Calculated Shielding Constants Research Articles

On the geometry dependence of the nuclear magnetic resonance chemical shift of mercury in thiolate complexes: A relativistic density functional theory study.

Thiolate containing mercury(II) complexes of the general formula [Hg(SR) ] have been of great interest since the toxicity of mercury was recognized. 199Hg nuclear magnetic resonance spectroscopy (NMR) is a powerful tool for characterization of mercury complexes. In this work, the Hg shielding constants in a series of [Hg(SR) ] complexes are therefore investigated computationally with particular emphasis on their geometry dependence. Geometry optimizations and NMR chemical shift calculations are performed at the density functional theory (DFT) level with both the zeroth-order regular approximation (ZORA) and four-component relativistic methods. The four exchange-correlation (XC) functionals PBE0, PBE, B3LYP, and BLYP are used in combination with either Dyall's Gaussian-type (GTO) or Slater-type orbitals (STOs) basis sets. Comparing ZORA and four-component calculations, one observes that the calculated shielding constants for a given molecular geometry have a constant difference of 1070 ppm. This confirms that ZORA is an acceptable relativistic method to compute NMR chemical shifts. The combinations of four-component/PBE0/v3z and ZORA/PBE0/QZ4P are applied to explore the geometry dependence of the isotropic shielding. For a given coordination number, the distance between mercury and sulfur is the key factor affecting the shielding constant, while changes in bond and dihedral angles and even different side groups have relatively little impact.

Accurate Prediction of Nuclear Magnetic Resonance Parameters via the XYG3 Type of Doubly Hybrid Density Functionals.

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and versatile tools in elucidating molecular structures. To eliminate ambiguities of experimental assignments, accurate calculations of NMR spectra are of great importance. Here, a method for theoretical evaluation of the NMR shielding constants by analytic derivatives using gauge including atomic orbitals (GIAO) has been implemented for the XYG3 type of doubly hybrid density functionals (xDH), namely, the GIAO-xDH method. Benchmark calculations on shielding constants and chemical shifts demonstrate the remarkable accuracy of the GIAO-xDH method, compared to the accurate CCSD(T) references. It is shown here that the XYGJ-OS functional is able to give a mean absolute deviation (MAD) of ∼3.0 ppm in the calculated shielding constants for 13C, 15N, 17O, 19F, while both XYGJ-OS and xDH-PBE0 functionals are able to provide a satisfactory estimation of chemical shifts with MADs of ∼0.03 and 1.0 ppm for 1H and 13C, respectively. The basis set influence upon the method has been examined and a computational scheme considering both accuracy and efficiency has been proposed and tested to predict the experimental 13C chemical shifts of five medium-sized natural product molecules, yielding a MAD of ∼1.0 ppm, which demonstrates the practical feasibility of the GIAO-xDH method.

The best density functional theory functional for the prediction of 1 H and 13 C chemical shifts of protonated alkylpyrroles.

The prediction of 13 C chemical shifts can be challenging with density functional theory (DFT). In this study 39 different functionals and three different basis sets were tested on three neutral alkylpyrroles and their corresponding protonated species. The calculated shielding constants were compared to experimental data and results from previous calculations at the MP2. We find that the meta-hybrid functional TPSSh with either the Pople style basis set 6-311++G(2d,p) or the polarization consistent basis set pcSseg-1 gives the best results for the 13 C chemical shifts, whereas for the 1 H chemical shifts it is the TPSSh functional with either the 6-311++G(2d,p) or pcSseg-2 basis set. Including an explicit solvent molecule hydrogen bonded to NH in the alkylpyrroles improves the results slightly for the 13 C chemical shifts. On the other hand, for 1 H chemical shifts the opposite is true. Compared to calculations at the MP2 level none of the DFT functionals can compete with MP2 for the 13 C chemical shifts but for the 1 H chemical shifts the investigated DFT functionals are shown to give better agreement with experiment than MP2 calculations.

Nuclear Magnetic Resonance parameters of mercury atom and water molecule complex: Relativistic calculations

Potential energy surface of the Hg⋯H2O dimer was studied by means of CCSD(T) and DFT methods. After that nuclear shielding constants and nuclear spin-spin coupling constants have been calculated at the DFT level. All the calculations were carried out with four-component relativistic Dirac-Coulomb Hamiltonian. DFT produces a global minimum with mercury atom on C2v axis of water molecule, but this is not confirmed by CCSD(T). All of the calculated shielding constants increase with the increase in the distance between mercury and oxygen atoms. σHg at equilibrium geometry is 12 ppm smaller than in isolated mercury atom.

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.

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density Functional Theory Calculations.

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solid-state 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state 45Sc NMR spectroscopy.

Direct Arylation of Adenine by Fluoro- and Chloronitrobenzenes: Effect of Microwaves

Direct arylation of adenine with fluoro- and chloronitrobenzenes leads to mixtures of N9 and N7 substituted adenines. After separation by column chromatography, the individual isomers can be efficiently hydrogenated on Pd to give the corresponding aminophenyladenines. A significant enhancement of the reaction rate by microwave irradiation was observed. This two-step procedure was found to be a feasible route to otherwise hardly available 7-aminophenyladenines. Correlation between calculated shielding constants and experimental values of chemical shifts in 13C and 15N NMR was used for assignment of the site of substitution. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]

Direct Arylation of Adenine by Fluoro- and Chloronitrobenzenes: Effect of Microwaves

Abstract Direct arylation of adenine with fluoro- and chloronitrobenzenes leads to mixtures of N9 and N7 substituted adenines. After separation by column chromatography, the individual isomers can be efficiently hydrogenated on Pd to give the corresponding aminophenyladenines. A significant enhancement of the reaction rate by microwave irradiation was observed. This two-step procedure was found to be a feasible route to otherwise hardly available 7-aminophenyladenines. Correlation between calculated shielding constants and experimental values of chemical shifts in 13C and 15N NMR was used for assignment of the site of substitution. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]

