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The use of computational methods that allow us to perform characterization on new compounds is not a novelty; nevertheless, the degree of complexity of the structures makes their study more challenging since new techniques and methods are required to adjust to the new structural model. The case of nuclear magnetic resonance characterization of boronate esters is fascinating because of its widespread use in materials science. In this paper, we use density functional theory to characterize the structure of the compound 1-[5-(4,5-Dimethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl]ethanonea by means of nuclear magnetic resonance. We studied the compound in its solid form with the PBE–GGA and PBEsol–GGA functionals, with a set of plane wave functions and an augmented wave projector, which included gauge in CASTEP and its molecular structure with the B3LYP functional using the package Gaussian 09. In addition, we performed the optimization and calculation of the chemical shifts and isotropic nuclear magnetic resonance shielding of 1 H, 13 C, and 11 B. Finally, we analyzed and compared the theoretical results with experimental diffractometric data observing a good approximation. [ABSTRACT FROM AUTHOR] more...

The Density Functional Theory (DFT) and Tomasi's Polarized Continuum Model (PCM) were used to investigate the effects of solvent dielectric constant on the structural stability and 15N NMR tensors of Medazepam (MDZ) drug. The results revealed that the structural stability of MDZ in polar protic solvents was higher than that in the polar aprotic and non-polar solvents; and its value depended on the solvent dielectric constant and its structure. so that in most cases, relative stability increased by increasing the solvent dielectric constant and the most stable structures were observed in water media at DFT level and in methanol at MP2 level. In this regard, natural bond orbital (NBO) interpretation showed that the tetravalent N1 nucleus of diazepine ring in the MDZ structure had the highest value of negative charge and the resonance energy related to LP (1) N1 → σ* and π* delocalizations among heteroatoms of MDZ structure in the tested solvents. The findings reported that with an increase in the solvent dielectric constant, the resonance energy related to LP (1) N1 → σ* and π* delocalizations increased and the highest value of resonance energy was observed in water media. Furthermore, NMR results represented that the N1 nucleus had a higher value of chemical shielding than the trivalent N4 nucleus in all of the tested media. However, it may be concluded that by increasing the accumulation of negative charge and lone pair electrons participation of nitrogen nuclei in the resonance delocalizations, isotropic chemical shielding around them increase. [ABSTRACT FROM AUTHOR] more...

Ab initio chemical shielding calculations greatly facilitate the interpretation of nuclear magnetic resonance (NMR) chemical shifts in biological systems, but the large sizes of these systems requires approximations in the chemical models used to represent them. Achieving good convergence in the predicted chemical shieldings is necessary before one can unravel how other complex structural and dynamical factors affect the NMR measurements. Here, we investigate how to balance trade-offs between using a better basis set or a larger cluster model for predicting the chemical shieldings of the substrates in two representative examples of protein-substrate systems involving different domains in tryptophan synthase: the N-(4'-trifluoromethoxybenzoyl)-2-aminoethyl phosphate (F9) ligand which binds in the α active site, and the 2-aminophenol quinonoid intermediate formed in the β active site. We first demonstrate that a chemically intuitive three-layer, locally dense basis model that uses a large basis on the substrate, a medium triple-zeta basis to describe its hydrogen-bonding partners and/or surrounding van der Waals cavity, and a crude basis set for more distant atoms provides chemical shieldings in good agreement with much more expensive large basis calculations. Second, long-range quantum mechanical interactions are important, and one can accurately estimate them as a small-basis correction to larger-basis calculations on a smaller cluster. The combination of these approaches enables one to perform density functional theory NMR chemical shift calculations in protein systems that are well-converged with respect to both basis set and cluster size. more...

The principal elements of the 199Hg chemical-shift (CS) tensors of the mercuric halides (HgX2, X = F, Cl, Br, and I) and the mercurous halides (Hg2X2, X = F and Cl) were determined from spectra of static polycrystalline powders and from magic-angle spinning (MAS) spectra. The CS tensors of both HgCl2 and Hg2Cl2 are axially symmetric (η = 0) within experimental error, differing from literature reports of η=0.12 and η=0.14, respectively. The principal elements of the axially symmetric CS tensor in HgBr2 were also measured using a static sample, and the wideline spectra of HgF2 and HgI2 (red polymorph) give chemical-shift tensors that suggest, within experimental error, that the mercury sits in sites of cubic symmetry. The 199Hg CS tensor for Hg2F2 is asymmetric. Experiments with static polycrystalline samples may allow the determination of the elements of the 199Hg CS tensors even when MAS fails to completely average the dipolar coupling of the spin-½ 199Hg and the quadrupolar halide nucleus. more...

