Chemical Shielding Anisotropy Research Articles

Effects of influence of carbon ring-doping on NMR parameters of boron phosphide nanotubes: A DFT study

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).

Dynamics Studies of HIV-1 CA Protein Assemblies by Solid-State MAS NMR Spectroscopy

The capsids of mature retroviruses perform the essential function of organizing the viral genome for efficient replication. In mature HIV-1 virions, the 25.6 kDa CA protein assemblies into a fullerene cone, a closed shell. Tyrosine residues in the capsid proteins of retroviruses play important roles in protein folding and assembly packing1. In HIV-1 CA protein, Y145, which is located in the linker region between N- and C-terminal domains might play a pivot role in HIV-1 capsid assembly.2,3 However, the molecular interactions involving Y145 that stabilize the assembly, including the dynamic behavior of this residue in the assembled and unassembled CA are unknown. We have recently determined that MAS NMR spectroscopy can yield atomic- resolution information on assembled CA capsids and reported partial resonance assignments and secondary structure analysis of conical assemblies.4 Herein, we present studies of the internal dynamics of the Tyr resiudes in CA as probed by various solid-state NMR methods. 15N/1H chemical shift anisotropies (CSA) and 1H-15N/1H-13C dipolar order parameters have been obtained in selectively labeled 13C, 15N -Tyr HIV-1 CA protein assemblies of conical morphologies, as well as in N- and C-terminal domains of CA. Our results demonstrate that Y145 exhibits reduced 15N and 1H chemical shift anisotropy, as well as smaller 1H-15N and 1H-13C dipolar order parameters, presumably due to its backbone mobility on nano- to microsecond timescales. Moreover, the NMR parameters of Y145 are strongly temperature dependent, another evidence of internal conformational dynamics. In contrast, the other three Tyr residues, Y130, Y164 and Y169, which are located in α-helical regions of CA, display rigid-limit chemical shielding anisotropies and dipolar order parameters.

DFT calculations of NMR properties for GaP nanotubes

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 (CSI and CSA) were calculated for 69Ga and 31P 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.

Influence of the Size Effect on the Chemical Shielding Tensors of SiO<sub>2</sub> Nanotubes

Finite length SiO2 nanotubes composed of a three-membered ring (3MR) cross section were investigated by density functional theory. Nuclear magnetic resonance (NMR) results reveal that the isotropic chemical shielding tensor (sigma(iso)) and anisotropic chemical shielding tensor (Delta sigma) of the SiO2 nanotubes oscillate from the terminal end to the intermediate part of the structures with an increase in nanotube lengths, and they tend to reach different stable values. When the length is greater than certain value the sigma(iso) and Delta sigma values obtained from the intermediate part of the finite length nanotube can be used for theoretical predictions of long or infinite length nanotubes. Hence, this investigation is a guide for the selection of reasonable finite length nanotubes to model infinite length nanotubes.

The influence of NH 3 -attaching on the NMR parameters in the zigzag BN nanotube

Two models of (10, 0) boron nitride nanotubes (BNNTs), perfect and Ammonia-attached, were studied in order to evaluate the influence of NH 3-attaching on the B-11 and N-15 nuclear magnetic resonance in the (10, 0) boron-nitride nanotube (BNNT) for the first time. At first, based on density functional theory (DFT) each of the structures was optimized using B3LYP/6-31G (d) model chemistry. At the next step, the chemical-shielding (CS) tensors were calculated using the B3LYP/6-31G (d, p) level of theory in both of the relaxed forms and were converted to experimentally measurable nuclear magnetic resonance (NMR) parameters, i.e. chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA). Our calculation revealed that in the NH 3-attached BNNT (the most stable model) the B atom chemically bonded to the NH 3 molecule has the largest chemical-shielding isotropic (CSI) and the smallest chemical-shielding anisotropic (CSA) values among the other boron nuclei. Additionally, the NMR parameters of those nuclei directly bonded to the boron dramatically change while those of the other B nuclei remain almost unchanged.

