Calculation of chemical shielding in C-doped zigzag BN nanotubes

Abstract  

Chemical shielding tensors at the sites of various ¹¹B and ¹⁵N nuclei were calculated to study the electronic structure properties of carbon-doped boron nitride nanotubes. The chemical shielding tensors were converted to isotropic and anisotropic chemical shielding parameters. The results reveal the significant effect of C-doping on the chemical shielding tensors at the sites of those ¹¹B and ¹⁵N nuclei located in the nearest neighborhood of the C-doped ring. The isotropic and anisotropic chemical shielding tensors of those B atoms directly bonded to C atoms reach values observed for B atoms placed at the B-mouth of the nanotube.     

Analysis of the bond‐valence method for calculating 29Si and 31P magnetic shielding in covalent network solids

²⁹Si and ³¹P magnetic‐shielding tensors in covalent network solids have been evaluated using periodic and cluster‐based calculations. The cluster‐based computational methodology employs pseudoatoms to reduce the net charge (resulting from missing co‐ordination on the terminal atoms) through valence modification of terminal atoms using bond‐valence theory (VMTA/BV). The magnetic‐shielding tensors computed with the VMTA/BV method are compared to magnetic‐shielding tensors determined with the periodic GIPAW approach. The cluster‐based all‐electron calculations agree with experiment better than the GIPAW calculations, particularly for predicting absolute magnetic shielding and for predicting chemical shifts. The performance of the DFT functionals CA‐PZ, PW91, PBE, rPBE, PBEsol, WC, and PBE0 are assessed for the prediction of ²⁹Si and ³¹P magnetic‐shielding constants. Calculations using the hybrid functional PBE0, in combination with the VMTA/BV approach, result in excellent agreement with experiment. © 2016 Wiley Periodicals, Inc.     

Probing local structure in zeolite frameworks: ultrahigh-field NMR measurements and accurate first-principles calculations of zeolite 29Si magnetic shielding tensors

The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.     

NMR spectra of free-base porphine, porphyrazine, phthalocyanine and naphthalocyanine as well as their metal complexes: density functional calculations

Nuclear magnetic shielding tensors of porphine have been calculated at density functional B3LYP and PBE level using the gauge independent atomic orbital (GIAO) method. The geometries used were optimized using the 6-31G(d) basis set and the NMR calculations were performed using 6-31G(d) and 6-311G(d,p) basis sets, respectively. The calculated NMR shielding tensors and chemical shifts of porphine are compared with previous calculations as well as experimental data and satisfying results are obtained. Further NMR calculations are extended to metal-free and metallo-porphyrazine, -phthalocyanine, and -naphthalocyanine for the first time and the results are compared with experimental data available. The chemical shifts of the atoms in these compounds are assigned according to the experimental data available.     

(13)C shielding tensors of crystalline amino acids and peptides: Theoretical predictions based on periodic structure models

Precise theoretical predictions of NMR parameters are helpful for the spectroscopic identification of complicated biological molecules, especially for the carbon shielding tensors in amino acids. The (13)C shielding tensors of various crystalline amino acids and peptides have been calculated using the gauge-including projector augmented wave (GIPAW) method based on two different periodic structure models, namely that deduced from available crystallographic data and that from theoretically optimized structures. The incorporation of surrounding lattice effects is found to be crucial in obtaining reliable predictions of (13)C shielding tensors that are comparable to the experimental data. This is accomplished by refining the experimental crystallographic data of the amino acids and peptides at the GGA/PBE level by which more accurate intramolecular C--H bond lengths and intermolecular hydrogen-bonding interactions are obtained. Accordingly, more accurate predictions of (13)C shielding tensors comparable to the experimental results (within a maximum deviation of +/-10 ppm) were achieved, rendering more explicit (13)C shielding tensors assignments for solid biological systems particularly for amino acids with multiple carboxyl carbons, such as asparagine, glutamine, and glutamic acid.     

