Magnetic Shielding Tensors Research Articles

Theoretical Analysis of Solvent Effect on the NMR Parameters in trans-(NHC)PtI2Py Complex: A Platinum-Based Anticancer Drug

In this study, quantum chemical calculations using MPW1PW91 method were applied to analyze the solvent effect on the NMR parameters for a platinum-based anticancer drug, namely complex trans-(NHC)PtI2Py. The solvent effects on isotropic magnetic shielding tensor data (σiso) for Pt and CNHC atoms and spin–spin coupling constants of Pt–C and Pt–N in the complex were studied by the self-consistent reaction field theory (SCRF) based on the Polarizable Continuum Model (PCM). The linear correlations between these parameters and dielectric constants of solvents were studied. The σiso (Pt) values in different solvents were correlated with the Kirkwood–Bauer–Magat equation (KBM).

Does induced current density explain the C-H and C-F Perlin effects?

Spin-spin coupling constant (SSCC) data may be useful in providing information on the stereochemistry and intramolecular interactions in molecules. One-bond C-H and C-F SSCCs (1JCH and 1JCF) are amongst the most important NMR parameters used to study the structure of alicyclic six-membered rings, because the Perlin effect, defined as 1JCHeq > 1JCHax, and the fluorine Perlin-like effect, defined as |1JCFeq| > |1JCFax|, are in wide currency to probe the conformations of these compounds. The origin of these effects has been usually attributed either to dipolar interactions or hyperconjugation, while the induced current density (ICD) has been recently correlated to the magnetic shielding tensors in some six-membered ring compounds, and then used to explain the Perlin effect. Accordingly, this work reports an analysis of the ICD as a descriptor of 1JCH and 1JCF in a series of six-membered rings to find out the role of the ICD in the conventional and fluorine Perlin effect. The atoms in molecules (AIM) magnetic responses obtained from density functional theory (DFT) calculations for the studied compounds did not show any relationship of the first-order electronic current density (ΔJ(1)) with the calculated Δ1JCHax-eq and Δ1JCFax-eq values. Consequently, the title effects cannot be precisely explained by ICD. Nevertheless, an interesting relationship between ΔJ(1) and Δdelocalization involving σCH* orbitals is observed.

Reconstruction of STO-3G Family Basis Set for the Accurate Calculation of Magnetic Properties

The new approach for the determination of orbital exponents and contracted coefficients for STO-3G family basis sets has been proposed. Calculations of the necessary coefficients have been performed using Mathcad program package with Minerr solving block. This approach has been used to perform the approximation of the Slater-type orbital (STO) by three Gaussian-type orbitals (GTO). The performance of such modified basis sets has been tested for the calculations of atomic energies using STO(0)-3G basis set and for nuclear magnetic shielding tensors using STO(1M)-3G basis set. The obtained atomic energies are characterized by lower values than those calculated using old parameters. The results for 1H and 13C chemical shifts calculations demonstrate better agreement with the experimental data compared to the data obtained using standard basis sets, such as 6-311G (2d, p), cc-pVDZ and pcS-1. Required time of calculations using the basic set suggested by us is less than the time spent on the calculation using standard basic sets with a similar number of basis functions. Physically adapted and at the same time small by size basic set STO(1M)-3G is perspective for the calculation of magnetic properties of big molecular systems. Proton and 13C chemical shifts have been calculated for molecules of adenosine monophosphate (AMP) and flavinadenine dinucleotide (FAD), that play an important role in various biological processes. For both molecules the results of the calculation have shown values close to the experimental data.

A Dft Investigation of the Interaction of B- And Al-Doped C60 Fullerenes with Cyclopropylpipezarine

Interactions and natural bond orbital analysis of cyclopropylpiperazine(cppp) and B- or Al-doped C60 fullerenes are investigated by quantum mechanical calculations. The structural and electronic properties, such as NH stretching vibrations, nuclear magnetic shielding tensors of 27Al and 11B nuclei or the energy gap are also investigated. All calculations are carried out in the gas phase and water media. Results of the bond order and the binding energy suggest that chemisorption occurs between the cppp nitrogen atom and boron or aluminum atoms of fullerenes. The Alcomplex in water is foundto have the highest binding energy. Further, AlC59…cppp complexes have a slightly higher kinetic stability and a low chemical reactivity. However, BC59…cppp works as a function-type sensor for cppp.

