| 1. |
- Abrahamsson, Christoffer, 1984, et al.
(författare)
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Magnetic orientation of nontronite clay in aqueous dispersions and its effect on water diffusion
- 2015
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 437, s. 205-210
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Tidskriftsartikel (refereegranskat)abstract
- The diffusion rate of water in dilute clay dispersions depends on particle concentration, size, shape, aggregation and water-particle interactions. As nontronite clay particles magnetically align parallel to the magnetic field, directional self-diffusion anisotropy can be created within such dispersion. Here we study water diffusion in exfoliated nontronite clay dispersions by diffusion NMR and time-dependant 1H-NMR-imaging profiles. The dispersion clay concentration was varied between 0.3 and 0.7 vol%. After magnetic alignment of the clay particles in these dispersions a maximum difference of 20% was measured between the parallel and perpendicular self-diffusion coefficients in the dispersion with 0.7 vol% clay. A method was developed to measure water diffusion within the dispersion in the absence of a magnetic field (random clay orientation) as this is not possible with standard diffusion NMR. However, no significant difference in self-diffusion coefficient between random and aligned dispersions could be observed.
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| 2. |
- Castro, Vasco, et al.
(författare)
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NMR investigations of interactions between anesthetics and lipid bilayers
- 2008
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Ingår i: Biochimica et Biophysica Acta - Biomembranes. - : ELSEVIER SCIENCE BV. - 0005-2736 .- 1879-2642. ; 1178:11, s. 2604-2611
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Tidskriftsartikel (refereegranskat)abstract
- Interactions between anesthetics (lidocaine and short chain alcohols) and lipid membranes formed by dimyristoylphosphatidylcholine (DMPC) were studied using NMR spectroscopy. The orientational order of lidocaine was investigated using deuterium NMR on a selectively labelled compound whereas segmental ordering in the lipids was probed by two-dimensional 1H–13C separated local field experiments under magic-angle spinning conditions. In addition, trajectories generated in molecular dynamics (MD) computer simulations were used for interpretation of the experimental results. Separate simulations were carried out with charged and uncharged lidocaine molecules. Reasonable agreement between experimental dipolar interactions and the calculated counterparts was observed. Our results clearly show that charged lidocaine affects significantly the lipid headgroup. In particular the ordering of the lipids is increased accompanied by drastic changes in the orientation of the P–N vector in the choline group.
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| 3. |
- Castro, Vasco, et al.
(författare)
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NMR studies of membranes composed of glycolipids and phospholipids
- 2007
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Ingår i: Biochimica et Biophysica Acta - Biomembranes. - : Elsevier BV. - 0005-2736 .- 1879-2642. ; 1768, s. 2432-2437:1768, s. 2432-2437
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Tidskriftsartikel (refereegranskat)abstract
- Lipid membranes composed of monogalactosyldiacylglycerol (MGDG) and dimyristoylphosphatidylcholine (DMPC) were studied by means of NMR spectroscopy. The macroscopic phase behaviour was investigated by P-31 NMR under stationary conditions, whereas microscopic properties such as segmental ordering were probed by two-dimensional H-1-C-13 separated local field experiments under magic-angle spinning conditions. Our results clearly show that ordering/disordering effects occur for the headgroups as well as for the acyl chains when the sample composition is varied. In particular, the H-1-C-13 dipolar couplings within the galactose headgroup of MGDG exhibited significant concentration dependence.
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| 4. |
- Cifelli, M., et al.
(författare)
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N-15-C-13 Dipole Couplings in Smectic Mesophase of a Thermotropic Ionic Liquid
- 2018
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Ingår i: Applied Magnetic Resonance. - : SPRINGER WIEN. - 0937-9347 .- 1613-7507. ; 49:6, s. 553-562
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Tidskriftsartikel (refereegranskat)abstract
- Unique combination of ionic conductivity and anisotropic physical properties in ionic liquid crystals leads to new dynamic properties exploited in modern technological applications. Structural and dynamics information at atomic level for molecules and ions in mesophases can be obtained by nuclear magnetic resonance (NMR) spectroscopy through the measurements of dipole-dipole spin couplings. While C-13-H-1 and N-15-H-1 dipolar NMR spectra can be routinely acquired in samples with natural isotopic abundance, recording N-15-C-13 dipolar NMR spectra is challenging because of the unfavourable combination of two rare isotopes. In the present study, an approach to measure N-15-C-13 dipole-dipole NMR spectra in static liquid crystalline samples with natural abundance is introduced. We demonstrate that well-resolved spectra can be recorded within 10 h of experimental time using a conventional NMR probe and a moderately strong magnetic field. The technique is applied to a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation.