NMR shielding tensors from auxiliary density functional theory

The working equations for the calculation of NMR shielding tensors in the framework of auxiliary density functional theory are derived. It is shown that in this approach the numerical integration over gauge-including atomic orbitals can be avoided without the loss of accuracy. New integral recurrence relations for the required analytic electric-field-type integrals are derived. The computational performance of the resulting formalism permits shielding tensor calculations of systems with more than 1000 atoms and 15,000 basis functions.     

Modeling NMR chemical shift: A survey of density functional theory approaches for calculating tensor properties

The NMR chemical shift, a six-parameter tensor property, is highly sensitive to the position of the atoms in a molecule. To extract structural parameters from chemical shifts, one must rely on theoretical models. Therefore, a high quality group of shift tensors that serve as benchmarks to test the validity of these models is warranted and necessary to highlight existing computational limitations. Here, a set of 102 13C chemical-shift tensors measured in single crystals, from a series of aromatic and saccharide molecules for which neutron diffraction data are available, is used to survey models based on the density functional (DFT) and Hartree-Fock (HF) theories. The quality of the models is assessed by their least-squares linear regression parameters. It is observed that in general DFT outperforms restricted HF theory. For instance, Becke's three-parameter exchange method and mpw1pw91 generally provide the best predicted shieldings for this group of tensors. However, this performance is not universal, as none of the DFT functionals can predict the saccharide tensors better than HF theory. Both the orientations of the principal axis system and the magnitude of the shielding were compared using the chemical-shift distance to evaluate the quality of the calculated individual tensor components in units of ppm. Systematic shortcomings in the prediction of the principal components were observed, but the theory predicts the corresponding isotropic value more accurately. This is because these systematic errors cancel, thereby indicating that the theoretical assessment of shielding predictions based on the isotropic shift should be avoided.     

Influence of Mn-doping on the chemical shielding tensors of Ga and N in the armchair GaN nanotubes: A DFT study

The chemical shielding (CS) tensors of Gallium-71 and nitrogen-15 are computed for the first time in order to investigate the influence of Mn-doping on the electronic properties of the (5, 5) Gallium nitride nanotube (GaNNT). A GaNNT consisting of 40 Ga and 40 N atoms and having a 1.2 nm length was considered. One portal of the nanotube was capped by ten hydrogen atoms and other-end was kept open. Additionally, two other forms of this model of Mn-doped GaNNT were considered where a Mn-atom was substituted for a Ga atom either in the first or in the second layer. The calculations reveal that in both models of Mn-doped GaNNTs, the N atoms that are directly connected to the Mn atom have the smallest isotropic chemical shielding among other N atoms. These calculations were performed at the level of the density functional theory (DFT) using GAUSSIAN 03 package. The basis sets for Ga and N atoms were chosen to be 6-31G (d) and those for Mn atom were chosen to be LanL2DZ.     

Carbon-13 NMR studies of diphenyl dichalcogenides, (C6H5)2X2 (X = S,Se, Te)

The ¹³C NMR chemical shifts and spin-lattice relaxation times for diphenyl disulfide, diselenide and ditelluride have been measured and assigned. The shielding effects experienced by the carbon atoms re qualitatively discussed in terms of the inductive and mesomeeric properties of the dichalcogen group. The attempt to calculate the magnetic shielding tensors for the ¹³C nuclei in terms of the Pople theory gave unsatisfactory results.     

Disiline-doped boron nitride nanotubes: A computational study

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^I) and chemical shielding anisotropic (CS^A) 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 ¹¹B and ¹⁵N 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.     

13C NMR investigations of the ferroelectric phase transition of tris-sarcosine calcium chloride

¹³C NMR spectra were measured for tris-sarcosine calcium chloride (TSCC) crystals in the paraelectric phase (at temperatures of ? 300 and 150 K) and in the ferroelectric phase (at temperatures of 119 K) by double-resonance techniques with proton decoupling. The ferroelastic single crystals were rotated around their three crystallographic axes to derive the tensors of magnetic shielding for the different carboxylic carbon atoms. There is no change of the eigenvalues and eigenvectors of the tensors at the phase transition. The results are consistent with the interpretation of former EPR data.     