The 199Hg chemical-shift tensor of solid HgCl2 was determined from spectra of polycrystalline materials, using static and magic-angle spinning (MAS) techniques at multiple spinning frequencies and field strengths. The chemical-shift tensor of solid HgCl2 is axially symmetric (η = 0) within experimental error. The 199Hg chemical-shift anisotropy (CSA) of HgCl2 in a frozen solution in dimethylsulfoxide (DMSO) is significantly smaller than that of the solid, implying that the local electronic structure in the solid is different from that of the material in solution. The experimental chemical-shift results (solution and solid-state) are compared with those predicted by density functional theory (DFT) calculations using the zeroth-order regular approximation (ZORA) to account for relativistic effects.199Hg spin-lattice relaxation of HgCl2 dissolved in DMSO is dominated by a CSA mechanism, but a second contribution to relaxation arises from ligand exchange. Relaxation in the solid state is independent of temperature, suggesting relaxation by paramagnetic impurities or defects. more...

In this research, geometrical structures of armchair single walled boron nitride nanotube (SWBNNT) and armchair single walled aluminum nitride nanotube (SWAlNNT) were optimized by Density Functional Theory (DFT) in the gas phase, both having the same length of 5 angstrom and n=9, m=9. B3LYP/6-31G* level of theory have been used to determine and compare electronic properties, natural charge and chemical shielding tensors of nanotubes. The chemical shielding tensors were calculated using GIAO method to obtain structural information and dynamic behavior for optimal boron nitride and aluminum nitride nanotube structures. Also, thermodynamic functions for the boron nitride nanotube (9, 9-5) and the aluminum nitride nanotube (9, 9-5) in the gas phase were carried out with using the B3LYP method and 6-31g* basis set. It is significant that all of NMR parameters and geometrical properties of both nanotubes were determined in 5 layers. [ABSTRACT FROM AUTHOR] more...

The g factor of paramagnetic defects in commercial high performance carbon fibers was determined by a double resonance experiment based on the Overhauser shift due to hyperfine coupled protons. Our carbon fibers exhibit a single, narrow and perfectly Lorentzian shaped ESR line and a g factor slightly higher than g free with g = 2.002644 = g free · ( 1 + 162 ppm ) with a relative uncertainty of 15 ppm . This precisely known g factor and their inertness qualify them as a high precision g factor standard for general purposes. The double resonance experiment for calibration is applicable to other potential standards with a hyperfine interaction averaged by a process with very short correlation time. [ABSTRACT FROM AUTHOR] more...

The electronic structure of boron phosphide nanotubes (BPNTs) and influence of carbon ring doping (C-doping) in the horizontal region (model A) and vertical region (model B) of BPNTs is studied by density functional theory (DFT). At first, each form was optimized at B3LYP level of theory using 6-31G∗ bases set. After, the computed chemical shielding (CS) tensors at the sites of 11B and 31P nuclei were converted to isotropic chemical shielding (CSI) and anisotropic chemical shielding (CSA). The calculated results reveal that the CS parameters of B and P nuclei in C-ring doped on vertical region (model B) undergo more significant changes than horizontal region (model A). more...

Complexes were formed pairing FX, FHY, FH2Z, and FH3T (X = Cl, Br, I; Y = S, Se, Te; Z = P, As, Sb; T = Si, Ge, Sn) with NH3 in order to form an A⋯N noncovalent bond, where A refers to the central atom. Geometries, energetics, atomic charges, and spectroscopic characteristics of these complexes were evaluated via DFT calculations. In all cases, the A−F bond, which is located opposite the base and is responsible for the σ-hole on the A atom, elongates and its stretching frequency undergoes a shift to the red. This shift varies from 42 to 175 cm−1 and is largest for the halogen bonds, followed by chalcogen, tetrel, and then pnicogen. The shift also decreases as the central A atom is enlarged. The NMR chemical shielding of the A atom is increased while that of the F and electron donor N atom are lowered. Unlike the IR frequency shifts, it is the third-row A atoms that undergo the largest change in NMR shielding. The change in shielding of A is highly variable, ranging from negligible for FSnH3 all the way up to 1675 ppm for FBr, while those of the F atom lie in the 55−422 ppm range. Although smaller in magnitude, the changes in the N shielding are still easily detectable, between 7 and 27 ppm. more...

