Nuclear spin-lattice (T1) and spin-spin (T2) relaxation times provide versatile information about... more Nuclear spin-lattice (T1) and spin-spin (T2) relaxation times provide versatile information about the dynamics and structure of substances, such as proteins, polymers, porous media, and so forth. Multidimensional experiments increase the information content and resolution of NMR relaxometry, but they also multiply the measurement time. To overcome this issue, we present an efficient strategy for a single-scan measurement of a 2D T1-T2 correlation map. The method shortens the experimental time by one to three orders of magnitude as compared to the conventional method, offering an unprecedented opportunity to study molecular processes in real-time. We demonstrate that, despite the tremendous speed-up, the T1-T2 correlation maps determined by the single-scan method are in good agreement with the maps measured by the conventional method. The concept of the single-scan T1-T2 correlation experiment is applicable to a broad range of other multidimensional relaxation and diffusion experiments.
Applications of liquid crystals (LCs) are based on controlling the orientational and translationa... more Applications of liquid crystals (LCs) are based on controlling the orientational and translational order of the medium. One important way of control is via confinement. In this work, uniaxial thermotropic LCs confined to nanosized cylindrical cavities are studied using isobaric parallel tempering (PT) Monte Carlo (MC) simulations. The LCs are modeled using the Gay-Berne (4.4, 20.0, 1, 1) (GB) potential in long, smooth-walled cavities. The chosen particle-wall interaction favours homogeneous planar anchoring - the alignment of molecules along the wall. We report the results for the phase structure appropriate to three different cavity sizes as well as comparison to the results of bulk simulations. Ensemble averages for orientational and translational order parameters as well as their local behavior as a function of the distance to the cavity wall is calculated by reweighting results from all the simulated temperatures. We find that the LC director tends to align strongly with the axis of the cylindrical cavity. The orientational order is enhanced and translational order suppressed by the walls of the cavity. Hence, there are notable differences between the local order close to the wall and near the cylinder axis. The position-dependent distributions of the order parameters result in smooth phase transitions in their respective system-wide averages. Particularly, the nematic-isotropic (N-I) transition is replaced by a continuous nematic-paranematic (N-PN) transition. This is caused by the core region of the cavities becoming isotropic at high temperatures, whereas near the wall the LC retains nematic order. In contrast to previous NVT ensemble simulations, we find the effect of confinement on the smectic (Sm) layering to be weak. Also, Sm-N and N-PN transitions are found to be both sharper and residing at higher temperatures than in the constant-volume simulations. At temperatures where the bulk LC is a solid, we observe a wall-induced density wave in the confined systems, which outweighs the self-organization of the LC to hexagonal in-plane order.
In this study, we made an attempt to qualitatively study the structures of few rare earth metal c... more In this study, we made an attempt to qualitatively study the structures of few rare earth metal complexes by employing solid state NMR, X-Ray Diffraction, and preliminary DFT calculations. High resolution 13C and 15N solid state CP/MAS NMR spectra were recorded for six diamagnetic polycrystalline rare earth dialkyldithiocarbamates of the general formula [(RE2S2CNR2)3 PHEN] (where RE=La or Y, R=C2H5, C3H7, and i-C3H7) [1]. Different isotropic 13C and 15N chemical shifts for the three dialkyldithiocarbamato groups were observed. Regulacio et al. (2005) inferred that irrespective of the alkyl chains, rare earth complexes of dialkyldithiocarbamates and phenanthroline (3:1) ligands always crystallize in a monoclinic system with a space P21/c group. However, comparative analysis of solid state 13C/15N CPMAS spectra of polycrystalline yttrium and lanthanum diethyldithiocarbamate complexes shows the presence of significant differences, indicating structural variations of these complexes. Al...
