript>19</Superscript>F NMR relaxation time studies in (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>ZrF<Subscript>6</Subscript> superionic conductor

¹H and ¹⁹F spin-lattice relaxation times in polycrystalline diammonium hexafluorozirconate have been measured in the temperature range of 10–400 K to elucidate the molecular motion of both cation and anion. Interesting features such as translational diffusion at higher temperatures, molecular reorientational motion of both cation and anion groups at intermediate temperatures and quantum rotational tunneling of the ammonium group at lower temperatures have been observed. Nuclear magnetic resonance (NMR) relaxation time results correlate well with the NMR second moment and conductivity studies reported earlier.     

A New Reactive 1,8-Naphthalimide Derivative for Highly Selective and Sensitive Detection of Hg<Superscript>2+</Superscript>

A 1,8-naphthalimide derivative with a reactive aliphatic hydroxyl was designed and synthesized as a fluorescent probe. Its structure was characterized by IR, ¹H NMR, ¹³C NMR, LC-MS and HPLC. The probe showed high selectivity and sensitivity to Hg²⁺ over other metal ions such as Pb²⁺, Na⁺, K⁺, Cd²⁺, Cr³⁺, Zn²⁺, Cu²⁺, Ni²⁺, Ca²⁺, Fe³⁺, Fe²⁺, Co²⁺, Mn²⁺ and Mg²⁺ in MeCN/H₂O (15/85, v/v). The increase in fluorescence intensity was linearly proportional to the concentration of Hg²⁺ in the range of 18–40 μM with a detection limit of 1.38 × 10^?7 mol/L. The probe could work in a pH span of 4.3–9.0 and respond to Hg²⁺ quickly with strong anti-interference ability. Job’s plot suggested a 1:2 complex of the probe and Hg²⁺.     

<Superscript>23</Superscript>Na spin-lattice relaxation in powder Rochelle salt

The temperature dependence of ²³Na spin-lattice relaxation in the polycrystalline Rochelle salt was studied by NMR within the range from 235 to 320 K covering both Curie points. The spin-relaxation time t ₁ versus temperature curve showed noticeable dips near the phase transitions against the background of the regular decrease in the relaxation time upon increasing temperature. The dips observed were ascribed to critical contributions to sodium spin-lattice relaxation caused by the slowdown of the correlation time for one of two relaxation modes in the Rochelle salt. The ²³Na NMR parameters were also measured for the melted Rochelle salt. This article was submitted by the authors in English.     

Search for superheavy hydrogen isotopes <Superscript>6</Superscript>H and <Superscript>7</Superscript>H in stopped π-absorption reactions

Experimental search for superheavy hydrogen isotopes ⁶H and ⁷H was performed in the stopped pion absorption on ⁹Be and ¹¹B nuclei. The structures in the missing-mass spectra were observed in the reaction channels ⁹Be(π^-, pd )X and ¹¹B(π^-, p⁴He)X . Four states of ⁶H were proposed. Evidences for ⁷H formation were obtained in the reaction channels ⁹Be(π^-, pp)X and ¹¹B(π^-, p³He)X .     

Spectroscopy of superheavy hydrogen isotopes <Superscript>4</Superscript>H and <Superscript>5</Superscript>H

The superheavy hydrogen isotopes ⁴H and ⁵H have been investigated in the stopped pion absorption on ⁹Be. Three states of ⁴H were proposed in the reaction channel ⁹Be( ^{{ - }}, dt)X. Four states of ⁵H were proposed in the reaction channels ⁹Be(^{{ - }}, pt)X and ⁹Be(^{{ - }}, dt)X. The excited states of ⁵H can decay into free nucleons.     