Influence of Structure on the Spectroscopic Properties of the Polymorphs of Piroxicam

The complete (13)C NMR chemical-shift tensors for the carbon sites of the two polymorphic forms (P(I) and P(II)) and the monohydrate form (PM) of the analgesic drug, piroxicam, are reported. The NMR parameters (isotropic chemical shifts, chemical-shielding anisotropies and asymmetries, and dipolar couplings), X-ray powder diffraction, and density functional calculations of piroxicam are analyzed in terms of hydrogen bonding and structure. The integration of all the data gives an improved model of the local solid-state structures of the polymorphs. In particular, the solid-state NMR spectra demonstrate that the asymmetric unit of the monohydrate, PM, contains two zwitterionic piroxicam molecules.

DFT study on influence of Si and Ge doping on the chemical shielding tensors in the armchair AlN nanotubes

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 1 nm 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.

A computational study of atomic oxygen-doped silicon carbide nanotubes

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.

The NMR parameters of the SiC-doped BN nanotubes: A DFT study

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.

Solid-state 27Al nuclear magnetic resonance investigation of three aluminum-centered dyes

We report the first solid-state 27Al NMR study of three aluminum phthalocyanine dyes: aluminum phthalocyanine chloride, AlPcCl (1); aluminum-1,8,15,22-tetrakis(phenylthio)-29H,31H-phthalocyanine chloride, AlPc(SPh)4Cl (2); and aluminum-2,3-naphthalocyanine chloride, AlNcCl (3). Each of these compounds contains Al3+ ions coordinating to four nitrogen atoms and a chlorine atom. Solid-state 27Al NMR spectra, including multiple-quantum magic-angle spinning (MQMAS) spectra and quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) spectra of stationary powdered samples have been acquired at multiple high magnetic field strengths (11.7, 14.1, and 21.1 T) to determine their composition and number of aluminum sites, which were analyzed to extract detailed information on the aluminum electric field gradient (EFG) and nuclear magnetic shielding tensors. The quadrupolar parameters for each 27Al site were determined from spectral simulations, with quadrupolar coupling constants (CQ) ranging from 5.40 to 10.0 MHz and asymmetry parameters (η) ranging from 0.10 to 0.50, and compared well with the results of quantum chemical calculations of these tensors. We also report the largest 27Al chemical shielding anisotropy (CSA), with a span of 120 ± 10 ppm, observed directly in a solid material. The combination of MQMAS and computational predictions are used to interpret the presence of multiple aluminum sites in two of the three samples.

High sensitivity of O17 NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, σ̂, in prototypical transition metal oxide ABO3 perovskite crystals. Our principal findings are (1) a large anisotropy of σ̂ between deshielded σx≃σy and shielded σz components (z along the Ti–O bond); (2) a nearly linear variation, across all the systems studied, of the isotropic σiso and uniaxial σax components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to σx and σy due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic δiso and uniaxial δax chemical shifts are in good agreement with recent BaTiO3 and SrTiO3 single crystal O17 NMR measurements. In PbTiO3 and PbZrO3, calculated δiso are also in good agreement with NMR powder spectrum measurements. In PbZrO3, δiso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of σ̂ on covalent O(2p)-B(nd) interactions seen in our calculations indicates that O17 NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.

Calculation of chemical shielding in C-doped zigzag BN nanotubes

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.

A magnetic resonance study of MoS2 fullerene-like nanoparticles

We report on the first nuclear magnetic resonance (NMR) and electronparamagnetic resonance (EPR) investigation of inorganic fullerene-likeMoS2 nanoparticles.Spectra of bulk 2H-MoS2 samples have also been measured for comparison. The similarity between the measured quadrupolecoupling constants and chemical shielding anisotropy parameters for bulk and fullerene-likeMoS2 reflects the nearly identical local crystalline environments of the Moatoms in these two materials. EPR measurements show that fullerene-likeMoS2 exhibits a larger density of dangling bonds carrying unpaired electrons, indicative ofthem having a more defective structure than the bulk sample. The latterobservation explains the increase in the spin–lattice relaxation rate observed inthe NMR measurements for this sample in comparison with the bulk 2H-MoS2 ones.