We present a method to measure self-diffusion acrossmembranes without the need for concentration ... more We present a method to measure self-diffusion acrossmembranes without the need for concentration or pressure gradients.Hyperpolarized xenon in combination with remote detection of NMR allowsthe measurement of membrane permeation, even in the gas phase. Theresulting images allow quantification of the amount of fluid diffusedthrough the membrane, and represent an alternative, potentially moreprecise way of measuring a membrane diffusion coefficient. The
Journal of the American Chemical Society, Jan 25, 2015
We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate... more We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate xenon in water with a binding constant of 16 M(-1). The observations pave the way for exploiting metallosupramolecular cages as economical means to extract rare gases as well as (129)Xe NMR-based bio-, pH, and temperature sensors. Xe in the Fe4L6 cage has an unusual chemical shift downfield from free Xe in water. The exchange rate between the encapsulated and free Xe was determined to be about 10 Hz, potentially allowing signal amplification via chemical exchange saturation transfer. Computational treatment showed that dynamical effects of Xe motion as well as relativistic effects have significant contributions to the chemical shift of Xe in the cage and enabled the replication of the observed linear temperature dependence of the shift.
The development of microfluidic processes requires information-rich detection methods. Here we in... more The development of microfluidic processes requires information-rich detection methods. Here we introduce the concept of remote detection exchange NMR spectroscopy (RD-EXSY), and show that, along with indirect spatial information extracted from time-of-flight data, it provides unique information about the active regions, reaction pathways, and intermediate products in a lab-on-a-chip reactor. Furthermore, we demonstrate that direct spatial resolution can be added to RD-EXSY efficiently by applying the principles of Hadamard spectroscopy.
The walls of solid matrix restrict the self-diffusion of a fluid absorbed in the matrix, and this... more The walls of solid matrix restrict the self-diffusion of a fluid absorbed in the matrix, and this is reflected in the echo amplitudes measured by PGSTE NMR. Hence, the pore size distribution of the matrix can be extracted from the echo amplitudes. We demonstrate that, when both liquids and gases (water and methane in this case) are used as probe fluids, the scale of the dimensions observable by PGSTE NMR may be over 4 orders of magnitude. This enables determining the dimensions of highly anisotropic pores. In the present case, the wood cell structures of Pinus sylvestris in three orthogonal directions were studied.
The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG)... more The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG) materials was investigated using 129Xe nuclear magnetic resonance (NMR) spectroscopy of xenon gas dissolved in the LC. The average pore diameters of the materials varied from 81 to 2917 A, and the measurements were carried out within a wide temperature range (approximately 185-370 K). The spectra contain lots of information about the effect of confinement on the phase of the LC. The theoretical model of shielding of noble gases dissolved in liquid crystals on the basis of pairwise additivity approximation was applied to the analysis of the spectra. When pore diameter is small, smaller than approximately 150 A, xenon experiences on average an isotropic environment inside the pore, and no nematic-isotropic phase transition is observed. When the size is larger than approximately 150 A, nematic phase is observed, and the LC molecules are oriented along pore axis. The orientational order parameter of the LC, S, increases with increasing pore size. In the largest pores, the orientation of the molecules deviates from the pore axis direction to magnetic field direction, which implies that the size of the pores (approximately 3000 A) is close to magnetic coherence length. The decrease of magnetic coherence length with increasing temperature is clearly seen from the spectra. When the sample is cooled rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, as they do during gradual freezing, but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. This makes it possible to study the effect of confinement on the solid phases. According to the measured 129Xe NMR spectra, possibly three different solid phases are observed from bulk liquid crystal in the used temperature region. The same is also seen from the samples containing larger pores (pore size larger than approximately 500 A), and the solid-solid phase-transition temperatures are the same. However, no first-order solid-solid phase transitions are observed from the smaller pores. Melting point depression, that is, the depression of solid-nematic transition temperature observed from the pores as compared with that in bulk LC, is seen to be very sensitive to the pore size, and it can be used for the determination of pore size of an unknown material.
