Novel electrorheological properties of a metal-organic framework Cu3(BTC)2

A metal-organic framework, Cu(3)(BTC)(2), was synthesized and applied as an electro-responsive electrorheological material dispersed in insulating oil. Powder of crystalline Cu(3)(BTC)(2) exhibited excellent chain-like structures and controllable rheological properties in an applied electric field.     

Argon adsorption on Cu3(benzene-1,3,5-tricarboxylate)2(H2O)3 metal-organic framework

Using volumetric adsorption techniques, we have measured the adsorption of argon on Cu3(BTC)2(H2O)3, (BTC = benzene-1,3,5-tricarboxylate), a microporous metal-organic framework structure, at temperatures between 66 and 143 K. In addition to the experiments, we have used Grand Canonical Monte Carlo simulations to calculate the adsorption isotherm of argon at 87 K. Our experimental and theoretical results are compared to those of previous studies. The experiments were performed using a high density of points, allowing us to obtain, in detail, the isosteric heat's coverage dependence. Our values from the simulations are in reasonable agreement with those obtained in the experiments.     

ReaxFF molecular dynamics simulation of thermal stability of a Cu3(BTC)2 metal-organic framework

The thermal stability of a dehydrated Cu(3)(BTC)(2) (copper(II) benzene 1,3,5-tricarboxylate) metal-organic framework was studied by molecular dynamics simulation with a ReaxFF reactive force field. The results show that Cu(3)(BTC)(2) is thermally stable up to 565 K. When the temperature increases between 600 K and 700 K, the framework starts to partially collapse. The RDF analysis shows that the long range correlations between Cu dimers disappear, indicating the loss of the main channels of Cu(3)(BTC)(2). When the temperature is above 800 K, we find the decomposition of the Cu(3)(BTC)(2) framework. CO is the major product, and we also observe the release of CO(2), O(2), 1,3,5-benzenetricarboxylate (C(6)H(3)(CO(2))(3), BTC) and glassy carbon. The Cu dimer is stable up to 1100 K, but we find the formation of new copper oxide clusters at 1100 K. These results are consistent with experimental findings, and provide valuable information for future theoretical investigations of Cu(3)(BTC)(2) and its application in adsorption, separation and catalytic processes.     

The crystalline structures of ethylene-dimethylaminoethyl methacrylate copolymers as studied by solid-state high-resolution 13C NMR spectroscopy

The crystalline structures of ethylene-dimethylaminoethyl methacrylate (EDAM) copolymers, which were either melt-quenched (mq) or isothermally crystallized (iso), were studied by solid-state high-resolution ¹³C NMR spectroscopy. It revealed that the crystalline structures of EDAM copolymers are greatly dependent on the comonomer content, crystallization condition and the storage time after treatment. The ratio of monoclinic to orthorhombic crystal (M/O) increases with the increase in the dimethylaminoethyl methacrylate content. Higher crystallinity and lower monoclinic content were observed for iso samples compared to the mq ones. The monoclinic crystal was found to melt at lower temperatures compared to the orthorhombic one during the heating process. The degree of crystallinity as well as the contents of monoclinic and orthorhombic crystals and the M/O value are found to increase after storage at room temperature for a month.     

Cu3(BTC)2: CO oxidation over MOF based catalysts

Crystalline and amorphized MOFs (Cu(3)(BTC)(2)) have been demonstrated to be excellent catalysts for CO oxidation. The catalytic activity can be further improved by loading PdO(2) nanoparticles onto the amorphized Cu(3)(BTC)(2).     

Order parameters based on (13)C(1)H, (13)C(1)H(2) and (13)C(1)H(3) heteronuclear dipolar powder patterns: a comparison of MAS-based solid-state NMR sequences

Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments. Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated, which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield-Rhim-Elleman-Vaughn (T-MREV) and Lee-Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled (13)C or (1)H nuclei, as well as due to B(1) inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B(1) field is highly homogeneous. The T-MREV experiment is far better compensated for B(1) inhomogeneity, and it also performs better in situations with limited signal.     

Conformation of drawn poly(trimethylene terephthalate) studied by solid-state 13C NMR

The change in the conformation of the flexible O-CH2-CH2-CH2-O segment of poly(trimethylene terephthalate) (PTT) monofilament caused by drawing was investigated by means of the gamma-gauche effect on the 13C solid-state NMR chemical shift of the internal methylene carbon, combined with the NMR relaxations. The conformation around the O-CH2 and CH2-O bonds for as-spun fiber was trans/trans. On drawing, followed by heat treatment, the conformation changed to gauche/gauche. The ratio of gauche/gauche to trans/trans for the drawn PTT fiber was determined quantitatively.     