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| 5. |
- Cifelli, M., et al.
(författare)
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NMR investigation of a thermotropic liquid crystal showing isotropic-isotropic'-(columnar)-cubic phase transitions
- 2017
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Ingår i: Molecular Crystals and Liquid Crystals. - : Taylor and Francis Inc.. - 1542-1406 .- 1563-5287. ; 649:1, s. 20-30
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we report a first detailed study of the mesophasic behaviour of the calamitic liquid crystal, a (4,4′-bis(4″-carboxybelzyloxy)biphenyl derivate, labelled 9/2 RS/RS, showing an isotropic-isotropic-(columnar)-cubic phase behaviour on cooling. 1H NMR diffusometry allowed to distinguish between the two isotropic phases in terms of diffusion characterized by different Arrhenius activation energy, while 13C NMR static measurements and 2H NMR investigations were of help in understanding the orientational ordering properties of the metastable columnar phase and confirmed the isotropic state of the sample in the cubic phase.
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| 6. |
- Cifelli, M., et al.
(författare)
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Study of Translational Diffusion Anisotropy of Ionic Smectogens by NMR Diffusometry
- 2015
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Ingår i: Molecular Crystals and Liquid Crystals. - : Informa UK Limited. - 1542-1406 .- 1563-5287. ; 614:1, s. 30-38
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Tidskriftsartikel (refereegranskat)abstract
- Thermotropic ionic liquid crystals are considered very promising for a wide range of applications, such as anisotropic conductors as well as electrolytes in dye-synthesized solar cells. Their potential comes from the unique combination of ionic conductivity and high polarizability. In this paper we present a study of the diffusional properties of the ionic smectic A phase formed by the cationic smectogen, N-docecyl-N'-methyl-imidazolium (C12mim), with two different counter ions. Experimental translational diffusion data measured by NMR diffusometry are collected both for cations and anions and discussed in terms of their different anisotropy.
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| 7. |
- Cifelli, M., et al.
(författare)
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The twist-bend nematic phase : translational self-diffusion and biaxiality studied by H-1 nuclear magnetic resonance diffusometry
- 2017
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Ingår i: Liquid crystals (Print). - : TAYLOR & FRANCIS LTD. - 0267-8292 .- 1366-5855. ; 44:1, s. 204-218
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Tidskriftsartikel (refereegranskat)abstract
- Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N-TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N-TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N-TB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1 '',7 ''-bis-4-(4'-cyanobiphenyl-4'-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase. [GRAPHICS] .
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| 8. |
- Cifelli, Mario, et al.
(författare)
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Translational self-diffusion in the smectic phases of ferroelectric liquid crystals : an overview
- 2012
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Ingår i: Phase Transitions. - : Informa UK Limited. - 0141-1594 .- 1029-0338. ; 85:10, s. 861-871
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present our recent results obtained in the field of translational self-diffusion studies by means of H-1 NMR diffusometry in smectic phases formed by calamitic chiral liquid crystals. In particular, the diffusional behavior of chiral smectic phases with different clinicity, such as the ferroelectric and antiferroelectric phases, will be discussed on the basis of the results obtained on three different chiral smectogens. This research demonstrates that, despite previous assertions, out-of-plane translational diffusion in smectic phases is poorly affected by the clinicity of the phases, showing no discontinuity at the transition from synclinic to anticlinic structures, typical of the smectic C* phases. We can conclude that diffusion basically reflects the layered smectic structure where in-plane diffusion is much faster and less hindered than the out-of-plane one.
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| 9. |
- Cifelli, Mario, et al.
(författare)
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Translational self-diffusion in the synclinic to anticlinic phases of a ferroelectric liquid crystal
- 2010
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Ingår i: SOFT MATTER. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 6:23, s. 5999-6003
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the first direct measurement of inter and intra-layer molecular self-diffusion in the smectic phases of a chiral smectogen (S)-2-methylbutyl-[4'-(4 ''-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')benzoyl)-propionyl)]-pro pionate (ZLL7/*) by means of pulsed field gradient NMR is reported. Self-diffusion coefficients as low as 10(-13) m(2) s(-1) have been measured by combining magic echo spin decoupling with a stimulated echo sequence. The transitions among synclinic-anticlinic smectic phases are associated with a change in the self-diffusion coefficients.
