Chemical shift referencing in MAS solid state NMR

Solid state 13C magic angle spinning (MAS) NMR spectra are typically referenced externally using a probe which does not incorporate a field frequency lock. Solution NMR shifts on the other hand are more often determined with respect to an internal reference and using a deuterium based field frequency lock. Further differences arise in solution NMR of proteins and nucleic acids where both 13C and 1H shifts are referenced by recording the frequency of the 1H resonance of DSS (sodium salt of 2,2-dimethyl-2-silapentane-5-sulphonic acid) instead of TMS (tetramethylsilane). In this note we investigate the difficulties in relating shifts measured relative to TMS and DSS by these various approaches in solution and solids NMR, and calibrate adamantane as an external 13C standard for solids NMR. We find that external chemical shift referencing of magic angle spinning spectra is typically quite reproducible and accurate, with better than +/-0.03 ppm accuracy being straight forward to achieve. Solid state and liquid phase NMR shifts obtained by magic angle spinning with external referencing agree with those measured using typical solution NMR hardware with the sample tube aligned with the applied field as long as magnetic susceptibility corrections and solvent shifts are taken into account. The DSS and TMS reference scales for 13C and 1H are related accurately using MAS NMR. Large solvent shifts for the 13C resonance in TMS in either deuterochloroform or methanol are observed, being +0.71 ppm and -0.74 ppm from external TMS, respectively. The ratio of the 13C resonance frequencies for the two carbons in solid adamantane to the 1H resonance of TMS is reported.     

15N chemical shift referencing in solid state NMR

Solid-state NMR spectroscopy has much advanced during the last decade and provides a multitude of data that can be used for high-resolution structure determination of biomolecules, polymers, inorganic compounds or macromolecules. In some cases the chemical shift referencing has become a limiting factor to the precision of the structure calculations and we have therefore evaluated a number of methods used in proton-decoupled ¹⁵N solid-state NMR spectroscopy. For ¹³C solid-state NMR spectroscopy adamantane is generally accepted as an external standard, but to calibrate the ¹⁵N chemical shift scale several standards are in use. As a consequence the published chemical shift values exhibit considerable differences (up to 22 ppm). In this paper we report the ¹⁵N chemical shift of several commonly used references compounds in order to allow for comparison and recalibration of published data and future work. We show that ¹⁵NH₄Cl in its powdered form (at 39.3 ppm with respect to liquid NH₃) is a suitable external reference as it produces narrow lines when compared to other reference compounds and at the same time allows for the set-up of cross-polarization NMR experiments. The compound is suitable to calibrate magic angle spinning and static NMR experiments. Finally the temperature variation of ¹⁵NH₄Cl chemical shift is reported.     

Theoretical Studies of the Electron Paramagnetic Resonance and Optical Spectral Data for Vanadium(IV) Ion in Yttrium Aluminum Garnet Crystal

Four spin-Hamiltonian parameters (g factors g _// and g _⊥ and hyperfine structure constants A _// and A _⊥) and two optical spectral band positions for vanadium(IV) (V⁴⁺) ion in the tetrahedral Al³⁺ site of yttrium aluminum garnet (Y₃Al₅O₁₂) crystals are calculated from the (third-order) perturbation theory method (PTM) and complete diagonalization (of energy matrix) method (CDM). Both methods are based on the cluster approach. The calculated results from both methods coincide and reasonably agree with the experimental values. The defect structure of V⁴⁺ center (which is different from the corresponding structure in the host crystal) in Y₃Al₅O₁₂ crystal is also obtained from the calculation.     

Potassium tetracyanoplatinate (II) trihydrate (K2Pt(CN)4·3H2O) studied by high resolution solid state C MAS NMR

Prudent analysis of the solid state ¹³C MAS NMR spectra of polycrystalline K₂Pt(CN)₄ · 3H₂O (KTCP)⁻ reveals that in crystals of this compound there are two types of carbon nuclei with slightly different ¹³C chemical shift tensors, contrary to what is found for the solution NMR spectrum and previous static powder NMR studies on this compound and the high resolution solid state NMR studies on other similar compounds. The ¹³C MAS spectra measured at different rotor spinning speeds are satisfactorily simulated though the use of a newly developed computer program based on a novel density matrix formulation. The present method is eminently successful even though the spectra are rather complicated because of (1) the relatively large anisotropies of the chemical shift tensors; (2) the high-order dipolar interactions between ¹³C and ¹⁴N nuclei because of the strong quadrupolar coupling constants of ¹⁴N nuclei; and (3) the indirect J-coupling between the ¹³C and ¹⁹⁵Pt. The principal elements as well as their orientations of the two ¹³C chemical shift tensors are evaluated from the spectral simulations.     

