Advanced liquid and gas NMR methods for probing topical materials

Abstract The present thesis exploits advanced liquid and gas NMR methods for the characterization of various interesting materials. The methods used to study the structural properties of thermally modified wood, ionic liquids, cements, shales, and porous organic cages include MRI, NMR cryoporometry, Laplace NMR, multidimensional Laplace NMR, as well as ¹²⁹Xe and ¹⁹F NMR. The commonality factor in all the studies is the usage of either inherent or introduced liquid or gas molecules to probe the topical materials. The MRI method was utilized to visualize the water absorption phenomena in the thermally modified pine wood. High-resolution images made it possible to observe the spatial distribution of free water and the changes in the rate of absorption of water in wood samples modified at different temperatures. The images also helped to resolve the individual resin channels. T₂ maps enabled us to observe the changes in the relaxation values of free water in thermally modified wood as compared to their unmodified reference wood samples. The multidimensional Laplace NMR methods were exploited to study the structural and dynamical properties of a novel halogen-free, boron-based ionic liquid (hf-BIL). NMR self-diffusion (D) experiments showed the presence of two coexisting dynamic phases in hf-BIL. Multidimensional D − T₂ correlation experiments made it possible to determine the T₂ relaxation times of the slow and fast diffusing phases. T₂ − T₂ relaxation exchange measurements allowed quantifying the exchange rates of anions and cations between the phases. Moreover, the theoretical modeling of the experimental data revealed that the slow diffusing phase was composed of anion-cation aggregates, while the fast diffusing phase was comprised of free anions and cations. ¹²⁹Xe NMR analysis of the xenon adsorbed in the cements and shales helped us to determine their porous structures. The method exploits the high sensitivity of the chemical shift of ¹²⁹Xe to its local environment. The chemical shift value of ¹²⁹Xe enabled us to estimate the size of the mesopores in the cement samples. The exchange spectroscopy (EXSY) measurements were used to determine the exchange rates between the free gas and mesopores of the cement samples. ¹²⁹Xe NMR spectra of the shale samples provided information about pore sizes and paramagnetic compounds. ¹H NMR cryoporometry measurements of the shale samples immersed in acetonitrile made it possible to analyze the pore size distribution ranging from 10 to over 100 nm. Moreover, T₂ − T₂ exchange measurements helped us to quantify the exchange rates of acetonitrile in the shale samples. Xenon and SF₆ were used as internal reporters to gain versatile information on adsorption phenomena in the cage and window cavities of the crystalline porous organic cages. ¹²⁹Xe NMR analysis of the adsorbed xenon helped us to determine the diffusion coefficients and activation energy of diffusion as well as thermodynamic parameters. With the help of T₂ relaxation time values, it was possible to estimate the exchange rates between cage and window cavities. Chemical exchange saturation transfer (CEST) experiments resolved a window cavity site, which arises from crystal defects in porous organic cages. In addition, ¹⁹F NMR analysis made it possible to estimate the relaxation rates and diffusion coefficients of SF₆ gas in porous organic cages. Modelling of the T₁, T₂ and diffusion data confirmed that the cage to window exchange is the completely dominating mechanism for ¹²⁹Xe T₂ relaxation. T₁ relaxation is dominated by diffusion modulated dipole-dipole relaxation (DDinter) and chemical shift anisotropy (CSA) relaxation due to local cavity mobility. Whereas, in case of SF₆ T₂ data, the dominating mechanism is diffusion modulated dipole-dipole relaxation and for T₁ the local tumbling of SF₆ in cage cavity is the key dynamics behind the dipole-dipole and CSA mechanisms

Characterization of the decay process of Scots pine caused by Coniophora puteana using NMR and MRI

