A thallium-205 NMR study of thallium acetate ion association in 2,2,2-trifluoroethanol and dimethylsulfoxide

The association of thallium acetate ion in 2,2,2-trifluoroethanol and dimethyl-sulfoxide has been investigated by thallium-205 NMR spectroscopy. Analysis of the association constants as a function of temperature indicates that in trifluoroethanol the association enthalpy and entropy are, respectively, 1.6 kcal-mol^–1 and 19.3 cal-mol^–1-K^–1 and in dimethylsulfoxide they are-0.99 kcal-mol^–1 and 18.0 cal-mol^–1-K^–1. Examination of the temperature dependence of the chemical shift of the ion-pair reveals that in dimethylsulfoxide the ion-pair exists as a contact species, while in trifluoroethanol the solvent-separated ion-pair is more likely.     

NMR shielding in helium-3 atoms modified by gaseous nitrogen and oxygen

Helium-3 nuclear magnetic resonance (³He NMR) measurements were carried out for the gaseous mixtures of helium-3 with pure nitrogen and synthetic air as the solvents. It was found that ³He shielding is linearly dependent on solvent density up to approx. 6 mol/L. At higher density of the gaseous solvent, the change of ³He shielding is nonlinear and especially distinct when helium-3 atoms can interact with two O₂ molecules. The interaction with paramagnetic oxygen molecules can induce two kinds of ³He shielding changes: (1) due to the isotropic Fermi contact interaction and (2) from the dipolar magnetic interaction between unpaired O₂ electrons and ³He nuclear magnetic dipole moment. The two paramagnetic effects in helium-3 shielding cannot be experimentally separated, although for such small molecular objects, they could be presumably modeled by advanced theoretical calculations.     

固体核磁碳结构参数的修正及其在煤结构分析中的应用

为消除~(13)C CP/M AS/TOSS NM R测试中碳核NOE效应,获得相对准确的碳结构参数,考察了不同模型化合物的碳核NOE效应强度。结果表明,不同模型化合物碳谱分峰拟合的测试值与样品碳结构参数的理论值之间存在明显误差,其中,脂肪碳在25%-125%、芳香碳为4%-50%,NOE效应在固体核磁碳谱测试中影响显著。为此,将模型化合物脂肪碳和芳香碳的实测值和理论值进行回归分析,得到非线性回归方程。用该方程对9,10-二甲基蒽进行碳结构修正,发现修正后脂肪碳实测值与理论值之间误差由不修正时的119.60%减小至7.84%,芳香碳误差为由不修正时的-17.10%到1.11%,修正后误差均在10%以内;同时用该回归方程修正了不同煤的碳结构参数,发现不同煤未修正的H/C原子比与其元素分析H/C原子比误差在45%-53%,修正后误差只有4%-13%,与元素分析结果具有一致性,表明非线性回归方程能够方便、准确地消除固体核磁NOE效应,为煤中碳结构分析提供新的技术支撑。     

Sodium-23, cesium-133 and thallium-205 NMR study of sodium,cesium and thallium complexes with large crown ethers in nonaqueous solutions

Formation constants for complexes of dibenzo-21-crown-7, dibenzo-24-crown-8 and dibenzo-27-crown-9 with the Na⁺, Cs⁺ and Tl⁺ ions have been determined by multinuclear NMR measurements in several nonaqueous solvents. The measurements of the cesium complexes were carried out at different temperatures and the enthalpy and entropy of complexation were determined from the temperature dependence of the formation constants.The stabilities of these complexes in solvents of low to medium donicity, —nitromethane, acetonitrile, acetone, methanol, and propylene carbonate, vary in the order Tl⁺>Cs⁺>Na⁺. Depending on the relative sizes of the cation and of the ligand cavity, either a three-dimensional wrap-around complex or a two-dimensional crown complex are formed. For the cesium complexes, the values of H _c ^o and S _c ^o are definitely solvent-dependent and in all cases the complexes are enthalpy stabilized but entropy destabilized. A compensating effect is observed in the variation of the enthalpy and entropy of complexation so that the overall influence of the above solvents on the stability of the complexes is rather limited.     

Effects of polymorphic differences for sulfanilamide, as seen through 13C and 15N solid-state NMR, together with shielding calculations

We recorded both carbon-13 and nitrogen-15 NMR spectra of the three solid forms of sulfanilamide most commonly known. This study led to an interpretation of the solid-state effects seen in cross-polarization magic angle spinning spectra. Relaxation times for the different forms were measured. These show different behaviour for the three forms, arising from mobility variations. To obtain information on local environments, static spectra and spinning sideband manifolds were recorded and analysed for the 15N resonances, using isotopically enriched samples. Shielding asymmetries and anisotropies for the two nitrogen nuclei were obtained, showing very different behaviour for the two sites. Shielding calculations were carried out for both 13C and 15N nuclei, and the results are discussed in relation to the experimental values.     