In light of the importance of developing novel corrosion inhibitors, a series of quantum chemical calculations were carried out to evaluate 15 N chemical shielding CS tensors as well as aromaticity indexes including NICS, HOMA, FLU, and PDI of three pyrazine derivatives, 2-methylpyrazine (MP), 2-aminopyrazine (AP) and 2-amino-5-bromopyrazine (ABP). The NICS parameters have been shown in previous studies to be paramount to the prediction of anti-corrosion properties, and have been combined here with HOMA, FLU and PDI and detailed wavefunction analysis to determine the effects from bromination and methylation on pyrazine. The results show that the electron density around the nitrogens, represented by CS tensors, can be good indicators of anti-corrosion efficiency. Additionally, the NICS, FLU and PDI, as aromaticity indicators of molecule, are well correlated with experimental corrosion inhibition efficiencies of the studied inhibitors. Bader sampling and detailed wavefunction analysis shows that the major effects from bromination on the pyrazine derivatives affect the Laplacian of the electron density of the ring, delocalizing the aromatic electrons of the carbon atoms into lone pairs and increasing polarization of the Laplacian values. This feature is well agreement with empirical studies, which show that ABP is the most efficient anti-corrosion compound followed by AP and MP, a property which can be attributed and predicted by derivation of the Laplacian of the electron density of the ring nuclei. This study shows the importance of devising DFT methods for development of new corrosion inhibitors, and the strength of electronic and nuclear analysis, and depicts most importantly how corrosion inhibitors composed of aromatic moieties may be modified to increase anti-corrosive properties. [ABSTRACT FROM AUTHOR] more...

The 17O NMR chemical shielding tensors and chemical shift for a set of substituted benzyl ethers derivatives containing (methyl, ethyl, isopropyl, t-butyl, brome and lithium) have been calculated. The molecular structures were fully optimized using B3LYP/6-31G(d,p). The calculation of the 17O shielding tensors employed the GAUSSIAN 98 implementation of the gauge-including atomic orbital (GIAO) and continuous set of gauge transformations (CSGT) by using 6-31G (d,p), 6-31++G(d,p) and 6-311++G(d,p) basis set methods at density functional levels of theories (DFT). The values determined using the GIAO and CSGT were found to give a good agreement with the experimental chemical shielding. more...

The electronic structure of boron phosphide nanotubes (BPNTs) and influence of carbon ring doping (C-doping) in the horizontal region (model A) and vertical region (model B) of BPNTs is studied by density functional theory (DFT). At first, each form was optimized at B3LYP level of theory using 6-31G ∗ bases set. After, the computed chemical shielding (CS) tensors at the sites of 11 B and 31 P nuclei were converted to isotropic chemical shielding (CSI) and anisotropic chemical shielding (CSA). The calculated results reveal that the CS parameters of B and P nuclei in C-ring doped on vertical region (model B) undergo more significant changes than horizontal region (model A). [ABSTRACT FROM AUTHOR] more...

The properties of the electronic structure of the Disiline-doped boron nitride nanotubes (Disiline-BNNTs) are investigated by a density functional theory (DFT) calculation. The structural forms are firstly optimized and the CS tensors calculated. Subsequently, the chemical-shielding isotropic (CS) and chemical shielding anisotropic (CS) parameters are found. The shielding values of boron (B) and nitrogen (N) atoms were calculated by Gauge-Including Atomic Orbital (GIAO), Continuous Set of Gauge Transformations (CSGT) and Individual Gauges for Atoms in Molecules (IGAIM) methods, using B3LYP/6-311+G*. The B3LYP level of theory with IGAIM was the best method to evaluate the theoretical chemical shifts for studied models. The results reveal a significant effect of Disiline doping on the chemical shielding tensors at the sites of those B and N nuclei located in the nearest neighborhood of the Disiline-doped ring. Furthermore, the values of dipole moments and HOMO-LUMO gaps change in the Disiline-doped models in comparison with the original pristine model. [ABSTRACT FROM AUTHOR] more...

Magnesium oxide nanotubes of finite length are investigated by the Density Functional Theory (DFT) at the B3LYP/6-31G (d) level. The (6, 0) zigzag and (4, 4) armchair of MgO nanotubes were considered and nuclear magnetic resonance properties including isotropic and anisotropic chemical shielding parameters (CSI and CSA) were calculated for 25Mg and 17O atoms of the optimized structures for the first time. The calculated CS parameters indicated that the Mg atoms cause slight changes of electronic environment in the MgONT structures, but the changes for the O atoms are more significant. Results indicated that the zigzag MgONTs could be considered a more reactive material than the armchair model for interactions with other atoms or molecules. [ABSTRACT FROM AUTHOR] more...