A DFT study of the nuclear magnetic response of the zigzag AlN–BN and BN–AlN nanotube junctions

The chemical shielding (CS) tensors of 15N, 27Al and 11B nuclei are calculated for the first time. The calculations were done on zigzag single-walled models of aluminum nitride–boron nitride (AlN–BN) and boron nitride–aluminum nitride (BN–AlN) nanotube (NT) junctions using B3LYP method and Gaussian 03 program. The computed CS tensors are converted to isotropic (ICS) and anisotropic chemical shielding (ACS). Our calculations show that there are significant changes in the NMR values for the N atoms as compared to such values for the Al and B atoms within AlN–BN and BN–AlN nanotubes.

Theoretical Study of the Effective Chemical Shielding Anisotropy (CSA) in Peptide Backbone, Rating the Impact of CSAs on the Cross-Correlated Relaxations in l-Alanyl-l-alanine

The dependence of the effective chemical shielding anisotropy (effective CSA, Deltasigma(eff)) on the phi and psi peptide backbone torsion angles was calculated in the l-alanyl-l-alanine (LALA) peptide using the DFT method. The effects of backbone conformation, molecular charge including the cation, zwitterion, and anion forms of the LALA peptide, and the scaling taking into account the length of the dipolar vector were calculated for the effective CSAs in order to assess their structural behaviors and to predict their magnitudes which can be probed for the beta-sheet and alpha-helix backbone conformations via measurement of the cross-correlated relaxation rates (CCR rates). Twenty different CSA-DD cross-correlation mechanisms involving the amide nitrogen and carbonyl carbon chemical shielding tensors and the C(alpha)H(alpha) (alpha-carbon group), NH(N) (amide group), C(alpha)H(N), NH(alpha), C'H(alpha), and C'H(N) (alpha = alpha1, alpha2) dipolar vectors were investigated. The X-C(alpha)H(alpha) (X = N, C'; alpha = alpha1, alpha2) cross-correlations, which had already been studied experimentally, exhibited overall best performance of the calculated effective CSAs in the LALA molecule; they spanned the largest range of values upon variation of the psi and phi torsions and depended dominantly on only one of the two backbone torsion angles. The X-NH(N) (X = N, C') cross-correlations, which had been also probed experimentally, depended on both backbone torsions, which makes their structural assignment more difficult. The N-NH(alpha2) and N-C'H(alpha1) cross-correlations were found to be promising for the determination of various backbone conformations due to the large calculated range of the scaled effective CSA values and due to their predominant dependence on the psi and phi torsions, respectively. The 20 calculated dependencies of effective CSAs on the two backbone torsion angles can facilitate the structural interpretation of CCR rates.

Impact of Reduction on the Properties of Metal Bisdithiolenes: Multinuclear Solid-State NMR and Structural Studies on Pt(tfd)2 and Its Reduced Forms

Transition-metal dithiolene complexes have interesting structures and fascinating redox properties, making them promising candidates for a number of applications, including superconductors, photonic devices, chemical sensors, and catalysts. However, not enough is known about the molecular electronic origins of these properties. Multinuclear solid-state NMR spectroscopy and first-principles calculations are used to examine the molecular and electronic structures of the redox series [Pt(tfd)(2)](z-) (tfd = S(2)C(2)(CF(3))(2); z = 0, 1, 2; the anionic species have [NEt(4)](+) countercations). Single-crystal X-ray structures for the neutral (z = 0) and the fully reduced forms (z = 2) were obtained. The two species have very similar structures but differ slightly in their intraligand bond lengths. (19)F-(195)Pt CP/CPMG and (195)Pt magic-angle spinning (MAS) NMR experiments are used to probe the diamagnetic (z = 0, 2) species, revealing large platinum chemical shielding anisotropies (CSA) with distinct CS tensor properties, despite the very similar structural features of these species. Density functional theory (DFT) calculations are used to rationalize the large platinum CSAs and CS tensor orientations of the diamagnetic species using molecular orbital (MO) analysis, and are used to explain their distinct molecular electronic structures in the context of the NMR data. The paramagnetic species (z = 1) is examined using both EPR spectroscopy and (13)C and (19)F MAS NMR spectroscopy. Platinum g-tensor components were determined by using solid-state EPR experiments. The unpaired electron spin densities at (13)C and (19)F nuclei were measured by employing variable-temperature (13)C and (19)F NMR experiments. DFT and ab initio calculations are able to qualitatively reproduce the experimentally measured g-tensor components and spin densities. The combination of experimental and theoretical data confirm localization of unpaired electron density in the pi-system of the dithiolene rings.