Spin-orbit effects on the 125Te magnetic-shielding tensor: A cluster-based ZORA/DFT investigation

Cluster-based calculations of 125Te magnetic-shielding tensors demonstrate that inclusion of spin-orbit effects is necessary to obtain the best agreement of theoretical predictions with experiment. The spin-orbit contribution to shielding depends on the oxidation state and stereochemistry of the 125Te site. Comparison of the performance of various density functionals indicates that GGA functionals behave similarly to each other in predicting NMR magnetic shielding. The use of hybrid functionals improves the predictive ability on average for a large set of 125Te-containing materials. The amount of Hartree-Fock exchange affects the predicted parameters. Inclusion of larger Hartree-Fock exchange contributions in hybrid functionals results in larger slopes of the correlation between calculated magnetic-shielding and experimental chemical-shift principal components, by 10–15% from the ideal value.

Efficient and Accurate Prediction of Nuclear Magnetic Resonance Shielding Tensors with Double-Hybrid Density Functional Theory.

Analytic calculation of nuclear magnetic resonance chemical shielding tensors, based on gauge-including atomic orbitals, is implemented for double-hybrid density functional theory (DHDFT), using the resolution of the identity (RI) approximation for its second order Møller-Plesset perturbation theory (MP2) correlation contributions. A benchmark set of 15 small molecules, containing 1H, 13C, 15N, 17O, 19F, and 31P nuclei, is used to assess the accuracy of the results in comparison to coupled cluster and empirical equilibrium reference data, as well as to calculations with MP2, Hartree-Fock, and commonly used pure and hybrid density functionals. Investigated are also errors due to basis set incompleteness, the frozen core approximation, different auxiliary basis sets for the RI approximation, and grids used for the chain-of-spheres exchange integral evaluation. The DSD-PBEP86 double-hybrid functional shows the smallest deviations from the reference data with mean absolute relative error in chemical shifts of 1.9%. This is significantly better than MP2 (4.1%), spin-component-scaled MP2 (3.9%), or the best conventional density functional tested, M06L (5.4%). A protocol (basis sets, grid sizes, etc.) for the efficient and accurate calculation of chemical shifts at the DHDFT level is proposed and shown to be routinely applicable to systems of 100-400 electrons, requiring computation times 1-2 orders of magnitude longer than for equivalent calculations with conventional (pure or hybrid) density functionals.

19F Magic Angle Spinning NMR Spectroscopy and Density Functional Theory Calculations of Fluorosubstituted Tryptophans: Integrating Experiment and Theory for Accurate Determination of Chemical Shift Tensors.

The 19F chemical shift is a sensitive NMR probe of structure and electronic environment in organic and biological molecules. In this report, we examine chemical shift parameters of 4F-, 5F-, 6F-, and 7F-substituted crystalline tryptophan by magic angle spinning (MAS) solid-state NMR spectroscopy and density functional theory. Significant narrowing of the 19F lines was observed under fast MAS conditions, at spinning frequencies above 50 kHz. The parameters characterizing the 19F chemical shift tensor are sensitive to the position of the fluorine in the aromatic ring and, to a lesser extent, the chirality of the molecule. Accurate calculations of 19F magnetic shielding tensors require the PBE0 functional with a 50% admixture of a Hartree-Fock exchange term, as well as taking account of the local crystal symmetry. The methodology developed will be beneficial for 19F-based MAS NMR structural analysis of proteins and protein assemblies.

Self-Consistent Field Calculation of Nuclear Magnetic Resonance Chemical Shielding Constants Using Gauge-Including Atomic Orbitals and Approximate Two-Electron Integrals.

The chain-of-spheres method (COS) for approximating two-electron integrals is applied to Hartree-Fock and density functional theory calculations of nuclear magnetic resonance chemical shielding tensors, based on gauge-including atomic orbitals. The accuracy of the approximation is compared to that of the resolution of the identity (RI) approach, using a benchmark test set of 15 small molecules. Reasonable auxiliary basis sets and grid sizes are selected on the basis of a careful investigation of how approximating each of the two-electron terms in the self-consistent field (SCF) and coupled perturbed SCF equations affects the calculated shielding constants. It is found that the errors are linearly additive but can have either sign. The mean absolute relative error due to applying the RI/COS approximations with the chosen settings to all two-electron terms is on the order of 0.01% and therefore negligible compared to the errors due to basis set incompleteness (∼1%) and the method used (10-50%). Several larger organic systems are used to assess the efficiency of the RI approximation for both Coulomb- and exchange-type integrals (RIJK) as well as a combination of RI for Coulomb and COS for exchange contributions (RIJCOSX). The RIJK approximation is more efficient for small molecules, while for systems of over 100 electrons and 1000 basis functions, the RIJCOSX approximation is superior.

