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Introduction
Published in Alexis T. Bell, Alexander Pines, NMR Techniques in Catalysis, 2020
Alexis T. Bell, Alexander Pines
Among the techniques that can be used for the characterization of catalysts and adsorbed species, NMR is unique in its ability to provide information about both structure and dynamics [3,4]. Modern solid-state NMR makes it possible to detect signals from distinguishable sites in molecules and materials and to monitor the connectivities, correlations, and dynamics of these sites. Furthermore, NMR spectroscopy is essentially noninvasive and can be carried out in the presence of gases or liquids over a wide range of temperatures and pressures. While the principal use of NMR spectroscopy is to obtain information about the chemical environment of elements in catalysts or species adsorbed on catalysts, the technique can also be used to characterize atomic and molecular motions.
Nuclear Magnetic Resonance
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
The advances in solid-state NMR spectra have enabled characterization of structure and dynamics at the molecular level. The previously mentioned, widespread application to the study of organic, inorganic, biological, and organometallic chemistry have included studies of organic and inorganic molecules, catalysts, soil samples, polymers, biomolecules, surfaces, and nanoparticles [122].
Synthesis and functionalization of chitosan built hydrogel with induced hydrophilicity for extended release of sparingly soluble drugs
Published in Journal of Biomaterials Science, Polymer Edition, 2018
Faheem Ullah, Fatima Javed, M. B. H. Othman, Abbas Khan, Rukhsana Gul, Zulkifli Ahmad, Hazizan Md. Akil1
Unlike liquid state NMR, the peaks in solid-state NMR spectra are very broad due to anisotropic and orientation-dependent interactions. The peak lines are highly affected by low sensitivity and poor resolution where several peaks overlap each other at low resolution. The experimentally-determined solid-state 13C NMR spectra of native and functionalized hydrogel samples coded as C1, C2 and C3 are shown in Figure 4, and the obtained peaks were confirmed with the reported literature as shown in Table 2 [5]. At 25–32 ppm, the increase in the resonance signal for –C–NH2 could be attributed to amine functionality in chitosan, which forms the basis of swelling on protonation at physiological pH. The resonance peak –C–OH (45–55 ppm), for –CH2 (48–52 ppm), –C–N (80–95), amides (155–165 ppm), carbonyl (180–190 ppm) could be attributed to the reaction between CS, AA and the chemical crosslinker MBA [22,23]. The effective functionalization of C3 hydrogel by 4-aminophenol has been verified by shifting of the peaks and appearance of the additional peak for phenyl functionality in the range of 205–210 ppm as shown in Figure 4(b). Thus, 13C-NMR spectra of C2 and C3 functionalized hydrogel revealed the effective functionalization and appearance of peaks for the induced functionalities. The splitting of the peak that was observed in the carbon resonance could be credited to the degree of crosslinking and induced hydrophilicity [24].
Hydrogen-bonded LC nanocomposites: characterisation of nanoparticle-LC interactions by solid-state NMR and FTIR spectroscopies
Published in Liquid Crystals, 2019
Mahdi Roohnikan, Kayla Cummings Premack, Brenda Guzman-Juarez, Violeta Toader, Alejandro Rey, Linda Reven
Solid-state NMR spectroscopy is widely used to characterise complex materials, such as pharmaceutical co-crystals and complexes that are formed via non-covalent interactions [16]. In particular, solid-state 1H and 13C NMR techniques can provide detailed structural information for hydrogen-bonded systems. The 1H and 13C chemical shifts of the COOH group of the different samples, listed in Table 3, vary with changes in the hydrogen bonding. In general, the chemical shift of the carboxylic acid proton increases with decreasing hydrogen-bond length [24]. (The 1H and 13C NMR spectra are shown in the Supplemental data, Figures S7–S12.) The following trend for the hydrogen-bond strengths is based on the dissociation energies: pyridine–COOH heterodimers > closed homodimers > open homodimers [22]. The proton chemical shifts of the COOH groups in Table 3 follow this trend with δH (6BA:BPy) > δH (4-BCHA:BPy) > δH (6BA) > δH (4-BCHA). The 13C chemical shifts of the carbonyl groups also reflect the hydrogen-bond strength, but the changes in the chemical shift anisotropy result in a lower frequency with increasing hydrogen-bond strength. Based on the 1H and 13C chemical shifts, the 6BA:4-BCHA heterodimer is comparable to or slightly stronger than the 4-BCHA homodimer, and the hydrogen bonds formed when the BPy is added to the mixture are weaker than either the 4-BCHA or 6BA complexes with BPy. These differences in the H-bonding strengths are also reflected in the noticeably smaller 1H and 13C NMR linewidths for 4-BCHA (see Supplemental data, Figures S7–S12) as compared to 6BA due to the considerable molecular mobility of 4-BCHA.
NMR of cellulose nanocrystals, mesoporous media, and liquid crystal assemblies
Published in Liquid Crystals, 2020
NMR studies of chiral mesoporous glasses (photonic crystals) produced by cellulose-based templating [1–3], and of thermotropic liquid crystals (LCs) acting as guests in the mesoporous films are surveyed. Mesoporous structures were derived from CNC-based templating to give either chiral nematic mesoporous silica (CNMS) or organosilica (CNMO) films. Tuning the optical properties of photonic crystals filled with suitable LC guests is possible by external stimuli such as applied fields, heat or light. Reversible control of optical responses in photonic crystals by external stimuli is crucial for applications such as reflective displays or sensors. Solid-state NMR is useful for probing the molecular arrangement and order of guest molecules within mesoporous environments. This enables one to examine how different molecular shapes of the confined guests can influence the behaviours of photonic crystals. Supramolecular liquid crystals (SMLCs) via hydrogen-bond (HB) provide an effective way to change the shape and structure of molecules and their mesomorphic phases. Hydrogen-bond assemblies (HBAs) have been synthesised based on the phloroglucinol core, which serves as an HB donator, while the HB accepting side chains can either be azo-pyridines (Ap) or/and trans-4-hydroxy-4ʹ-stilbazole (St). The orientational order in bulk HBAs studied by solid-state NMR is detailed. Moreover, the molecular shape and/or packing of HBAs in their mesophases have been investigated. Introducing chiral centres in the HB accepting side chains, HBAs are found to exhibit chiral phases which include chiral smectic A (SmA*), twist grain boundary (TGB) phase and/or blue phase (BP). Moreover, a BPI with a broad temperature range had been found in a series of chiral HBAs and was studied by NMR. The order parameter S had been measured for the first time in a BPI in which the conventional Haller relation could be used to describe the S temperature dependency. The pseudo-exponential β in the Haller relation was determined in BPI for the first time.