A density functional theory study is carried out to investigate the geometries and electronic structure of pristine and carbon-doped (8, 0) single-walled boron nitride nanotubes (BNNTs). In order to understand the effect of impurities or doping on (8, 0) single-walled BNNT, we simulated C-doping in six different ways. Geometry optimizations reveal that in the considered models, B-N bond lengths are not significantly influenced by C-doping. Based on the quantum theory of atoms in molecules analysis, charge density accumulation for axial B-N bond critical points (BCPs) of pristine BNNT is slightly larger than zigzag ones. However, due to C-doping at the B- or N-tips, the evaluated electron density tends to decrease slightly at both axial and zigzag B-N BCPs. Besides, results indicate that influence of C-doping on properties of the (8, 0) BNNT could be also detected by values of chemical shielding isotropy ( σ) and anisotropy (Δ σ). [ABSTRACT FROM AUTHOR] more...

A quantum chemistry investigation was carried out to examine hydrogen- and halogen-bonds properties in crystalline dichloroacetic acid (DCAA). We reported a systematic density functional theory study of the O, Cl, and H nuclear magnetic resonance (NMR) parameters in DCAA. Our results indicated that for those nuclei participated in the hydrogen- and halogen-bonding interactions; NMR parameters exhibit considerable changes on going from the isolated molecule model to the crystalline DCAA. Of course, the magnitude of these changes at each nucleus depends directly on its amount of contribution to the interactions. The topology of the electron density of O-H···O, C-H···O, Cl···Cl, and Cl···O interactions in solid DCAA was characterized using quantum theory of atoms in molecules (QTAIM). Based on QTAIM results, a partial covalent character is attributed to the O-H···O hydrogen bonds in DCAA, whereas all C-H···O, Cl···O, and Cl···Cl intermolecular contacts are weak and basically electrostatic in nature. Moreover, an approximate linear relationship seems to exist for each of the proton chemical shifts and anisotropies as a function of ρ. [ABSTRACT FROM AUTHOR] more...

NMR parameters consisting of isotropic and anisotropic chemical shielding parameters have been calculated based on density functional theory (DFT) to investigate the properties of the electronic structures of the aluminum doped (Al-doped) carbon nanotubes (CNTs). The structures are allowed to relax by performing all atomic optimization. Then the chemical shielding (CS) tensors are calculated for 13C and 27Al nuclei in the Al-doped forms and also pristine models of the (4,4) armchair and (6,0) zigzag CNTs. The calculated CS tensors in the optimized structures are converted to chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA) parameters. In comparison to the pristine model, the CSI parameters of the C atoms around the Al-doped are significantly influenced by the doping process. Density functional theory (DFT) calculations are performed using Gaussian 98 suite of programs. [ABSTRACT FROM AUTHOR] more...

Abstract: 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. [Copyright &y& Elsevier] more...

Electronic polarization induced by the interaction of a reference molecule with a liquid environment is expected to affect the magnetic shielding constants. Understanding this effect using realistic theoretical models is important for proper use of nuclear magnetic resonance in molecular characterization. In this work, we consider the pyridine molecule in water as a model system to briefly investigate this aspect. Thus, Monte Carlo simulations and quantum mechanics calculations based on the B3LYP/6-311++G (d,p) are used to analyze different aspects of the solvent effects on the N magnetic shielding constant of pyridine in water. This includes in special the geometry relaxation and the electronic polarization of the solute by the solvent. The polarization effect is found to be very important, but, as expected for pyridine, the geometry relaxation contribution is essentially negligible. Using an average electrostatic model of the solvent, the magnetic shielding constant is calculated as −58.7 ppm, in good agreement with the experimental value of −56.3 ppm. The explicit inclusion of hydrogen-bonded water molecules embedded in the electrostatic field of the remaining solvent molecules gives the value of −61.8 ppm. [ABSTRACT FROM AUTHOR] more...

