Nuclear Magnetic Shielding Tensors Research Articles

Quantum Effects of Hydrogen Nuclei on the Nuclear Magnetic Shielding Tensor of Ice Ih.

Ice is the most fundamental hydrogen-bonded system in which the hydrogen nuclear quantum effect significantly impacts the structure and relevant thermochemical and spectroscopic properties. While ice was experimentally investigated using proton nuclear magnetic resonance spectroscopy more than 40 years ago, the corresponding theoretical investigations have been rarely reported due to the difficulty in evaluating how the proton nuclear quantum effect influences the spectral characteristics of such a condensed material. In this study, we applied a combination of the ONIOM and multicomponent molecular orbital (MC_MO) methods for calculating the anisotropic and isotropic components of the nuclear magnetic shielding tensor of the hexagonal ice crystal to quantify the effects of nuclear quantum fluctuations on the spectroscopic properties of ice. The nuclear magnetic shielding values computed by incorporating the hydrogen nuclear quantum effect reasonably agree with the experimental values. The nuclear quantum effects were found to increase the anisotropic component of the magnetic shielding tensor while decreasing the isotropic component. Such a difference can be explained by their distinct dependence on the electrostatic field and hydrogen-bonding structural parameters.

A relativistic relationship between parity-violating nuclear spin-rotation tensors and parity-violating NMR shielding tensors.

The nuclear-spin-dependent parity-violation contributions to the nuclear magnetic resonance shielding and nuclear spin-rotation tensors (σPV and MPV, respectively) are known to be formally related to one another in the non-relativistic regime. In this work, the polarization propagator formalism and the linear response within the elimination of small components model are used to show a new and more general relationship between them, which is valid within the relativistic framework. The full set of the zeroth- and first-order relativistic contributions to σPV and MPV are also given here for the first time, and these results are compared with previous findings. According to four-component relativistic calculations, the electronic spin-orbit effects are the most significant ones for the isotropic values of σPV and MPV in the H2X2 series of molecules (with X = O, S, Se, Te, and Po). When only scalar relativistic effects are taken into account, the non-relativistic relationship between σPV and MPV does hold. However, when the spin-orbit effects are taken into consideration, this old non-relativistic relationship breaks down, and therefore, the new one must be considered.

Reference interaction site model self-consistent field with constrained spatial electron density approach for nuclear magnetic shielding in solution.

We propose a new hybrid approach combining quantum chemistry and statistical mechanics of liquids for calculating the nuclear magnetic resonance (NMR) chemical shifts of solvated molecules. Based on the reference interaction site model self-consistent field with constrained spatial electron density distribution (RISM-SCF-cSED) method, the electronic structure of molecules in solution is obtained, and the expression for the nuclear magnetic shielding tensor is derived as the second-order derivative of the Helmholtz energy of the solution system. We implemented a method for calculating chemical shifts and applied it to an adenine molecule in water, where hydrogen bonding plays a crucial role in electronic and solvation structures. We also performed the calculations of 17O chemical shifts, which showed remarkable solvent dependence. While converged results could not be sometimes obtained using the conventional method, in the present framework with RISM-SCF-cSED, an adequate representation of electron density is guaranteed, making it possible to obtain an NMR shielding constant stably. This introduction of cSED is key to extending the method's applicability to obtain the chemical shift of various chemical species. The present demonstration illustrates our approach's superiority in terms of numerical robustness and accuracy.

1-(4-Chlorophenyl) piperazine: FT-IR, Raman, NMR and Theoretical studies

This paper were investigated spectroscopic studies of 1-(4-Chlorophenyl) piperazine (14CPP) with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and Raman techniques. The conformational analysis, vibrational spectra, vibrational assignments and nuclear magnetic shielding tensors of title molecule were examined by the density functional theory (DFT), using the Becke-3-Lee-Yang-Parr (B3LYP) functional and the 6-311++G(d,p) basis sets. Moreover, energy gap values (HOMO-LUMO) of title molecule was performed using TD-DFT/B3LYP/6-311++G(d,p) method. There is a good agreement between the experimentally obtained data and theoretically obtained data.

Magnetically Induced Current Density Spatial Domains.

The pseudostagnation graphs of the current density induced by a magnetic field perpendicular to the plane of the carbon atoms of benzene and cyclopropane, chosen as archetypal of π- and σ-aromatic molecules, have been worked out. Saddle nodes forming the saddle stagnation lines of the pseudostagnation graph have been connected by means of trajectories of the current density, which delimit the border of four different sectors of flow around each saddle line. The merging of all such boundary trajectories provides local zero-flux surfaces in the pseudocurrent density, i.e., separatrices, which split the induced current density field in a number of three-dimensional domains of flow. As a result, a new partition scheme, corresponding to the physical flow that one could observe, has been obtained. For benzene and cyclopropane, by integration of the current density over the various spatial domains, the parallel component of the nuclear magnetic shielding and magnetizability tensors has been decomposed into contributions that are hard to reconcile with any atomic models of partition.

