| 1. |
- Dvinskikh, Sergey V., et al.
(författare)
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Anisotropic self-diffusion in thermotropic liquid crystals studied by H-1 and H-2 pulse-field-gradient spin-echo NMR
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:6
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Tidskriftsartikel (refereegranskat)abstract
- The molecular self-diffusion coefficients in nematic and smectic-A thermotropic liquid crystals are measured using stimulated-echo-type H-2 and H-1 pulse-field-gradient spin-echo nuclear magnetic resonance (PGSE NMR) combined with multiple-pulse dipolar decoupling and slice selection. The temperature dependence of the principal components of the diffusion tensor in the nematic phase follows a simple Arrhenius relationship except in the region of nematic-isotropic phase transition where it reflects, merely, the decrease of the molecular orientational order. The average of the principal diffusion coefficients in the isotropic-nematic phase transition region is close to the diffusion coefficient in the isotropic phase. At the nematic-smectic-A phase transition the diffusion coefficients change continuously. The results in nematic phase are best described in terms of the affine transformation model for diffusion in nematics formed by hard ellipsoids. In the smectic-A phase the data are interpreted using a modified model for diffusion in presence of a periodic potential along the director.
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| 2. |
- Dvinskikh, Sergey V., et al.
(författare)
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C-13-detected H-1-H-2 separated local field NMR spectroscopy
- 2003
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 382:04-mar, s. 410-417
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Tidskriftsartikel (refereegranskat)abstract
- We present a new NMR method for measuring H-1-H-2 dipolar couplings in macroscopically oriented media. To overcome the lack of dipolar resolution in 1D H-1 and H-2 spectra of deuterated molecules, we use a 2D heteronuclear correlation experiment where H-1 chemical shifts and H-1-H-2 dipolar interactions in the first dimension are. correlated with C-13 chemical shifts and H-2-C-13 dipolar interactions in the second dimension. The technique is demonstrated on a columnar liquid-crystalline phase.
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| 3. |
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| 4. |
- Dvinskikh, Sergey V., et al.
(författare)
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Deuterium stimulated-echo-type PGSE NMR experiments for measuring diffusion : Application to a liquid crystal
- 2001
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 153:1, s. 83-91
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Tidskriftsartikel (refereegranskat)abstract
- The accessibility of molecular self-diffusion coefficients in anisotropic materials, such as liquid crystals or solids, by stimulated-echo-type H-2 PGSE NMR is examined. The amplitude and phase modulation of the signal in the stimulated-echo-type sequence by the static quadrupole coupling during the encoding/decoding delays is suppressed by adjusting the pulse flip angles and the phase cycle. For nuclei that experience both nonnegligible quadrupole and dipole couplings, the application of magic echoes during the evolution periods of stimulated echo is demonstrated as a helpful technique in the case of slow diffusion. These findings are demonstrated by experimental results in the thermotropic liquid crystal of partially deuterated 8CB. The obtained diffusion coefficients are also compared to data obtained by a H-1 homonuclear-decoupling-type PGSE NMR method in the same material.
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| 5. |
- Dvinskikh, Sergey V., et al.
(författare)
-
Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences
- 2003
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 164:1, s. 165-170
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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.
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| 6. |
- Dvinskikh, Sergey V., et al.
(författare)
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Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-, phase-, and frequency-modulated Lee-Goldburg cross-polarization
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 122:4
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a theoretical, numerical, and experimental study of phase- and frequency-switched Lee-Goldburg cross-polarization (FSLG-CP) under magic-angle spinning conditions. It is shown that a well-defined amplitude modulation of one of the two radio-frequency (rf) fields in the FSLG-CP sequence results in highly efficient heteronuclear dipolar recoupling. The recoupled dipolar interaction is gamma-encoded and, under ideal conditions, the effective spin Hamiltonian is equivalent to that in continuous-wave Lee-Goldburg CP. In practice, however, FSLG-CP is less susceptible to rf field mismatch and inhomogeneity, and provides better suppression of H-1 spin diffusion. The performance of FSLG-CP is experimentally demonstrated on liquid-crystalline samples exhibiting motionally averaged dipolar couplings.
