Nuclear Magnetic Shielding Research Articles

Performance of the LRESC Model on top of DFT Functionals for Relativistic NMR Shielding Calculations.

The linear response within the elimination of the small component model (LRESC) is an insightful and computationally efficient method for including relativistic effects on molecular properties like the nuclear magnetic shielding constants, spin-rotation constant, g-tensor, and electric field gradient of heavy atom containing molecules with atoms belonging up to the sixth row of the periodic table. One of its main advantages is its capacity to analyze the electronic origin of the different relativistic correcting terms. Until now, it was always applied on top of Hartree-Fock ground-state wave functions (LRESC/HF) to calculate and analyze NMR shieldings. In this work, we show the performance of the LRESC formalism on top of some density functional theory (DFT) functionals to compute tin shielding constants in SnX4 (X = H, F, Cl, Br, I) molecular systems. We analyze the performance of each LRESC/DFT scheme on reproducing the electronic mechanisms of the shieldings, taking as a benchmark the results of relativistic calculations at the RPA level of approach (4c/RPA). As in previous works, we divide the LRESC relativistic correcting terms into two groups: core-dependent and ligand-dependent contributions. It is shown here that core-dependent corrections are well-reproduced for the selected DFT functionals, but some differences arise in the ligand-dependent ones. We focus on the performance of different functionals, including the same electron correlation part but containing different amounts of HF exchange. The best results are obtained for the BHandHLYP functional (50% of HF exchange) and the worst for BLYP (0%). When the percentage of HF exchange increases, ligand-dependent contributions are better described, and the final LRESC/DFT results are closer to those obtained with LRESC/HF and 4c/RPA methods. The spin-orbit correction to the shielding constant is one of the main ligand-dependent contributions (there are two more) with total value depending on the amount of HF exchange included in the functional. When the amount of HF exchange decreases, the spin-orbit contribution becomes larger, overestimating the shielding constant even when nonrelativisitc DFT values are much smaller than the nonrelativistic HF ones, as it happens for the heaviest molecular system studied here (SnI4).

Multinuclear Magnetic Resonance Study of Sodium Salts in Water Solutions

The small amounts of gaseous 3He dissolved in low concentrated water solutions of NaCl, NaNO3 and NaClO4 were prepared and examined by 3He-, 23Na-, 35Cl- and 15N-NMR spectroscopy. This experimental data, along with new theoretical shielding factors, was used to measure the 23Na nuclear magnetic moment against that of helium-3 μ(23Na) = +2.2174997(111) in nuclear magnetons. The standard relationship between NMR frequencies and nuclear magnetic moments of observed nuclei was used. The nuclear magnetic shielding factors of 23Na cation were verified against that of counter ions present in water solutions. Very good agreement between shielding constants σ(3He), σ(23Na+), σ(35Cl‒), σ(35ClO4‒), σ(15NO3‒) in water at infinite dilution and nuclear magnetic moments was observed for all magnetic nuclei. It can be used as a reference nucleus for calculating a few other magnetic moments of different nuclei by the NMR method. An analysis of new and former μ(23Na) experimental data obtained by the atomic beam magnetic resonance method (ABMR) and other NMR measurements shows good replicability of all specified results. The composition of sodium water complexes was discussed in terms of chemical equilibria and NMR shielding scale.

Investigation of 205Tl NMR shielding, structural, and electronical properties in thallium halides by applying PBE‐GGA, YS‐PBE0 and mBJ functionals

We report the structural, electronical, and heavy nuclear 205 Tl Nuclear Magnetic Resonance (NMR) shielding properties of thallium halides TlX (X = F, Cl, Br, and I) by the first principles calculation. The Perdew-Burke-Ernzerhof Generalized Gradient Approximation, Yukawa Screened-PBE0 hybrid functional, and modified Becke-Johnson (mBJ) functionals including the relativistic and spin-orbit coupling effects are applied for calculation of the exchange-correlation potentials. Calculated PDOS spectra display that the valence band is composed of the X-s, Tl-5d, X-p, and Tl-6s states, and these states play an important role in 205 Tl NMR shielding. Our findings indicate that the nuclear magnetic shielding parameters depend on the electronic properties. Obtained results by mBJ show that there is a close agreement between the experimental and the calculated NMR parameters.