Phase transition of L-Ser monohydrate crystal studied by (13)C solid-state NMR

We used gravimetric analysis (GA) and (13)C solid-state nuclear magnetic resonance (NMR) to study solid-phase transition from the transparent single crystal of L-serine (L-Ser) monohydrate to a turbid powder. We found that L-Ser monohydrate loses water molecules and transforms into an anhydrate, thus experimentally demonstrating Frey's assumption. Application of a handmade cross-polarization (CP) NMR probe with a saddle-type coil to the oriented crystal of the L-Ser monohydrate revealed the dehydration mechanism. Furthermore, the chemical shift tensor components of the carboxyl carbon in L-Ser monohydrate were determined. The difference in the tensor component of delta(22) between the monohydrate and anhydrate forms was more than 7 ppm, probably owing to differences in the hydrogen-bonding structure of each form.     

Electron spin density distribution of a series of nitroxide radicals with five-member ring as studied by solid-state 1H, 2H and 13C NMR

Stable nitroxide radicals are useful to construct molecular magnetic systems. Particularly, radicals substituted by –COOH and –CONH₂ can be coordinated to magnetic metal ions and may be used as cladding reagents of gold nano-particles for modifying magnetism. Nitroxide molecules with unsaturated five-member ring have almost planner structure and electron spin delocalization may be expected. We determined the hyperfine coupling constants (hfcc) of ¹H, ²H and ¹³C of a series of nitroxide radicals with five-member ring. Experimental values of hfcc were compared with those deduced from calculations based on density functional theory. The electron spin density distribution at β position of ring was sensitive to the ring structure, although the electron spin density at β position is small compared with N–O site. Magnetic susceptibility and UV–Vis absorption spectra were also measured and discussed.     

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy

Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.     

Polymer electrolyte poly(ethylene oxide)/LiCF3SO3 studied by solid-state 13C NMR spectroscopy and ab initio calculations

Solid-state ¹³C NMR spectra and ab initio calculations of ¹³C NMR chemical shifts show that poly(ethylene oxide) (PEO) carbons in complex with LiCF₃SO₃ (both in crystalline and amorphous phase) are more shielded in comparison with neat PEO, due to the coordination to the Li⁺ cation. The results obtained from ¹³C NMR cross-polarization dynamics are in agreement with the published X-ray crystal structure of the PEO/LiCF₃SO₃ complex. The mobility of PEO in the crystalline complex is lower than in neat crystalline PEO.     

Protonation of pyrimidotriazinediones by1H and13C NMR spectroscopy

On the basis of an analysis of the changes in the chemical shifts of the signals in the¹H and¹³C NMR spectra on the pyrimidotriazinedione and trifluoric acid concentrations in CDCl₃ it was established that the protonation of rheumycin and fervenulin takes place at the N(₂) atom, whereas the protonation of isofervenulin takes place competitively at the N(₁), N(₂), and O(₆) atoms. The equilibrium constants of the investigated protonation processes were measured.A study of the chemical peculiarities of the behavior of Ia, b in aqueous acidic media (H₂O-HCl, D₂O-DCl) was described in [6].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1532–1538, November, 1988.     

Identifying guanosine self assembly at natural isotopic abundance by high-resolution 1H and 13C solid-state NMR spectroscopy

By means of the (1)H chemical shifts and the proton-proton proximities as identified in (1)H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)(2) and dG(C3)(2) derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by (15)N refocused INADEQUATE spectra in a previous study of (15)N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc.2005, 127, 16018). The NH (1)H chemical shift is observed to be higher (13-15 ppm) for ribbon-like self-assembly as compared to 10-11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH(2)(1)H chemical shifts is also inverted, with the NH(2) proton closest in space to the NH proton having a higher or lower (1)H chemical shift than that of the other NH(2) proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)(2) derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, (14)N-(1)H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH(2) chemical shifts for the dG(C3)(2) derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.     

Water-protein interactions in microcrystalline crh measured by 1H-13C solid-state NMR spectroscopy

Using solid-state NMR carbon-proton dipolar correlation spectroscopy, we observed hydrogen exchange on the millisecond time scale between water molecules and protein protons in a solid sample. These interactions are shown to be related to important structural features of the protein such as hydrogen-bonding or salt-bridge networks.     

