| 1. |
- McIntyre, Deane D., et al.
(författare)
-
Nitrogen-14 NMR relaxation, and reorientation behavior of dissolved dinitrogen
- 1989
-
Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856 .- 0022-2364. ; 83:2, s. 377-382
-
Tidskriftsartikel (refereegranskat)abstract
- In recent years, nitrogen- 14 and - 15 NMR spectroscopy has become an important technique in organic chemistry and in biochemistry ( Z-4). A frequent occurrence in the NMR spectra of both nuclei is the presence of a signal at approximately -7 1.5ppm (referred to neat nitromethane; -66 ppm referred to aqueous nitrate). This resonance, which has been observed in water (4) and a range of organic solvents (.5-7), has been the subject of some confusion in the literature. In the case of nitrogen-14 NMR spectra, the signal for the quadrupolar nucleus has a remarkably narrow linewidth (about 25 Hz) compared to those recorded for a wide variety of other substances. This indicates a fairly high degree of electronic symmetry about the nitrogenatom and/or a very short correlation time ( 7,). In a recent report (8)) this signal has been assigned to dissolved dinitrogen on the basis that it could be removed by degassing´the solution; however, no reference was made to the fact that it had a remarkably narrow linewidth compared to other 14N NMR resonances. When detected in the 15NCIDNP spectra of the decomposition products of diazonium ions (5, 6) as well as azo compounds ( 7)) the signal has been assigned either to dinitrogen (6, 7) or to a terminal diazonium nitrogen (5). This work is in general agreement with our own observations over a period of years which indicate that the signal arises from dissolved dinitrogen. We have measured the 14N chemical shift of dissolved N2 in a number of solvents at 25°C and have also determined the T, and T2 relaxation times under a variety of conditions in solvents of different viscosity. This Note is concerned with a discussion of these 14N NMR observations and with the determination of the correlation time of dissolved dinitrogen which permits the determination of both the enthalpy and the entropy of activation via the Eyring equation. These results will be compared with earlier reported data regarding the chemical shift and relaxation of liquid nitrogen obtained under a variety of conditions ( 9-12). All spectra were obtained on a Bruker AM-400 wide-bore NMR spectrometer operating in the FT mode at a frequency of 28.9 MHz for 14N and 40.5 MHz for 15N, using a 10 mm broadband probe. Typical conditions for the acquisition of 14N spectra
|
|
| 2. |
- Lindh, Erik L., et al.
(författare)
-
Site-resolved H-2 relaxation experiments in solid materials by global line-shape analysis of MAS NMR spectra
- 2016
-
Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856 .- 1064-1858 .- 1096-0864. ; 268, s. 18-24
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate a way one can achieve good spectral resolution in H-2 MAS NMR experiments. The goal is to be able to distinguish between and study sites in various deuterated materials with small chemical shift dispersion. We show that the H-2 MAS NMR spectra recorded during a spin-relaxation experiment are amenable to spectral decomposition because of the different evolution of spectral components during the relaxation delay. We verify that the results are robust by global least-square fitting of the spectral series both under the assumption of specific line shapes and without such assumptions (COmponent-REsolved spectroscopy, CORE). In addition, we investigate the reliability of the developed protocol by analyzing spectra simulated with different combinations of spectral parameters. The performance is demonstrated in a model material of deuterated poly(methacrylic acid) that contains two H-2 spin populations with similar chemical shifts but different quadrupole splittings. In H-2-exchanged cellulose containing two H-2 spin populations with very similar chemical shifts and quadrupole splittings, the method provides new site-selective information about the molecular dynamics.
|
|
| 3. |
- Johansson, Edvin, et al.
(författare)
-
Gradient echo imaging of flowing hyperpolarized nuclei : theory and phantom studies on 129Xe dissolved in ethanol
- 2002
-
Ingår i: Journal of Magnetic Resonance. - 1090-7807 .- 1096-0856. ; 159:1, s. 68-75
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of flip angle and flow velocity on the signal intensity achieved when imaging a hyperpolarized substance with a spoiled gradient echo sequence was investigated. The study was performed both theoretically and experimentally using hyperpolarized xenon dissolved in ethanol. Analytical expressions regarding the optimal flip angle with respect to signal and the corresponding signal level are presented and comparisons with thermally polarized substances are made. Both experimentally and theoretically, the optimal flip angle was found to increase with increasing flow velocity. Numerical calculations showed that the velocity dependence of the signal differs between the cases of hyperpolarized and thermally polarized substances.
|
|
| 4. |
- Dvinskikh, Sergey V., et al.
