The 15N chemical shifts in mixed NB2Si and NBSi2 environments of Si3B3N7--a theoretical investigation

Nuclear magnetic resonance (NMR) chemical shifts in solids may be calculated by ab initio methods approximating the solid state by molecular clusters. We employed this technique to obtain estimates of (15)N chemical shifts in NB(2)Si and NBSi(2) environments in the solid state. Such nitrogen environments are found in amorphous (Si/B/N-)ceramics which exhibit very interesting features such as high thermal and mechanical stability. We based our calculations on cutouts of hypothetical Si(3)B(3)N(7) crystals suggested by Kroll and Hoffmann [Silicon boron nitrides: hypothetical polymorphs of Si(3)B(3)N(7), Angew. Chem. Int. Ed. 37 (1998) 2527]. Taking the systematic errors of our calculations into account we expect the chemical shifts in NBSi(2) environments around -293+/-5ppm. Chemical shifts in NB(2)Si environments are expected at -272+/-6ppm. The range of the calculated chemical shifts in NBSi(2) environments coincides with experimental chemical shifts in molecular compounds. Experimental chemical shifts of NB(2)Si nitrogen in molecules appear at lower field than our calculated chemical shifts in the solid state.     

Measurement of 15N chemical shift anisotropy in a protein dissolved in a dilute liquid crystalline medium with the application of magic angle sample spinning

The chemical shifts of nuclei that have chemical shielding anisotropy, such as the 15N amide in a protein, show significant changes in their chemical shifts when the sample is altered from an isotropic state to an aligned state. Such orientation-dependent chemical shift changes provide information on the magnitudes and orientation of the chemical shielding tensors relative to the molecule's alignment frame. Because of the extremely high sensitivity of the chemical shifts to the sample conditions, the changes in chemical shifts induced by adding aligned bicelles do not arise only from the protein alignment but should also include the accumulated effects of environmental changes including protein-bicelle interactions. With the aim of determining accurate 15N chemical shielding tensor values for solution proteins, here we have used magic angle sample spinning (MAS) to observe discriminately the orientation-dependent changes in the 15N chemical shift. The application of MAS to an aligned bicelle solution removes the torque that aligns the bicelles against the magnetic field. Thus, the application of MAS to a protein in a bicelle solution eliminates only the molecular alignment effect, while keeping all other sample conditions the same. The observed chemical shift differences between experiments with and without MAS therefore provide accurate values of the orientation-dependent 15N chemical shifts. From the values for ubiquitin in a 7.5% (w/v) bicelle medium, we determined the 15N chemical shielding anisotropy (CSA) tensor. For this evaluation, we considered uncertainties in measuring the 1H-15N dipolar couplings and the 15N chemical shifts and also structural noise present in the reference X-ray structure, assuming a random distribution of each NH bond vector in a cone with 5 degrees deviation from the original orientation. Taking into account these types of noise, we determined the average 15N CSA tensor for the residues in ubiquitin as Delta sigma=-162.0+/-4.3 ppm, eta=0.18+/-0.02, and beta=18.6+/-0.5 degrees, assuming a 1H-15N bond length of 1.02 A. These tensor values are consistent with those obtained from solid-state NMR experiments.     

19F spectroscopy and relaxation behavior of trifluorovinyldichloroborane

The 19F NMR spectra and spin-lattice relaxation rate, R1, of trifluorovinyldichloroborane as shown in were studied as a function of temperature, T, and magnetic field, B. All logR1 vs 1/T plots show a minimum at 299K indicating the presence if dipolar relaxation at lower T and spin-rotation relaxation at higher T. The R1 values increase with increasing B due to chemical shift anisotropy relaxation. Estimates of the fluorine chemical shift values for F3 (cf. Fig. 1) suggest that there is pi character in the F-C bond. The other two C-F bonds are largely single bonded. No evidence was found for intermolecular exchange of the trifluorovinyl group. Two of the three fluorine atoms show large increases in their NMR linewidth with increasing temperature while the third shows only a small increase but the activation energy for the process is the same for all. The increase in linewidths is due to scalar coupling to the boron atoms. The boron linewidths were measured between 253 and 363K and decreased with increasing temperature. A plot of logR2, where R2 is the linewidth of the boron as a function of 1/T shows some curvature indicating a second relaxation mechanism. This is ascribed to spin-rotation but not enough data are available to be conclusive. In all cases there is a second small set of fluorine peaks that are due to 10B interactions separated from the 11B peaks by amounts varying from 1 to 4 ppm depending on the field and fluorine atom.     

