Solid-state F NMR investigation of poly(vinylidene fluoride) with high-power proton decoupling

The use of high-power proton decoupling has enabled highly-resolved spectra of fluorine polymers to be recorded, as is exemplified herein for semicrystalline poly(vinylidene fluoride) (PVDF). By means of high MAS speeds (up to 17 kHz), the spinning sidebands are removed from the whole of the relevant chemical shift range. For spectra of the crystalline regions of the polymer, the high-power decoupling is necessary, though its effect is not large. Various relaxation techniques have been used to examine the semicrystallinity and the polymorphism of PVDF, with special pulse sequences used to discriminate between the various domains. Different chemical shifts have been observed for the signals of the amorphous and crystalline phases. Those of the more immobile parts cover a substantial range.     

Multiple-rotor-cycle 2D PASS experiments with applications to (207)Pb NMR spectroscopy

Thetwo-dimensional phase-adjusted spinning sidebands (2D PASS) experiment is a useful technique for simplifying magic-angle spinning (MAS) NMR spectra that contain overlapping or complicated spinning sideband manifolds. The pulse sequence separates spinning sidebands by their order in a two-dimensional experiment. The result is an isotropic/anisotropic correlation experiment, in which a sheared projection of the 2D spectrum effectively yields an isotropic spectrum with no sidebands. The original 2D PASS experiment works best at lower MAS speeds (1-5 kHz). At higher spinning speeds (8-12 kHz) the experiment requires higher RF power levels so that the pulses do not overlap. In the case of nuclei such as (207)Pb, a large chemical shift anisotropy often yields too many spinning sidebands to be handled by a reasonable 2D PASS experiment unless higher spinning speeds are used. Performing the experiment at these speeds requires fewer 2D rows and a correspondingly shorter experimental time. Therefore, we have implemented PASS pulse sequences that occupy multiple MAS rotor cycles, thereby avoiding pulse overlap. These multiple-rotor-cycle 2D PASS sequences are intended for use in high-speed MAS situations such as those required by (207)Pb. A version of the multiple-rotor-cycle 2D PASS sequence that uses composite pulses to suppress spectral artifacts is also presented. These sequences are demonstrated on (207)Pb test samples, including lead zirconate, a perovskite-phase compound that is representative of a large class of interesting materials.     

Angular dependence of the207Pb NMR linewidth in Pb(NO3)2 due to14N—207Pb direct dipole-dipole interaction

NMR signals of²⁰⁷Pb²⁺ have been measured in a single crystal of Pb(NO₃)₂. For rotation axes perpendicular to the static magnetic field and perpendicular to the (100) or (011) plane the linewidths of these signals vary by a factor 2 for different rotation angles of the single crystal. This dependence of the linewidth on the orientation has been explained by a¹⁴N —²⁰⁷Pb direct dipole-dipole coupling and it has been used to assign the different Pb²⁺ NMR signals to the different Pb²⁺ sites in the cubic unit cell of Pb(NO₃)₂.     

High-resolution solid-state NMR spectroscopy of protons with homonuclear dipolar decoupling schemes under magic-angle spinning

High-resolution NMR spectroscopy of ¹H spins in the solid state is normally rendered difficult due to the strong homonuclear ¹H–¹H dipolar couplings. Even under very high-speed magic-angle spinning (MAS) at ca. 60–70 kHz, these couplings are not completely removed. An appropriate radiofrequency pulse scheme is required to average out the homonuclear dipolar interactions in combination with MAS to get high-resolution ¹H NMR spectrum in solid state. Several schemes have been introduced in the recent past with a variety of applications also envisaged. Development of some of these schemes has been made possible with a clear understanding of the underlying spin physics based on bimodal Floquet theory. The utility of these high-resolution pulse schemes in combination with MAS has been demonstrated for spinning speeds of 10–65 kHz in a range of ¹H Larmor frequencies from 300 to 800 MHz.     

