Amorphization of crystalline Co and Sn multilayers by solid state reaction

Co_xSn_1?x amorphous alloys have been formed by solid state reaction at room temperature, from polycrystalline multilayers of Co and Sn deposited at 77 K. The forming ability range, temperature variation of the resistance, crystallization temperature of the amorphous alloys obtained by this new technique are very similar to those of amorphous Co_xSn_1?x alloys formed by vapour quenching.     

NMR study of solid state reaction in Co-Sn multilayers

Amorphization by solid state reaction at room temperature in Co-Sn multilayers with periodicities of 65, 130 and 195 Å and relative Co to Sn ratio varying as 4, 3, 2 has been studied by zero field NMR method. The distribution of⁵⁹Co hyperfine fields and that of NMR enhancement factors were used as a probe of the ferromagnetic part of a sample and its evolution with the annealing time. The critical role of the interface during the first hours of annealing is pointed out. On the long time scale the diffusion process is slowed down by the creation of Kirkendall voids and after 3 months of annealing some crystalline Co is still present.     

Carbon-13 NMR studies of camphene in the plastic crystalline phase

The¹³C NMR of camphene in the solid plastic crystalline phase was studied at room temperature. Chemical shifts, spin-lattice relaxation times (T₁), nuclear Overhauser enhancement factors and ¹³C-¹H coupling constants were measured and compared with the corresponding values obtained in solution.     

Dipolar recoupling in solid state NMR by phase alternating pulse sequences

We describe some new developments in the methodology of making heteronuclear and homonuclear recoupling experiments in solid state NMR insensitive to rf-inhomogeneity by phase alternating the irradiation on the spin system every rotor period. By incorporating delays of half rotor periods in the pulse sequences, these phase alternating experiments can be made γ encoded. The proposed methodology is conceptually different from the standard methods of making recoupling experiments robust by the use of ramps and adiabatic pulses in the recoupling periods. We show how the concept of phase alternation can be incorporated in the design of homonuclear recoupling experiments that are both insensitive to chemical shift dispersion and rf-inhomogeneity.     

An accurate and simple method for setting the magic angle for solid state NMR studies

Use of the deuterium NMR spectrum of a deuteriated solute dissolved in the nematic phase of a liquid crystal is suggested for setting the magic angle in solid state NMR. The use of the method as a convenient and accurate means of setting the angle is demonstrated. Copyright 2000 Academic Press.     

Kubo-Anderson oscillator and NMR of solid state

The analytical solution for the Kubo-Anderson oscillator with a fluctuating frequency omega for arbitrary distribution function p(omega) has been obtained. The obtained theoretical expression has been applied to consideration of some dynamical problems of solid state NMR, namely (1) dynamical transformation of NMR line shape and spin-echo signal and (2) the temperature transformation of the second moment of NMR line for the case, when the potential barrier for the mobility of magnetic nuclei is a stochastic function of time.     

Tuning the phase diagrams: the miscibility studies of multilactate liquid crystalline compounds

Design of binary and multicomponent liquid crystalline mixtures is a very powerful tool to reach the desired self-assembling properties. Beyond many advantages, this method has a distinct negativity – it is very material-consuming. While working with unique chiral materials in the research laboratory, this problem can be solved by applying miscibility study by the contact preparation method. In this work, the miscibility studies of lactic acid derivatives and non-chiral/chiral liquid crystalline molecules of different structure have been done in order to establish the phase diagrams. Special attention is focused on the ferro(antiferro)electric smectic phases.     

Phosphorus-31 NMR studies of stabilized phosphorus ylids in the solid state.

Phosphorus-31 powder NMR spectra and high-resolution MAS spectra have been obtained for a number of stabilized phosphorus ylids under conditions of high-power proton decoupling and cross-polarization. The 31P CP/MAS spectra are compared to those obtained from isotropic solutions. The variation of chemical shift anisotropy and of the principal components of the 31P chemical shift tensor determined from 31P powder NMR line shapes are discussed in terms of the relative importance of accepted valence bond resonance structures. The results indicate that the invariance of the isotropic chemical shift, delta iso, observed in previous 31P NMR investigations of phosphorus ylids in solution is due to fortuitous cancellation of opposing changes in the principal components, delta 11 and delta 33, of the 31P chemical shift tensor. The 31P dipolar NMR powder spectrum of a typical stabilized ylid, (C6H5)3(31)P-13CHC(O)OCH2CH3, is analyzed in order to obtain the orientation of the 31P chemical shift tensor with respect to the 31P-13C alpha dipolar vector.     

Dipolar 31P NMR spectroscopy of crystalline inorganic phosphorus compounds.