On the calculation of magnetic properties of nucleic acids in liquid water with the sequential QM/MM method

We present calculations of nuclear magnetic shieldings of the nucleic acids adenine, cytosine, guanine e thymine in liquid water environment in normal thermodynamic condition. The study is based on a sequential approach, where a classical Monte Carlo simulation of the solvated system is performed to generate the different solute–solvent configurations to be used in the following quantum mechanics calculations of magnetic properties. We find that the environment polarization effect shields the unprotonated nitrogens, with solvent shifts for the magnetic shieldings between +10.52 and +48.95 ppm. Protonated and amino nitrogens are less shielded by amounts of −17.77 and +0.22 ppm. The results obtained from supermolecule calculations show that protonated and amino nitrogen magnetic shieldings are particularly sensitive to the hydrogen bond interactions, with solvent shifts between −5.48 and −33.42 ppm, when the first microsolvation shell is explicitly considered. In addition, the supermolecular calculations also show a marked deshielding effect for the hydrogens of the protonated nitrogens, with solvent shifts between −2.43 and −5.12 ppm. There is an overall agreement between the theoretical predictions for the 1H and 15N chemical shifts and the available experimental results.

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.

NMR studies of daidzein and puerarin: active anti-oxidants in traditional Chinese medicine

Radix puerariae—a popular traditional Chinese medicine—is used for the treatment of diarrhea, acute dysentery, deafness, and cardiovascular diseases. It can also be used as an effective antioxidant and has been tested as an anticancer drug. Daidzein and puerarin are its main active compounds. The present contribution was focused on experimental and theoretical studies of the 1H and 13C NMR chemical shifts and nuclear magnetic shielding parameters of daidzein and puerarin. Experimental data were gained by exploring standard one-dimensional spectra and a set of two-dimensional measurements: COSY, HSQC, and HMBC. The theoretical gauge independent atomic orbital density functional theory supporting studies, were performed to determine shielding constants and chemical shifts in vacuum and methanol-d4 solvent. The correlation between experimental and theoretical data was fairly good, especially when the DFT/PBE0 approach was used. The molecular properties of daidzein and puerine related to antiradical activity were studied in the context of a single-step hydrogen atom transfer mechanism and its correlation with 13C NMR chemical shifts.Graphical abstractElectron spin density distribution for daidzein radical

Sublinear scaling quantum chemical methods for magnetic shieldings in large molecules.

It is shown that quantum chemical calculations of the nuclear magnetic shieldings of a group of vicinal nuclei in a large molecule can be made sublinear scaling [i.e., O(1)] with respect to the molecular size. This is achieved by making both the diamagnetic and paramagnetic terms translation invariant [following the work of Ochsenfeld et al. J. Chem. Phys. 134, 074102 (2011)] and by making use of a local representation that is independent of the molecular size. Although only Hartree-Fock results are reported here, the idea can readily be extended to correlated wave function methods as well as the relativistic domain.

The 13C nuclear magnetic shielding constants at the singlet excited states in CH2CCH2, CH2O, CH3CHO, CH3NH2, and CO

Abstract Quantum-chemical calculations of excited state nuclear shielding constants are reported for singlet states in CH2CCH2, CH2O, CH3CHO, CH3NH2, and CO. The multiconfigurational self-consistent field method is used to account for static correlation in the excited states. With one exception, the molecules considered had larger shielding constants in their excited states than in their ground states. This tendency towards deshielding is rationalized using the sum-over-states expression for magnetic response.

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.

Method Calibration or Data Fitting?

We investigate by explicit parameter optimization to what extent basis sets of polarized double-ζ quality can introduce compensating errors in five different density functional methods. It is shown that minor changes in the contraction coefficients of the valence functions in the basis sets can have a significant impact and allow different density functional methods to achieve very similar performances. This holds for nuclear magnetic shielding constants and for isomerization energies, barrier heights, and noncovalent interactions. It is furthermore shown that errors due to neglect of vibrational and solvent effects can be absorbed in the combined method and basis set errors. These findings hold for data sets consisting of 50-150 data points. This raises the question of whether the common practice of identifying combinations of density functional methods and basis sets that have a good performance against a selected set of reference data should be considered as data fitting in the combined parameter space spanned by the method and basis set.

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.