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| 10. |
- Dahlberg, Carina, et al.
(författare)
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Polymer mobilization and drug release during tablet swelling. A 1H NMR and NMR microimaging study
- 2007
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Ingår i: Journal of Controlled Release. - : Elsevier BV. - 0168-3659 .- 1873-4995. ; 122, s. 199-205
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this study was to investigate the swelling characteristics of a hydroxypropyl methylcellulose (HPMC) matrix incorporating the hydrophilic drug antipyrine. We have used this matrix to introduce a novel analytical method, which allows us to obtain within one experimental setup information about the molecular processes of the polymer carrier and its impact on drug release. Nuclear magnetic resonance (NMR) imaging revealed in situ the swelling behavior of tablets when exposed to water. By using deuterated water, the spatial distribution and molecular dynamics of HPMC and their kinetics during swelling could be observed selectively. In parallel, NMR spectroscopy provided the concentration of the drug released into the aqueous phase. We find that both swelling and release are diffusion controlled. The ability of monitoring those two processes using the same experimental setup enables mapping their interconnection, which points on the importance and potential of this analytical technique for further application in other drug delivery forms.
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| 11. |
- Dahlberg, Carina, et al.
(författare)
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Polymer Swelling, Drug Mobilization and Drug Recrystallization in Hydrating Solid Dispersion Tablets Studied by Multinuclear NMR Microimaging and Spectroscopy
- 2011
-
Ingår i: Molecular Pharmaceutics. - : American Chemical Society (ACS). - 1543-8384 .- 1543-8392. ; 8:4, s. 1247-1256
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Tidskriftsartikel (refereegranskat)abstract
- Despite the advantages offered by solid dispersions, the marketed products based on this technology are few. The most frequent concern is the stability of the amorphous drug. The state of the drug in solid dispersions is, in general, poorly characterized as the number of characterization techniques available to monitor nanometer-sized drug particles embedded in a matrix are limited. Here we present a combination of localized NMR spectroscopic and NMR imaging techniques which allow in situ monitoring of the state of the drug during tablet disintegration and dissolution. (19)F NMR relaxation is shown to be sensitive to both the crystalline/amorphous state and the size of the model nanoparticles made of the drug substance flutamide. The time course of drug mobilization and recrystallization is detected with spatial resolution within swelling solid dispersion tablets. Comparing results from spatially resolved (19)F, (2)H and (1)H NMR experiments, recrystallization is related to its enabling factors such as local hydration level and local mobility of the polymer matrix. The initially amorphous drug may recrystallize either by nanoparticle coalescence or by ripening of crystalline grains.
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| 12. |
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| 13. |
- Dai, Jing, et al.
(författare)
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Molecular and Segmental Orientational Order in a Smectic Mesophase of a Thermotropic Ionic Liquid Crystal
- 2019
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Ingår i: Crystals. - : MDPI. - 2073-4352. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate conformational dynamics in the smectic A phase formed by the mesogenic ionic liquid 1-tetradecyl-3-methylimidazolium nitrate. Solid-state high-resolution C-13 nuclear magnetic resonance (NMR) spectra are recorded in the sample with the mesophase director aligned in the magnetic field of the NMR spectrometer. The applied NMR method, proton encoded local field spectroscopy, delivers heteronuclear dipolar couplings of each C-13 spin to its H-1 neighbours. From the analysis of the dipolar couplings, orientational order parameters of the C-H bonds along the hydrocarbon chain were determined. The estimated value of the molecular order parameter S is significantly lower compared to that in smectic phases of conventional non-ionic liquid crystals.
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| 14. |
- Dai, Jing, et al.
(författare)
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NMR Spectroscopic Study of Orientational Order in Imidazolium-Based Ionic Liquid Crystals
- 2019
-
Ingår i: Crystals. - : MDPI. - 2073-4352. ; 9:10
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Tidskriftsartikel (refereegranskat)abstract
- We report on molecular and local orientational order of a series of imidazolium-based ionic liquid crystals exhibiting layered smectic A mesophase. Materials constituting of 1-dodecyl-3-methylimidazolium cation, and different counter-ions, were investigated. We apply two-dimensional C-13-H-1 dipolar NMR spectroscopy to quantify orientational order of C-H bonds of the organic cation. The experimental data supported the structural model of the interdigitated chains aligned with the smectic layer normal. Molecular order parameter S was found to increase in the anion sequence BF4- < I- < Br- < Cl-. This trend correlates well with ionic radius, negative charge delocalization, and hydrogen-bonding properties of the anions.