Magnetic order in the structurally disordered helicoidal magnet Cr<Subscript>1/3</Subscript>NbS<Subscript>2</Subscript>: NMR at <Superscript>53</Superscript>Cr nuclei

The Cr_1/3NbS₂ magnet is studied by nuclear magnetic resonance (NMR) at ⁵³Cr nuclei in a zero applied magnetic field. The following two frequency ranges are distinguished in the ⁵³Cr NMR spectrum at T = 4.2 K: ν ₁ = 64–68 MHz and ν ₂ = 49–51 MHz. They can be related to two valence states of chromium ions, namely, Cr⁴⁺ and Cr³⁺. The components of the electric field gradient, the hyperfine fields, and the magnetic moment at chromium atoms are determined. The NMR data demonstrate that the magnetic moments of chromium lie in plane ab and form a magnetic structure consisting of regions with a helicoidal magnetic order and regions where this order is broken.     

Analysis of the electronic,IR, and <Superscript>1</Superscript>H NMR spectra of conjugated oligomers based on 4,4'-triphenylamine vinylene

Two types of conjugated oligomers based on 4,4'-triphenylamine vinylene have been synthesized and characterized by the methods of IR, UV–visible, and ¹H NMR spectroscopy. The corresponding spectra have also been simulated theoretically at the density functional theory level with application of the B3LYP and BMK hybrid exchange-correlation functionals. A comparative analysis of the experimental and theoretical spectra of polymers and oligomers has revealed regularities of the manifestation of spectral signals depending on the conjugation chain length and the presence of a substituent in the triphenylamine core. It has been established, in particular, that the absolute intensity of IR bands satisfies a linear dependence with increase in the degree of polymerization; however, no frequency shift is observed at the same time. The position of the main peak in electron absorption spectra demonstrates the bathochromic shift with an increase in the oligomeric chain length due to the narrowing of the energy gap between the boundary molecular orbitals. Based on the theoretical estimation of the hydrogen atoms chemical shifts, the signals of various protons types in the strongly broadened experimental ¹H NMR spectra of the bis-(4-iodine phenyl)-phenylamine and N,N-bis-(4-iodine phenyl)-4'-(phenylethynyl)-phenylamine polymerization products have also been identified.     

Crystalline Aluminium Borates with the Mullite Structure: A <Superscript>11</Superscript>B and <Superscript>27</Superscript>Al Solid-State NMR Study

²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at 8.45, 14.1 and 16.45 T for a series of aluminium borates with the mullite structure (Al_6−x B _x O₉, where x has nominal values of 1 to 4) augmented with ²⁷Al multiple-quantum MAS NMR spectra at 8.45 T. Even though the ²⁷Al NMR spectra are complex, simulation of the combined set of data produced a relatively well-defined set of parameters (e.g., quadrupolar interaction, isotropic chemical shift, etc.) for each site. The ¹¹B MAS NMR spectra of the same compounds were also acquired at 14.1 T. Linear changes in the X-ray a-, b- and c-cell parameters with composition suggest that these compounds constitute a continuous series. Based on a Rietveld structural refinement of the compound synthesized as Al₄B₂O₉, the resulting site occupancies and relative site distortions allow the identification of particular sites with specific NM resonances. Changes in the ²⁷Al and ¹¹B MAS NMR spectra of the related compounds with x = 1–4 show at the lowest Al contents a greater degree of asymmetry in the Al sites of the octahedral chains. A fairly distorted cross-linking tetrahedral site, which persists throughout the composition range, is accompanied in the lower Al compositions by two 5-fold coordinated Al–O units which are replaced by two more-regular tetrahedral Al–O sites as the Al content increases. In the compounds of lowest Al composition (i.e., highest B content) both the tetrahedral and trigonal cross-linking sites are distinguishable, but as the Al content increases, the BO₄ units progressively disappear. Authors' address: Kenneth J. D. MacKenzie, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand     

<Superscript>57</Superscript>Fe NMR study of multiferroic BiFeO<Subscript>3</Subscript>

The effects of the ⁵⁷Fe isotope content and high-frequency magnetic field amplitude h ₁ on the shape of the NMR spectrum of multiferroic BiFeO₃ at T = 4.2 K are studied by pulsed nuclear magnetic resonance. The NMR spectrum shape and transverse relaxation time T ₂ are found to depend strongly on the ⁵⁷Fe isotope content and h ₁ in multiferroic BiFeO₃ in the presence of a spatial spin-modulated structure of a cycloid type. In a sample with a high ⁵⁷Fe isotope content, the Suhl-Nakamura interaction contributes substantially to T ₂. When these dynamic effects are taken into account for analysis of the NMR spectrum shape, an undisturbed (without an anharmonicity effect) spatial spin-modulated structure of a cycloid type is shown to exist in BiFeO₃.     