129Xe NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica gels w... more 129Xe NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica gels with nominal pore diameters of 40, 60, and 100 A have been measured over the temperature range 170-245 K. The spectra consist of a number of lines, which contain detailed information on the system. The most interesting result is that the chemical shift of a particular signal observed below the melting point of confined acetonitrile is highly sensitive to the pore size, and hence its shape is sensitive to the pore size distribution function. This signal originates from the xenon atoms sited in very small cavities built up inside the pores during the freezing transition. It can be used to determine the size or even the size distribution function of the pores. In addition, the emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. The difference in the chemical shifts of two other signals, which arise from xenon dissolved in bulk and confined acetonitrile, provides still another novel method for determining the size of the pores.
In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spe... more In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spectra of xenon dissolved in acetonitrile confined into mesoporous materials give detailed information on the system, especially about the pore sizes. A resonance signal originating from xenon atoms sited in very small cavities built up inside the pores during the freezing transition (referred to as signal D) turned out to be highly sensitive to the pore size. The emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. In addition, the difference in the chemical shifts of two other signals arising from xenon dissolved in bulk and confined acetonitrile (B and C) provides another method for determining the pore sizes. In the present work, the observed correlations have been investigated using an extensive set of measurements with a variety of porous materials (silica gels and controlled pore glasses) with the mean pore diameters ranging from 43 to 2917 A. The usefulness of the correlations has been demonstrated by calculating the pore size distributions from the spectral data. The distributions are in agreement with those reported by the manufacturers, when the mean pore diameter is smaller than approximately 500 A. In addition, it has been shown that the porosity of the materials can be determined by comparing the intensities of the signals arising from the bulk and confined liquid. When acetonitrile is replaced by cyclohexane in the sample, the dependence of the chemical shift difference between the B and C signals on the pore size becomes more sensitive, but no D signal appears below the freezing point. In addition, the influence of xenon gas on the melting points of bulk and confined acetonitrile has been studied by 1H NMR cryoporometry. The measurements show that the temperature of the latter transition lowers slightly more, and consequently affects the pore sizes calculated by means of the difference in the phase transition temperatures. Hysteresis in the phase transitions in a cooling-warming cycle has also been studied as a function of the temperature stabilization time by 129Xe NMR of xenon dissolved in acetonitrile.
Self-diffusion of a fluid absorbed in a solid matrix is restricted by the walls of the matrix. We... more Self-diffusion of a fluid absorbed in a solid matrix is restricted by the walls of the matrix. We demonstrate that the local self-diffusion probability densities (propagators) of fluid molecules can be measured by position-exchange nuclear magnetic resonance spectroscopy (POXSY), and analysis of the shape of the propagators reveals the local size-distributions of the voids in the matrix. We also show that, in the case of rectangular voids, size-distribution can be calculated in a long diffusion-time limit without any assumptions about the shape of the distribution. Pinus sylvestris pine wood was used as a sample material in the experiments, and the results show that this method gives detailed information about the structure of wood.
Xenon porometry is a new method for characterization of porous materials. In this method, the mat... more Xenon porometry is a new method for characterization of porous materials. In this method, the material is immersed in a medium, and its properties are studied by means of 129Xe NMR spectra of xenon dissolved in the sample. The method is particularly suitable for the determination of pore size distribution of the material, since the spectra display two signals whose chemical shift is dependent on the pore size. A prerequisite for an accurate determination is the fact that the diffusion of xenon between different pores is slow enough. The diffusion is studied in this work using two-dimensional exchange spectroscopy (2-D EXSY). The spectra measured as a function of the mixing time imply that the exchange is really slow as compared with the NMR time scale, and therefore the distribution of the resonance frequencies indeed represents the pore size distribution.
Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nucl... more Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nuclear shielding in CX(2) (X = O, S, Se, Te) are calculated taking into account the relativistic spin-orbit (SO) interaction. The SO effect is considered for the first time for the secondary isotope shifts. The nuclear shielding hypersurface in terms of nuclear displacements is calculated by using a density-functional theory method. Ab initio multiconfiguration self-consistent field calculations are done at the equilibrium geometry for comparison. (13)C NMR measurements are carried out for CS(2). The calculated results are compared with both present and earlier experimental data on CO(2), CS(2), and CSe(2). The heavy-atom SO effects on the rovibrational corrections of (13)C shielding are shown to be significant. For CSe(2) and CTe(2), reliable prediction of secondary isotope effects and their temperature dependence requires the inclusion of the SO corrections. In particular, earlier discrepancies of theory and experiment for CSe(2) are fully resolved by taking the SO interactions into account.