The ability of the 90 degrees-t1-180 degrees pulse sequence to produce accurate dipole-dipole coupling information in solids is investigated. To this end, the experimental 31P spin echo decays are measured for eighteen crystalline phosphides and phosphorus chalcogenides and compared with simulations, based on the known internuclear distances in these compounds. The experimental results are generally found accurate in compounds where the dominant contribution to the dipole-dipole coupling arises from nuclei in structurally inequivalent sites with large chemical shift anisotropies. For this situation, the quantum mechanical "flip-flop" term in the dipolar Hamiltonian is suppressed and the dipole-dipole coupling is entirely heteronuclear in character. All of those compounds that do not obey this condition show accelerated spin echo decays due to a fractional contribution of the flip-flop term and possibly incomplete refocusing of chemical shift terms on the time scale of the experiment. The results confirm on an empirical basis that the spin echo NMR technique can provide accurate dipole-dipole coupling information (and thus distance distributions) in disordered solids and glasses.     

Studies of heterogeneous catalysis using high-resolution solid state NMR

Recent results obtained at the institute of Catalysis from studies of heterogeneous catalysis using high-resolution¹H,¹³C,¹⁵N,²⁷Al,²⁹Si,³¹P and⁵¹V solid state NMR have been summarized. Emphasis is made on: (1) structural studies of active in catalysis compounds and sites in supported oxides, hydrides and inorganic acids; (2) studies of structures and properties of surface OH groups active in Bronsted acid catalysis; (3) studies of Lewis acidity of heterogeneous catalysts using¹⁵N NMR of adsorbed N₂O and (4) studies of adsorption mechanisms for molecules of reactants over various catalysts.     

The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses

Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current level of understanding about crystalline polymorphism in these materials. Emphasis is placed on observations from solid state 13C nuclear magnetic resonance (NMR), which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled Ialpha and Ibeta. Historical background is presented, highlighting the structural controversies which mainly arose because different native celluloses were used, each one representing a different mixture of allomorphs. Input from Raman, infrared (IR) and electron diffraction data is included in the discussion of our current understanding of polymorphism in native celluloses. Also noted is the input from more recently studied celluloses (e.g., Halocynthia) as well as from newer processes that convert the Ialpha to the Ibeta form. On the basis of Raman and IR observations, it is argued that the Ialpha and Ibeta allomorphs differ in hydrogen bonding patterns only and that backbone conformations are nearly identical. Also, the point is made that the absence of correlation field splittings in the Raman spectra calls into question (although it does not disprove) whether the normal two-chain-per-unit-cell, monoclinic Ibeta allomorph really possesses two equivalent chains. Considerable discussion is devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses are reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of self-consistency of inferences that are based on lineshape analyses for different carbons (particularly C1 and C4). It is concluded that higher plant celluloses most likely possess a minor amount of the Ialpha allomorph where the Ialpha/Ibeta ratio is probably less than 0.25.     

The solid state phase transformation of potassium sulfate

Potassium sulfate single crystals that are grown from aqueous solutions lose, upon the first heating, up to 1% of mass that is assumed to be water. This mass loss occurs in the vicinity of the PT from orthorhombic to hexagonal K₂SO₄. Only in the first heating run of K₂SO₄ that has not yet released water, pretransitional thermal effects can be observed in the DTA curve. If K₂SO₄ crystals are grown from solutions containing 4 wt% Cd, Cu, or Fe, only Cu or Fe can be incorporated significantly with concentrations of several 0.1%. The phase transformation temperature measured for such solid solutions depends on the heating rate. For pure K₂SO₄, the phase transformation temperature is independent of heating rate 581.3 ^°C and the enthalpy of transformation is (5.8±0.2) kJ/mol.     

Precision and sensitivity optimization of quantitative measurements in solid state NMR

This work presents a methodology for optimizing the precision, accuracy and sensitivity of quantitative solid state NMR measurements based on the external reference method. It is shown that the sample must be exclusively located within and completely span the coil region where the NMR response is directly proportional to the sample amount. We describe two methods to determine this "quantitative" coil volume, based on whether the probe is equipped or not with a gradient coil. In addition, to improve the sensitivity and the accuracy, an optimum rotor packing design is described, which allows the sample volume of the rotor to be matched to the quantitative coil volume. Experiments conducted on adamantane and NaCl, which are representative of a soft and hard material, respectively, show that one order of magnitude increase in experimental precision can be achieved with this methodology. Interestingly, the precision can be further improved by using the ERETIC method in order to compensate for most instrumental instabilities.     

Preparation of protein nanocrystals and their characterization by solid state NMR