NMR of residual protons in partly deuterated anisotropic materials with phase-alternated decoupling of deuterium spins

A new method is presented for decoupling spins with S = 1 like deuterium in anisotropic media while observing other spins such as residual protons in partly deuterated samples. The carrier frequency of a weak radiofrequency (RF) field is applied near the center of the doublet arising from the quadrupolar interaction of the S = 1 spins. The phase of the RF field is periodically reversed with intervals matching the reciprocal of the magnitude of the quadrupolar splitting. It is shown by theory and experiment that, even when the irradiating field is quite weak, the efficiency of this phase-alternated decoupling scheme is much better than for simple continuous-wave irradiation at the center of the doublet, an established technique which is usually referred to as double quantum decoupling. The phase-alternated experiment makes it possible to decouple large quadrupolar interactions with a weak RF field. A theoretical analysis and numerical simulations are presented to demonstrate the decoupling performance. Proton spectra of partly deuterated dibromobutane in a liquid crystalline solvent have been recorded to illustrate the efficiency of phase-alternated deuterium decoupling.     

Phase transition in Pb5Al3F19

A study of the PbF₂-AlF₃ system has resulted in a new material Pb₅Al₃F₁₉ with a tetragonal structure at room temperature closely related to that of Pb₅Cr₃F₁₉. A phase transition in Pb₅Al₃F₁₉ has been detected by means of X-ray diffraction, calorimetric and dielectric measurements. The coexistence of the high and low-temperature Pb₅Al₃F₁₉ phases over the range 270 to 300 K and the abrupt 0.7% increase in the a-axis and 1.5% decrease in the c-axis, on heating through the phase transition, are characteristic of a first-order phase change. The increase in symmetry from point group 4 below to point group 4/m above the phase transition is consistent with a ferroelectric to paraelectric transition, as found previously in Pb₅Cr₃F₁₉.     

PAD investigation of19F in fullerites

Perturbed angular distribution experiments on¹⁹F implanted into fullerites revealed a large fraction of fluorine atoms which are not subject to static quadrupole interactions in contrast to other allotropic forms of carbon.     

Measurement of dipolar couplings in partially oriented molecules by local field NMR spectroscopy with low-power decoupling

Low-power phase-modulated Lee–Goldburg homonuclear decoupling was used to record PDLF spectra of fluorine-substituted benzene derivatives dissolved in nematic thermotropic liquid crystalline solvents. The low-power decoupling minimizes sample heating during RF irradiation while still achieving highly resolved PDLF spectra. The method is illustrated by recording spectra for 1,3-dichloro-4-fluoro-5-nitrobenzene, 1,3-dichloro-4-fluorobenzene, and 1,2-difluorobenzene dissolved in different nematic solvents.     

Experimental aspects of proton NMR spectroscopy in solids using phase-modulated homonuclear dipolar decoupling

In this paper we demonstrate experimentally that the continuously phase-modulated homonuclear decoupling sequence DUMBO-1 is suitable for high-resolution proton NMR spectroscopy of rigid solids. Over a wide range of experimental conditions, we show on the model sample L-alanine as well as on small peptides that proton linewidths of less than 0.5 ppm can be obtained under DUMBO-1 decoupling. In particular the DUMBO-1 sequence yields well resolved proton spectra both at slow and fast MAS. The DUMBO-1 decoupling scheme can in principle be inserted in any multi-nuclear or multi-dimensional solid-state NMR experiment which requires a high-resolution 1H dimension. An example is provided with the 13C-1H MAS-J-HMQC experiment.     