We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin sy... more We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin systems, can provide unique quantitative information about anisotropic reorientational motions and molecular geometry. Relevant theory for AX(2) and AX(3) spin systems is presented, assuming the dipole-dipole and random field relaxation mechanisms of (13)C nucleus, and demonstrated on methyl iodide solution in chloroform. Agreement with experimental intensities of all the six independent peaks is very good in the whole range of mixing times (up to 45 s).
We demonstrate that time-of-flight (TOF) remote detection (RD) magnetic resonance imaging (MRI) p... more We demonstrate that time-of-flight (TOF) remote detection (RD) magnetic resonance imaging (MRI) provides detailed information about physical changes in wood due to thermal modification that is not available with conventional nuclear magnetic resonance (NMR) based techniques. In the experiments, xenon gas was forced to flow through Pinus sylvestris pine wood samples, and the flow paths and dispersion of gas atoms were observed by measuring (129)Xe TOF RD MRI images from the samples. MRI sensitivity of xenon was boosted by the spin exchange optical pumping (SEOP) method. Two different samples were studied: a reference sample, dried at low temperature, and a modified sample, which was thermally modified at 240 degrees C after the drying. The samples were taken next to each other from the same wood plank in order to ensure the comparability of the results. The most important conclusion is that both the smaller dispersion observed in all the TOF RD experiments independent of each other and the decreased amount of flow paths shown by the time projection of z-encoded TOF RD MRI experiment imply that a large amount of pits connecting tracheid cells are closed in thermal modification. Closed pits may be one reason for reduced moisture content and improved dimensional stability of wood achieved in thermal modification. This is the first time biological samples have been investigated by TOF RD MRI.
Nuclear spin-lattice (T1) and spin-spin (T2) relaxation times provide versatile information about... more Nuclear spin-lattice (T1) and spin-spin (T2) relaxation times provide versatile information about the dynamics and structure of substances, such as proteins, polymers, porous media, and so forth. Multidimensional experiments increase the information content and resolution of NMR relaxometry, but they also multiply the measurement time. To overcome this issue, we present an efficient strategy for a single-scan measurement of a 2D T1-T2 correlation map. The method shortens the experimental time by one to three orders of magnitude as compared to the conventional method, offering an unprecedented opportunity to study molecular processes in real-time. We demonstrate that, despite the tremendous speed-up, the T1-T2 correlation maps determined by the single-scan method are in good agreement with the maps measured by the conventional method. The concept of the single-scan T1-T2 correlation experiment is applicable to a broad range of other multidimensional relaxation and diffusion experiments.
Applications of liquid crystals (LCs) are based on controlling the orientational and translationa... more Applications of liquid crystals (LCs) are based on controlling the orientational and translational order of the medium. One important way of control is via confinement. In this work, uniaxial thermotropic LCs confined to nanosized cylindrical cavities are studied using isobaric parallel tempering (PT) Monte Carlo (MC) simulations. The LCs are modeled using the Gay-Berne (4.4, 20.0, 1, 1) (GB) potential in long, smooth-walled cavities. The chosen particle-wall interaction favours homogeneous planar anchoring - the alignment of molecules along the wall. We report the results for the phase structure appropriate to three different cavity sizes as well as comparison to the results of bulk simulations. Ensemble averages for orientational and translational order parameters as well as their local behavior as a function of the distance to the cavity wall is calculated by reweighting results from all the simulated temperatures. We find that the LC director tends to align strongly with the axis of the cylindrical cavity. The orientational order is enhanced and translational order suppressed by the walls of the cavity. Hence, there are notable differences between the local order close to the wall and near the cylinder axis. The position-dependent distributions of the order parameters result in smooth phase transitions in their respective system-wide averages. Particularly, the nematic-isotropic (N-I) transition is replaced by a continuous nematic-paranematic (N-PN) transition. This is caused by the core region of the cavities becoming isotropic at high temperatures, whereas near the wall the LC retains nematic order. In contrast to previous NVT ensemble simulations, we find the effect of confinement on the smectic (Sm) layering to be weak. Also, Sm-N and N-PN transitions are found to be both sharper and residing at higher temperatures than in the constant-volume simulations. At temperatures where the bulk LC is a solid, we observe a wall-induced density wave in the confined systems, which outweighs the self-organization of the LC to hexagonal in-plane order.