Abstract: In this work, an armchair model of the (4,4) boron phosphide nanotubes (BPNTs) with a 1-nm length and consisting of 32 B and 32 P atoms is considered to study the influence of doping three atoms of aluminum in sites of boron (B3AlPNTs) and three atoms of nitrogen in sites of phosphors (BP3NNTs) on the electrostatic structure properties. The mouths of nanotubes are capped by hydrogen atoms in order to saturate the dangling bonds of the boundaries and to decrease the calculation time. The structures of BPNTs, B3AlPNTs and BP3NNTs are optimized by performing the level of density functional theory (DFT) using 6-31G⁎ basis set. The optimized structures are used for calculating the chemical shielding (CS) tensors and nuclear magnetic resonance parameters such as isotropic chemical shielding (CSI) and anisotropic chemical shielding (CSA). The results reveal that in both models of B3AlPNTs and BP3NNTs by doping N atoms the chemical shielding parameters of P and B atoms, which are directly connected to the Al and N atoms decreased and the other sites significantly changed. [Copyright &y& Elsevier] more...

A computational investigation was carried out to characterize the O and H chemical shielding (CS) tensors in crystalline aspirin. It was found that O-H⋯O and C-H⋯O hydrogen bonds around the aspirin molecule in the crystal lattice have a different influence on the calculated O and H CS eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the BLYP, B3LYP, and M06 functionals employing 6-311++G(d,p) standard basis set. Calculated CS tensors were used to evaluate the O and H chemical shift isotropy (δ) and anisotropy (Δσ) in crystalline aspirin, which are in reasonable agreement with available experimental data. The difference between the calculated NMR parameters of the monomer and molecular clusters shows how much hydrogen-bonding interactions affect the CS tensors of each nucleus. [ABSTRACT FROM AUTHOR] more...

Density functional theory calculations were applied to investigate C chemical shielding tensors in cryptolepine and its bromo-substituted analogs, 2-bromocryptolepine and 2,7-dibromocryptolepine. The fact that bromo-substituted cryptolepine shows higher antiplasmodial activity than cryptolepine raises the question of whether this effect can be related to the electronic properties around carbon atoms. The results show that changes to the principal components of the shielding tensors upon substitution are significant. In particular, σ is the most affected tensor for carbons in the substituted ring, which could be related to the increased antiplasmodial activity of bromosubstituted cryptolepine. The analyses were also focused on atomic charges and dipole moment. [ABSTRACT FROM AUTHOR] more...

A density functional theory (DFT) study was performed to examine intra- and inter-molecular hydrogen bond (HB) properties in crystalline salicylic acid (SA). BLYP, B3LYP, and M06 functionals with 6-311++G** basis set were employed to calculate NMR chemical shielding isotropy ( σiso) and anisotropy (Δ σ) at the sites of the 17O and 1H nuclei of SA. From this study, it appears that the intra- and inter-molecular O-H···O as well as C-H···O HBs around the SA molecule in the crystal lattice have a major influence on the chemical shielding tensors and more specifically on the carbonyl 17O isotropy value. The quantum theory of atoms in molecules (QTAIM) analysis was also employed to elucidate the interaction characteristics in SA H-bonded network. Based on QTAIM results, a partial covalent character is attributed to the intra- and inter-molecular O-H···O HBs in SA. [ABSTRACT FROM AUTHOR] more...

Abstract: It has been known for over 70years that nuclear spins couple to molecular rotation via a Zeeman interaction. This spin–rotation coupling can be observed as a discrete splitting in molecular beam magnetic resonance experiments, but is quenched by molecular collisions at higher pressures. We show that because of differential thermal population of MJ levels at high magnetic fields, the spin rotation coupling retains a small isotropic component at high field. For all but the smallest molecules at very low temperature, the residual coupling is temperature independent and linear in the magnetic field; it therefore closely mimics the chemical shift. The ‘super spin rotation’ shift may in the future be a necessary correction to ultra – high precision computations of the NMR chemical shielding of small molecules in gases and liquids. [Copyright &y& Elsevier] more...

Nuclear magnetic resonance (NMR) parameters including isotropic and anisotropic chemical shielding parameters and electronic structures were calculated using density functional theory (DFT) for silicon-carbide-doped boron phosphide nanotubes. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 program suite. The isotropic and anisotropic chemical shielding parameters were calculated for the sites of various C, Si, B, and also P atoms in pristine and SiC-doped (6,0) zigzag boron phosphide nanotube models. The calculations indicated that doping of B and P atoms by C and Si atoms had a more significant influence on the calculated shielding tensors than did doping of the B and P atoms by Si and C atoms. In comparison with the pristine model, Si- and C-doping of P and B sites of the zigzag nanotubes reduces the energy gaps of the nanotubes and increases their electrical conductance. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR] more...