A polarizable QM/MM description of environment effects on NMR shieldings: from solvated molecules to pigment–protein complexes

We present an investigation on the role of polarization effects within hybrid QM/MM approaches in determining the nuclear magnetic shieldings of molecular systems either in solution or embedded in a protein matrix. An induced dipole formulation is used to account for the mutual polarization between the quantum and the classical subsystems. The two applications presented show that the inclusion of a polarizable embedding can indeed lead to an improvement in the description of the environment effects on the magnetic property especially when hydrogen bonding effects are possible between the nucleus of interest and some of the closest atoms of the environment.

NMR measurements and DFT studies of nuclear magnetic shielding in emodin and chuanxiongzine molecules

Emodin and chuanxiongzine are important active compounds of plants widely used in traditional Chinese medicine because of its well-known pharmacological and biological activity. In the present work the two compounds are studied using 1H, 13C and 14N NMR experiments and DFT calculations of nuclear magnetic shielding in the gaseous phase and in solution. The 1H and 13C experimental shielding values of emodin are directly compared with the appropriate results of calculations performed for an isolated molecule. It is shown that unknown inter- and intramolecular effects in shielding are significant and can be estimated within the range of ±6 ppm for carbons and ±3 ppm for hydrogen atoms. The similar comparison is also presented for the theoretical and experimental 1H and 13C chemical shifts of emodin. The 1H, 13C and 14N shieldings in chuanxiongzine molecule are analyzed by the same DFT calculations and NMR measurements as well. As shown the correlation between calculated and experimental shielding parameters is satisfactory especially when the PBE0/aug-cc-pVTZ approach is applied.

Current density tensors.

It is shown that nonsymmetric second-rank current density tensors, related to the current densities induced by magnetic fields and nuclear magnetic dipole moments, are fundamental properties of a molecule. Together with magnetizability, nuclear magnetic shielding, and nuclear spin-spin coupling, they completely characterize its response to magnetic perturbations. Gauge invariance, resolution into isotropic, deviatoric, and antisymmetric parts, and contributions of current density tensors to magnetic properties are discussed. The components of the second-rank tensor properties are rationalized via relationships explicitly connecting them to the direction of the induced current density vectors and to the components of the current density tensors. The contribution of the deviatoric part to the average value of magnetizability, nuclear shielding, and nuclear spin-spin coupling, uniquely determined by the antisymmetric part of current density tensors, vanishes identically. The physical meaning of isotropic and anisotropic invariants of current density tensors has been investigated, and the connection between anisotropy magnitude and electron delocalization has been discussed.

Influence of Solvent Representation on Nuclear Shielding Calculations of Protonation States of Small Biological Molecules.

In this study, we assess the influence of solvation on the accuracy and reliability of isotropic nuclear magnetic shielding calculations for amino acids in comparison to experimental data. We focus particularly on the performance of solvation methods for different protonation states, as biological molecules occur almost exclusively in aqueous solution and are subject to protonation with pH. We identify significant shortcomings of current implicit solvent models and present a hybrid solvation approach that improves agreement with experimental data by taking into account the presence of direct interactions between amino acid protonation state and water molecules.

Gauge-origin independent formalism of two-component relativistic framework based on unitary transformation in nuclear magnetic shielding constant.

This article proposes a gauge-origin independent formalism of the nuclear magnetic shielding constant in the two-component relativistic framework based on the unitary transformation. The proposed scheme introduces the gauge factor and the unitary transformation into the atomic orbitals. The two-component relativistic equation is formulated by block-diagonalizing the Dirac Hamiltonian together with gauge factors. This formulation is available for arbitrary relativistic unitary transformations. Then, the infinite-order Douglas-Kroll-Hess (IODKH) transformation is applied to the present formulation. Next, the analytical derivatives of the IODKH Hamiltonian for the evaluation of the nuclear magnetic shielding constant are derived. Results obtained from the numerical assessments demonstrate that the present formulation removes the gauge-origin dependence completely. Furthermore, the formulation with the IODKH transformation gives results that are close to those in four-component and other two-component relativistic schemes.

Convergence of nuclear magnetic shieldings and one-bond 1 J(11 B1 H) indirect spin-spin coupling constants in small boron molecules.

Self-consistent field Hartree-Fock, density functional theory, and coupled-cluster calculations of the nuclear magnetic shielding constants of BH and BH3 molecules have been conducted to characterize the convergence of individual results obtained with correlation-consistent and polarization-consistent basis sets. The individual 11 B and 1 H NMR parameters were estimated in the complete basis set limit and compared with benchmark results. Only the KT3 density functional accurately reproduced 11 B shielding in BH molecule.

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.