(författare)
-
Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences
- 2003
-
Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 164:1, s. 165-170
-
Tidskriftsartikel (refereegranskat)abstract
- A pulse sequence is described for the recoupling of heteronuclear dipolar interactions under MAS. The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two radio-frequency fields. The technique is used for measurements of H-1-C-13 dipolar couplings in unoriented solid and liquid-crystalline samples.
|
|
| 5. |
- Dvinskikh, Sergey V., et al.
(författare)
-
Separated local field spectroscopy of columnar and nematic liquid crystals
- 2003
-
Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 163:1, s. 46-55
-
Tidskriftsartikel (refereegranskat)abstract
- We are in this work comparing the efficiencies of various H-1-C-13 separated local field (SLF) experiments when applied to columnar and nematic liquid crystals. In particular, the performances of the conventional SLF, proton-detected local field (PDLF), and polarization inversion spin exchange at the magic angle (PISEMA) methods in terms of spectral resolution, robustness, and ability to measure long-range couplings are investigated. The PDLF sequence provides in most cases the best dipolar resolution. This is especially obvious for weakly coupled H-1-C-13 spin pairs.
|
|
| 6. |
- Ivchenko, Nickolay V., et al.
(författare)
-
Application of cogwheel phase cycling to sideband manipulation experiments in solid-state NMR
- 2003
-
Ingår i: Journal of magnetic resonance. - 1090-7807 .- 1096-0856. ; 164:2, s. 286-293
-
Tidskriftsartikel (refereegranskat)abstract
- Cogwheel phase-cycling schemes are applied to sideband suppression and sideband separation experiments in solid-state NMR. It is shown that cogwheel phase cycles lead to the elimination of most pulse imperfection effects, while using far fewer experimental signal acquisitions than conventional phase-cycling methods.
|
|
| 7. |
- Ivchenko, Nickolay V., et al.
(författare)
-
Multiplex phase cycling
- 2003
-
Ingår i: Journal of magnetic resonance. - 1090-7807 .- 1096-0856. ; 160:1, s. 52-58
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss a new class of phase cycling procedures, in which a set of individual phase-shifted transients are stored separately in the computer and processed afterwards to yield the separated NMR signals from two or more coherence transfer pathways. In the case of two-dimensional double-quantum spectroscopy, this multiplex acquisition procedure allows the acquisition of pure-absorption spectra in only 62.5% of the time needed by previous methods.
|
|
| 8. |
- Larsson, Erik G., et al.
(författare)
-
SNR-optimality of sum-of-squares reconstruction for phased-array magnetic resonance imaging
- 2003
-
Ingår i: Journal of magnetic resonance. - : Elsevier. - 1090-7807 .- 1096-0856. ; 163:1, s. 121-123
-
Tidskriftsartikel (refereegranskat)abstract
- We consider the commonly used “Sum-of-Squares” (SoS) reconstruction method for phased-array magnetic resonance imaging with unknown coil sensitivities. We show that the signal-to-noise ratio (SNR) in the image produced by SoS is asymptotically (as the input SNR→∞) equal to that of maximum-ratio combining, which is the best unbiased reconstruction method when the coil sensitivities are known. Finally, we discuss the implications of this result.
|
|
| 9. |
- Price, W. S., et al.
(författare)
-
Determination of pore space shape and size in porous systems using NMR diffusometry. Beyond the short gradient pulse approximation
- 2003
-
Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 160:2, s. 139-143
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of finite length gradient pulses on NMR diffusion experiments on liquids confined to diffuse between two parallel planes is investigated. It is experimentally verified that the pore size decreases when determined using finite gradient pulses if the results are analyzed within the short gradient pulse approximation. The results are analyzed using the matrix formulation. The observed minima in the echo decay profiles are considerably less sharp than theoretical analysis would indicate and we suggest that this is due to the presence of a distribution of pore sizes in the sample. In addition, effects due to the presence of background gradients are discussed. It is argued that effects due to the finite length gradient pulses are relatively minor and in realistic applications the effects due to inhomogeneities in pore sizes and effects due to background gradients will constitute more serious problems in pore size determinations by means of NMR diffusometry.
|
|
| 10. |
|
|