Abstract: We performed density functional (DFT) calculations to investigate the properties of nitrogen-doped (N-doped) models of silicon carbide nanotubes (SiCNTs). To this end, we optimized the structures of representative zigzag and armchair SiCNTs and then calculated chemical shielding (CS) parameters for the Si-29, N-15 and C-13 atoms. Our results indicated that the influence of N-doping in zigzag nanotube is more significant than that in armchair nanotube, which is achieved near the N-doped atom. The effect of N-doping on the structure is more obvious in the wider SiCNTs than the narrower ones. Moreover, N-doping of Si atom causes more changes to the SiCNTs than N-doping of C atom. The N atom can function as a good electron donor in the N-doped SiCNTs where the C atom is doped by the N atom. [Copyright &y& Elsevier] more...

We investigated the properties of atomic oxygen-doped (O-doped) models of representative (6,0) and (4,4) silicon carbide nanotubes (SiCNTs) by density functional theory (DFT) calculations of isotropic and anisotropic chemical shielding (CS) parameters of Si-29, O-17 and C-13 atoms for the optimized structures. The calculated parameters indicated the effects of O-doping on the electronic environments of the first neighboring atoms of the doped sites. Comparing the results of the zigzag and armchair models also indicated that the latter model detects more effects of the O-doping than the former one. [ABSTRACT FROM AUTHOR] more...

Abstract: The principal elements of the 199Hg chemical-shift (CS) tensors of the mercuric halides (HgX2, X=F, Cl, Br, and I) and the mercurous halides (Hg2X2, X=F and Cl) were determined from spectra of static polycrystalline powders and from magic-angle spinning (MAS) spectra. The CS tensors of both HgCl2 and Hg2Cl2 are axially symmetric (η =0) within experimental error, differing from literature reports of η =0.12 and η =0.14, respectively. The principal elements of the axially symmetric CS tensor in HgBr2 were also measured using a static sample, and the wideline spectra of HgF2 and HgI2 (red polymorph) give chemical-shift tensors that suggest, within experimental error, that the mercury sits in sites of cubic symmetry. The 199Hg CS tensor for Hg2F2 is asymmetric. Experiments with static polycrystalline samples may allow the determination of the elements of the 199Hg CS tensors even when MAS fails to completely average the dipolar coupling of the spin-½ 199Hg and the quadrupolar halide nucleus. [Copyright &y& Elsevier] more...

Density functional theory calculations were performed to investigate representative models of (6,0) zigzag and (4,4) armchair gallium phosphide nanotubes (GaPNTs). Nuclear magnetic resonance properties including isotropic and anisotropic chemical shielding parameters (CS and CS) were calculated for Ga and P atoms of the optimized structures. The calculated CS parameters indicated that the P atoms detect slight changes of electronic environment in the GaPNT structures, but the changes for the Ga atoms are more significant. Moreover, armchair GaPNTs could be considered a more reactive material than the zigzag model for interactions with other atoms or molecules. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR] more...

Abstract: To investigate the influence of Si and Ge doping on electrostatic structure properties, we considered an armchair model of (4, 4) aluminum nitride nanotubes (AlNNTs) of 1nm length, consisting of 32 Al and 32 N atoms. The mouths of the nanotubes are capped by hydrogen atoms in order to saturate the dangling bonds of the boundaries, to simulate a longer tube and to decrease calculation time. The calculated chemical shielding (CS) tensors are converted to isotropic and anisotropic CS. The results show that in both models of Si- and Ge-doped AlNNTs, among all N atoms those that are directly connected to Si and Ge atoms have the smallest isotropic and anisotropic chemical shielding. The calculations are performed at the level of the density functional theory (DFT) using a Gaussian 03 package of program. [ABSTRACT FROM AUTHOR] more...

Abstract: The properties of boron (B)-doped models of representative (4,4) armchair and (6,0) zigzag silicon carbide nanotubes were investigated by density functional theory (DFT) calculations of chemical shielding (CS) parameters. The calculated results indicated that by B-doping of Si atom of armchair SiCNT, the CS parameters of C atoms only detect the effects but by B-doping of C atom, the CS parameters of both Si and C atoms detect the effects of doping. The CS parameters of both Si and C atoms detect notable changes by B-doping of either Si or C atoms of zigzag SiCNT. The role of electron receptor of B atom was more significant for the structure of (6,0) SiCNT in which C atom is doped by B atom. [Copyright &y& Elsevier] more...

