NMR Probe (nmr + probe)

Distribution by Scientific Domains

Selected Abstracts

Chemical shift imaging (CSI) by precise object displacement,,

Sebastien Leclerc
Abstract A mechanical device (NMR lift) has been built to displace vertically an object (typically an NMR sample tube) inside the NMR probe with an accuracy of 1 m. A series of single pulse experiments are performed for incremented vertical positions of the sample. With a sufficiently spatially selective radio-frequency (r.f.) field, one obtains chemical shift information along the displacement direction (one-dimensional chemical shift imaging (CSI)). Knowing the vertical r.f. field profile (the amplitude of the r.f. field along the vertical direction), one can reconstruct the spectrum associated with all the slices corresponding to consecutive sample positions and improve the spatial resolution, which is simply related to the accuracy of the displacement device. Beside tests performed on phantoms, the method has been applied to solvent penetration in polymers and to benzene diffusion in a heterogeneous zeolite medium. Copyright 2006 John Wiley & Sons, Ltd. [source]

Phase transition of L -Ser monohydrate crystal studied by 13C solid-state NMR,

Tsunenori Kameda
Abstract We used gravimetric analysis (GA) and 13C solid-state nuclear magnetic resonance (NMR) to study solid-phase transition from the transparent single crystal of L -serine (L -Ser) monohydrate to a turbid powder. We found that L -Ser monohydrate loses water molecules and transforms into an anhydrate, thus experimentally demonstrating Frey's assumption (Acta Cryst., B29, 876, 1973). Application of a handmade cross-polarization (CP) NMR probe with a saddle-type coil to the oriented crystal of the L -Ser monohydrate revealed the dehydration mechanism. Furthermore, the chemical shift tensor components of the carboxyl carbon in L -Ser monohydrate were determined. The difference in the tensor component of ,22 between the monohydrate and anhydrate forms was more than 7 ppm, probably owing to differences in the hydrogen-bonding structure of each form. Copyright 2006 John Wiley & Sons, Ltd. [source]

The design of a multi-dimensional LC-SPE-NMR system (LC2 -SPE-NMR) for complex mixture analysis,

A. J. Alexander
Abstract In this communication, we describe the design of an online multi-chromatographic approach to the routine NMR analyses of low-level components (,0.1%) in complex mixtures. The technique, termed LC2 -SPE-NMR, optimally combines multi-dimensional liquid chromatography with SPE technology for isolating, enriching and delivering trace analytes to the NMR probe. The fully automated LC2 -SPE-NMR system allows for maximal loading capacity (in the first, preparative LC dimension), close to optimal peak resolution (in the second, analytical LC dimension) and enhanced sample concentration (through SPE). Using this system, it is feasible to conveniently conduct a wide range of NMR experiments on, for example, drug impurities at the low microgram per milliliter level, even for components poorly resolved in the first dimension. Such a sensitivity gain significantly elevates the analytical power of online NMR technology in terms of the level at which substances of pharmaceutical significance can be structurally characterized. Copyright 2006 John Wiley & Sons, Ltd. [source]

HPLC,SPE,NMR hyphenation in natural products research: optimization of analysis of Croton membranaceus extract,

Maja Lambert
Abstract The HPLC,SPE,NMR technique was used for the analysis of a root-bark extract of Croton membranaceus. The components of the extract were separated on an analytical-size reversed-phase HPLC column, the chromatographic peaks were trapped on SPE (solid-phase extraction) cartridges after post-column dilution of the eluate with water and the compounds were eluted from the cartridges with acetonitrile- d3 into a 30 l 600 MHz NMR probe in a fully automated procedure. The trapping efficiency of scopoletin (1), the major extract constituent, was much higher on a GP (general phase, a polystyrene-type polymer) SPE phase than on a C18 phase. Thus, under the conditions used, up to 100 g of scopoletin per cartridge could be accumulated linearly after repeated trappings. The maximum achievable NMR signal-to-noise ratio using the GP cartridges was at least four times higher than that achievable with the C18 cartridges. It was shown that excessively long T1 relaxation times may compromise experiments in which acetonitrile- d3 is used as the cartridge eluent. Nevertheless, the sensitivity gain provided by the HPLC,SPE,NMR technique through repeated peak trappings allowed the acquisition of good-quality proton-detected 2D NMR spectra without the need for solvent suppression. Copyright 2005 John Wiley & Sons, Ltd. [source]

Computational NMR Spectroscopy of Transition-Metal/Nitroimidazole Complexes: Theoretical Investigation of Potential Radiosensitizers

The computed chemical shifts of transition-metal complexes with dimetridazole (=,1,2-dimethyl-5-nitro-1H -imidazole; 1), a prototypical nitro-imidazole-based radiosensitizer, are reported at the GIAO-BP86 and -B3LYP levels for BP86/ECP1-optimized geometries. These complexes comprise [MCl2(1)2] (M,=,Zn, Pd, Pt), [RuCl2(DMSO)2(1)2], and [Rh2(O2CMe)4(1)2]. Available ,(1H) and ,(15N) values, and ,,(1H) and ,,(15N) coordination shifts are well-reproduced theoretically, provided solvation and relativistic effects are taken into account by means of a polarizable continuum model and suitable methods including spin,orbit (SO) coupling, respectively. These effects are particularly important for the metal-coordinated N-atom, where the contributions from solvation and relativity can affect ,(15N) and ,,(15N) values up to 10,20,ppm. The 195Pt chemical shifts of cis - and trans -[PtCl2(1)2] are well-reproduced using the zero-order regular approximation including SO coupling (ZORA-SO). Predictions are reported for 99Ru and 103Rh chemical shifts, which suggest that these metal centers could be used as additional, sensitive NMR probes in their complexes with nitro-imidazoles. [source]

Probing the Local Structure of Pure Ionic Liquid Salts with Solid- and Liquid-State NMR,

CHEMPHYSCHEM, Issue 1 2010
Peter G. Gordon
Abstract Room-temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid-state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second-order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of ,10,8 sec or longer, RTILs behave as isotropic liquids without residual order. [source]