The Bloch equation with terms induced by an electric field.

The Bloch equation of the nuclear magnetization of spin-1/2 nuclei in molecules, which have permanent electric dipole moments μe that are placed simultaneously in a magnetic field B and an electric field E, is derived. It is shown that if the principal components of the nuclear magnetic shielding tensor σ and the dipole moment μe are known, then the measurement of the transverse component to the magnetic field B of the nuclear magnetization, which is induced by the application of the electric field oscillating at the half of the spin precession frequency, allows determining the orientation of the dipole moment μe with respect to the principal axis system of the symmetric part of the tensor σ. Four-component relativistic density functional theory computations, which have been performed for several molecules containing heavy nuclei, i.e., 207Pb, 205Tl, 199Hg, 195Pt, and 125Te, indicate that coefficients of the relaxation matrix perturbed by the electric field E are in favorable cases of the order of 1000 pm2 V-2 T-2. Therefore, the spin dynamics is perturbed at experimentally observable levels for the strengths of electric and magnetic fields E = 5 kV/mm and B = 10 T, respectively.

Magnetic Shielding Studies of C2 and C2 H2 Support Higher than Triple Bond Multiplicity in C2.

The carbon-carbon bonds in the ground states of C2 and C2 H2 , at their equilibrium geometries, are compared by analysing the changes in the off-nucleus magnetic shielding tensor within the space surrounding each of these molecules. A wide range of quantum-chemical approaches, including full-valence CASSCF-GIAO, CCSD(T)-GIAO and CCSDT-GIAO, all with the cc-pVQZ basis set, as well as HF-GIAO and MP2-GIAO, with the cc-pVQZ, cc-pV5Z and cc-pV6Z basis sets, show that the surroundings of the carbon-carbon bond in C2 are more shielded than those of the carbon-carbon bond in C2 H2 . The additional shielding of the carbon-carbon bond in C2 is found to be due to a larger paramagnetic contribution to the component of the shielding tensor which is perpendicular to the molecular axis. The analysis of the off-nucleus shielding data indicates that the carbon-carbon bond in C2 is "bulkier" and, therefore, of a higher multiplicity, but weaker than the corresponding bond in C2 H2 . According to the results of the shielding calculations, the carbon nuclei in C2 should be much more shielded than those in C2 H2 , with 13 C isotropic magnetic shieldings in the ranges of around 224-227 ppm and around 123-125 ppm for C2 and C2 H2 , respectively.

Two-dimensional honeycomb (A7) and zigzag sheet (ZS) type nitrogen monolayers. A first principles study of structural, electronic, spectral, and mechanical properties

Two single-bonded 2D nitrogen allotropes of the honeycomb (A7) and zigzag sheet (ZS) topology have been calculated using density functional theory (DFT). The optical (vibrational, absorption, nuclear magnetic resonance), thermodynamic and elastic properties of the A7 and ZS sheets have been calculated for the first time. The band structure calculation have revealed a semiconducting nature of the ZS sheet with a direct gap of 1.246eV, while the A7 monolayer behaves as an insulator with an indirect gap of 3.842eV. Phonon dispersion calculations have justified these structures as vibrationally stable 2D materials. The IR spectroscopy completely failed in the characterization of the studied materials, while the Raman spectroscopy can be effectively applied for the experimental spectral identification. The absorption spectra demonstrate complete opacity of the A7 and ZS monolayers to the UV irradiation only above ca. 9 and 6eV, respectively. Thus, the studied materials are expected to be transparent to the visible light. The electron arrangement of the nitrogen nuclei in the studied polynitrogen sheets is denser compared to the N2 molecule which follows from the calculation of the values of magnetic shielding tensors. The elastic constants reveal a robust mechanical stability of the studied 2D nitrogen allotropes. The Young moduli values are only twice as lower than that of the graphene molecule.

Interaction mechanisms and structural properties of MC19 (M = Si and Al) fullerenes with chlorophenylpiperazine isomers

Abstract A great deal of attention has been paid recently to the fullerene based complex systems for their possible applications as drug delivery vehicles and sensor applications. In the framework of this research, Si and Al-doped C20 fullerenes and physical interaction mechanisms of doped fullerenes with ortho, meta and para chlorophenylpiperazines were examined based on the computational density functional methods. Energetically stable complexes were searched with M06-2X and B3LYP functionals with 6-31G(d) basis set and some important geometrical structures including interatomic distances and dihedral angles were reported. All calculations were carried out in the gas phase and water media. It was observed that stabilities of the investigated systems were enhanced in the water phase calculations and upon adsorption besides the changes in NH stretching values, nuclear magnetic shielding tensors of 27 Al and 29 Si nuclei were considerably changed. In order to further analyse the electronic, physical and chemical properties, binding and band gap energies, electrophilicity indexes and chemical hardness values were also calculated.