Abstract Wood decay is an economically significant process, as it is one of the major causes of wood deterioration in buildings. In this study, the decay process of Scots pine (Pinus sylvestris) samples caused by cellar fungus (Coniophora puteana) was followed by nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) methods. Altogether, 30 wood sample pieces were exposed to fungus for 10 weeks. Based on the decrease of the dry mass, the samples were categorized into three classes: decomposed (mass decrease 50–70%), slightly decomposed (10–50%), and nondecomposed (<10%). MRI made it possible to identify the active regions of fungus inside the wood samples based on the signal of free water brought by the fungus and arisen from the decomposition of wood carbohydrates. MRI implies that free water is not only created by the decay process, but fungal hyphae also transports a significant amount of water into the sample. Two-dimensional ¹H T₁-T₂ relaxation correlation NMR measurements provided detailed information about the changes in the microstructure of wood due to fungal decomposition. Overall, this study paves the way for noninvasive NMR and MRI detection of fungal decay at early stages as well as the related structural changes

Determination of pore structures and dynamics of fluids in hydrated cements and natural shales by various 1H and 129Xe NMR methods

Abstract Cements and shales play a vital role in the construction and energy sectors. Here, we use a set of advanced NMR methods to characterize the porous networks and dynamics of fluids in hydrated cement and shale samples. We compare the properties of cements from two different manufacturers, BASF and Portland, as well as shales brought from USA and China. 129Xe NMR spectra of xenon gas adsorbed in the samples indicate that the capillary mesopores are smaller and the exchange between free and confined gas is slower in the Portland than in the BASF cement samples. The pores probed by xenon in the shale samples from USA are significantly smaller than in the cement samples, partially in the micropore region. There is a substantial difference in between the 129Xe spectra of shales from USA and China. Whereas the latter show a clear signature of paramagnetic impurities by exhibiting large negative 129Xe chemical shifts (referenced to the free gas), the samples from USA lack the negative chemical shifts but feature large positive shift values, which may indicate the presence of micropores and/or paramagnetic defects. 1H NMR cryoporometry measurements using acetonitrile as probe liquid allowed the observation of mesopores in the shale samples as well, and T2-T2 relaxation exchange experiment enabled the quantification of the exchange rates between free and confined acetonitrile

NMR relaxation and modelling study of the dynamics of SF6 and Xe in porous organic cages

Abstract The porous solid formed from organic CC3 cage molecules has exceptional performance for rare gas separation. NMR spectroscopy provides a way to reveal the dynamical details by using experimental relaxation and diffusion measurements. Here, we investigated T₁ and T₂ relaxation as well as diffusion of ¹²⁹Xe and SF₆ gases in the CC3-R molecular crystal at various temperatures and magnetic field strengths. Advanced relaxation modelling made it possible to extract various important dynamical parameters for gases in CC3-R, such as exchange rates, activation energies and mobility rates of xenon, occupancies of the cavities, rotational correlational times, effective relaxation rates, and diffusion coefficients of SF₆

Structure and dynamics elucidation of ionic liquids using multidimensional Laplace NMR

Abstract We demonstrate the ability of multidimensional Laplace NMR (LNMR), comprising relaxation and diffusion experiments, to reveal essential information about microscopic phase structures and dynamics of ionic liquids that is not observable using conventional NMR spectroscopy or other techniques

Inside information on xenon adsorption in porous organic cages by NMR

Abstract A solid porous molecular crystal formed from an organic cage, CC3, has unprecedented performance for the separation of rare gases. Here, xenon was used as an internal reporter providing extraordinarily versatile information about the gas adsorption phenomena in the cage and window cavities of the material. 129Xe NMR measurements combined with state-of-the-art quantum chemical calculations allowed the determination of the occupancies of the cavities, binding constants, thermodynamic parameters as well as the exchange rates of Xe between the cavities. Chemical exchange saturation transfer (CEST) experiments revealed a minor window cavity site with a significantly lower exchange rate than other sites. Diffusion measurements showed significantly reduced mobility of xenon with loading. 129Xe spectra also revealed that the cage cavity sites are preferred at lower loading levels, due to more favourable binding, whereas window sites come to dominate closer to saturation because of their greater prevalence