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| 15. |
- Dvinskikh, Sergey, et al.
(författare)
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13C Nmr Studies Of Columnar Liquid Crystals
- 2007
-
Ingår i: New Research on Magnetic Resonance. - New York : Nova Science Publishers, Inc.. - 1600211402 ; , s. 137-186
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Bokkapitel (refereegranskat)
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| 16. |
- Dvinskikh, Sergey, 1963-
(författare)
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Chapter 13 : Characterization of Liquid-crystalline Materials by Separated Local Field Methods
- 2018
-
Ingår i: New Developments in NMR. - Cambridge : Royal Society of Chemistry. ; , s. 391-423
-
Bokkapitel (refereegranskat)abstract
- A unique feature of liquid crystals is a high degree of molecular mobility combined with orientational and positional order. Solid-state NMR contributes to fundamental understanding of diverse molecular organizations and complex dynamic processes in these exciting materials. The focus of this chapter is on the development and application of advanced solid-state NMR methodologies for liquid crystal studies, with emphasis on techniques for measuring anisotropic spin couplings. The discussion centers on applications of separated dipolar local field NMR spectroscopy, which is used in an increasing number of directions in studies of novel liquid-crystalline materials for emerging technological applications.
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| 17. |
- Dvinskikh, Sergey, 1963-, et al.
(författare)
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Moisture content profiles and uptake kinetics in wood cladding materials evaluated by a portable nuclear magnetic resonance spectrometer
- 2011
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Ingår i: Wood Material Science & Engineering. - : Taylor & Francis Group. - 1748-0272 .- 1748-0280. ; 6:3, s. 119-127
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Tidskriftsartikel (refereegranskat)abstract
- We have evaluated the capability of nuclear magnetic resonance (NMR) technology based on small portable magnets for in situ studies of the local moisture content in wood. Low field and low resolution 1H NMR with a unilateral permanent magnet was used to monitor and map the moisture content of wood cladding materials of various types in a spatially resolved manner. The results show that portable NMR equipment based on small open-access permanent magnets can be successfully used for non-invasive monitoring of the moisture content in various extended wood specimens. The moisture content was measured with a depth resolution of 0.2 mm and a maximum penetration depth of 3 mm. This makes the technique suitable for e.g. in situ local moisture content measurements beneath a coating layer in the claddings and it is also possible to relate the moisture level to specific properties of the wood material.
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| 18. |
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Dvinskikh, Sergey V., et al.
(författare)
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A multinuclear magnetic resonance imaging (MRI) study of wood with adsorbed water : Estimating bound water concentration and local wood density
- 2011
-
Ingår i: Holzforschung. - 0018-3830 .- 1437-434X. ; 65:1, s. 103-107
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between moisture and the macromolecular wood tissue is of critical importance to wood properties. In this context, magnetic resonance imaging (MRI) is very promising as this method could deliver molecular information on the submillimeter scale (i.e., along concentration gradients) about both free and adsorbed water and the cell wall polymers. In the present study, it is demonstrated for the first time that wood containing adsorbed heavy water ((H2O)-H-2) can be studied by MRI based on separated images due to water (H-2 MRI) and cell wall polymers (H-1 MRI). Data confirm that in specimens equilibrated at controlled humidity there is a direct correlation between bound water content and relative density of the polymers in wood tissue; there is a strong variation across annual rings.
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| 23. |
- Dvinskikh, Sergey V., et al.
(författare)
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Anisotropic self-diffusion in nematic, smectic-A, and reentrant nematic phases
- 2012
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 86:3, s. 031704-
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the reentrant nematic phase has been actively investigated both experimentally and theoretically during the past few decades. Most studies concluded that, as concerning molecular dynamics, a reentrant nematic phase is essentially analogous to a conventional nematic one. Recent computer simulations [Mazza et al., Phys. Rev. Lett. 105, 227802 (2010)], however, predicted molecular translational self-diffusion along the phase director that was dominated by a collective transport mode and was, relative to that observed in a conventional nematic phase, enhanced by an order of magnitude. In the present work, the principal components of the diffusion tensor in a reentrant nematic phase are determined experimentally and compared to those in conventional nematic and smectic-A phases. We find that the temperature dependence of the translational diffusion in the two nematic phases, within experimental error, follows a uniform trend and can be adequately described in terms of available diffusion models in nematics. Hence, we find no evidence for enhanced diffusion but confirm instead the similarity of conventional and reentrant nematic phases with respect to molecular translational dynamics.
