From Free Radicals and Spin-Chemistry Over Spin-Dynamics and Hyperpolarization to Biology and Materials Science

This special issue of ZPC is dedicated to the 80th anniversary of Kev Salikhov, a prominent scientist who has made seminal contributions to spin chemistry, chemical kinetics, spin hyperpolarization and magnetic resonance. Altogether there are 41 scientific papers; for this reason, the guest editors have decided to publish three subsequent issues. Each issue is focused on a particular field, which is closely related to research interests of Salikhov or was even seeded by his work. As the following papers clearly demonstrate, the initial seed of Salikhov fell on a very fruitful soil and has grown to a huge and impressive scientific tree with branches ranging from fundamental science to applications in biochemistry and materials science

NMR Studies on the Temperature-Dependent Dynamics of Confined Water

We use $^2$H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that $\alpha$ process-like dynamics is probed above the crossover temperature, whereas $\beta$ process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the $\beta$ process of supercooled liquids during the glass transition, suggesting a value of $T_g\!\approx\!185$ K for interface-affected liquid water. In the high-temperature range, $T\!>\!225$ K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the $\alpha$ process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of $E_a\!\approx\!0.5\$eV in the low-temperature range, $T\!<\!180$K, revealing that the confinement size plays a minor role for the $\beta$ process of water.Comment: 12 pages, 9 figure

Small Molecules, Non-Covalent Interactions, and Confinement

This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems

Coherent manipulation of non-thermal spin order in optical nuclear polarization experiments

Time resolved measurements of Optical Nuclear Polarization (ONP) have been performed on hyperpolarized triplet states in molecular crystals created by light excitation. Transfer of the initial electron polarization to nuclear spins has been studied in the presence of radiofrequency excitation; the experiments have been performed with different pulse sequences using different doped molecular systems. The experimental results clearly demonstrate the dominant role of coherent mechanisms of spin order transfer, which manifest themselves in well pronounced oscillations. These oscillations are of two types, precessions and nutations, having characteristic frequencies, which are the same for the different molecular systems and the pulse sequences applied. Hence, precessions and nutations constitute a general feature of polarization transfer in ONP experiments. In general, coherent manipulation of spin order transfer creates a powerful resource for improving the performance of the ONP method, which paves the way to strong signal enhancement in nuclear magnetic resonance

Solid-State Nuclear Magnetic Resonance as a Versatile Tool To Identify the Main Chemical Components of Epoxy-Based Thermosets

Cross-linked thermosets are investigated by ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy to determine their structure and to distinguish important epoxy resins and hardening agents. In addition to the epoxy resin and hardening agent, the identification of phosphorus-containing flame retardants is demonstrated by ³¹P solid-state NMR. These studies provide a spectral database for routine use, which is finally applied to analyze commercial products containing an unknown multicomponent system

Water and small organic molecules as probes for geometric confinement in well- ordered mesoporous carbon material

Mesoporous carbon materials were synthesized employing polymers and silica gels as structure directing templates. The basic physico-chemical properties of the synthetic mesoporous materials were characterized by 1H and 13C MAS solid-state NMR, X-ray diffraction, transmission electron microscopy (TEM) and nitrogen adsorption measurements. The confinement effects on small guest molecules such as water, benzene and pyridine and their interactions with the pore surface were probed by a combination of variable temperature 1H-MAS NMR and quantum chemical calculations of the magnetic shielding effect of the surface on the solvent molecules. The interactions of the guest molecules depend strongly on the carbonization temperature and the pathway of the synthesis. All the guest-molecules, water, benzene and pyridine, exhibited high-field shifts by the interaction with the surface of carbon materials. The geometric confinement imposed by the surface causes a strong depression of the melting point of the surface phase of water and benzene. The theoretical calculation of 1H NICS maps shows that the observed proton chemical shifts towards high-field values can be explained as the result of electronic ring currents localized in aromatic groups on the surface. The dependence on the distance between the proton and the aromatic surface can be exploited to estimate the average diameter of the confinement structures

Reconciling local structure disorder and the relaxor state in (Bi1/2Na1/2)TiO3 -BaTiO3

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO(3) (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (Delta QIS(width)) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in Na-23 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.ope

Nanoscale phase quantification in lead-free (Bi1/2Na1/2)TiO3-BaTiO3 relaxor ferroelectrics by means of Na 23 NMR

We address the unsolved question on the structure of relaxor ferroelectrics at the atomic level by characterizing lead-free piezoceramic solid solutions (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT) (for x=1,4,6, and 15). Based on the relative intensity between spectral components in quadrupolar perturbed Na23 nuclear magnetic resonance, we present direct evidence of the coexistence of cubic and polar local symmetries in these relaxor ferroelectrics. In addition, we demonstrate how the cubic phase vanishes whenever a ferroelectric state is induced, either by field cooling or changing the dopant amount, supporting the relation between this cubic phase and the relaxor state.open0

Theoretical description of hyperpolarization formation in the SABRE-relay method

SABRE (Signal Amplification By Reversible Exchange) has become a widely used method for hyper-polarizing nuclear spins, thereby enhancing their Nuclear Magnetic Resonance (NMR) signals by orders of magnitude. In SABRE experiments, the non-equilibrium spin order is transferred from parahydrogen to a substrate in a transient organometallic complex. The applicability of SABRE is expanded by the methodology of SABRE-relay in which polarization can be relayed to a second substrate either by direct chemical exchange of hyperpolarized nuclei or by polarization transfer between two substrates in a second organometallic complex. To understand the mechanism of the polarization transfer and study the transfer efficiency, we propose a theoretical approach to SABRE-relay, which can treat both spin dynamics and chemical kinetics as well as the interplay between them. The approach is based on a set of equations for the spin density matrices of the spin systems involved (i.e., SABRE substrates and complexes), which can be solved numerically. Using this method, we perform a detailed study of polarization formation and analyze in detail the dependence of the attainable polarization level on various chemical kinetic and spin dynamic parameters. We foresee the applications of the present approach for optimizing SABRE-relay experiments with the ultimate goal of achieving maximal NMR signal enhancements for substrates of interest