Self- and transport diffusion coefficients from NMR experiments

Self-diffusion coefficients give insight into the mobility of molecular species and serve as benchmark variables for molecular modeling. Nuclear magnetic resonance (NMR) spectroscopy is an excellent method for the measurement of self-diffusion coefficients: In the present work, a pulsed gradient stimulated echo (PGSTE) pulse sequence was employed for their determination in binary and ternary liquid systems at 298 K and 1 bar. To ensure a high quality of the measurements, a preliminary gradient mapping was carried out. The experimental results are compared to molecular simulation data by Guevara-Carrion et al. [1] for binary systems (acetone/toluene, acetone/ethanol and acetone/cyclohexane) and to molecular simulation data that was determined in the present work for the ternary system acetone/toluene/cyclohexane

Rapid hyperpolarization and purification of the metabolite fumarate in aqueous solution

Hyperpolarized fumarate is a promising biosensor for carbon-13 magnetic resonance metabolic imaging. Such molecular imaging applications require nuclear hyperpolarization to attain sufficient signal strength. Dissolution dynamic nuclear polarization is the current state-of-the-art methodology for hyperpolarizing fumarate, but this is expensive and relatively slow. Alternatively, this important biomolecule can be hyperpolarized in a cheap and convenient manner using parahydrogen-induced polarization. However, this process requires a chemical reaction, and the resulting solutions are contaminated with the catalyst, unreacted reagents, and reaction side-product molecules, and are hence unsuitable for use in vivo. In this work we show that the hyperpolarized fumarate can be purified from these contaminants by acid precipitation as a pure solid, and later redissolved to a desired concentration in a clean aqueous solvent. Significant advances in the reaction conditions and reactor equipment allow for formation of hyperpolarized fumarate at ¹³C polarization levels of 30–45%

Xenon NMR with spectroscopic, spatial, and temporal resolution

129Xe NMR has found many applications in material sciences and medicine because of two useful properties of Xenon atoms for NMR: the sensitivity to their environment due to their highly polarizable electron cloud, which results in a wide range of chemical shifts, and the ability of being hyperpolarized, which overcomes the problem of the low signal-to-noise ratio of thermally polarized Xenon. In this work a variety of different experiments were performed that combine NMR measurements with spectroscopic, spatial, and temporal resolution thereby exploiting the large non-equilibrium magnetization of hyperpolarized Xenon to monitor dynamic processes. In the first part of this work a new analytical model was introduced which allows the quantitative evaluation of the Xenon chemical shift in gas and liquid phases and of Xenon dissolved in organic solvents. Extensive measurements of the chemical shifts of thermally polarized Xenon were performed as a function of temperature (150-295 K) and Xenon density (1- 400 amagat). A simplified phenomenological model was developed, which quantitatively describes the dependence of the Xenon chemical shift on temperature and density of the interacting atoms or molecules. The chemical shift can be divided into a part which depends on the interaction of the solvent molecules and the Xenon atoms and into a part which describes the Xenon-Xenon interaction in the solvent. The second part of this work reports for the first time an application of hyperpolarized 129Xe NMR spectroscopy to analyze polymerization processes in real-time which is a challenge in polymer engineering. It has been successfully demonstrated that the chemical shift of 129Xe dissolved in the reaction bulk monitors quantitatively the mole fraction of the monomer allowing the calculation of the reaction constant. The capability of this method to follow not only the living cationic polymerization of THF but also the free radical polymerization of styrene suggests an exciting potential for the monitoring of various kinds of reactions. In the next chapter the possibility to store large quantities of hyperpolarized Xenon in a liquid was evaluated. Therefore, the dynamics of melting, migration, and dissolution of hyperpolarized Xenon ice into ethanol and an ethanol/water mixture were investigated with the method of time resolved two-dimensional MRI and one-dimensional CSI starting from the initial condition of a Xenon ice layer on top of the frozen solvent. A wealth of different physical phenomena, such as the observation of phase transitions, the position dependent line narrowing of Xenon ice, the creation of pores, and the existence of a dense liquid Xenon layer in ethanol have been observed. It has been shown that the dilution of ethanol with water and the subsequent increase of the melting point lead to a very ineffective incorporation of hyperpolarized Xenon. However, the dense liquid Xenon/ethanol mixture is promising for the injection of Xenon into biochemical or biological systems in vitro. The last chapter of this work was devoted to the multi-dimensional imaging of hyperpolarized Xenon produced in the continuous flow mode of the hyperpolarizer. It has been successfully demonstrated that the monitoring of dynamic processes like the dissolution of Xenon in an organic solvent or the penetration of Xenon through a filter medium is viable even with a very small number of hyperpolarized spins (0.07 bar in the gas phase). The penetration of hyperpolarized Xenon in a filter material was followed by time-resolved imaging indicating the possibility to model the dynamics of important gas/solid reactions or the efficiency of filtration processes with hyperpolarized Xenon. It has been also shown that hyperpolarized 129Xe NMR can be used to determine non-invasively the pore size distribution and pore connectivity of porous materials by three-dimensional imaging within reasonable experimental time