钛－镍－硅固相反应形成的镍硅化物/n-硅(100)接触的I-V-T研究

在不同退火温度下，有一薄层钛覆盖层的镍－硅经过固相反应生成了镍硅化物/n-硅(100)接触，研究了其在80K到室温的电流－电压（I-V）特性。低温I-V曲线在低偏压区的电流显著地比传统的热电子发射（TE）模型预计的要大。用基于Tung的夹断模型简化得到的双肖特基势垒模型分析了实测的I-V曲线，从中可以得到肖特基势垒不均匀性的量度。较高温度退火导致较大的势垒不均匀性，意味着硅化物薄膜均匀性的变坏。钛覆盖薄层可以稍微提高硅化镍的相转变温度，以及形成的一硅化镍的热稳定性。     

Two-dimensional solid state NMR characterization of physisorbed siloxane polymer (OV-225) on silica

Physisorbed cyanopropyl-methyl-phenyl-methyl-siloxane polymer on a silica surface was characterized by one- and two-dimensional solid state NMR techniques including heteronuclear proton-silicon correlation spectroscopy. Spin-lattice relaxations of protons of the siloxane polymer exhibited only small changes upon anchoring to the silica surface indicating somewhat altered molecular dynamics of proton moieties that contribute to the relaxation process. However, the same relaxation rates of the siloxane polymer’s silica atoms were reduced due to restricted mobility of the polymer. Proton-silicon heteronuclear correlation spectroscopy (HETCOR) revealed strong correlations of silanol protons with both Q³ and Q⁴ sites of the silica surface. In addition, a correlation between methyl protons and the Q³ site of the silica surface was observed when HETCOR experiments with very small mixing time (5 ms) were performed. The presence of these correlations is indicative of the coherent magnetization transfer mainly through dipolar mechanisms. Since magnetization transfer through the dipolar mechanism is 1/r³ dependent, methyl protons must lie in close proximity to the silica surface. Hydrogen bonding of the silica surface’s hydroxyl protons with the bridging oxygen of the siloxane polymer is most likely responsible for positioning the methyl protons closer to the surface. Additional correlations between ²⁹Si nuclei and methylene protons next to cyano group was also observed with mixing time indicating the closer proximity of these protons to the silica surface as well. This juxtaposition of methylene protons is most likely due to hydrogen bonding of the siloxane polymer through the cyano moiety. Furthermore, the hydrogen bonding through the cyano group is most likely to be in parallel orientation to the surface. Finally, the aromatic protons exhibited weak correlations only with Q³ sites, indicating that these protons must also lie in close proximity of the silica surface.     

Sensitivity enhancement in solid state (15)N NMR by indirect detection with high-speed magic angle spinning

Enhancement of sensitivity in solid state (15)N NMR by indirect detection through (1)H NMR signals under high-speed magic angle spinning and high-field conditions is demonstrated experimentally on two (15)N-labeled peptides, polycrystalline AlaGlyGly and the helix-forming, 17-residue peptide MB(i + 4)EK in lyophilized form. Sensitivity enhancement factors ranging from 2.0 to 3.2 are observed experimentally, depending on the (15)N and (1)H linewidths and polarization transfer efficiencies. The (1)H-detected two-dimensional (1)H/(15)N correlation spectrum of AlaGlyGly illustrates the possibility of increased spectral resolution and resonance assignments in indirectly detected experiments, in addition to the sensitivity enhancement.     

13C solid state NMR investigation of structural relaxation of the polymer backbone in poly (ethylmethacrylate)

Geometry and time scale of structural relaxation of poly(n-alkylmethacrylates) above the glass transition is studied by temperature dependent one- and two-dimensional 13C-NMR spectroscopy. The geometry of the isotropization of the polymer backbone as deduced from detailed analysis of spectral line shapes is identified as random angular jumps. Analysis of echo decays confirms that at a given temperature this isotropization can adequately be described with a single correlation time. The results are discussed in terms of conformational memory and local structure recently identified in these polymeric glasses.     