Abstract: Density functional theory (DFT) calculations have been performed to investigate the electronic and structural properties of sulfur (S) terminated models of zigzag boron nitride (BN) nanotube. Four models including pristine, boron (B) tip terminated by S, nitrogen (N) tip terminated by S, and both of B and N tips terminated by S have been considered for optimizations and chemical shielding (CS) parameters calculations. The results indicate that the B–N bond lengths do not detect any changes due to the S-termination but the band gaps and dipole moments detect notable changes especially for the model of the N-tip terminated by the S atoms. The CS parameters also indicate that the atoms of the models are divided into layers with similar parameters in each layer. In the model of the B-tip terminated by the S atoms, the CS parameters indicate strong chemical bonding of N- and S-layers; however, only some attractions between the B- and S-layers of the model of the N-tip terminated by the S atoms have been detected. In the model of B and N tips terminated by the S atoms, the most significant changes among the models are detected. [Copyright &y& Elsevier] more...

Abstract: NMR parameters consisting of isotropic and anisotropic chemical shielding parameters have been calculated based on density functional theory (DFT) to investigate the properties of the electronic structures of the silicon–carbon (SiC) doped boron nitride nanotubes (BNNTs). The structures have been allowed to relax by performing all atomic optimization. Afterwards, the chemical shielding (CS) parameters have been calculated for the B-11, C-13, N-15, and Si-29 atoms in the pristine and the SiC-doped models of the (4,4) armchair and (6,0) zigzag BNNTs. The results indicated that the doping of the B and N atoms by the C and Si atoms, respectively, has more significant influence on the calculated NMR parameters than the doping of the B and N atoms by the Si and C atoms, respectively. In comparison to the pristine model, the band gaps detect more changes in the former model of the doping process. [Copyright &y& Elsevier] more...

Abstract: Behavior of a single NH3 molecule adsorbed on external surface of H-capped (5,5), (6,6), (5,0), and (8,0) single-walled carbon nanotubes (SWCNTs) is studied via DFT calculations. Binding energies clearly exhibit adsorption dependence on tube diameter. 13C, 15N and 1H chemical shielding tensors are calculated at the B3LYP level using GIAO method. NMR calculations reveal that 13C chemical shielding of (8,0) is more sensitive to NH3 adsorption compared to (5,5), (6,6) and (5,0) tubes. 15N and 1H chemical shielding correlate noticeably with diameter of the nanotubes. 14N and 2H nuclear quadrupole coupling constants, CQ , and asymmetry parameter, η, reveal the remarkable effect of NH3 adsorption on electronic structure of the SWCNTs. [Copyright &y& Elsevier] more...

A computational study was conducted to examine hydrogen bond (HB) properties of peptide group in four derivatives of acetamide by density functional theory (DFT) calculations of nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) parameters at the sites of oxygen and nitrogen nuclei of peptide groups. The available crystalline structures of four derivatives; 2,2,2-trifluoro-N-(2-hydroxy-5-nitrophenyl)acetamide,N-(2-acetylphenyl)acetamide, 2-chloro-N-(4-nitrophenyl) acetamide, and N-(4-fluorophenyl)acetamide were obtained from literature. Following the influence of HB interactions, calculations were done on non-hydrogen bonded (single) and hydrogen bonded (cluster) models of derivatives. The results revealed different behaviors of peptide group in contributing to HB interactions in different derivative structures. HB interactions are the strongest in 2-chloro-N-(4-nitrophenyl)acetamide. However, the strengths of HB interactions in all of the four derivatives are still less than that of acetamide. The calculations are done at the level of B3LYP method and 6-31 1++G** standard basis set using GAUSSIAN 98 package of program. [ABSTRACT FROM AUTHOR] more...

Abstract: We have investigated the properties of the electronic structure of the aluminum doped (Al-doped) boron nitride nanotubes (BNNTs) by density functional theory (DFT) calculations. The structures are allowed to relax by performing all atomic optimization. Afterwards, the chemical shielding (CS) tensors are calculated for B-11, N-15, and Al-27 nuclei in the Al-doped forms and also pristine models of the (4,4) armchair and (6,0) zigzag BNNTs. The results indicated that the doping of B atom by Al atom, the properties of the electronic structure were less influenced regarding to the pristine model than the case of doping of N atom by Al atom. Furthermore, doping of N atom by Al atom yields the equivalent calculated values of the band gaps for the Al-doped forms of the armchair and zigzag models of BNNTs. [Copyright &y& Elsevier] more...