Study of hydrogen bonds in N-methylacetamide by DFT calculations of oxygen, nitrogen, and hydrogen solid-state NMR parameters

Solid-state nuclear magnetic resonance (NMR) parameters of ¹⁷O, ¹⁴N/¹⁵N, and ²H/¹H nuclei were evaluated in two available neutron crystalline structures of N-methylacetamide (NMA) at 250 and 276 K, NMA-I and NMA-II, respectively. Density functional theory calculations were performed by B3LYP method and 6-311++G** and IGLO-II type basis sets to calculate the electric field gradient (EFG) and chemical shielding (CS) tensors at the sites of mentioned nuclei. In order to investigate hydrogen bonds (HBs) effects on NMR tensors, calculations were performed on four-model systems of NMA: an optimized isolated gas-phase, crystalline monomers, crystalline dimers, and crystalline trimers. Comparing the calculated results reveal the influence of N–H···O=C and C–H···O=C HB types on the NMR tensors which are observable by the evaluated parameters including quadrupole coupling constant, C _Q, and isotropic CS, σ _iso. Furthermore, the results demonstrate more influence of HB on the EFG and CS tensors of NMA at 276 K rather than that of 250 K.     

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the ¹H and ¹³C NMR spectra of 1‐(2‐selenophenyl)‐1‐alkanone oximes, the ¹H, the ¹³C‐3 and ¹³C‐5 signals of the selenophene ring are shifted by 0.1–0.4, 2.5–3.0 and 5.5–6.0 ppm, respectively, to higher frequencies, whereas those of the ¹³C‐1, ¹³C‐2 and ¹³C‐4 carbons are shifted by 4–5, ～11 and ～1.7 ppm to lower frequencies on going from the E to Z isomer. The ¹⁵N chemical shift of the oximic nitrogen is larger by 13–16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the ⁷⁷Se resonance positions is revealed in the studied oxime isomers, the ⁷⁷Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the ¹H, ¹³C, ¹⁵N and ⁷⁷Se shielding constants in the energy‐favorable conformation with the syn orientation of the? C?N? O? H group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd.     

Study of stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the configurational isomers of the selenophene-2-carbaldehyde azine by NMR spectroscopy and MP2-GIAO calculations

In the (1)H and (13)C NMR spectra of selenophene-2-carbaldehyde azine, the (1)H-5, (13)C-3 and (13)C-5 signals of the selenophene ring are shifted to higher frequencies, whereas those of the (1)H-1, (13)C-1, (13)C-2 and (13)C-4 are shifted to lower frequencies on going from the EE to ZZ isomer or from the E moiety to the Z moiety of EZ isomer. The (15)N chemical shift is significantly larger in the EE isomer relative to the ZZ isomer and in the E moiety relative to the Z moiety of EZ isomer. A very pronounced difference (60-65 mg/g) between the (77)Se resonance positions is revealed in the studied azine isomers, the (77)Se peak being shifted to higher frequencies in the ZZ isomer and in the Z moiety of EZ isomer. The trends in the changes of the measured chemical shifts are reasonably reproduced by the GIAO calculations at the MP2 level of the (1)H, (13)C, (15)N and (77)Se shielding constants in the energy-favorable conformation with the syn orientation of both selenophene rings relative to the C = N groups. The NBO analysis suggests that such an arrangement of the selenophene rings may take place because of a higher energy of some intramolecular interactions.     

Investigation of structural and magnetoelectronic properties of new half-metallic Heusler alloys Ru2VGexSb1−x (x = 0, 0.5 and 1): A density functional theory study

We performed an ab initio study using a method named linearized augmented plane wave with a full potential (FP-LAPW) based on the density functional theory. We predicted the physical properties of Ru₂VGe_xSb_1−x (x = 0, 0.5 and 1) Heusler alloys in L2₁ structure. We computed the magnetic and structural properties using the general gradient approximation. The modified Becke-Johnson scheme was used to study the electronic structure of these compounds. The obtained results show that the lattice constants and the spin magnetic moments are in favorable agreement compared with theoretical values and experimental data. The computed densities of state (DOSs) of these compounds indicate a half-metallic behavior with a real gap for the ternary materials, which gives perfect spin polarization, while for the quaternary one, the DOS indicate a nearly half-metallic character with a pseudogap in the minority spin close to the Fermi level E_F.     