Preparation of proteins in their crystalline state has been found to be important in producing stable therapeutic protein formulations, cross-linked enzyme crystals for application in industrial processes, generating novel porous media for separations, and of course in structure elucidation. Of these applications only X-ray crystallography requires large crystals, defined here as being crystals 100s of microns or greater in size. Smaller crystals have attractive attributes in many instances, and are just as useful in structure determination by solid state NMR (ssNMR) as are large crystals. In this paper we outline a simple set of procedures for preparing nanocrystalline protein samples for ssNMR or other applications and describe the characterization of their crystallinity by ssNMR and X-ray powder diffraction. The approach is demonstrated in application to five different proteins: ubiquitin, lysozyme, ribonuclease A, streptavidin, and cytochrome c. In all instances the nanocrystals produced are found to be highly crystalline as judged by natural abundance 13C ssNMR and optical and electron microscopy. We show for ubiquitin that nanocrystals prepared by rapid batch crystallization yield equivalent 13C ssNMR spectra to those of larger X-ray diffraction quality crystals. Single crystal and powder X-ray diffraction measurements are made to compare the degree of order present in polycrystalline, nanocrystalline, and lyophilized ubiquitin. Solid state 13C NMR is also used to show that ubiquitin nanocrystals are thermally robust, giving no indication of loss of local order after repeated temperature cycling between liquid nitrogen and room temperature. The methods developed are rapid and should scale well from the tenths of milligram to multi-gram scales, and as such should find wide utility in the preparation of protein nanocrystals for applications in catalysis, separations, and especially in sample preparation for structural studies using ssNMR.     

Crystal-blocking perspectives for nuclear and solid state studies on heavy-ion beam

The mechanism of ion-crystal interaction is discussed and a conclusion is formulated that the opposite tendencies manifest themselves both in the influence of ion-bombardment on the lattice and the lattice potential on ion trajectory. The experimental examples of regular and paradoxical behaviour are described. New experiments are proposed for the studies in nuclear and solid state physics using a crystal-blocking technique.     

13C high-resolution solid state NMR studies of the structure and dynamics of 1,6:3,4-dianhydro-2-O-tosyl-beta-D-galactopyranose

13C CP/MAS, dipolar dephasing MAS and theoretical GIAO calculations were employed to assign 13C resonances to the molecular structure of 1,6:3,4-dianhydro-2-O-tosyl-beta-D-galactopyranose 1. From spinning sideband intensities, employing the graphical method of Herzfeld and Berger the 13C delta(ii) parameters for aromatic residue were calculated. The experimental data were compared with computed results obtained by means of the B3PW91 hybrid method and 6-311G (df, p) basis set. The X-ray geometry of 1 with the correlated position of hydrogen atoms was taken as input data for theoretical calculations. As concluded from Cambridge Crystallographic Database (CSD) search, there are two reports describing the X-ray studies of 1 that show the slightly different geometry of the compound under investigation. This work shows that such discrepancies in geometry can generate differences between computed 13C delta(ii) parameters up to 6 ppm. 13C T1 and 1H T1rho relaxation times reveal that 1 is very rigid in crystal lattice. This structure is characterized by extremely long 1H T1rho, found to be in range ca. 200 ms.     

Comparative studies of the NMR of thulium in Tm123 and Tm124 compounds: Evidence for structural and electronic phase separation

The nuclear magnetic relaxation of ¹⁶⁹Tm in TmBa₂Cu₃O_6+x (x=0.1–1.0, Δ x=0.1) and TmBa₂Cu₄O₈ is studied at temperatures below 5 K. In all the samples, the Tm spin-lattice relaxation proceeds via intrinsic paramagnetic centers (PCs) like Cu²⁺ or copper-oxygen spin-polarized clusters. The experimental data for TmBa₂Cu₃O_6+x support the idea of the structural (chemical) micro-phase separation in oxygen-deficient 123 compounds. Apparently, the samples with x⩾0.4 contain hole-poor nonsuperconducting regions, enriched with PCs, and hole-rich (PC-poor) superconducting regions. The volume fraction f _n of the PC-rich phase reaches a maximum value of 0.85 at x=0.4 and decreases monotonically with increasing x (f _n=0.5, 0.3, and 0.25 at x=0.5, 0.6, and 0.7, respectively). The Tm spin-lattice relaxation in the underdoped TmBa₂Cu₄O₈ compound indicates that this sample, in contrast to oxygen-deficient TmBa₂Cu₃O_6+x , has a homogeneous composition. However, the Tm spin-spin relaxation measurements reveal two sorts of the Tm nuclear spins in Tm124, having different NMR spectra and different relaxation times T ₂. The latter result is evidence of electronic phase separation in CuO₂ phases. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 365–370 (10 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Methodology for solid state NMR off-resonance study of molecular dynamics in heteronuclear systems

Methodology for the study of dynamics in heteronuclear systems in the laboratory frame was described in the previous paper [1]. Now the methodology for the study of molecular dynamics in the solid state heteronuclear systems in the rotating frame is presented. The solid state NMR off-resonance experiments were carried out on a homemade pulse spectrometer operating at the frequency of 30.2 MHz for protons. This spectrometer includes a specially designed probe which contains two independently tuned and electrically isolated coils installed in the coaxial position on the dewar. A unique probe design allows working at three slightly differing frequencies off and on resonance for protons and at the frequency of 28.411 MHz for fluorine nuclei with complete absence of their electrical interference. The probe allows simultaneously creating rf magnetic fields at off-resonance frequencies within the range of 30.2–30.6 MHz and at the frequency of 28.411 MHz. Presented heteronuclear cross-relaxation off-resonance experiments in the rotating frame provide information about molecular dynamics.