The carbon-doped (4,4) boron nitride nanotube: A computational NMR approach

Boron-11 and nitrogen-15 chemical shielding (CS) tensors are calculated to investigate the influence of carbon doping on the electronic structure properties of the (4,4) boron nitride nanotube. The calculated CS tensors are converted to isotropic and anisotropic chemical shielding. The results reveal that the properties of those boron and nitrogen atoms directly connected to carbon atoms are significantly changed while those of other atoms are almost remained unchanged. But some changes are also viewed for those nitrogen atoms located in the C-doped ring. The calculations are performed at the level of the density functional theory (DFT) using GAUSSIAN 98 package of program.

A computational NMR study on zigzag aluminum nitride nanotubes

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.

Characterizing Nitrilimines with Nuclear Magnetic Resonance Spectroscopy. A Theoretical Study

The 13C chemical shifts in selected nitrilimines, nitriles, acetylenes, allenes, and singlet carbenes have been calculated using density-functional theory [PBE0/6-311++G(2df,pd)] and the gauge including atomic orbital (GIAO) method. The effects of substitution on the 13C chemical shifts in nitrilimines, R1-CNN-R2, have been examined. The carbon nucleus is generally found to be deshielded by substituents in the order CH3 < NH2 < OH < F. Comparison with nitriles, acetylenes, and allenes shows that this effect is related to the presence of the cumulated functionality, C=N=N. Terminal N-substitution is found to have a larger effect than C-substitution due to a large increase in chemical shielding anisotropy. The electronic structure of nitrilimines has recently been shown to possess a carbene component whose resonance contribution varies widely with substitution, and, as previously reported, insight into the electronic structure can be gained by an analysis of the shielding tensor, especially for carbenes. Accordingly, the components of the diagonalized 13C shielding tensor for nitrilimines and stable singlet carbenes have been examined. This analysis suggests that diaminonitrilimine, H2N-CNN-NH2, may be a stable carbene, and, to the best of our knowledge, it would be the first acyclic, unsaturated stable carbene ever reported. Finally, a detailed analysis of the 13C chemical shifts shows that an increase in the dipolar character of nitrilimines induces a shielding at the carbon nucleus, while an increase in allenic or carbenic character tends to cause a deshielding.

Solid-State63Cu and65Cu NMR Spectroscopy of Inorganic and Organometallic Copper(I) Complexes

Solid-state 63Cu and 65Cu NMR experiments have been conducted on a series of inorganic and organometallic copper(I) complexes possessing a variety of spherically asymmetric two-, three-, and four-coordinate Cu coordination environments. Variations in structure and symmetry, and corresponding changes in the electric field gradient (EFG) tensors, yield 63/65Cu quadrupolar coupling constants (CQ) ranging from 22.0 to 71.0 MHz for spherically asymmetric Cu sites. These large quadrupolar interactions result in spectra featuring quadrupolar-dominated central transition patterns with breadths ranging from 760 kHz to 6.7 MHz. Accordingly, Hahn-echo and/or QCPMG pulse sequences were applied in a frequency-stepped manner to rapidly acquire high S/N powder patterns. Significant copper chemical shielding anisotropies (CSAs) are also observed in some cases, ranging from 1000 to 1500 ppm. 31P CP/MAS NMR spectra for complexes featuring 63/65Cu-31P spin pairs exhibit residual dipolar coupling and are simulated to determine both the sign of CQ and the EFG tensor orientations relative to the Cu-P bond axes. X-ray crystallographic data and theoretical (Hartree-Fock and density functional theory) calculations of 63/65Cu EFG and CS tensors are utilized to examine the relationships between NMR interaction tensor parameters, the magnitudes and orientations of the principal components, and molecular structure and symmetry.