19F-decoupling of half-integer spin quadrupolar nuclei in solid-state NMR: application of frequency-swept decoupling methods

In solid-state NMR studies of minerals and ion conductors, quadrupolar nuclei like ⁷Li, ²³Na or ¹³³Cs are frequently situated in close proximity to fluorine, so that application of ¹⁹F decoupling is beneficial for spectral resolution. Here, we compare the decoupling efficiency of various multi-pulse decoupling sequences by acquiring ¹⁹F-decoupled ²³Na-NMR spectra of cryolite (Na₃AlF₆). Whereas the MAS spectrum is only marginally affected by application of ¹⁹F decoupling, the 3Q-filtered ²³Na signal is very sensitive to it, as the de-phasing caused by the dipolar interaction between sodium and fluorine is three-fold magnified. Experimentally, we find that at moderate MAS speeds, the decoupling efficiencies of the frequency-swept decoupling schemes SW_f-TPPM and SW_f-SPINAL are significantly better than the conventional TPPM and SPINAL sequences. The frequency-swept sequences are therefore the methods of choice for efficient decoupling of quadrupolar nuclei with half-integer spin from fluorine.     

207Pb NMR of minium, Pb3O4: evidence for the [Pb2]4+ ion and possible relativistic effects in the Pb-Pb bond

Solid Pb3O4 has been studied with 207Pb nuclear magnetic resonance (NMR) spectroscopy. The 207Pb NMR chemical-shift tensor of the Pb2+ site has principal values of delta11 = 1980 +/- 5 ppm, delta22 = 1540 +/- 5 ppm, and delta33 = -1108 +/- 10 ppm; delta(iso) = 804 +/- 10 ppm. The chemical-shift tensor of the Pb4+ site is axial, with principal values delta(parallel) = -1009 +/- 3 ppm and delta(perpendicular) = 1132 +/- 3 ppm; delta(iso) = -1091 +/- 3 ppm. The Pb4+-Pb2+ scalar coupling constant J(Pb-Pb) = 2.3 +/- 0.1 kHz. The main contribution to the Pb2- chemical-shift anisotropy is proposed to arise from an exchange interaction in the Pb2+-Pb2+ pairs, conventionally regarded as molecular [Pb2]4+ ions.     

19F NMR技术在蛋白质研究中的应用

因为¹⁹F核自身的特性,如自旋为1/2;丰度高(100%),具有与¹H相当的灵敏度;化学位移分布广且对环境敏感;绝大部分生物体内都不含¹⁹F因而无背景干扰等;¹⁹F NMR自诞生以来就成为非常有吸引力的研究手段. 现在,¹⁹F NMR已被广泛用于核磁共振成像、药物化学及生物大分子的研究,各个方面均有较多的相关文献综述. 该综述将集中讨论¹⁹F NMR在蛋白质研究中的应用,包括蛋白质¹⁹F标记方法,¹⁹F NMR在蛋白质结构、 动力学、蛋白质折叠、蛋白质与药物的相互作用及In-cell NMR中的应用.     

An investigation of range distribution parameters of implanted 19F ions in Au,Ag and Cu noble metals

Depth profiles of fluorine implanted in gold, silver and copper metal have been accurately measured using the ¹⁹F(p,αγ)¹⁶O resonance nuclear reaction at E_R = 872.1 keV. A proper deconvolution calculation method was used to extract the depth profile of fluorine from the experimental excitation yield curves. The range parameters, R_p, ΔR_p, and SK obtained in this work were compared with the theoretical calculations. The comparison shows that for all materials studied here the experimental R_p values are in good agreement with the results obtained by the TRIM96 code, while the experimental range stragglings, ΔR_p, are systematically larger than the TRIM simulation results.     

Measurement of the principal values of the anisotropic diffusion tensor in an unoriented sample by exploiting the chemical shift anisotropy: (19)F PGSE NMR with homonuclear decoupling

NMR methods (S. V. Dvinskikh et al., J. Magn. Reson. 142, 102-110 (2000) and S. V. Dvinskikh and I. Furó, J. Magn. Reson. 144, 142-149 (2000)) that combine PGSE with dipolar decoupling are extended to polycrystalline solids and unoriented liquid crystals. Decoupling suppresses dipolar dephasing not only during the gradient pulses but also under signal acquisition so that the detected spectral shape is dominated by the chemical shift tensor of the selected nucleus. The decay of the spectral intensity at different positions in the powder spectrum provides the diffusion coefficient in sample regions with their crystal axes oriented differently with respect to the direction of the field gradient. Hence, one can obtain the principal values of the diffusion tensor. The method is demonstrated by (19)F PGSE NMR with homonuclear decoupling in a lyotropic lamellar liquid crystal.     