1-(2-NITROPHENYL)PIPERAZINE: NMR, RAMAN, FTIR AND DFT STUDIES

The structure of 1-(2-nitrophenyl)piperazine (NPP, C 10 H 13 N 3 O 2 ) was characterized by nuclear magnetic resonance (NMR), Fourier Transform infrared (FTIR) and Raman techniques. The conformational analysis, nuclear magnetic shielding tensors, normal mode frequencies and corresponding vibrational assignments of NPP were examined using the density functional theory (DFT), with the Becke-3-Lee-Yang-Parr (B3LYP) functional and the 6-31G(d) and 6-311++G(d,p) basis sets. Reliable vibrational assignments were investigated by the total energy distributions (TED) obtained with scaled quantum mechanical (SQM) method. The hydrogen of NH group in piperazine and the phenyl fragment of NPP equatorially oriented relative to piperazine. There is a good agreement between the experimentally determined nuclear magnetic shielding tensors and vibrational frequencies of NPP and those predicted theoretically.

13 C and 15 N NMR spectra of high-energy polyazidocyanopyridines.

13 C and 15 N NMR spectra of high-energy 2,4,6-triazidopyridine-3,5-dicarbonitrile, 2,3,5,6-tetraazidopyridine-4-carbonitrile and 3,4,5,6-tetraazidopyridine-2-carbonitrile are reported. The assignment of signals in the spectra was performed on the basis of density functional theory calculations. The molecular geometries were optimized using the M06-2X functional with the 6-311+G(d,p) basis set. The magnetic shielding tensors were calculated by the gauge-independent atomic orbital method with the Tao-Perdew-Staroverov-Scuseria hybrid functional known as TPSSh. In all the calculations, a polarizable continuum model was used to simulate solvent effects. This approach provided accurate predictions of the 13 C and 15 N chemical shifts for all the three compounds despite complications arising due to non-coplanar arrangement of the azido groups in the molecules. It was found that the 15 N chemical shifts of the Nα atoms in the azido groups of 2,4,6-triazidopyridines correlate with the 13 C chemical shifts of the carbon atoms attached to these azido groups. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis of the bond-valence method for calculating (29) Si and (31) P magnetic shielding in covalent network solids.

(29) Si and (31) P 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 (29) Si and (31) P 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.

Spin–orbit effects on the 119Sn magnetic-shielding tensor in solids: a ZORA/DFT investigation

Periodic-boundary and cluster calculations of the magnetic-shielding tensors of (119)Sn sites in various co-ordination and stereochemical environments are reported. The results indicate a significant difference between the predicted NMR chemical shifts for tin(ii) sites that exhibit stereochemically-active lone pairs and tin(iv) sites that do not have stereochemically-active lone pairs. The predicted magnetic shieldings determined either with the cluster model treated with the ZORA/Scalar Hamiltonian or with the GIPAW formalism are dependent on the oxidation state and the co-ordination geometry of the tin atom. The inclusion of relativistic effects at the spin-orbit level removes systematic differences in computed magnetic-shielding parameters between tin sites of differing stereochemistries, and brings computed NMR shielding parameters into significant agreement with experimentally-determined chemical-shift principal values. Slight improvement in agreement with experiment is noted in calculations using hybrid exchange-correlation functionals.

Structure analysis and spectroscopic characterization of 2-Fluoro-3-Methylpyridine-5-Boronic Acid with experimental (FT-IR, Raman, NMR and XRD) techniques and quantum chemical calculations

Possible stable conformers, geometrical molecular structures, vibrational properties as well as band assignments, nuclear magnetic shielding tensors of 2-Fluoro-3-Methylpyridine-5-Boronic Acid (2F3MP5BA) were studied experimentally and theoretically using FT-IR, Raman, (CP/MAS) NMR and XRD spectroscopic methods. FT-IR and Raman spectra were evaluated in the region of 3500–400 cm−1, and 3200–400 cm−1, respectively. The optimized geometric structures, vibrational wavenumbers and nuclear magnetic shielding tensors were examined using Becke-3–Lee–Yang–Parr (B3LYP) hybrid density functional theory method with 6–311++G(d, p) basis set. 1H, 13C NMR chemical shifts were calculated using the gauge invariant atomic orbital (GIAO) method. 1H, 13C, APT and HETCOR NMR experiments of title molecule were carried out in DMSO solution. 13C CP/MAS NMR measurement was done with 4 mm zirconium rotor and glycine was used as an external standard. Single crystal of 2F3MP5BA was also prepared for XRD measurements. Assignments of vibrational wavenumbers were also strengthened by calculating the total energy distribution (TED) values using scaled quantum mechanical (SQM) method.