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| 24. |
- Dvinskikh, Sergey V., et al.
(författare)
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Anisotropic self-diffusion in the nematic phase of a thermotropic liquid crystal by H-1-spin-echo nuclear magnetic resonance
- 2001
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 115:4, s. 1946-1950
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Tidskriftsartikel (refereegranskat)abstract
- The orientation-dependent molecular diffusion in the nematic liquid crystal 4-pentyl-4'-cyanobiphenyl is measured to high accuracy. The applied nuclear magnetic resonance method combines multiple-pulse homonuclear dipolar decoupling with pulsed-field-gradient stimulated echo and slice selection. The obtained temperature dependencies of the principal diffusion coefficients are not described by a simple Arrhenius relationship but reflect the decrease of the molecular orientational order close to the nematic-to-isotropic phase transition. The geometric average of the principal diffusion coefficients is continuous with the diffusion coefficient in the isotropic phase. The results are best described in terms of the affine transformation model of diffusion in hard-ellipsoid nematics.
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| 25. |
- 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
-
Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:6
-
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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| 26. |
- 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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| 27. |
- Dvinskikh, Sergey V., et al.
(författare)
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C-13 PGSE NMR experiment with heteronuclear dipolar decoupling to measure diffusion in liquid crystals and solids
- 2000
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 142:1, s. 102-110
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Tidskriftsartikel (refereegranskat)abstract
- A new PGSE NMR experiment, designed to measure molecular diffusion coefficients in systems with nonvanishing static dipolar coupling, is described. The fast static dipolar dephasing of the single-quantum C-13 coherences is removed by multiple-pulse heteronuclear decoupling. The resulting slow dephasing of the C-13 coherences allows for inserting appropriate gradient pulses into the pulse sequence. The presence of the large magnetic field gradient reduces the efficiency of the decoupling sequences which is compensated for by introducing a scheme of sequential slice selection across the sample. The method is demonstrated by F-19-decoupled C-13 PGSE NMR experiments in a lyotropic nematic and lamellar liquid crystal,
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| 28. |
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| 29. |
- Dvinskikh, Sergey V., et al.
(författare)
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Combining PGSE NMR with homonuclear dipolar decoupling
- 2000
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 144:1, s. 142-149
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Tidskriftsartikel (refereegranskat)abstract
- A new robust approach for combining multiple-pulse homonuclear decoupling and ROSE NMR is introduced for accurately measuring molecular diffusion coefficients in systems with nonvanishing static homonuclear dipolar couplings. Homonuclear decoupling suppresses dipolar dephasing during the gradient pulses but its efficiency and scaling factor for the effective gradient vary across the sample because of the large variation of the frequency offset caused by the gradient. The resulting artifacts are reduced by introducing a slice selection scheme. The method is demonstrated by F-19 PGSE NMR experiments in a lyotropic liquid crystal.
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| 30. |
- Dvinskikh, Sergey V., et al.
(författare)
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Cross-polarization with radio-frequency field phase and amplitude modulation under magic-angle spinning conditions
- 2006
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Ingår i: Journal of Experimental and Theoretical Physics. - : Pleiades Publishing Ltd. - 1063-7761 .- 1090-6509. ; 102:1, s. 91-101
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Tidskriftsartikel (refereegranskat)abstract
- Nucleus cross-polarization technique in a rotating frame of reference is analyzed as applied to NMR experiments with sample magic-angle spinning. The concept of simultaneous phase and amplitude modulation is suggested. According to this suggestion, the form of the Hamiltonian of recoupled dipolar interaction remains unchanged if phase inversion is accompanied by inversion of the difference of radio-frequency (RF) field amplitudes. A theoretical treatment is given in terms of the average Hamiltonian theory. The concept is demonstrated experimentally and by numerical analysis for several particular cases. Periodic phase inversion in cross-polarization experiments was shown to have the practically important advantage of suppressing chemical shift interactions and the effect of inaccurate tuning of RF field parameters.
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| 31. |
- Dvinskikh, Sergey V., et al.