Background signal in solid state 13C NMR spectra of soil organic matter (SOM)-quantification and minimization

13C background signal, obtained for an empty rotor, was shown by spin counting experiments to be equivalent to 1 mg of observable carbon for cross-polarization (CP) spectra and 69 mg of observable C for Bloch decay (BD) spectra. The BD background was mainly due to Kel-F in the stator. with minimal signal detected from the Kel-F end-caps. The CP background was attributed to non-Kel-F components of the stator, probe, or probe supports. The BD background signal was eliminated by using a modified dipolar dephasing pulse sequence in which the absence of 19F decoupling (rather than the absence of 1H decoupling) causes selective elimination of the Kel-F signal.     

Solid state NMR studies and chemical shift calculations of a gold(I) complex with a diphosphacyclobutadiene cobaltate sandwich anion

The ³¹P and ¹³C solid state nuclear magnetic resonance spectra of a neutral gold(I) complex with bis(diphosphacyclobutadiene) cobaltate anions, [Au{Co(P₂C₂tPent₂)₂}₂(PMe₃)₂], are reported. Complete ³¹P resonance assignments have been derived from saturation transfer, radio-frequency driven recoupling (RFDR) and RTOBSY experiments and confirmed further by ab-initio calculations of magnetic shielding tensors by density functional theory, with consideration of relativistic effects. Coordination of the diphosphacyclobutadiene ring with gold(I) results in a high-frequency shift of the ³¹P signal of the directly coordinated P atom, whereas a low-frequency shift is observed for the P atom at the opposite end of that ring. Based on these results, a previous assignment made for the complex salts [Au(PMe₃)₄][Au{Co(P₂C₂Ad₂)₂}₂] and [K(18-crown-6)(thf)₂][Au{Co(P₂C₂Ad₂)₂}₂] (Ad=adamantyl) must be corrected.     

Quantification of composition and domain sizes of industrial poly(phthalamide)/poly(dimethylsiloxane) block copolymers using different 1H solid state NMR methods

We have applied several 1H solid state NMR techniques to obtain structural information on industrial silicone copolymers as thermoplastic elastomers that proved difficult to be analysed by conventional techniques. Static measurements were applied to achieve the separation between mobile and rigid parts. To get structural information fast magic angle spinning at 25 kHz in combination with a high field of 700 MHz was used. 1H spin diffusion was applied to determine the domain sizes of the micro phase-separated polymer components.     

Optical ac Stark Effect in ~(23)Na NMR Spectra

1.ＩＮＴＲＯＤＵＣＴＩＯＮＯｐｔｉｃａｌａｃＳｔａｒｋｅｆｆｅｃｔ(ａｌｓｏｓｉｍｐｌｙｃａｌｌｅｄａｓｌｉｇｈｔｓｈｉｆｔ),ａｓａｎｉｎｔｅｒｅｓｔｉｎｇｆｉｅｌｄｏｆｉｎｖｅｓｔｉｇａｔｉｏｎｆｏｒｔｅｓｔｉｎｇｔｈｅｆｕｎｄａｍｅｎｔａｌｔｈｅ?..     

Spectral properties of rare-earth ions in nanocrystalline YAG:Re (Re=Ce, Pr, Tb)

Bulk and nanoscale powders of YAG:Re (Re=Ce, Pr, Tb) were synthesized by solid-state and sol-gel method. The changes of spectra and energy level were studied. Compared with the bulk YAG:Re (Re=Ce, Pr, Tb) crystals, the lattice parameter of YAG:Re (Re=Ce, Pr, Tb) nanocrystals decreases. It is also found that the excitation peaks of 5d energy levels shift in nanocrystals. The physical reason for spectral and energy level changes is a comprehensive result from the shift of energy centroid of the 5d orbit, the Coulomb interaction between 4f and 5d electrons and the crystal field splitting of the 5d energy level.     