Xenon NMR with spectroscopic, spatial, and temporal resolution

129Xe NMR has found many applications in material sciences and medicine because of two useful properties of Xenon atoms for NMR: the sensitivity to their environment due to their highly polarizable electron cloud, which results in a wide range of chemical shifts, and the ability of being hyperpolarized, which overcomes the problem of the low signal-to-noise ratio of thermally polarized Xenon. In this work a variety of different experiments were performed that combine NMR measurements with spectroscopic, spatial, and temporal resolution thereby exploiting the large non-equilibrium magnetization of hyperpolarized Xenon to monitor dynamic processes. In the first part of this work a new analytical model was introduced which allows the quantitative evaluation of the Xenon chemical shift in gas and liquid phases and of Xenon dissolved in organic solvents. Extensive measurements of the chemical shifts of thermally polarized Xenon were performed as a function of temperature (150-295 K) and Xenon density (1- 400 amagat). A simplified phenomenological model was developed, which quantitatively describes the dependence of the Xenon chemical shift on temperature and density of the interacting atoms or molecules. The chemical shift can be divided into a part which depends on the interaction of the solvent molecules and the Xenon atoms and into a part which describes the Xenon-Xenon interaction in the solvent. The second part of this work reports for the first time an application of hyperpolarized 129Xe NMR spectroscopy to analyze polymerization processes in real-time which is a challenge in polymer engineering. It has been successfully demonstrated that the chemical shift of 129Xe dissolved in the reaction bulk monitors quantitatively the mole fraction of the monomer allowing the calculation of the reaction constant. The capability of this method to follow not only the living cationic polymerization of THF but also the free radical polymerization of styrene suggests an exciting potential for the monitoring of various kinds of reactions. In the next chapter the possibility to store large quantities of hyperpolarized Xenon in a liquid was evaluated. Therefore, the dynamics of melting, migration, and dissolution of hyperpolarized Xenon ice into ethanol and an ethanol/water mixture were investigated with the method of time resolved two-dimensional MRI and one-dimensional CSI starting from the initial condition of a Xenon ice layer on top of the frozen solvent. A wealth of different physical phenomena, such as the observation of phase transitions, the position dependent line narrowing of Xenon ice, the creation of pores, and the existence of a dense liquid Xenon layer in ethanol have been observed. It has been shown that the dilution of ethanol with water and the subsequent increase of the melting point lead to a very ineffective incorporation of hyperpolarized Xenon. However, the dense liquid Xenon/ethanol mixture is promising for the injection of Xenon into biochemical or biological systems in vitro. The last chapter of this work was devoted to the multi-dimensional imaging of hyperpolarized Xenon produced in the continuous flow mode of the hyperpolarizer. It has been successfully demonstrated that the monitoring of dynamic processes like the dissolution of Xenon in an organic solvent or the penetration of Xenon through a filter medium is viable even with a very small number of hyperpolarized spins (0.07 bar in the gas phase). The penetration of hyperpolarized Xenon in a filter material was followed by time-resolved imaging indicating the possibility to model the dynamics of important gas/solid reactions or the efficiency of filtration processes with hyperpolarized Xenon. It has been also shown that hyperpolarized 129Xe NMR can be used to determine non-invasively the pore size distribution and pore connectivity of porous materials by three-dimensional imaging within reasonable experimental time

High Flow-Rate Benchtop NMR Spectroscopy Enabled by Continuous Overhauser DNP

Analysis of a fast-flowing liquid with NMR spectroscopy is challenging because short residence times in the magnetic field of the spectrometer result in inefficient polarization buildup and thus poor signal intensity. This is particularly problematic for benchtop NMR spectrometers because of their compact design. Therefore, in the present work, different methods to counteract this prepolarization problem in benchtop NMR spectroscopy were studied experimentally. The tests were carried out with an equimolar acetonitrile + water mixture flowing through a capillary with a 0.25 mm inner diameter at flow rates up to 2.00 mL min–1, corresponding to mean velocities of up to 0.7 m s–1. Established approaches gave only poor results at high flow rates, namely, using a prepolarization magnet, using a loopy flow cell, and using a T1 relaxation agent. To overcome this, signal enhancement by Overhauser dynamic nuclear polarization (ODNP) was used, which is based on polarization transfer from unpaired electron spins to nuclear spins and happens on very short time scales, resulting in high signal enhancements, also in fast-flowing liquids. A corresponding setup was developed and used for the studies: the line leading to the 1 T benchtop NMR spectrometer first passes through a fixed bed with a radical matrix placed in a Halbach magnet equipped with a microwave cavity to facilitate the spin transfer. With this ODNP setup, excellent results were obtained even for the highest studied flow rates. This shows that ODNP is an enabler for fast-flow benchtop NMR spectroscopy

