The structural and dynamic properties of 1‐bromodecane in urea inclusion compounds investigated by solid‐state 1H, 13C and 2H NMR spectroscopy

For asymmetric guest molecules in urea, the end‐groups of two adjacent guest molecules may arrange in three different ways: head–head, head–tail and tail–tail. Solid‐state ¹H and ¹³C NMR spectroscopy is used to study the structural properties of 1‐bromodecane in urea. It is found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1‐bromodecane in urea inclusion compounds are probed by variable‐temperature solid‐state ²H NMR spectroscopy (line shapes, spin–spin relaxation: T₂, spin‐lattice relaxation: T_1Z and T_1Q) between 120 K and room temperature. The comparison between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in a quantitative way using a non‐degenerate three‐site jump process in the low‐temperature phase and a degenerate three‐site jump in the high‐temperature phase, in combination with the small‐angle wobbling motion. The kinetic parameters can be derived from the simulation. Copyright © 2011 John Wiley & Sons, Ltd.     

氨基甲酸酯型脱氧胆酸分子钳对中性分子的识别性能研究

利用差紫外光谱法考察了新型分子钳1～6对苯胺、对硝基苯胺、对甲氧基苯胺等中性分子的识别性能, 测定了25 ℃下, 在CHCl₃中主客体间的结合常数(K_a)和自由能变化(ΔG⁰). 结果表明, 所有分子钳主体对所考察的客体分子显示良好的识别作用, 主客体间形成1∶1型主客体络合物. 识别作用的主要推动力为多重氢键和π-π重叠等的作用. 讨论了主客体间形状、大小匹配和几何互补及识别模式等因素对识别能力的影响, 并利用核磁共振氢谱与计算机模拟作为辅助手段对实验结果进行了解释.     

Hopping dynamics and diffusion of atoms,molecules, and ions in nanoporous solids by exchange NMR spectroscopy

Molecular diffusion in nanoporous materials can be understood as series of dynamic hopping or exchange motions of molecules between different discrete sites. Exchange NMR offers the spectral resolution to distinguish between these different sites, based on isotropic and anisotropic NMR interactions that manifest differences in the local chemical or structural environments of molecules at different sites or their local orientations. Such interactions facilitate the observation of distinct adsorption environments and provide insights on the number and distributions of distinct types of environments and the geometries and motional correlation times of local hopping events between different sites. The temporal range accessible by exchange NMR is governed by the time required for the observation of the NMR signal (<?1 ms) and the return of the nuclear magnetic polarization to thermal equilibrium (typically several seconds). Over such timescales, this permits slow molecular exchange processes between local environments to be probed in great quantitative detail. The resulting insights on dynamic exchange or hopping of atoms, molecules, or ions in nanoporous solids provide a basis for understanding processes that occur over longer length and time scale, which ultimately account for their macroscopic diffusion properties.

     

Spectroscopic and Computational Insight into the Intermolecular Interactions between Zwitter‐Type Ionic Liquids and Water Molecules

Geometric and conformational changes of zwitter‐type ionic liquids (ZILs) due to hydrogen‐bonding interactions with water molecules are investigated by density functional theory (DFT), two‐dimensional IR correlation spectroscopy (2D IR COS), and pulsed‐gradient spin‐echo NMR (PGSE NMR). Simulation results indicate that molecular structures in the optimized states are strongly influenced by hydrogen bonding of water molecules with the sulfonate group or imidazolium and pyrrolidinium rings of 3‐(1‐methyl‐3‐imidazolio)propanesulfonate ( 1 ) and 3‐(1‐methyl‐1‐pyrrolidinio)propanesulfonate ( 2 ), respectively. Concentration‐dependent 2D IR COS reveals kinetic conformational changes of the two ZIL–H₂O systems attributable to intermolecular interactions, as well as the interactions of sulfonate groups and imidazolium or pyrrolidinium rings with water molecules. The dramatic changes in the ¹H self‐diffusion coefficients elucidate the formation of proton‐conduction pathways consisting of ZIL networks. In ZIL domains, protons are transferred by a Grotthuss‐type mechanism through formation, breaking, and restructuring of bonds between ZILs and H₂O, leading to an energetically favorable state. The simulation and experimental investigations delineated herein provide a perspective to understanding the interactions with water from an academic point of view as well as to designing ILs with desired properties from the viewpoint of applications.     