(författare)
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Cross-relaxation effects in stimulated-echo-type PGSE NMR experiments by bipolar and monopolar gradient pulses
- 2000
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 146:2, s. 283-289
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Tidskriftsartikel (refereegranskat)abstract
- Exchange of longitudinal spin polarization by dipolar cross relaxation between nonequivalent spins results in a modulation of the stimulated echo signal on increasing the encoding/decoding delays and in a multiexponential decap on increasing the diffusion time. These artifacts are suppressed by 180 degrees pulses inserted in the middle of the gradient encoding/decoding periods. The efficiency of the gradient encoding is preserved if bipolar gradient pulses are used instead of monopolar pulses. The behavior of the different pulse sequences is demonstrated by F-19 PGSE NMR experiments in a lyotropic liquid crystal in both isotropic micellar and oriented nematic phases.
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| 32. |
- 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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| 33. |
- Dvinskikh, Sergey V., et al.
(författare)
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Domain structure in an unoriented lamellar lyotropic liquid crystal phase studied by H-2 NMR
- 2001
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 17:21, s. 6455-6460
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Tidskriftsartikel (refereegranskat)abstract
- The spatial variation of the phase director in an unoriented lamellar lyotropic liquid crystal is investigated by H-2 two-dimensional and one-dimensional exchange and PGSE NMR. Exchange NMR probes the single-particle orientational correlation function of D2O molecules that diffuse among regions with different director orientations. The obtained correlation time and the water diffusion coefficient, measured by H-2 PGSE NMR, provide the persistence length of director orientation that is defined as domain size. The nature of spatial variation is revealed by the decay of the H-2 stimulated echo signal recorded with different evolution times. The persistence length of the director is found to be strongly dependent on the rate of cooling the sample from its isotropic phase.
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| 34. |
- Dvinskikh, Sergey V., et al.
(författare)
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Efficient solid-state NMR methods for measuring heteronuclear dipolar couplings in unoriented lipid membrane systems
- 2005
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 7:4, s. 607-613
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Tidskriftsartikel (refereegranskat)abstract
- Recently (Dvinskikh et al., J. Magn. Reson., 2003, 164, 165 and Dvinskikh et al., J. Magn. Reson., 2004, 168, 194), some of us introduced two efficient solid-state NMR techniques for the determination of heteronuclear dipolar couplings under magic-angle spinning (MAS). These two-dimensional (2D) recoupling methods have been applied previously to simple amino acids, and to columnar systems with high positional and orientational order. In this work, we show that the 2D MAS sequences produce unparalleled H-1-C-13 dipolar resolution in unoriented lipid membranes. The recoupling experiments were applied to hydrated dimyristoylphosphatidylcholine (DMPC) in the liquid-crystalline L-alpha phase, and the results agreed well with previous NMR investigations using specifically deuterated phospholipids.
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| 35. |
- Dvinskikh, Sergey V., et al.
(författare)
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Frequency offset refocused PISEMA-type sequences
- 2005
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 175:1, s. 163-169
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Tidskriftsartikel (refereegranskat)abstract
- The popular PISEMA experiment is highly sensitive to the H-1 chemical shift dispersion and the choice of the H-1 carrier frequency. This is due to the off-resonance H-1 irradiation in the FSLG-CP sequence employed during the dipolar evolution period. In the modified approach described in this work, the interfering frequency offset terms are suppressed. In the new pulse schemes, conventional FSLG-CP is intercalated with 180 degrees pulses applied Simultaneously to both frequency channels, and with phases set orthogonal to those of the spin-lock fields. The technique is demonstrated on a nematic liquid-crystalline sample. Extensions to amplitude-modulated FSLG-CP recoupling under MAS are also presented.
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| 36. |
- Dvinskikh, Sergey V., et al.
(författare)
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Heating caused by radiofrequency irradiation and sample rotation in C-13 magic angle spinning NMR studies of lipid membranes
- 2004
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Ingår i: Magnetic Resonance in Chemistry. - : Wiley. - 0749-1581 .- 1097-458X. ; 42:10, s. 875-881
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Tidskriftsartikel (refereegranskat)abstract
- Application of rapid sample rotation and radiofrequency irradiation in magic angle spinning (MAS) NMR of lipid bilayers can significantly increase the sample temperature. In this work, we studied the extent of heating during the acquisition of H-1-decoupled C-13 MAS spectra of hydrated dimyristoylphosphatidyl-choline (DMPC) in the L. phase. First, we describe a simple procedure for determining the increase in temperature by observing the shift of the H-1 water signal. The method is then used to identify and assess the various factors that contribute to the sample heating. The important factors discussed in this paper include: (i) the spinning speed, (ii) the variable-temperature gas pressure, (iii) the rotor geometry, (iv) the power, duration and frequency of the radiofrequency irradiation and (v) the hydration level. A comparison of different heteronuclear decoupling schemes in terms of their ability to produce highly resolved C-13 spectra of DMPC is also reported.