Structural and spectral assignment by 2D NMR of a new prenylated benzopyrancarboxylic acid and structural reassignment of a related compound

A new prenylated benzopyrancarboxylic acid, 1a (3,4‐dihydro‐5‐hydroxy‐2,7‐dimethyl‐8‐(2‐methyl‐2‐butenyl)‐ 2‐(4‐methyl‐1, 3‐pentadienyl)‐2H‐1‐benzopyran‐6‐carboxylic acid) was isolated from Peperomia amplexicaulis and fully characterized by 1D and 2D NMR and high‐resolution mass spectrometry. In the course of this investigation, the structure of a related compound (minus the carboxylic acid group) which was previously assigned as 2b was corrected to structure 1b . Copyright © 2003 John Wiley & Sons, Ltd.     

Comparative analysis of 13C shielding constants stereospecificity in the silicon and germanium derivatives of acetylenic aldehyde and ketone oximes based on the 13C NMR spectroscopy and GIAO calculations

In the acetylenic aldehyde oximes with substituents containing silicon and germanium, the ¹³C NMR signal of the C‐2 carbon of triple bond is shifted by 3.5 ppm to lower frequency and that of the C‐3 carbon is moved by 7 ppm to higher frequency on going from E to Z isomer. A greater low‐frequency effect of 5.5 ppm on the C‐2 carbon signal and a greater high‐frequency effect of 11 ppm on the C‐3 carbon signal are observed in the analogous acetylenic ketone oximes. The carbon chemical shift of the C?N bond is larger by 4 ppm in E isomer relative to Z isomer for the aldehyde and ketone oximes. The ²⁹Si chemical shifts in the silicon containing acetylenic aldehyde and ketone oximes are almost the same for the diverse isomers. The trends in changes of the measured chemical shifts are well reproduced by the gauge‐including atomic orbital (GIAO) calculations of the ¹³C and ²⁹Si shielding constants. Copyright © 2009 John Wiley & Sons, Ltd.     

A review on the mechanical,electrical and EMI shielding properties of carbon nanotubes and graphene reinforced polycarbonate nanocomposites

Recently, it has been found that carbon nanotubes (CNTs) and graphene could prove to be the most promising carbonaceous fillers in polymers nanocomposites field because of their better structural and functional properties. Their uniform dispersion in polymer matrix leads to significant improvements in their several properties. This paper reviews the effect of nanofillers, ie, CNTs, derivatized CNTs, and graphene on the polycarbonate nanocomposite and its application in aerospace, automobile, sports, electronic sectors, and various industries. The comparative analysis of carbon‐based fillers on the different properties of polycarbonate nanocomposites is also included.     

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.     

Simulation of structural, elastic, and electronic properties of new cubic crystals of carbon and BN nanotubes

Models of new cubic crystals from carbon and boron-nitrogen (BN) nanotubes are proposed. Within electronic density functional theory, their structural, elastic, and electronic properties are studied. These isotropic nanotubular crystals are found to have extremely high elastic modules B (～490–650 GPa) and low compressibility β (～0.0020–0.0015 1/GPa) and maintain the conductivity typical of their “building blocks,” i.e. isolated carbon and BN nanotubes.     

Bifurcated hydrogen-bonding effect on the shielding and coupling constants in trifluoroacetyl pyrroles as studied by 1H, 13C and 15N NMR spectroscopy and DFT calculations

According to the (1)H, (13)C and (15)N NMR spectroscopic data and DFT calculations, bifurcated N--H...N and N--H...O intramolecular hydrogen bond is shown to be present in 2-trifluoroacetyl-5-(2'-pyridyl)-pyrrole. This bifurcated hydrogen bond causes an increase in the absolute size of the (1)J(N,H) coupling constant by about 6 Hz, and the deshielding of the bridge proton by 2 ppm. DFT calculations show that the influence of the N--H...N and N--H...O intramolecular hydrogen bonds on the (1)J(N,H) coupling and proton shielding is almost additive, although the components of the bifurcated hydrogen bond slightly weaken each other. In 2-trifluoroacetyl-5-(2'-pyridyl)-pyrrole, the coupling constants involving the fluorine and the N--H covalent bond nuclei depend dramatically on the spatial position of the pyridine ring. The pyridine ring rotation operates as a quantum switch controlling the spin information transfer between the (19)F and (15)N nuclei, as well as the proton.     