A solid-state 17O NMR study of L -phenylalanine and L -valine hydrochlorides

We have presented an experimental investigation of the oxygen-17 chemical shielding (CS) and electric-field-gradient (EFG) tensors for alpha-COOH groups in polycrystalline amino acid hydrochlorides. The 17O CS and EFG tensors including the relative orientations between the two NMR tensors are determined in [17O]-L-phenylalanine hydrochloride and [17O]-L-valine hydrochloride by the analysis of the 17O magic-angle-spinning (MAS) and stationary NMR spectra obtained at 9.4, 11.7, 16.4, and 21.8 T. The quadrupole coupling constants (CQ) and the span of the CS tensors are found to be 8.41-8.55 MHz and 7.35-7.41MHz, and 548-570 ppm and 225-231 ppm, for carbonyl and hydroxyl oxygen atoms, respectively. Extensive quantum chemical calculations using density functional theory (DFT) have been also carried out for a hydrogen-bonding model. It is demonstrated that the behavior of the dependence of hydrogen-bond distances on 17O NMR tensors for the halogen ions is different from those for the water molecule.     

Theoretical study of the in-plane components of the 13C shielding tensors in condensed aromatic hydrocarbons

The Pople model for chemical shielding is applied to calculate the in-plane components of the ¹³C shielding tensors of condensed aromatic hydrocarbons. The wave functions are evaluated using the MNDO method and the calculated results are supported by the very good agreement with the experimental results in the few cases in which experimental information is available.The relationship found between the calculated bond orders and the in-plane components of the ¹³C shielding tensors suggest that the experimental study of the ¹³C shielding tensors in these compounds may provide a powerful technique for studying aromaticity. The in-plane components are found to be directly affected by the degree of delocalization of the -electrons in the adjacent bonds. Rules are given for estimating the orientation of the two in-plane components of the shielding tensor.     

Influence of N-H...O and C-H...O hydrogen bonds on the 17O NMR tensors in crystalline uracil: computational study

We report a computational study for the 17O NMR tensors (electric field gradient and chemical shielding tensors) in crystalline uracil. We found that N-H...O and C-H...O hydrogen bonds around the uracil molecule in the crystal lattice have quite different influences on the 17O NMR tensors for the two C=O groups. The computed 17O NMR tensors on O4, which is involved in two strong N-H...O hydrogen bonds, show remarkable sensitivity toward the choice of cluster model, whereas the 17O NMR tensors on O2, which is involved in two weak C-H...O hydrogen bonds, show much smaller improvement when the cluster model includes the C-H...O hydrogen bonds. Our results demonstrate that it is important to have accurate hydrogen atom positions in the molecular models used for 17O NMR tensor calculations. In the absence of low-temperature neutron diffraction data, an effective way to generate reliable hydrogen atom positions in the molecular cluster model is to employ partial geometry optimization for hydrogen atom positions using a cluster model that includes all neighboring hydrogen-bonded molecules. Using an optimized seven-molecule model (a total of 84 atoms), we were able to reproduce the experimental 17O NMR tensors to a reasonably good degree of accuracy. However, we also found that the accuracy for the calculated 17O NMR tensors at O2 is not as good as that found for the corresponding tensors at O4. In particular, at the B3LYP/6-311++G(d,p) level of theory, the individual 17O chemical shielding tensor components differ by less than 10 and 30 ppm from the experimental values for O4 and O2, respectively. For the 17O quadrupole coupling constant, the calculated values differ by 0.30 and 0.87 MHz from the experimental values for O4 and O2, respectively.     