Orientation-dependent (19)F dipolar couplings within a trifluoromethyl group are revealed by static multipulse NMR in the solid state

The homonuclear dipolar coupling between the three equivalent (19)F-spins of a trifluoromethyl group, rotating about its threefold symmetry axis, was studied by multipulse solid-state NMR. A modified CPMG sequence was used first to resolve the dipolar splitting of a powder sample, and then to follow its orientation-dependence in uniaxially aligned samples. Our aim is to employ the CF(3)-group as a highly sensitive reporter to describe the mobility and spacial alignment of (19)F-labeled molecules in biomembranes. As an example, the fluorinated anti-inflammatory drug, flufenamic acid, was embedded as a guest compound in lipid bilayers. Undistorted (19)F dipolar spectra of its CF(3)-group were obtained without (1)H-decoupling, revealing a sharp triplet lineshape. When an oriented membrane sample was tilted in the magnetic field, the change in dipolar splittings confirmed that the guest molecule is motionally averaged about the membrane normal, as expected. A different behavior of flufenamic acid, however, was observed under conditions of low bilayer hydration. From this set of orientation-dependent lineshapes we conclude that the axis of motional averaging becomes aligned perpendicular to the sample normal. It thus appears that flufenamic acid induces a hexagonal phase in the membrane at low hydration. Finally, the dipolar (19)F NMR experiments were extended to frozen samples, where no molecular diffusion occurs besides the fast rotation about the CF(3)-axis. Also under these conditions, the CPMG experiment with composite pulses could successfully resolve the dipolar coupling between the three (19)F-nuclei.     

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C,15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C,15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions (omega1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.     

Sensitivity enhancement in (13)C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing (1)H T(1) relaxation

We discuss a simple approach to enhance sensitivity for (13)C high-resolution solid-state NMR for proteins in microcrystals by reducing (1)H T(1) relaxation times with paramagnetic relaxation reagents. It was shown that (1)H T(1) values can be reduced from 0.4-0.8s to 60-70 ms for ubiquitin and lysozyme in D(2)O in the presence of 10 mM Cu(II)Na(2)EDTA without substantial degradation of the resolution in (13)C CPMAS spectra. Faster signal accumulation using the shorter (1)H T(1) attained by paramagnetic doping provided sensitivity enhancements of 1.4-2.9 for these proteins, reducing the experimental time for a given signal-to-noise ratio by a factor of 2.0-8.4. This approach presented here is likely to be applicable to various other proteins in order to enhance sensitivity in (13)C high-resolution solid-state NMR spectroscopy.     

绿豆(Vigna Radiata)代谢物组成的核磁共振定量分析

绿豆(Vigna radiata)是重要的经济作物之一,具有抗氧化、抗炎和抗菌作用.然而迄今为止,绿豆代谢物组成信息仍不完善.为了系统地定量研究绿豆代谢物组成,该研究利用DOSY,J-RES,COSY,TOCSY,HSQC,HMBC和1D 1H NMR谱图在鄂绿4号(ElüNo.4)绿豆水相提取物中鉴定和归属了48种代谢物,确定了绿豆中棉籽糖家族寡糖的构成,并且利用全弛豫谱对其中44种代谢物进行了定量分析.     

Reassignment of the C-Cl stretching frequency in the Ã(A″) state of CBrCl

In a recent theoretical paper, vibrational frequencies were reported for CBrCl in the and à states. These values agree well with our previously published experimental frequencies, except in the case of , where the theoretical value of 968.3 cm⁻¹ is significantly higher than the experimental value of 712.6 cm⁻¹. This discrepancy prompted a re-examination of the assignments for transitions involving ν₁ in the laser induced fluorescence spectrum. If the progression ν₁ + nν₂ has the assignment of n reduced by 1 (the origin is not observed), a new experimental value of results. The close accord with the theoretical value lends weight to this re-assignment. Consequently we propose that the experimental frequency be changed to .     

Radially dependent antishielding factor γ(r) of the Sc3+ ion in the solid state

We report the results of our calculation of γ(r), the radially dependent antishielding factor of Sc³⁺ ion in the crytalline environment. Watson sphere model is used to approximately represent the crystal potential. Differential equations resulting from non-orthogonal Hartree-Fock perturbation theory are solved to obtain perturbations to core electron states. Contribution to γ(r) from electron self-consistency effect has been evaluated by using the latter wave-functions in the many-body expression of Schmidtet al.