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.

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.

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.

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.

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.

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.

Chiral discrimination in NMR spectroscopy.

Nuclear magnetic resonance is the most important form of molecular spectroscopy in chemistry and biochemistry but it is normally blind to chirality. It was predicted in 2004 that precessing nuclear spins in chiral molecules in a liquid in a strong magnetic field induce a rotating electric polarization that is of opposite sign for enantiomers. This polarization arises from the distortion of the electronic structure by the nuclear magnetic moment in the presence of the strong magnetic field and is equivalent to the linear effect of an electric field on the nuclear shielding tensor. The polarization is strongly enhanced in dipolar molecules through the partial orientation of the permanent dipole through the antisymmetric part of the nuclear magnetic shielding tensor. Alternatively, an applied electric field will induce a chirally sensitive magnetization perpendicular to the field and to the nuclear spin. Progress towards the experimental realization of chiral discrimination by NMR is assessed.

Four-Component Relativistic Density-Functional Theory Calculations of Nuclear Spin-Rotation Constants: Relativistic Effects in p-Block Hydrides.

We present an implementation of the nuclear spin-rotation (SR) constants based on the relativistic four-component Dirac-Coulomb Hamiltonian. This formalism has been implemented in the framework of the Hartree-Fock and Kohn-Sham theory, allowing assessment of both pure and hybrid exchange-correlation functionals. In the density-functional theory (DFT) implementation of the response equations, a noncollinear generalized gradient approximation (GGA) has been used. The present approach enforces a restricted kinetic balance condition for the small-component basis at the integral level, leading to very efficient calculations of the property. We apply the methodology to study relativistic effects on the spin-rotation constants by performing calculations on XHn (n = 1-4) for all elements X in the p-block of the periodic table and comparing the effects of relativity on the nuclear SR tensors to that observed for the nuclear magnetic shielding tensors. Correlation effects as described by the density-functional theory are shown to be significant for the spin-rotation constants, whereas the differences between the use of GGA and hybrid density functionals are much smaller. Our calculated relativistic spin-rotation constants at the DFT level of theory are only in fair agreement with available experimental data. It is shown that the scaling of the relativistic effects for the spin-rotation constants (varying between Z(3.8) and Z(4.5)) is as strong as for the chemical shieldings but with a much smaller prefactor.

A theoretical study of potentially observable chirality-sensitive NMR effects in molecules.

Two recently predicted nuclear magnetic resonance effects, the chirality-induced rotating electric polarization and the oscillating magnetization, are examined for several experimentally available chiral molecules. We discuss in detail the requirements for experimental detection of chirality-sensitive NMR effects of the studied molecules. These requirements are related to two parameters: the shielding polarizability and the antisymmetric part of the nuclear magnetic shielding tensor. The dominant second contribution has been computed for small molecules at the coupled cluster and density functional theory levels. It was found that DFT calculations using the KT2 functional and the aug-cc-pCVTZ basis set adequately reproduce the CCSD(T) values obtained with the same basis set. The largest values of parameters, thus most promising from the experimental point of view, were obtained for the fluorine nuclei in 1,3-difluorocyclopropene and 1,3-diphenyl-2-fluoro-3-trifluoromethylcyclopropene.

Relativistic theory of nuclear spin-rotation tensor with kinetically balanced rotational London orbitals.

Both kinetically balanced (KB) and kinetically unbalanced (KU) rotational London orbitals (RLO) are proposed to resolve the slow basis set convergence in relativistic calculations of nuclear spin-rotation (NSR) coupling tensors of molecules containing heavy elements [Y. Xiao and W. Liu, J. Chem. Phys. 138, 134104 (2013)]. While they perform rather similarly, the KB-RLO Ansatz is clearly preferred as it ensures the correct nonrelativistic limit even with a finite basis. Moreover, it gives rise to the same "direct relativistic mapping" between nuclear magnetic resonance shielding and NSR coupling tensors as that without using the London orbitals [Y. Xiao, Y. Zhang, and W. Liu, J. Chem. Theory Comput. 10, 600 (2014)].

Revisiting NMR through-space J(FF) spin-spin coupling constants for getting insight into proximate F---F interactions.

At present times it is usual practice to mark biological compounds replacing an H for an F atom to study, by means of (19)F NMR spectroscopy, aspects such as binding sites and molecular folding features. This interesting methodology could nicely be improved if it is known how proximity interactions on the F atom affect its electronic structure as gauged through high-resolution (19)F NMR spectroscopy. This is the main aim of the present work and, to this end, differently substituted peri-difluoronaphthalenes are chosen as model systems. In such compounds are rationalized some interesting aspects of the diamagnetic and paramagnetic parts of the (19)F nuclear magnetic shielding tensor as well as the transmission mechanisms for the PSO and FC contributions to (4)JF1F8 indirect nuclear spin-spin coupling constants.