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| 37. |
- Dvinskikh, Sergey V., et al.
(författare)
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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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| 38. |
- 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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| 39. |
- Dvinskikh, Sergey V., et al.
(författare)
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Heteronuclear isotropic mixing separated local field NMR spectroscopy
- 2006
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 125:3
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a theoretical, numerical, and experimental study of a new class of separated local field (SLF) techniques. These techniques are based on the heteronuclear isotropic mixing leading to spin exchange via the local field (HIMSELF). It is shown that highly efficient and robust SLF experiments can be designed based on double channel windowless homonuclear decoupling sequences. Compared to rotating frame techniques based on Hartmann-Hahn cross polarization, the new approach is less susceptible to the frequency offset and chemical shift interaction and can be applied in the structural studies of macromolecules that are uniformly labeled with isotopes such as C-13 and N-15. Furthermore, isotropic mixing sequences allow for transfer of any magnetization component of one nucleus to the corresponding component of its dipolar coupled partner. The performance of HIMSELF is studied by analysis of the average Hamiltonian and numerical simulation and is experimentally demonstrated on a single crystalline sample of a dipeptide and a liquid crystalline sample exhibiting motionally averaged dipolar couplings.
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| 40. |
- Dvinskikh, Sergey V., et al.
(författare)
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High-resolution 2D NMR spectroscopy of bicelles to measure the membrane interaction of ligands
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:4, s. 794-802
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Tidskriftsartikel (refereegranskat)abstract
- Magnetically aligned bicelles are increasingly being used as model membranes in solution- and solid-state NMR studies of the structure, dynamics, topology, and interaction of membrane-associated peptides and proteins. These studies commonly utilize the PISEMA pulse sequence to measure dipolar coupling and chemical shift, the two key parameters used in subsequent structural analysis. In the present study, we demonstrate that the PISEMA and other rotating-frame pulse sequences are not suitable for the measurement of long-range heteronuclear dipolar couplings, and that they provide inaccurate values when multiple protons are coupled to a C-13 nucleus. Furthermore, we demonstrate that a laboratory-frame separated-local-field experiment is capable of overcoming these difficulties in magnetically aligned bicelles. An extension of this approach to accurately measure C-13-P-31 and H-1-P-31 couplings from phospholipids, which are useful to understand the interaction of molecules with the membrane, is also described. In these 2D experiments, natural abundance C-13 was observed from bicelles containing DMPC and DHPC lipid molecules. As a first application, these solid-state NMR approaches were utilized to probe the membrane interaction of an antidepressant molecule, desipramine, and its location in the membrane.
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| 41. |
- Dvinskikh, Sergey V., et al.
(författare)
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High-resolution characterization of liquid-crystalline 60 fullerenes using solid-state nuclear magnetic resonance spectroscopy
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:39, s. 12347-12353
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Tidskriftsartikel (refereegranskat)abstract
- Liquid-crystalline materials containing fullerenes are valuable in the development of supramolecular switches and in solar cell technology. In this study, we characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic compounds using solid-state natural abundance C-13 NMR experiments under stationary and magic angle spinning sample conditions. Chemical shifts spectra were measured in isotropic, liquid-crystalline nematic and smectic A and crystalline phases using one-dimensional 13C experiments, while two-dimensional separated local-field experiments were used to measure the H-1-C-13 dipolar couplings in mesophases. Chemical shift and dipolar coupling parameters were used to characterize the structure and dynamics of the liquid-crystalline dyads. NMR data of fullerene-containing thermotropic liquid crystals are compared to that of basic mesogenic unit and mesomorphic promoter compounds. Our NMR results suggest that the fullerene-ferrocene dyads form highly dynamic liquid-crystalline phases in which molecules rotate fast around the symmetry axis on the characteristic NMR time scale of similar to 10(-1) s.
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| 42. |
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| 43. |
- Dvinskikh, Sergey V., et al.