In this study, we made an attempt to qualitatively study the structures of few rare earth metal c... more In this study, we made an attempt to qualitatively study the structures of few rare earth metal complexes by employing solid state NMR, X-Ray Diffraction, and preliminary DFT calculations. High resolution 13C and 15N solid state CP/MAS NMR spectra were recorded for six diamagnetic polycrystalline rare earth dialkyldithiocarbamates of the general formula [(RE2S2CNR2)3 PHEN] (where RE=La or Y, R=C2H5, C3H7, and i-C3H7) [1]. Different isotropic 13C and 15N chemical shifts for the three dialkyldithiocarbamato groups were observed. Regulacio et al. (2005) inferred that irrespective of the alkyl chains, rare earth complexes of dialkyldithiocarbamates and phenanthroline (3:1) ligands always crystallize in a monoclinic system with a space P21/c group. However, comparative analysis of solid state 13C/15N CPMAS spectra of polycrystalline yttrium and lanthanum diethyldithiocarbamate complexes shows the presence of significant differences, indicating structural variations of these complexes. Al...
We present a method to measure self-diffusion acrossmembranes without the need for concentration ... more We present a method to measure self-diffusion acrossmembranes without the need for concentration or pressure gradients.Hyperpolarized xenon in combination with remote detection of NMR allowsthe measurement of membrane permeation, even in the gas phase. Theresulting images allow quantification of the amount of fluid diffusedthrough the membrane, and represent an alternative, potentially moreprecise way of measuring a membrane diffusion coefficient. The
Journal of the American Chemical Society, Jan 25, 2015
We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate... more We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate xenon in water with a binding constant of 16 M(-1). The observations pave the way for exploiting metallosupramolecular cages as economical means to extract rare gases as well as (129)Xe NMR-based bio-, pH, and temperature sensors. Xe in the Fe4L6 cage has an unusual chemical shift downfield from free Xe in water. The exchange rate between the encapsulated and free Xe was determined to be about 10 Hz, potentially allowing signal amplification via chemical exchange saturation transfer. Computational treatment showed that dynamical effects of Xe motion as well as relativistic effects have significant contributions to the chemical shift of Xe in the cage and enabled the replication of the observed linear temperature dependence of the shift.
The development of microfluidic processes requires information-rich detection methods. Here we in... more The development of microfluidic processes requires information-rich detection methods. Here we introduce the concept of remote detection exchange NMR spectroscopy (RD-EXSY), and show that, along with indirect spatial information extracted from time-of-flight data, it provides unique information about the active regions, reaction pathways, and intermediate products in a lab-on-a-chip reactor. Furthermore, we demonstrate that direct spatial resolution can be added to RD-EXSY efficiently by applying the principles of Hadamard spectroscopy.
The walls of solid matrix restrict the self-diffusion of a fluid absorbed in the matrix, and this... more The walls of solid matrix restrict the self-diffusion of a fluid absorbed in the matrix, and this is reflected in the echo amplitudes measured by PGSTE NMR. Hence, the pore size distribution of the matrix can be extracted from the echo amplitudes. We demonstrate that, when both liquids and gases (water and methane in this case) are used as probe fluids, the scale of the dimensions observable by PGSTE NMR may be over 4 orders of magnitude. This enables determining the dimensions of highly anisotropic pores. In the present case, the wood cell structures of Pinus sylvestris in three orthogonal directions were studied.