Characterization of hydrogen bond lengths in Watson-Crick base pairs by cross-correlated relaxation

Hydrogen bond lengths in Watson-Crick base pairs can be characterized by cross-correlated relaxation between 1H chemical shift anisotropy and dipole-dipole coupling of 1H and its hydrogen bond acceptor 15N. As a reference, the cross-correlated relaxation between 1H chemical shift anisotropy and dipole-dipole coupling of 1H and its hydrogen bond donor 15N is used. With the two measured cross-correlated relaxation rates, an apparent hydrogen bond length can be determined, which is composed by the hydrogen bond length multiplied by a term representing the amplitude of inter-base motions. Data are presented for the 15N3-1H3...15N1 hydrogen bonds in A=T base pairs of the Antennapedia homeodomain-DNA complex with a correlation time of global rotational diffusion of 20 ns.     

Magnitudes and orientations of the 15N chemical shift tensor of [1-15N]-2'-deoxyguanosine determined on a polycrystalline sample by two-dimensional solid-state NMR spectroscopy.

The magnitudes and orientations of the 15N chemical shift tensor of [1-15N]-2'-deoxyguanosine were determined from a polycrystalline sample using the two-dimensional PISEMA experiment. The magnitudes of the principal values of the 15N chemical shift tensor of the N1 nitrogen of [1-15N]-2'-deoxyguanosine were found to be sigma11 = 54 ppm, sigma22 = 148 ppm, and sigma33 = 201 ppm with respect to (15NH4)2SO4 in aqueous solution. Comparisons of experimental and simulated two-dimensional powder pattern spectra show that sigma33N is approximately collinear with the N-H bond. The tensor orientation of sigma33N for N1 of [1-15N]-2'-deoxyguanosine is similar to the values obtained for the side chain residues of 15Nepsilon1-tryptophan and 15Npi-histidine even though the magnitudes differ significantly.     

Tunable enhanced spatial shifts of reflective beam on the surface of a twisted bilayer of hBN

We investigated Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts of a reflective beam on a twisted bilayer of hexagonal boron nitride (hBN), where a left circularly polarized beam was incident on the surface. Our results demonstrate that the twist angle between the two optical axes plays an important role in obtaining large shifts with a high reflectivity. The GH shift with 10λ₀ is achieved, while the reflectivity is near 100% by tuning the twist angle. The maximum of the IF shift is found in the certain condition satisfied by the reflective coefficients, and the shift strongly depends on the twist angle between the optical axes of the two slabs. The spatial shifts obtained directly from the GH and IF shift definitions were provided, which indicate that the theoretical results from the stationary phase method are believable. These results may open up a new way for developing the nano-optical devices.     

Is it possible to use the 31P chemical shifts of phosphines to measure hydrogen bond acidities (HBA)? A comparative study with the use of the 15N chemical shifts of amines for measuring HBA

The geometries, energies, and nuclear magnetic resonance (NMR) chemical shifts of 3 bases (trimethylphosphine, trimethylamine, and trimethylphosphine oxide), their 3 protonated cations, and 15 hydrogen‐bonded complexes (corresponding to the HF, HNC, HCN, HCCH, H₂O, and CH₃OH Brønsted acids) have been calculated at the B3LYP/6‐311++G(d,p) level. The determination of hydrogen bond acidities by NMR is classically performed using the ³¹P chemical shifts Me₃PO. This method is more reliable than the use of the ¹⁵N NMR chemical shifts of Me₃N. This work shows that the ³¹P NMR chemical shifts of Me₃P cannot be used. The raison of the difference between Me₃P on one hand and Me₃PO and Me₃N on the other will be discussed.     

The local physical structure of amorphous hydrogenated boron carbide: insights from magic angle spinning solid-state NMR spectroscopy

Magic angle spinning solid-state nuclear magnetic resonance spectroscopy techniques are applied to the elucidation of the local physical structure of an intermediate product in the plasma-enhanced chemical vapour deposition of thin-film amorphous hydrogenated boron carbide (B(x)C:H(y)) from an orthocarborane precursor. Experimental chemical shifts are compared with theoretical shift predictions from ab initio calculations of model molecular compounds to assign atomic chemical environments, while Lee-Goldburg cross-polarization and heteronuclear recoupling experiments are used to confirm atomic connectivities. A model for the B(x)C:H(y) intermediate is proposed wherein the solid is dominated by predominantly hydrogenated carborane icosahedra that are lightly cross-linked via nonhydrogenated intraicosahedral B atoms, either directly through B-B bonds or through extraicosahedral hydrocarbon chains. While there is no clear evidence for extraicosahedral B aside from boron oxides, ～40% of the C is found to exist as extraicosahedral hydrocarbon species that are intimately bound within the icosahedral network rather than in segregated phases.     