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| 7. |
- Dvinskikh, Sergey V., et al.
(författare)
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Measurements of motionally averaged heteronuclear dipolar couplings in MAS NMR using R-type recoupling
- 2004
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 168:2, s. 194-201
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Tidskriftsartikel (refereegranskat)abstract
- A novel MAS NMR approach is presented for the determination of heteronuclear dipolar couplings in unoriented materials. The technique is based on the proton-detected local field (PDLF) protocol, and achieves dipolar recoupling by R-type radio-frequency irradiation. The experiment, which is called R-PDLF spectroscopy, is demonstrated on solid and liquid-crystalline systems. For mobile systems, it is shown that the R-PDLF scheme provides better dipolar resolution as compared to techniques combining conventional separated local field (SLF) spectroscopy with R-type recoupling.
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| 8. |
- Dvinskikh, Sergey V., et al.
(författare)
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Molecular characterization of hexaoctyloxy-rufigallol in the solid and columnar phases : A local field NMR study
- 2003
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 107:9, s. 1969-1976
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Tidskriftsartikel (refereegranskat)abstract
- Separated local field NMR spectroscopy was used to obtain H-1-C-13 dipolar interactions in the solid and columnar phases of the discotic compound 1,2,3,5,6,7-hexaoctyloxy-rufigallol (RufH8O). The couplings were measured and assigned by a variety of advanced NMR techniques performed under both static and magic-angle spinning conditions. The analysis of the dipolar couplings shows that the three nonequivalent aliphatic chains in RufH8O exhibit vastly different structural and dynamical characteristics. It was also found that one of the alpha methylene signals exhibits two H-1-C-13 dipolar splittings at low temperatures in the columnar phase. Similar pairs of doublets have previously been observed in a H-2 NMR study of RufH8O-H-2(alpha) and were there interpreted in terms of dynamic packing fluctuations along the columns. We will present an alternative explanation and show that these two doublets most likely originate from nonequivalent protons (or deuterons) in the alpha methylene group and not from density modulations.
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| 9. |
- Dvinskikh, Sergey V., et al.
(författare)
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Molecular self-diffusion in a columnar liquid crystalline phase determined by deuterium NMR
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:5
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Tidskriftsartikel (refereegranskat)abstract
- We report translational-diffusion coefficients in a columnar phase of a discotic liquid crystal formed by a triphenylpne-based compound. The experiments were performed using H-2 stimulated-echo-type pulsed-field-gradient spin-echo NMR applied to a chain-deuterated sample. The diffusion coefficients were found in the range of 1 x 10(-14)-4 x 10(-14) m(2)/s, three orders of magnitude lower than in the isotopic phase of the same compound. This, together with the high activation energy obtained in, columnar phase, indicates that the diffusion is dominated by solidlike jump processes.
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| 10. |
- Dvinskikh, Sergey V., et al.
(författare)
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Molecular structure and order of hexaoctyloxy-rufigallol in the solid and columnar phases : Analysis of H-2-C-13 dipolar and C-13 chemical-shift interactions
- 2003
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 119:1, s. 413-422
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Tidskriftsartikel (refereegranskat)abstract
- We report a C-13 NMR investigation of 1,2,3,5,6,7-hexaoctyloxy-rufigallol in the solid and columnar phases using various H-2- and C-13-labeled isotopomers. The measurements were performed under both static and magic-angle spinning conditions. The conformations of the aliphatic side chains were derived from an analysis of H-2-C-13 dipolar and C-13 chemical-shift interactions. The nonequivalent chains exhibit significantly different structural behavior in both solid and columnar phase. It emerged that one of the side chains adopts an in-plane structure, while the other two chains prefer antiparallel out-of-plane conformations. Quantum-chemical calculations of the C-13 chemical-shift tensors were also carried out. These calculations support the experimental findings.
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