The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG)... more The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG) materials was investigated using 129Xe nuclear magnetic resonance (NMR) spectroscopy of xenon gas dissolved in the LC. The average pore diameters of the materials varied from 81 to 2917 A, and the measurements were carried out within a wide temperature range (approximately 185-370 K). The spectra contain lots of information about the effect of confinement on the phase of the LC. The theoretical model of shielding of noble gases dissolved in liquid crystals on the basis of pairwise additivity approximation was applied to the analysis of the spectra. When pore diameter is small, smaller than approximately 150 A, xenon experiences on average an isotropic environment inside the pore, and no nematic-isotropic phase transition is observed. When the size is larger than approximately 150 A, nematic phase is observed, and the LC molecules are oriented along pore axis. The orientational order parameter of the LC, S, increases with increasing pore size. In the largest pores, the orientation of the molecules deviates from the pore axis direction to magnetic field direction, which implies that the size of the pores (approximately 3000 A) is close to magnetic coherence length. The decrease of magnetic coherence length with increasing temperature is clearly seen from the spectra. When the sample is cooled rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, as they do during gradual freezing, but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. This makes it possible to study the effect of confinement on the solid phases. According to the measured 129Xe NMR spectra, possibly three different solid phases are observed from bulk liquid crystal in the used temperature region. The same is also seen from the samples containing larger pores (pore size larger than approximately 500 A), and the solid-solid phase-transition temperatures are the same. However, no first-order solid-solid phase transitions are observed from the smaller pores. Melting point depression, that is, the depression of solid-nematic transition temperature observed from the pores as compared with that in bulk LC, is seen to be very sensitive to the pore size, and it can be used for the determination of pore size of an unknown material.
129Xe NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica gels w... more 129Xe NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica gels with nominal pore diameters of 40, 60, and 100 A have been measured over the temperature range 170-245 K. The spectra consist of a number of lines, which contain detailed information on the system. The most interesting result is that the chemical shift of a particular signal observed below the melting point of confined acetonitrile is highly sensitive to the pore size, and hence its shape is sensitive to the pore size distribution function. This signal originates from the xenon atoms sited in very small cavities built up inside the pores during the freezing transition. It can be used to determine the size or even the size distribution function of the pores. In addition, the emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. The difference in the chemical shifts of two other signals, which arise from xenon dissolved in bulk and confined acetonitrile, provides still another novel method for determining the size of the pores.
In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spe... more In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spectra of xenon dissolved in acetonitrile confined into mesoporous materials give detailed information on the system, especially about the pore sizes. A resonance signal originating from xenon atoms sited in very small cavities built up inside the pores during the freezing transition (referred to as signal D) turned out to be highly sensitive to the pore size. The emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. In addition, the difference in the chemical shifts of two other signals arising from xenon dissolved in bulk and confined acetonitrile (B and C) provides another method for determining the pore sizes. In the present work, the observed correlations have been investigated using an extensive set of measurements with a variety of porous materials (silica gels and controlled pore glasses) with the mean pore diameters ranging from 43 to 2917 A. The usefulness of the correlations has been demonstrated by calculating the pore size distributions from the spectral data. The distributions are in agreement with those reported by the manufacturers, when the mean pore diameter is smaller than approximately 500 A. In addition, it has been shown that the porosity of the materials can be determined by comparing the intensities of the signals arising from the bulk and confined liquid. When acetonitrile is replaced by cyclohexane in the sample, the dependence of the chemical shift difference between the B and C signals on the pore size becomes more sensitive, but no D signal appears below the freezing point. In addition, the influence of xenon gas on the melting points of bulk and confined acetonitrile has been studied by 1H NMR cryoporometry. The measurements show that the temperature of the latter transition lowers slightly more, and consequently affects the pore sizes calculated by means of the difference in the phase transition temperatures. Hysteresis in the phase transitions in a cooling-warming cycle has also been studied as a function of the temperature stabilization time by 129Xe NMR of xenon dissolved in acetonitrile.