(författare)
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Measurement of the principal values of the anisotropic diffusion tensor in an unoriented sample by exploiting the chemical shift anisotropy : F-19 PGSE NMR with homonuclear decoupling
- 2001
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 148:1, s. 73-77
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Tidskriftsartikel (refereegranskat)abstract
- NMR methods (S. V. Dvinskikh et al. J. Magn. Reson. 142, 102-110 (2000) and S. V. Dvinskikh and I. Furo, 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 F-19 PGSE NMR with homonuclear decoupling in a lyotropic lamellar liquid crystal.
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| 44. |
- Dvinskikh, Sergey V., et al.
(författare)
-
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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| 45. |
- Dvinskikh, Sergey V., et al.
(författare)
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Mesomorphism in columnar phases studied by solid-state nuclear magnetic resonance
- 2006
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Ingår i: Physical Review E. - : American Physical Society. - 1539-3755. ; 74:2, s. 021703-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present C-13 and H-1 NMR investigations of 2, 3, 6, 7, 10, 11-hexahexyl-thiotriphenylene (HHTT). The measurements were carried out under both static and magic-angle spinning conditions. The phase diagram of HHTT is K <-> H <-> D-hd <-> I, where H is a helical phase and D-hd is a columnar liquid crystal. The motivation was to characterize the molecular order and dynamics and to investigate differences at the molecular level between the two mesophases: H and D-hd. It is shown that D-hd is a conventional columnar liquid crystal, where the molecular core undergoes fast rotation about the symmetry axis. The orientational order in this mesophase is lower and the temperature dependence of the order parameter is steeper than in other triphenylene-based compounds. On the other hand, in the helical phase the core, similarly to the solid phase, is essentially rigid. The difference between the solid and helical phases is mainly manifested in an increased mobility of the aliphatic chains observed in the latter phase. In addition, the sample exhibits thermal history effects, which are observed in the different behavior upon cooling and heating.
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| 46. |
- Dvinskikh, Sergey V., et al.
(författare)
-
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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| 47. |
- 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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| 48. |
- Dvinskikh, Sergey V., et al.
(författare)
-
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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| 49. |
- Dvinskikh, Sergey V., et al.
(författare)
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MRI profiles over very wide concentration ranges : Application to swelling of a bentonite clay
- 2009
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 198:2, s. 146-150
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Tidskriftsartikel (refereegranskat)abstract
- In MRI investigation of soils, clays, and rocks, mainly mobile water is detected, similarly to that in biological and medical samples. However, the spin relaxation properties of water in these materials and/or low water concentration may make it difficult to use standard MRI approaches. Despite these limitations, one can combine MRI techniques developed for solid and liquid states and use independent information on relaxation properties of water, interacting with the material of interest, to obtain true images of both water and material content. We present procedures for obtaining such true density maps and demonstrate their use for studying the swelling of bentonite clay by water. A constant time imaging protocol provides 1D mapping of the clay distribution in regions with clay concentration above 10 vol%. T-1 relaxation time imaging is employed to monitor the clay content down to 10(-3) vol%. Data provided by those two approaches are in good agreement in the overlapping range of concentrations. Covering five orders of magnitude of clay concentration, swelling of sodium-exchanged bentonite clays from pre-compacted pellets into a gel phase is followed in detail.
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| 50. |
- Dvinskikh, Sergey V., et al.
(författare)
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NMR imaging study and multi-Fickian numerical simulation of moisture transfer in Norway spruce samples
- 2011
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Ingår i: Engineering structures. - : Elsevier BV. - 0141-0296 .- 1873-7323. ; 33:11, s. 3079-3086
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Tidskriftsartikel (refereegranskat)abstract
- Wood has potential as a renewable material for a large variety of applications that often call for improved properties such as dimensional stability, moisture insensitivity, and durability. Moisture migration in wood is a particularly important factor in determining the cost-effective service life of wooden construction. Within the present research, proton NMR imaging was applied for recording the moisture spatial distribution of various samples of Norway Spruce. Moisture distribution along the radial, tangential and longitudinal directions in wood was monitored at different times upon three consecutive changes of relative humidity: (1) from 65% to 94%; (2) from 94% to 33%; (3) from 33% to 65%. Uncoated samples and specimens treated with different types of surface coatings were studied. The experiments were numerically simulated by using the multi-Fickian model. The model describes the moisture transport process in wood which is characterized by three phenomena: (a) bound water diffusion, (b) water vapor diffusion and (c) coupling between the two phases through sorption. The model is implemented into the Abaqus FEM code. The numerical results are found to be in agreement with the experimental data.
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