Self- and transport diffusion coefficients from NMR experiments

Self-diffusion coefficients give insight into the mobility of molecular species and serve as benchmark variables for molecular modeling. Nuclear magnetic resonance (NMR) spectroscopy is an excellent method for the measurement of self-diffusion coefficients: In the present work, a pulsed gradient stimulated echo (PGSTE) pulse sequence was employed for their determination in binary and ternary liquid systems at 298 K and 1 bar. To ensure a high quality of the measurements, a preliminary gradient mapping was carried out. The experimental results are compared to molecular simulation data by Guevara-Carrion et al. [1] for binary systems (acetone/toluene, acetone/ethanol and acetone/cyclohexane) and to molecular simulation data that was determined in the present work for the ternary system acetone/toluene/cyclohexane

Self- and transport diffusion coefficients from NMR experiments

Self-diffusion coefficients give insight into the mobility of molecular species and serve as benchmark variables for molecular modeling. Nuclear magnetic resonance (NMR) spectroscopy is an excellent method for the measurement of self-diffusion coefficients: In the present work, a pulsed gradient stimulated echo (PGSTE) pulse sequence was employed for their determination in binary and ternary liquid systems at 298 K and 1 bar. To ensure a high quality of the measurements, a preliminary gradient mapping was carried out. The experimental results are compared to molecular simulation data by Guevara-Carrion et al. [1] for binary systems (acetone/toluene, acetone/ethanol and acetone/cyclohexane) and to molecular simulation data that was determined in the present work for the ternary system acetone/toluene/cyclohexane

Mutual Diffusion Coefficients from NMR Imaging

In the present work, we use nuclear magnetic resonance imaging (MRI) to measure the mutual diffusion coefficient of methane in toluene at 298 K. The concentration field obtained upon dissolving gaseous methane in liquid toluene was monitored with two-dimensional MRI. To cope with the low concentration of methane, a chemical shift-selective pulse sequence was employed. The diffusion coefficient was determined from the resulting temporally and spatially resolved concentration data based on Fick’s second law. The resulting diffusion coefficient is in good agreement with reference data. We conclude that MRI experiments are well-suited for quantitative studies of mutual diffusion in liquid mixtures, also in challenging applications as the one studied here

Singlet Spin Order Originating from Para-H2

Parahydrogen induced singlet spin order provides an excellent tool for many NMR/MRI applications in the natural sciences and medicine. The huge benefit of para-H2 for NMR experiments is two-fold: it represents a pure spin state that can be very efficiently used in hyperpolarization experiments, and its singlet spin order may make it long-lived because it is immune to many relaxation mechanisms. By the method of Parahydrogen Induced Polarization (PHIP), para-H2 can be introduced in certain molecules giving rise to hyperpolarized long-lived singlet spin states under favourable experimental conditions. In this chapter, the quantum mechanical properties of H2 are briefly introduced, the procedure for para-H2 enrichment is explained and the basic concept of para-H2 induced polarization (PHIP) is presented. The application of PHIP for the generation of hyperpolarized long-lived states in low and high magnetic fields is summarized. Because singlet spin states are NMR inactive, a special focus must be set on strategies for a controlled conversion of the singlet state into an NMR detectable triplet state. The advantages and disadvantages of several methods for accomplishing this goal are discussed.Fil: Münnemann, Kerstin. University of Kaiserslautern; AlemaniaFil: Buljubasich Gentiletti, Lisandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomia y Física. Sección Física. Grupo de Resonancia Magnética Nuclear; ArgentinaFil: Franzoni, Maria Belen. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomia y Física. Sección Física. Grupo de Resonancia Magnética Nuclear; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentin

Estimating Activity Coefficients of Target Components in Poorly Specified Mixtures with NMR Spectroscopy and COSMO-RS

Poorly specified mixtures are common in process engineering, especially in bioprocess engineering. The properties of such mixtures of unknown composition cannot be described using conventional thermodynamic models. The NEAT method, which has recently been developed in our group, enables the calculation of activity coefficients of known target components in such poorly specified mixtures. In NEAT, the group composition of the mixture is determined by NMR spectroscopy and a thermodynamic group contribution method is used for calculating the activity coefficients. In all previous studies with NEAT, the UNIFAC group contribution method was used. In the present work, we demonstrate that NEAT can also be applied with another important method for predicting activity coefficients: COSMO-RS. COSMO-RS (OL) developed in Oldenburg together with its group contribution version GC-COSMO-RS (OL) is used here. The new version of NEAT was successfully tested. For a variety of aqueous mixtures excellent agreement of the NEAT predictions, for which only information on the target component was used, with results that were obtained using the full knowledge on the composition of the mixture was found. The results demonstrate the generic nature of the idea of NEAT and the broad applicability of the method