A DFT/B3LYP study was performed to calculate 15N chemical shielding tensors in (benzamide)n = 1-6 clusters. We found that N–H⋯O hydrogen bonds around the benzamide molecule in crystalline lattice have significant influences on the 15N chemical shielding tensors. For (benzamide)n clusters, the n-dependent trend in 15N chemical shielding appears to be correlated with cooperative effects in R[N–H⋯O] bond distance. Natural bonding orbital (NBO) analysis was used to rationalize the chemical shielding results in terms of $n \to \sigma_{{\rm N-H}}^\ast$ charge delocalization effects in the benzamide clusters. This suggests that 15N chemical shielding measurements can provide a useful probe of electron delocalization phenomena in both gaseous and condensed media. [ABSTRACT FROM AUTHOR] more...

A preliminary study involving solid-state 77Se NMR spectroscopy and first principles calculations of 77Se NMR parameters in Se–N heterocycles is reported. 77Se CP/MAS NMR spectra of the ring systems reveal expansive selenium chemical shift (CS) tensors, which are extremely sensitive to molecular geometry, symmetry, ligand substitution, and intermolecular contacts. For systems with known crystal structures, hybrid density functional theory (DFT) calculations of selenium nuclear magnetic shielding (NMS) tensors were carried out, and tensor orientations in the molecular frames examined. Additional DFT calculations of selenium NMS tensors are presented, along with a detailed analysis of pairs of occupied and virtual molecular orbitals that give rise to the Se NMS tensors. A new naturalized local molecular orbital (NLMO) analysis under the same DFT framework is also discussed. Collectively, the NMR data and first principles calculations provide understanding of the influences of electronic structure, bonding, and intermolecular interactions on the selenium NMS tensors, allowing for (i) prediction of unknown molecular structures and (ii) insight into the positions of the stereochemically active selenium lone pairs. [ABSTRACT FROM AUTHOR] more...

Abstract: The 199Hg chemical-shift tensor of solid HgCl2 was determined from spectra of polycrystalline materials, using static and magic-angle spinning (MAS) techniques at multiple spinning frequencies and field strengths. The chemical-shift tensor of solid HgCl2 is axially symmetric (η =0) within experimental error. The 199Hg chemical-shift anisotropy (CSA) of HgCl2 in a frozen solution in dimethylsulfoxide (DMSO) is significantly smaller than that of the solid, implying that the local electronic structure in the solid is different from that of the material in solution. The experimental chemical-shift results (solution and solid state) are compared with those predicted by density functional theory (DFT) calculations using the zeroth-order regular approximation (ZORA) to account for relativistic effects. 199Hg spin–lattice relaxation of HgCl2 dissolved in DMSO is dominated by a CSA mechanism, but a second contribution to relaxation arises from ligand exchange. Relaxation in the solid state is independent of temperature, suggesting relaxation by paramagnetic impurities or defects. [Copyright &y& Elsevier] more...

Abstract: A computational nuclear magnetic resonance (NMR) study is performed to investigate the electronic structure properties of the single-walled zigzag aluminum nitride nanotubes (AlNNTs). The chemical-shielding (CS) tensors are calculated at the sites of Al-27 and N-15 nuclei in three structural forms of AlNNT including H-saturated, Al-terminated, and N-terminated ones. The structural forms are firstly optimized and then the calculated CS tensors in the optimized structures are converted to chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA) parameters. The calculated parameters reveal that various Al-27 and N-15 nuclei are divided into some layers with equivalent electrostatic properties; furthermore, Al and N can act as Lewis base and acid, respectively. In the Al-terminated and N-terminated forms of AlNNT, in which one mouth of the nanotube is terminated by aluminum and nitrogen nuclei, respectively, just the CS tensors of the nearest nuclei to the mouth of the nanotube are significantly changed due to removal of saturating hydrogen atoms. Density functional theory (DFT) calculations are performed using GAUSSIAN 98 package of program. [Copyright &y& Elsevier] more...

Abstract: Density functional calculations of 207Pb NMR shielding in PbX2 (X=F, Br, Cl and I) anionic fragments suggest that in solid PbX2, the observed variation of chemical shift with halide is dominated by the paramagnetic contribution to the chemical shielding, with a lesser effect by the spin–orbit contribution. The calculations include relativistic effects at the level of the zero-order regular approximation (ZORA). The present observation contrasts with previous calculations for the molecular system, PbX4, in which the spin–orbit contribution has been shown to be the major factor in the variation of the NMR chemical shift. [Copyright &y& Elsevier] more...