Self-diffusion of a fluid absorbed in a solid matrix is restricted by the walls of the matrix. We... more Self-diffusion of a fluid absorbed in a solid matrix is restricted by the walls of the matrix. We demonstrate that the local self-diffusion probability densities (propagators) of fluid molecules can be measured by position-exchange nuclear magnetic resonance spectroscopy (POXSY), and analysis of the shape of the propagators reveals the local size-distributions of the voids in the matrix. We also show that, in the case of rectangular voids, size-distribution can be calculated in a long diffusion-time limit without any assumptions about the shape of the distribution. Pinus sylvestris pine wood was used as a sample material in the experiments, and the results show that this method gives detailed information about the structure of wood.
Xenon porometry is a new method for characterization of porous materials. In this method, the mat... more Xenon porometry is a new method for characterization of porous materials. In this method, the material is immersed in a medium, and its properties are studied by means of 129Xe NMR spectra of xenon dissolved in the sample. The method is particularly suitable for the determination of pore size distribution of the material, since the spectra display two signals whose chemical shift is dependent on the pore size. A prerequisite for an accurate determination is the fact that the diffusion of xenon between different pores is slow enough. The diffusion is studied in this work using two-dimensional exchange spectroscopy (2-D EXSY). The spectra measured as a function of the mixing time imply that the exchange is really slow as compared with the NMR time scale, and therefore the distribution of the resonance frequencies indeed represents the pore size distribution.
Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nucl... more Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nuclear shielding in CX(2) (X = O, S, Se, Te) are calculated taking into account the relativistic spin-orbit (SO) interaction. The SO effect is considered for the first time for the secondary isotope shifts. The nuclear shielding hypersurface in terms of nuclear displacements is calculated by using a density-functional theory method. Ab initio multiconfiguration self-consistent field calculations are done at the equilibrium geometry for comparison. (13)C NMR measurements are carried out for CS(2). The calculated results are compared with both present and earlier experimental data on CO(2), CS(2), and CSe(2). The heavy-atom SO effects on the rovibrational corrections of (13)C shielding are shown to be significant. For CSe(2) and CTe(2), reliable prediction of secondary isotope effects and their temperature dependence requires the inclusion of the SO corrections. In particular, earlier discrepancies of theory and experiment for CSe(2) are fully resolved by taking the SO interactions into account.
We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin sy... more We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin systems, can provide unique quantitative information about anisotropic reorientational motions and molecular geometry. Relevant theory for AX(2) and AX(3) spin systems is presented, assuming the dipole-dipole and random field relaxation mechanisms of (13)C nucleus, and demonstrated on methyl iodide solution in chloroform. Agreement with experimental intensities of all the six independent peaks is very good in the whole range of mixing times (up to 45 s).
We demonstrate that time-of-flight (TOF) remote detection (RD) magnetic resonance imaging (MRI) p... more We demonstrate that time-of-flight (TOF) remote detection (RD) magnetic resonance imaging (MRI) provides detailed information about physical changes in wood due to thermal modification that is not available with conventional nuclear magnetic resonance (NMR) based techniques. In the experiments, xenon gas was forced to flow through Pinus sylvestris pine wood samples, and the flow paths and dispersion of gas atoms were observed by measuring (129)Xe TOF RD MRI images from the samples. MRI sensitivity of xenon was boosted by the spin exchange optical pumping (SEOP) method. Two different samples were studied: a reference sample, dried at low temperature, and a modified sample, which was thermally modified at 240 degrees C after the drying. The samples were taken next to each other from the same wood plank in order to ensure the comparability of the results. The most important conclusion is that both the smaller dispersion observed in all the TOF RD experiments independent of each other and the decreased amount of flow paths shown by the time projection of z-encoded TOF RD MRI experiment imply that a large amount of pits connecting tracheid cells are closed in thermal modification. Closed pits may be one reason for reduced moisture content and improved dimensional stability of wood achieved in thermal modification. This is the first time biological samples have been investigated by TOF RD MRI.
Uploads
Papers by Ville-veikko Telkki