A solid-state NMR application of the anomeric effect in carbohydrates: galactosamine, glucosamine, and N-acetyl-glucosamine

Simple 2D 13C/15N heteronuclear correlation solid-state NMR spectroscopy was implemented to resolve the 15N resonances of the alpha and beta anomers of three amino monosaccharides: galactosamine (GalN), glucosamine hydrochloride (GlcN), and N-acetyl-glucosamine (GlcNAc) labeled specifically with 13C1/15N spin pairs. Although the 15N resonances could not be distinguished in normal 1D spectra, they were well resolved in 2D double CP/MAS correlation spectra by taking advantage of the 13C spectral resolution. The alpha and beta resonances shifted apart by 3-5 ppm in their 13C chemical shifts, and differed by 1-2 ppm in the extended 15N dimension. Aside from this, the detection of other 13C/15N correlations over short distances was also achieved arising from the C2, C3 and CO carbons present in natural abundance. 2D double CP/MAS chemical shift correlation NMR spectroscopy is a simple and powerful technique to characterize the anomeric effect of amino monosaccharides. Applications of the 2D method reveal well-resolved 15N and 13C chemical shifts might be useful for structural determination on carbohydrates of biological significance, such as glycopeptide or glycolipids.     

六角氮化硼纳米带的几何和电子结构理论研究

本文采用密度泛函B3LYP方法在6-31G*水平上,对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加,纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性;锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡,且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布,呈现出向边界偏移的现象。     

六角氮化硼纳米带的几何和电子结构理论研究

本文采用密度泛函B3LYP方法在6-31G*水平上，对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加，纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性；锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡，且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布，呈现出向边界偏移的现象。     

Hydrogen bonding effects on the (15)N and (1)H shielding tensors in nucleic acid base pairs

The results of systematic ab initio calculations of (15)N and (1)H chemical shielding tensors in the GC base pair as a function of hydrogen bond length are presented for the first time. The hydrogen bond length characterized by the distance r(N...N) between purine N1 and pyrimidine N3 was varied between 2.57 and 3.50 A and the chemical shift tensors were calculated by the sum-over-states density functional perturbation theory. It is shown that the hydrogen bond length has a strong effect on the chemical shielding tensor of both imino proton and nitrogen, on their orientation, and, as a consequence, on the relaxation properties of both nuclei. For a nitrogen nucleus not involved in hydrogen bonding, the shielding tensor is nearly axially symmetric and almost collinear with the bond vector. As the length of the hydrogen bond decreases, the least shielding component sigma(11) deflects from the N-H vector and the shielding tensor becomes increasingly asymmetric. The significance of the presented results for the analysis of relaxation data and the efficiency of TROSY effects together with a summary of the relevant shielding parameters are presented and discussed.     

Chemical shielding properties for BN,BP, AlN,and AlP nanocones: DFT studies

The properties of boron nitride (BN), boron phosphide (BP), aluminum nitride (AlN), and aluminum phosphide (AlP) nanocones were investigated by density functional theory (DFT) calculations. The investigated structures were optimized and chemical shielding (CS) properties including isotropic and anisotropic CS parameters were calculated for the atoms of the optimized structures. The magnitudes of CS parameters were observed to be mainly dependent on the bond lengths of considered atoms. The results indicated that the atoms could be divided into atomic layers due to the similarities of their CS properties for the atoms of each layer. The trend means that the atoms of each layer detect almost similar electronic environments. Moreover, the atoms at the apex and mouth of nanocones exhibit different properties with respect to the other atomic layers.     

Energetics and structural properties of carbon and oxygen doped hexagonal boron nitride sheets

Energetics and structural properties of carbon and oxygen doped hexagonal boron nitride sheets have been investigated by performing density functional theory calculations. Substitutional doping model has been considered in the neutral charge state. C and O atoms replaced either B or N site in the system as impurities. A systematic study has been performed to see the effect of cell size on the calculated quantities, such as formation energy, relaxation energy, charge and bond length. It has been found that substitution of O atom on the N site in the hexagonal BN sheet is more favorable.     

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C_70?2x(BN)_x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated ¹³C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. ¹³C Chemical shifts (δ_iso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from ?25.29 to ?8.80) demonstrate that all the substituted species are aromatic, but less than C₇₀ (?27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral ³He NMR chemical shifts.     