In vivo NMR spectroscopy: in situ 15N pulse labelling NMR spectroscopy with photoautotrophic microorganisms

For studying the nitrogen metabolism in plants ¹⁵N NMR spectroscopy can be used. For in vivo ¹⁵N NMR (natural abundance of ¹⁵N: 0.37%) enrichment of the sample with the isotope ¹⁵N is compulsory. The detection of time courses of ¹⁵N assimilation from cells, which are enriched in culture is restricted in scope. Here, a method, the ¹⁵N pulse labelling NMR spectroscopy, is demonstrated, which permits labelling of different nitrogen compounds in photoautotrophic microorganisms during the NMR spectroscopic measurement. Using an effective illumination system it is possible to maintain photosynthesis in plant samples of high biomass densities in the magnet necessary for ammonia assimilation. The technique thus enables to directly observe ammonia assimilation pathways by application of a ¹⁵NO₃ ^? or ¹⁵NH₄ ^? pulses.

Für das Studium des Stickstoffstoffwechsels der Pflanzen kann die ¹⁵N-NMR-Spektroskopie herangezogen werden. Hierzu ist bei der in-vivo-¹⁵N-NMR (natürliche Häufigkeit von ¹⁵N: 0.37%) eine Anreicherung der Probe mit dem Isotop ¹⁵N unerläßlich. Eine Verfolgung der ¹⁵N-Assimilationskinetik mit Zellen, die in der Kultur angereichert wurden, ist jedoch nur bedingt möglich. In dieser Arbeit wird die ¹⁵N-Pulsmarkierungs-NMR-Spektroskopie als eine Methode vorgestellt, die es erlaubt, eine Markierung von Stickstoffverbindungen in photoautotrophen einzelligen Mikroorganismen während der NMR-Messung im Magneten vorzunehmen. Es wird ein spezielles Beleuchtungssystem verwendet, das eine für die Stickstoffassimilation ausreichende Photosyntheseleistung der Zellen unter NMR-Bedingungen bei hoher Biomassedichte ermöglicht. Diese Technik erlaubt durch die Applikation eines ¹⁵NO₃ ^?-oder ¹⁵NH₄ ⁺-Pulses eine direkte Verfolgung von Ammonium-Assimilationswegen.     

Oxygen-sensitive 19F NMR imaging of the vascular system in vivo

The fluorine nuclear magnetic resonance spin-lattice relaxation rate (1/T1) of the perfluorochemical blood substitute perfluorotripropylamine (FTPA) is very sensitive to oxygen tension. This presents the possibility of measuring blood oxygen tension by 19F MR imaging. We obtained oxygen-sensitive 19F NMR images of the circulatory system of rats infused with emulsified FTPA. Blood oxygenation was assessed under conditions of both air- and 100% O2-breathing. T1 relaxation times were derived from MR images using a partial saturation pulse sequence. The T1 times were compared with a phantom calibration curve to calculate average blood pO2 values in the lung, liver, and spleen. The results showed marked, organ-specific increases in blood oxygen tension when the rat breathed 100% O2 instead of air.     

Hydrogen Bond and 19F NMR Chemical Shift Akisotropy in Apatite

Abstract

The fluoroapatite minerals, Ca₅-F(PO₄)₃ usually contain some hydroxyis, instead of fluorine. From infrared spectra it was concluded, that there are weak hydrogen bonds OH … F along apatite hexagonal axis¹. In order to receive more detailed information about the character of F^?and OH^?bonds with the nearest atoms in apatites we studied the nuclear magnetic resonance (NMR) spectra of protons and fluorines in natural apatite single crystal, in which approximately 15% of fluorines is substituted by hydroxyis.