Metal-ligand interplay in blue copper proteins studied by 1H NMR spectroscopy: Cu(II)-pseudoazurin and Cu(II)-rusticyanin

The blue copper proteins (BCPs), pseudoazurin from Achromobacter cycloclastes and rusticyanin from Thiobacillus ferrooxidans, have been investigated by (1)H NMR at a magnetic field of 18.8 T. Hyperfine shifts of the protons belonging to the coordinated ligands have been identified by exchange spectroscopy, including the indirect detection for those resonances that cannot be directly observed (the beta-CH(2) of the Cys ligand, and the NH amide hydrogen bonded to the S(gamma)(Cys) atom). These data reveal that the Cu(II)-Cys interaction in pseudoazurin and rusticyanin is weakened compared to that in classic blue sites (plastocyanin and azurin). This weakening is not induced by a stronger interaction with the axial ligand, as found in stellacyanin, but might be determined by the protein folding around the metal site. The average chemical shift of the beta-CH(2) Cys ligand in all BCPs can be correlated to geometric factors of the metal site (the Cu-S(gamma)(Cys) distance and the angle between the CuN(His)N(His) plane and the Cu-S(gamma)(Cys) vector). It is concluded that the degree of tetragonal distortion is not necessarily related to the strength of the Cu(II)-S(gamma)(Cys) bond. The copper-His interaction is similar in all BCPs, even for the solvent-exposed His ligand. It is proposed that the copper xy magnetic axes in blue sites are determined by subtle geometrical differences, particularly the orientation of the His ligands. Finally, the observed chemical shifts for beta-CH(2) Cys and Ser NH protons in rusticyanin suggest that a less negative charge at the sulfur atom could contribute to the high redox potential (680 mV) of this protein.     

Stability improvement of Cu3(BTC)2 metal-organic frameworks under steaming conditions by encapsulation of a Keggin polyoxometalate

Cu(3)(BTC)(2) with an incorporated Keggin polyoxometalate was demonstrated to be stable under steaming conditions up to 483 K, while the isostructural HKUST-1 degrades and transforms into [Cu(2)OH(BTC)(H(2)O)](n)·2nH(2)O from 343 K onwards.     

Aromatic imine stereochemistry as studied by 13C and 1H NMR of 15N-enriched materials

A series of thirteen ¹⁵N-enriched N-aryl imines derived from benzaldehydes and acetophenones have been prepared and examined by ¹³C and ¹H NMR. Preferred conformations of the C-arvl and N-aryl rings have been deduced from ¹³C chemical shifts and ¹³C-¹⁵N couplings. Evidence for steric inhibition of resonance in O-alkylated materials is presented. Relative signs of ¹J(CN) and ²J(CCN) have been determined in some compounds. An E: Z interconversion barrier of 21.6 kcal mol^?1 has been obtained from ¹H NMR coalescence data.     

Structural analysis of 1-vinylbenzazoles by 1H and 13C NMR spectroscopy

Analysis of ¹H and ¹³C NMR spectral parameters for 1-vinylbenzazoles has shown that the vinyl group has a predominantly trans orientation with respect to the condensed benzene ring. Additional evidence is given for a long range effect of the nitrogen lone pair electrons on the one bond ¹³C-¹H and ¹H-¹H spin-spin couplings and on the ¹H chemical shifts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 499–501, April, 1988.     

Prediction of adsorption isotherms of C3H6/C3H8 on hierarchical porous HP–Cu–BTC

The isotherms of C₃H₆ and C₃H₈ on three distinct HP–Cu–BTCs were determined using the static volumetric capacity technique across the pressure range 0–100 kPa, and the resulting experimental data set was regressed using the dual–site sips (DSS) model. The kinetics and thermodynamics of C₃H₆ and C₃H₈ on HP–Cu–BTCs were studied. The results show that the adsorption kinetics of the three samples conform to the pseudo–first–order kinetic model, indicating that the adsorption process of the HP–Cu–BTCs samples is the adsorption process with physical adsorption as the control step. The thermodynamic analysis results show that the adsorption of propylene and propane on the surface of HP–Cu–BTCs are a spontaneous exothermic process, because the transition of propylene and propane from three–dimensional motion to two–dimensional motion leads to a decrease in the system entropy. In addition, the isosteric heat of adsorption ($Q_{st}$) was used to predict the isotherms of C₃H₆ and C₃H₈ at 298 and 303 K, respectively. When the predicted and experimental values are compared, the predicted isotherms are shown to be fully associated with the experimental values, with mean relative errors (MRE%) of less than 2%. Additionally, the C₃H₆ and C₃H₈ adsorption isotherms and selectivity for C₃H₆ adsorption were predicted using a combination of the DSS model and ideal adsorbed solution theory (IAST). The findings suggest that the overall adsorption capacity of the mixes rose as the mole fraction of C₃H₆ increased, but the adsorption capacity of the equimolar C₃H₆ and C₃H₈ in the three HP–Cu–BTC combinations was smaller than the pure component. Additionally, an undetectable shift in C₃H₆/C₃H₈ selectivity was seen when the molar percentage of C₃H₆ increased.