15N chemical shift tensor magnitude and orientation in the molecular frame of uracil determined via MAS NMR

The potential of heteronuclear MAS NMR spectroscopy for the characterization of (15)N chemical shift (CS) tensors in multiply labeled systems has been illustrated, in one of the first studies of this type, by a measurement of the chemical shift tensor magnitude and orientation in the molecular frame for the two (15)N sites of uracil. Employing polycrystalline samples of (15)N(2) and 2-(13)C, (15)N(2)-labeled uracil, we have measured, via (15)N-(13)C REDOR and (15)N-(1)H dipolar-shift experiments, the polar and azimuthal angles (θ, psi) of orientation of the (15)N-(13)C and (15)N-(1)H dipolar vectors in the (15)N CS tensor frame. The (θ(NC), psi(NC)) angles are determined to be (92 +/- 10 degrees, 100 +/- 5 degrees ) and (132 +/- 3 degrees, 88 +/- 10 degrees ) for the N1 and N3 sites, respectively. Similarly, (θ(NH), psi(NH)) are found to be (15 +/- 5 degrees, -80 +/- 10 degrees ) and (15 +/- 5 degrees, 90 +/- 10 degrees ) for the N1 and N3 sites, respectively. These results obtained based only on MAS NMR measurements have been compared with the data reported in the literature.     

Orientational information from TEDOR spectral sidebands.

In a dipolar-coupled spin-1/2 network of the type 15N1-(13)C-15N2, an assessment of the sensitivity of the N --> C and C --> N TEDOR sideband intensities to the Euler angles defining the orientation of the two heteronuclear dipolar vectors in the 13C and 15N chemical shift (CS) tensor principal axes system has been carried out via numerical calculations. The results clearly indicate the potential of TEDOR MAS NMR spectroscopy for the characterization of the CS tensor orientation in the molecular frame. The efficacy of the method has been experimentally illustrated by TEDOR studies on a polycrystalline sample of [1, 3-(15)N2, 2-(13)C]uracil, which is one of the four bases in RNA.     

Ammonia adsorption on the C30B15N15 heterofullerene: DFT study of nuclear magnetic shielding and electric field gradient tensors of N and B nuclei

Ammonia adsorption on the external surface of C₃₀B₁₅N₁₅ heterofullerene was studied using density functional calculations. Three models of the ammonia-attached C₃₀B₁₅N₁₅ together with the perfect model were optimized at the B3LYP/6-31G^? level. The optimization process reveals that dramatic influences occurred for the geometrical structure of C₃₀B₁₅N₁₅ after ammonia adsorption; the B atom relaxes outwardly and consequently the heterofullerene distorts from the spherical form in the adsorption sites. The chemical shielding (CS) tensors and nuclear quadrupole coupling constants of B and N nuclei were calculated at the B3LYP/6-311G^** level. Our calculations reveal that the B atom is chemically bonded to NH₃ molecule. The B atom in the NH₃-attached form has the largest chemical shielding isotropic (CSI) value among the other boron nuclei. The C_Q parameters of B nuclei at the interaction sites are significantly decreased after ammonia adsorption.     

Isotopic phonon effects in β-rhombohedral boron--non-statistical isotope distribution

On the basis of the spectra of IR- and Raman-active phonons, the isotopic phonon effects in β-rhombohedral boron are analysed for polycrystalline (10)B- and (11)B-enriched samples of different origin and high-purity (nat)B single crystals. Intra- and inter-icosahedral B-B vibrations are harmonic, hence meeting the virtual crystal approximation (VCA) requirements. Deviations from the phonon shift expected according to the VCA are attributed to the anharmonic share of the lattice vibrations. In the case of icosahedral vibrations, the agreement with calculations on α-rhombohedral boron by Shirai and Katayama-Yoshida is quite satisfactory. Phonon shifts due to isotopic disorder in (nat)B are separated and determined. Some phonon frequencies are sensitive to impurities. The isotopic phonon effects yield valuable specific information on the nature of the different phonon modes. The occupation of regular boron sites by isotopes deviates significantly from the random distribution.     

B-C-N Compound Synthesized Under High Temperature and High Pressure

Crystalline hexagonal B(N 1 m x C x ) and cubic B-C-N compounds have been synthesized from a precursor produced from melamine and boric acid by application of high temperature and high pressure. The synthesized products were characterized by X-ray diffraction. The lattice parameters for the hexagonal crystal are a=2.506 Å, c=6.657 Å, and that for the cubic crystal is a=3.596 Å. The X-ray photoelectron spectra of the B-C-N compound indicate the presence of B-N, C-N, C-C, and B-C bonds, which suggests that boron, carbon, and nitrogen atoms all bond with one another and that the B-C-N crystal is a compound in which the three kinds of atoms are mixed atomically. The composition of the B-C-N compound is B 0.47 C 0.23 N 0.30 . A strong absorption band at 1000～1120 cm m 1 attributable to the cubic B-C-N phase is observed in the infrared spectrum. The photoluminescence spectrum of hexagonal B-C-N powder measured at room temperature features a broad peak centered at 374 nm, corresponding to the band-edge emission of h-B-C-N, and is similar to that of w-GaN.