Investigation of ion-pairing phenomenon in BaF2 aqueous solution: Experimental and theoretical studies

The ion-pair association constant values, related to the reaction Ba²⁺ + F⁻ ? [BaF]⁺, are determined by means of NMR spectroscopy. The values for thermodynamic functions of the ion-pairing process are calculated on the basis of the NMR results. In addition, the association entropy has been found to be dependent on temperature. Comparing the experimental data and Fuoss theory, it is found that [BaF]⁺ contact ion-pair is formed in the BaF₂ aqueous solution. Also, hydration of barium-fluoride ion-pair is investigated by the DFT method. The hydration number of barium-fluoride ion-pair is determined by comparing the experimental and theoretical results. The effect of number of water molecules on the properties of ion-pairs is investigated by determining NQR and NMR parameters. Also, the relation between the chemical shifts and the energy gap between the highest occupied molecular orbital (HOMO) and low-lying virtual molecular orbital (LUMO) is investigated.     

Conformational Preferences of a β‐Octapeptide as Function of Solvent and Force‐Field Parameters

The ability to design properly folded β‐peptides with specific biological activities requires detailed insight into the relationship between the amino acid sequence and the secondary and/or tertiary structure of the peptide. One of the most frequently used spectroscopic techniques for resolving the structure of a biomolecule is NMR spectroscopy. Because only signal intensities and frequencies are recorded in the experiment, a conformational interpretation of the recorded data is not straightforward, especially for flexible molecules. The occurrence of conformational and/or time averaging, and the limited amount and accuracy of experimental data hamper the precise conformational determination of a biomolecule. In addition, the relation between experimental observables with the underlying conformational ensemble is often only approximately known, thereby aggravating the difficulty of structure determination of biomolecules. The problematic aspects of structure refinement based on NMR nuclear Overhauser effect (NOE) intensities and ³J‐coupling data are illustrated by simulating a β‐octapeptide in explicit MeOH and H₂O as solvents using three different force fields. NMR Data indicated that this peptide would fold into a 3₁₄‐helix in MeOH and into a hairpin in H₂O. Our analysis focused on the conformational space visited by the peptide, on structural properties of the peptide, and on agreement of the MD trajectories with available NMR data. We conclude that 1) although the 3₁₄‐helical structure is present when the peptide is solvated in MeOH, it is not the only relevant conformation, and that 2) the NMR data set available for the peptide, when solvated in H₂O, does not provide sufficient information to derive a single secondary structure, but rather a multitude of folds that fulfill the NOE data set.     

Vanadium pentoxide gels: From “chimie douce” to “matière molle”

This article presents a review of the colloidal and liquid-crystalline properties of vanadium pentoxide suspensions from a physicist's perspective. The processes occurring during the synthesis of these suspensions are first discussed. Then, the liquid-crystalline properties of V₂O₅ sols and gels are described. These nematic phases are easily aligned by weak magnetic fields or by alternative electric fields. The delicate interplay between repulsive hardcore and electrostatic interactions and van der Waals attractions defines the (concentration, ionic strength) phase diagram that includes an isotropic phase, a uniaxial nematic phase, a biaxial nematic gel state and a flocculated state. Deuterium NMR spectroscopy of D₂O molecules gives information on the rotational dynamics of the nematic phase. Finally, various applications of these colloidal suspensions in the fields of hybrid organic/inorganic materials, mesoporous solids, and of the structures of biomolecules, are reviewed.     

NMR identification of C11 to C40 branched hydrocarbons derived from the decomposition of polyisobutylene

The composition of nonvolatile fluids obtained from thermally cracking and hydrogenating polyisobutylene was determined by using a combination of gas chromatography and nuclear magnetic resonance spectroscopy (NMR). This work involved the separation and characterization of a homologous series, C₁₁–C₄₀, of sixteen branched hydrocarbon species consisting of repeating isobutylene structures. As a result of this investigation, useful correlations between NMR spectra and molecular structure for highly branched hydrocarbons were developed. The data demonstrate that these hydrocarbons are unique species characterized by “crowded” and sterically hindered geminal methyl and isolated methylene groups. NMR solvent shift studies in benzene solutions indicate that it is possible to differentiate between maximally crowded geminal methyl groups and between maximally crowded methylene groups in these structures. Results of the benzene-induced solvent effects are discussed with respect to the stereochemistry of these molecules and related to existing solvent shift data. These results suggest that these hydrocarbons are polar or nearly polar materials. Successive losses of isobutylene units from stabilized tertiary radicals can account for the formation of the major species identified in these fluids. Higher carbon numbered species have lower refractive indices and densities and higher molal volumes than predicted by calculations.     