Calculation of magnetic properties of HF,H2O,NH3, and CH4 molecules using a longitudinal gauge for the vector potential

A transformation of the transverse Coulomb vector potential was implemented to calculate molecular magnetic properties via the random-phase approximation (RPA) within the framework of a “longitudinal gauge.” In this gauge, the diamagnetic contribution to magnetic susceptibility is a tensor with equal diagonal components as in atoms, irrespective of molecular symmetry, whereas diagonal and average diamagnetic contributions to the nuclear magnetic shielding are the same as in the Coulomb gauge. Near-Hartree–Fock magnetic susceptibility and nuclear magnetic shielding tensors were evaluated for a set of small molecules, HF, H₂O, NH₃, and CH₄, employing extended Gaussian basis sets. The peculiar features of the longitudinal gauge, and the fulfillment of a series of sum rules involving the virial operator, which must be satisfied to guarantee gauge invariance of total magnetic tensors, were exploited to check the degree of convergence of theoretical values and to estimate the corresponding Hartree–Fock limit for the properties. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 31–45, 1998     

31P MAS-NMR an Phosphoroxidsulfiden — experimentelle Bestimmung und quantenchemische Berechnung der Tensoren der chemischen Verschiebung

³¹P MAS-NMR of Phosphorus Oxide Sulfides — Experimental Determination and Quantumchemical Calculation of Chemical Shift Tensors By high resolution solid state ³¹P MAS NMR and analysis of spinning sidebands the principal values of the chemical shift tensors in the series P₄O₆S_n with n = 0–4 have been determined. The orientations of the corresponding principal axes within the molecules have been derived. All magnetic shielding tensors show axial symmetry within the limits of experimental error. Thus the orientation of the shielding tensor within the molecules can be deduced indirectly. This information is usually not accessible for polycrystalline samples. The principal values of the tensor of the trivalent phosphorus atoms in P₄O₆S seem to deviate considerably from those of the other compounds with respect to anisotropy and axiality. The reason is a dynamic effect: the rotation of the molecule about the PS bond. All experimental results are confirmed by ab-initio calculations using the IGLO method.     

A combined experimental and quantum chemistry study of selenium chemical shift tensors

A comprehensive investigation of selenium chemical shift tensors is presented. Experimentally determined chemical shift tensors were obtained from solid-state 77Se NMR spectra for several organic, organometallic, or inorganic selenium-containing compounds. The first reported indirect spin-spin coupling between selenium and chlorine is observed for Ph(2)SeCl(2) where 1J(77Se,35Cl)iso is 110 Hz. Selenium magnetic shielding tensors were calculated for all of the molecules investigated using zeroth-order regular approximation density functional theory, ZORA DFT. The computations provide the orientations of the chemical shift tensors, as well as a test of the theory for calculating the magnetic shielding interaction for heavier elements. The ZORA DFT calculations were performed with nonrelativistic, scalar relativistic, and scalar with spin-orbit relativistic levels of theory. Relativistic contributions to the magnetic shielding tensor were found to be significant for (NH4)2WSe4 and of less importance for organoselenium, organophosphine selenide, and inorganic selenium compounds containing lighter elements.     

A comparison of quantum chemical models for calculating NMR shielding parameters in peptides: mixed basis set and ONIOM methods combined with a complete basis set extrapolation

This article compares several quantum mechanical approaches to the computation of chemical shielding tensors in peptide fragments. First, we describe the effects of basis set quality up to the complete basis set (CBS) limit and level of theory (HF, MP2, and DFT) for four different atoms in trans N-methylacetamide. For both isotropic shielding and shielding anisotropy, the MP2 results in the CBS limit show the best agreement with experiment. The HF values show quite a different tendency to MP2, and even in the CBS limit they are far from experiment for not only the isotropic shielding of carbonyl carbon but also most shielding anisotropies. In most cases, the DFT values differ systematically from MP2, and small basis-set (double- or triple-zeta) results are often fortuitously in better agreement with the experiment than the CBS ones. Second, we compare the mixed basis set and ONIOM methods, combined with CBS extrapolation, for chemical shielding calculations at a DFT level using various model peptides. From the results, it is shown that the mixed basis set method provides better results than ONIOM, compared to CBS calculations using the nonpartitioned full systems. The information studied here will be useful in guiding the selection of proper quantum chemical models, which are in a tradeoff between accuracy and cost, for shielding studies of peptides and proteins.