Effect of cluster polarization on the spectrum of the <Superscript>7</Superscript>Li nucleus and on the reaction <Superscript>6</Superscript>Li(<Emphasis Type="Italic">n</Emphasis>, <Superscript>3</Superscript>H)<Superscript>4</Superscript>He

The effect of polarization of the ⁶Li and ³H clusters on the parameters of states of the ⁷Li nucleus and on the cross section for the reaction ⁶Li(n,³H)⁴He was studied. The ⁴He + d + n three-cluster configuration was invoked for this purpose, and cluster dynamics was determined within a microscopic model where the relative motion of the clusters was described in terms of Faddeev amplitudes. The ⁴He + d + n three-cluster configuration made it possible to consider the ⁴He + ³H and ⁶Li + n binary channels, which are dominant in ⁷Li, and to take simultaneously into account the cluster polarization of ⁶Li as a two-cluster subsystemin the ⁴He + d representation and the cluster polarization of ³Has a two-cluster subsystem in the d + n representation.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

<Superscript>23</Superscript>Na NMR in binary lithium-sodium cobaltite

Li_0.48Na_0.35CoO₂ lithium-sodium cobaltite was studied by means of wide-line ²³Na and ⁷Li NMR. A series of quantum-chemical calculations allowed to us determine the optimum positions of Li and Na atoms, to construct a map of the densities of electronic states near the Fermi level, and to estimate the electric field gradient on the Na nuclei. The results from these calculations are compared with experimental data from NMR and X-ray photoelectron spectroscopy     

<Superscript>1</Superscript>H NMR studies of cyclohexane confined in mesoporous solids: Melting point depression and pore size distribution

The pore size distributions of four controlled pore glasses and two silica gels with nominal diameters ranging from 6–24 nm were determined by measuring the¹H nuclear magnetic resonance (NMR) signal from the nonfrozen fraction of confined cyclohexane as a function of temperature, in steps of ca. 0.1–1 K. The intensity curves of the liquid component are well represented by a sum of two error functions. The mean melting point depression of cyclohexane confined in pores with radiusR follows the simplified Gibbs-Thompson equation δT=k _p/R with ak _p value of 72.4 Knm. To our knowledge, this is the first time that thek _p value of cyclohexane has been directly and accurately calibrated by NMR. As expected, thek _p value mainly determines the position of the pore size distribution curve, i.e., the mean pore radius. The overall pore size distributions determined by NMR are in reasonable agreement with the results specified by the manufacturer, or measured by us by the N₂ sorption technique. Although the melting point depression of confined cyclohexane is found to be less than previously assumed, this compound is still one of the most suitable candidates for NMR-based pore size determinations. However, pore sizes larger than approximately 50 nm in diameter will be difficult to determine accurately by NMR unless adsorbates undergoing larger melting point depressions than cyclohexane can be found.     

Spectroscopy of the <Superscript>5</Superscript>H superheavy hydrogen isotope

The formation of the ⁵H superheavy hydrogen isotope was experimentally sought in the reactions induced by stopped π^? mesons absorbed by ⁹Be nuclei. Peaks in missing-mass spectra were observed in two reaction channels, ⁹Be(π^?, pt)X and ⁹Be(π^?, dd)X, and were attributed to the ⁵H resonance states. The lowest state has parameters Er=5.5±0.2 MeV and Г=5.4±0.5 MeV [E_r is the resonance energy measured from the (triton + two neutrons) threshold]. Therefore, ⁵H is bound more weakly than ⁴H. Excited states of ⁵H were also observed. All three resonance levels (E_1r=10.6±0.3 MeV, Г_1r=6.8±0.5 MeV; E_2r=18.5±0.4 MeV, Г_2r=4.8±1.3 MeV; E_3r=26.7±0.4 MeV, Г_3r=3.6±1.3 MeV) can decay into five free nucleons.