High-resolution protein hydration NMR experiments: probing how protein surfaces interact with water and other non-covalent ligands

High-resolution solution NMR experiments are extremely useful to characterize the location and the dynamics of hydrating water molecules at atomic resolution. However, these methods are severely limited by undesired incoherent transfer pathways such as those arising from exchange-relayed intra-molecular cross-relaxation. Here, we review several complementary exchange network editing methods that can be used in conjunction with other types of NMR hydration experiments such as magnetic relaxation dispersion and ¹J_NC′ measurements to circumvent these limitations. We also review several recent contributions illustrating how the original solution hydration NMR pulse sequence architecture has inspired new approaches to map other types of non-covalent interactions going well beyond the initial scope of hydration. Specifically, we will show how hydration NMR methods have evolved and have been adapted to binding site mapping, ligand screening, protein-peptide and peptide-lipid interaction profiling.     

MC-determination in elastomers by 1H-NMR relaxation and 2H-NMR spectroscopy

We show how ¹H–NMR transversal relaxation and ²H–NMR spectroscopy can be used for the determination of the number average molecular mass M_C in typical elastomers at temperatures well above the glass transition temperature. M_C-results of the different NMR methods are compared among one another and with M_C-results of other common independent methods. Moreover, the NMR measurements provide a number of additional useful parameters: correlation times, portions of dangling chain ends and of sol molecules, molecular order.     

Cooperative and Diminutive Interplay Between Lithium and Dihydrogen Bonding in F3YLi…︁NCH…︁HMH and F3YLi…︁HMH…︁HCN Triads (Y=C,Si; M=Be,Mg)

The F₃YLi…NCH…HMH and F₃YLi…HMH…HCN triads (Y=C, Si; M=Be, and Mg) are connected by lithium and dihydrogen bonds. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries, binding energies, infrared spectra and NMR properties of monomers, dyads, and triads are investigated at the MP2/6‐311++G** computational level. Particular attention is paid to parameters, such as cooperative energies, and many‐body interaction energies. Triads with the HMH molecule located at the end of the chain, show energetic cooperativity ranging between ?3.66 to ?7.59 kJ mol^‐1. When the HMH molecule is located in the middle, the obtained cluster is diminutive with an energetic effect between 3.49 to 5.17 kJ mol^‐1. The electronic properties of the complexes are analyzed using parameters derived from the atoms in molecules (AIM) methodology.     

Nuclear magnetic resonance chemical shifts and quadrupole couplings for different hydrogen-bonding cases occurring in liquid water: a computational study

Nuclear magnetic resonance (NMR) parameters are determined theoretically for the oxygen and hydrogen/deuterium nuclei of differently hydrogen-bonded water molecules in liquid water at 300 K. The parameters are the chemical shift, the shielding anisotropy, the asymmetry parameter of shielding, the nuclear quadrupole coupling constant, and the asymmetry parameter of the nuclear quadrupole coupling. We sample instantaneous configurations from a Car-Parrinello molecular dynamics simulation and feed nuclear coordinates into a quantum chemical program for the calculation of NMR parameters using density-functional theory with the three-parameter hybrid exchange-correlation (B3LYP) functional. In the subsequent analysis, molecules are divided into groups according to the number of hydrogen bonds they possess, and the full average NMR tensors are calculated separately for each group. The classification of the hydrogen-bonding cases is performed using a simple distance-based criterion. The analysis reveals in detail how the NMR tensors evolve as the environment changes gradually from gas to liquid upon increasing the number of hydrogen bonds to the molecule of interest. Liquid-state distributions of the instantaneous values of the NMR properties show a wide range of values for each hydrogen-bonding species with significant overlap between the different cases. Our study shows how local changes in the environment, along with classical thermal averaging, affect the NMR parameters in liquid water. For example, a broken or alternatively extra hydrogen bond induces major changes in the NMR tensors, and the effect is more pronounced for hydrogen or deuterium than for oxygen. The data sheds light on the usefulness of NMR experiments in investigating the local coordination of liquid water.     

Is NMR the tool to characterize the structure of C20 isomers?

We investigate the feasibility of using nuclear magnetic resonance (NMR) chemical shift calculations as a tool to provide structural information for C₂₀ fullerene type molecules. NMR chemical shifts are extremely sensitive to the local chemical environment of an atom, reflecting unambiguously its bond lengths and angles as well as its hybridization. Thus, they can distinguish between the different isomers that are candidates for the ground state of this molecule. We calculate the NMR shifts for several C₂₀ isomers and show that NMR constitutes a potential tool to discriminate and identify experimentally a particular C₂₀ molecular conformation, and also the level of theory which best describes the experimental structure.     