Recent Advances in Theoretical and Physical Aspects of NMR Chemical Shifts

In the first part of this review, theoretical aspects of nuclear magnetic shielding include (a) general theory, for example, newly developed approaches in relativistic theory of nuclear shielding, the relation between the spin-rotation tensor and shielding in relativistic theory, ab initio methods for treating open shell systems and a complete theory of chemical shifts in paramagnetic systems, the link between the definitions of the elusive concepts aromaticity and anti-aromaticity and the magnetic properties: the magnetizability tensor and the nuclear magnetic shielding tensor via delocalized electron currents and electron current maps, (b) ab initio and DFT calculations, both relativistic and non-relativistic, for various nuclei in various molecular systems using various levels of theoretical treatment. Physical aspects include (a) anisotropy of the shielding tensor, usually from solid state measurements, and calculations to support these, (b) shielding surfaces and rovibrational averaging, paying special attention to the sensitive relationship between shielding and bond angles or torsion angles that makes shielding such a powerful tool for structural/conformational determination in macromolecules, (c) chemical shifts that arise from isotopic substitution of NMR nucleus or neighboring nuclei, (d) intermolecular effects on nuclear shielding, and (e) absolute shielding scales.

Critical Analysis of Cluster Models and Exchange-Correlation Functionals for Calculating Magnetic Shielding in Molecular Solids.

Calculations of the principal components of magnetic-shielding tensors in crystalline solids require the inclusion of the effects of lattice structure on the local electronic environment to obtain significant agreement with experimental NMR measurements. We assess periodic (GIPAW) and GIAO/symmetry-adapted cluster (SAC) models for computing magnetic-shielding tensors by calculations on a test set containing 72 insulating molecular solids, with a total of 393 principal components of chemical-shift tensors from 13C, 15N, 19F, and 31P sites. When clusters are carefully designed to represent the local solid-state environment and when periodic calculations include sufficient variability, both methods predict magnetic-shielding tensors that agree well with experimental chemical-shift values, demonstrating the correspondence of the two computational techniques. At the basis-set limit, we find that the small differences in the computed values have no statistical significance for three of the four nuclides considered. Subsequently, we explore the effects of additional DFT methods available only with the GIAO/cluster approach, particularly the use of hybrid-GGA functionals, meta-GGA functionals, and hybrid meta-GGA functionals that demonstrate improved agreement in calculations on symmetry-adapted clusters. We demonstrate that meta-GGA functionals improve computed NMR parameters over those obtained by GGA functionals in all cases, and that hybrid functionals improve computed results over the respective pure DFT functional for all nuclides except 15N.

Structural characterization, solvent effects on nuclear magnetic shielding tensors, experimental and theoretical DFT studies on the vibrational and NMR spectra of 3-(acrylamido)phenylboronic acid

Structural elucidation of 3-(acrylamido)phenylboronic acid (C9H10BNO3) was carried out with 1H, 13C and HETCOR NMR techniques. Solvent effects on nuclear magnetic shielding tensors were examined with deuterated dimethyl sulfoxide, acetone, methanol and water solvents. The correct order of appearance of carbon and hydrogen atoms on NMR scale from highest magnetic field region to the lowest one were investigated using different types of theoretical levels and the details of the levels were presented in this study. Stable structural conformers and vibrational band analysis of the title molecule (C9H10BNO3) were studied both experimental and theoretical viewpoints using FT-IR, Raman spectroscopic methods and density functional theory (DFT). FT-IR and Raman spectra were obtained in the region of 4000–400 cm−1, and 3700–10 cm−1, respectively. Becke-3-Lee-Yang-Parr (B3LYP) hybrid density functional theory method with 6-31++G(d, p) basis set was included in the search for optimized structures and vibrational wavenumbers. Experimental and theoretical results show that after application of a suitable scaling factor density functional B3LYP method resulted in acceptable results for predicting vibrational wavenumbers except OH and NH stretching modes which is most likely arising from increasing unharmonicity in the high wave number region and possible intra and inter molecular interaction at OH edges those of which are not fully taken into consideration in theoretical processes. To make a more quantitative vibrational assignments, potential energy distribution (PED) values were calculated using VEDA 4 (Vibrational Energy Distribution Analysis) program.