Relationships between Fe NMR, Mössbauer parameters, electrochemical properties and the structures of ferrocenylketimines

A comparative study of the electrochemical properties, ⁵⁷Fe NMR and Mössbauer spectroscopic data of compounds [(η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-C(R¹)N-R²}] {R¹ = H, R² = CH₂-CH₂OH (1a), CH(Me)-CH₂OH (1b), CH₂C₆H₅ (1c), C₆H₄-2Me (1d), C₆H₄-2SMe (1e) or C₆H₄-2OH (1f) and R¹ = C₆H₅, R² = C₆H₄-2Me (2d)} is reported. The X-ray crystal structure of [(η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-CHN-C₆H₄-2OH}] (1f) is also described. Density functional theoretical (DFT) studies of these systems have allowed us to examine the effects induced by the substituents of the “-C(R¹)N-R²” moiety or the aryl rings (in 1d-1f) upon the electronic environment of the iron(II) centre.     

Synthesis, characterization and nonlinear optical properties in a series of new chiral organotin(IV) Schiff base complexes

Four new chiral organotin derivatives are reported with their crystal structure. They were synthesized by reaction of diphenyltin oxide and four different ligands obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde and (1R,2S)-(+)-norephedrine, (R)-(−)-phenylglycinol, (R)-(−)-1-amino-2-propanol and (1S,2R)-2-amino-1,2-diphenylethanol. Their nonlinear optical properties were investigated experimentally in solid state and with the electric field induced second harmonic (EFISH) technique. In particular, the compound obtained with (R)-(−)-phenylglycinol exhibits an efficiency 11 times that of urea in second harmonic generation at 1.907 μm. The properties are discussed in relation with computational studies conducted within the framework of the DFT theory.     

Cr concentration driving the structural,mechanical, and thermodynamic properties of Cr-Al compounds from first-principles calculations

Cr-Al binary compounds are regarded as the potential high-temperature structural materials. However, the structure and important properties of Cr-Al compounds are not completely unclear. Here, we report on the influence of Cr concentration on the structural, mechanical, and thermodynamic properties of Cr-Al compounds by using the first-principles calculations. Four novel Cr-Al compounds, Cr₃Al₈ with monoclinic structure (C2/m), Cr₃Al₅ with hexagonal structure (P63mc), Cr₂Al₃ with tetragonal structure (I4/mmm), and Cr₃Al with cubic structure (Pm-3 m), are predicted. The calculated elastic modulus of Cr-Al compounds gradually increases with increasing Cr concentration. Compared to other Cr-Al compounds, our predicted Cr₃Al with cubic structure exhibits a strong deformation resistance and high hardness due to symmetrical Cr Al bonds. However, the Debye temperature of Cr₇Al₃ is larger than that of other Cr-Al compounds. The calculated phonon density of state shows that the high-temperature thermodynamic properties of Cr-Al compounds are attributed to the vibration of Al atom and Cr Al bond.     

Thallium-205 NMR spectroscopy. Solvation of the Tl(l) ion in the binary solvent systems water/pyridine,water/dimethyl sulfoxide,and pyridine/dimethyl sulfoxide

Solvation of the Tl⁺ ion in 0.005M solutions of water/pyridine, water/dimethyl sulfoxide, and pyridine/dimethyl sulfoxide was studied with ²⁰⁵ Tl NMR spectroscopy as a function of solvent composition and anion (NO ₃ ^– and ClO ₄ ^t- ). Dimethyl sulfoxide solvated the Tl⁺ ion more strongly than did pyridine, despite the latter's greater electron-donating ability. This was explained in terms of structural effects, which were found to be large for all three binary solvent systems. Ion pairing was evident in the DMSO/pyridine and water/pyridine solvent systems in which the pyridine mole fraction was greater than 0.8.     

DFT computational schemes for 1H and 13C NMR chemical shifts of natural products,exemplified by strychnine

A number of computational schemes based on different Density Functional Theory (DFT) functionals in combination with a number of basis sets were tested in the calculation of ¹H and ¹³C NMR chemical shifts of strychnine, as a typical representative of the vitally important natural products, and used as a challenging benchmark and a rigorous test for such calculations. It was found that the most accurate computational scheme, as compared with experiment, was PBE0/pcSseg-4//pcseg-3 characterized by a mean absolute error of 0.07 ppm for the range of about 7 ppm for ¹H NMR chemical shifts and that of only 1.13 ppm for ¹³C NMR chemical shifts spread over the range of about 150 ppm. For more practical purposes, including investigation of larger molecules from this series, a much more economical computational scheme, PBE0/pcSseg-2//pcseg-2, characterized by almost the same accuracy and much less computational demand, was recommended.