Relationship between calculated NMR data and intermolecular hydrogen bond properties in X-pyridine⋯HF

The effect of different substituents in para and metapositions on the NMR data of X-pyridine?HF complex has been studied at B3LYP/6-311++G(d,p) level of theory. The relationship between NMR data and electron donation of substituents has been investigated. The results of topological properties of electron charge density calculated using atoms in molecules (AIM) analysis can be used to predict some NMR data. The magnetism-based indices, nucleus independent chemical shift NICS(1) and its z component NICS(1)_ZZ, were used to investigate the ring aromaticity changes on complexation. A linear correlation between Hammett coefficients and some NMR data could be found with a good correlation coefficient.     

Ordering and Clathrate Hydrate Formation in Co‐deposits of Xenon and Water at Low Temperatures

Local ordering in co‐deposits of water and xenon atoms produced at low temperatures can be followed uniquely by ¹²⁹Xe NMR spectroscopy. In water‐rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ～140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder Xray diffraction. There is evidence that Xe ? 20 H₂O clusters are prominent in the early stages of crystallization. In xenon‐rich deposits at 77 K there is evidence of xenon atoms trapped in Xe ? 20 H₂O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.     

Gas transport properties and molecular motions of 6FDA copolyimides

The gas transport properties of 6FDA copolyimide membranes have been studied by examining their interaction such as the charge transfer complex between donor and acceptor molecules or the π–π aromatic stacking and their molecular motion in the solid-state. The interaction and the molecular motion in the membranes have been measured using fluorescence spectroscopy and solid-state ¹³C NMR spectroscopy, respectively. The gas permeability and selectivity of CO₂, O₂, N₂, and CH₄ for the membranes have been measured at 35°C and at pressures up to 10 atm. The gas permeability of the copolyimide membranes was significantly dependent on the gas diffusion, and the diffusion selectivity was a principal factor that dominates the determination of the gas selectivity in the membranes. A good correlation was found between the gas transport properties and the carbon spin–lattice relaxation time associated with the molecular motion of the CF₃ group in the copolyimide membranes.     

芳酰胺-吖啶分子钳对中性分子的识别性能. II. 新型吖啶类分子钳的研究

采用差紫外谱法研究了新型芳酰胺-吖啶分子钳(1～7)对苯胺、苯二胺(邻, 间, 对)等中性分子的识别性能. 测定了结合常数(K_a)和自由能变化(ΔG°), 结果表明, 所有的分子钳受体与所考察的客体分子均形成1∶1型超分子配合物. 识别作用的主要推动力为多重氢键、van der Waals等的协同作用. 主客体间尺寸/形状匹配、几何互补等因素对识别性能均有重要的影响. 利用核磁氢谱与计算机模拟作为辅助手段对主要的实验结果与现象进行了解释.     

Electric field effects on nuclear spin�Cspin coupling tensors and chiral discrimination via NMR spectroscopy

Nuclear magnetic resonance spectrometers presently available are unable to recognize the two mirror-image forms of a chiral molecule, because in the absence of a chiral solvent, the NMR spectral parameters (chemical shifts and spin?Cspin coupling constants) are identical for the two enantiomers. This paper discusses how chirality may nevertheless, at least in theory, be recognized in liquid-state NMR spectroscopy by applying strong d.c. electric fields and measuring a pseudoscalar contribution to nuclear spin?Cspin coupling polarizability. Calculations are reported for medium-size chiral molecules, (2R)-N-methyloxaziridine, (R _a )-1,3-dimethylallene, and (2R)-2-methyloxirane. The very small contributions provided by the pseudoscalar of nuclear spin?Cspin coupling polarizability seem rather difficult to detect via NMR experiments in disordered phase.     

Aqueous solutions of sodium methylsulfate by Raman scattering,NMR, ultrasound,and density measurements

Dynamic properties of sodium methylsulfate were investigated by means of NMR and Raman spectroscopies. The concentration dependence of the spin-lattice relaxation times of²³Na in aqueous solutions of sodium methylsulfate show no detectable specific interactions between sodium and methylsulfate ions. Raman band profiles of the S-O stretching mode of methylsulfate ion in aqueous solution show asymmetry above about 3.0 mol-dm^–3. Furthermore, the proton spin-lattice relaxation rates of the methylsulfate ion in D₂O increase linearly up to about 3.0 mol-dm^–3, but above this concentration they deviate from linearity.These experimental results indicate that dynamic properties of methylsulfate ion are affected by ion-water interactions. Raman band asymmetry is attributed to the non-uniformity of the distribution of water molecules around the methylsulfate ion. This interpretation is supported by the theory developed by Knapp and Fischer. The distortion of the distribution of water molecules around ions is also discussed on the basis of the overlap of the hydration layer around methylsulfate ion which is estimated by the measurement of sound velocity and density.