1H, 13C MAS NMR and DFT GIAO study of quercetin and its complex with Al(III) in solid state

Quercetin (Q) as a pentahydroxy flavonoid, has three possible chelating sites competing in complexation processes. 1H and 13C MAS NMR spectra were recorded for solid quercetin and its Al(III) complex (AlQ). 1H MAS spectrum of quercetin shows a broad resonance at ca. 12ppm that confirms the existence of intramolecular C5OH … OC4 hydrogen bond. Such a signal is absent in the spectrum of AlQ, which is in accordance with other spectroscopic data and the suggested model for the solid-state structure of the complex. DFT GIAO calculations were used to verify the experimental 13C CPMAS NMR data and to suggest the best model structure for the complex AlQ. The calculated shielding constants for different conformers of isolated quercetin molecules, quercetin trimer as taken from the X-ray data, and different model structures for possible Al(III) complexes were compared with the 13C CPMAS NMR experimental values. The results demonstrate the importance of intermolecular interactions when dealing with structures in solid state and the successful application of the combined DFT GIAO and 13C CPMAS NMR approach. All data confirm that the chelating site of Q in the solid complex AlQ involves the deprotonated C5OH and the C4O groups at ring C, in contrast to the available studies performed in solution.

Solvent Dependence of (14)N Nuclear Magnetic Resonance Chemical Shielding Constants as a Test of the Accuracy of the Computed Polarization of Solute Electron Densities by the Solvent.

Although continuum solvation models have now been shown to provide good quantitative accuracy for calculating free energies of solvation, questions remain about the accuracy of the perturbed solute electron densities and properties computed from them. Here we examine those questions by applying the SM8, SM8AD, SMD, and IEF-PCM continuum solvation models in combination with the M06-L density functional to compute the (14)N magnetic resonance nuclear shieldings of CH3CN, CH3NO2, CH3NCS, and CH3ONO2 in multiple solvents, and we analyze the dependence of the chemical shifts on solvent dielectric constant. We examine the dependence of the computed chemical shifts on the definition of the molecular cavity (both united-atom models and models based on superposed individual atomic spheres) and three kinds of treatments of the electrostatics, namely the generalized Born approximation with the Coulomb field approximation, the generalized Born model with asymmetric descreening, and models based on approximate numerical solution schemes for the nonhomogeneous Poisson equation. Our most systematic analyses are based on the computation of relative (14)N chemical shifts in a series of solvents, and we compare calculated shielding constants relative to those in CCl4 for various solvation models and density functionals. While differences in the overall results are found to be reasonably small for different solvation models and functionals, the SMx models SM8, and SM8AD, using the same cavity definitions (which for these models means the same atomic radii) as those employed for the calculation of free energies of solvation, exhibit the best agreement with experiment for every functional tested. This suggests that in addition to predicting accurate free energies of solvation, the SM8 and SM8AD generalized Born models also describe the solute polarization in a manner reasonably consistent with experimental (14)N nuclear magnetic resonance spectroscopy. Models based on the nonhomogeneous Poisson equation show slightly reduced accuracy. Scaling the intrinsic Coulomb radii to larger values (as has sometimes been suggested in the past) does not uniformly improve the results for any kind of solvent model; furthermore it uniformly degrades the results for generalized Born models. Use of a basis set that increases the outlying charge diminishes the accuracy of continuum models that solve the nonhomogeneous Poisson equation, which we ascribe to the inability of the numerical schemes for approximately solving the nonhomogeneous Poisson equation to fully account for the effects of electronic charge outside the solute cavity.

NMR shielding constants in BF3 and magnetic dipole moments of B11 and B10 nuclei

Gas-phase NMR spectra of (11)B, (10)B, and (19)F in BF(3) are reported, and high-level ab initio calculations of the corresponding NMR shielding constants are described. Extrapolation of the measured resonance frequencies to the zero-density limit ensures that the results correspond to the ab initio values for an isolated molecule. Simultaneous measurements of (3)He resonance frequencies and application of the calculated shielding constants allow us to determine improved values of the nuclear magnetic dipole moments of (11)B and (10)B. The magnetic moments of both isotopes are also determined independently by comparing with the (19)F spectral parameters (frequencies and shielding constants). The separately derived nuclear magnetic moments are in good agreement, whereas the literature moments of both (11)B and (10)B are noticeably less accurate.

Mg-25 ultra-high field solid state NMR spectroscopy and first principles calculations of magnesium compounds

Due to sensitivity problems, (25)Mg remains a largely under-explored nucleus in solid state NMR spectroscopy. In this work at an ultrahigh magnetic field of 21.1 T, we have studied at natural abundance the (25)Mg solid state (SS) NMR spectra for a number of previously unreported magnesium compounds with known crystal structures. Some previously reported compounds have been revisited to clarify the spectra that were obtained at lower fields and were either not sufficiently resolved, or misinterpreted. First principles calculations of the (25)Mg SS NMR parameters have been carried out using plane wave basis sets and periodic boundary conditions (CASTEP) and the results are compared with experimental data. The calculations produce the (25)Mg absolute shielding scale and give us insight into the relationship between the NMR and structural parameters. At 21.1 T the effects of the quadrupolar interactions are reduced significantly and the sensitivity and accuracy in determining chemicals shifts and quadrupole coupling parameters improve dramatically. Although T(1) measurements were not performed explicitly, these proved to be longer than assumed in much of the previously reported work. We demonstrate that the chemical shift range of magnesium in diamagnetic compounds may approach 200 ppm. Most commonly, however, the observed shifts are between -15 and +25 ppm. Quadrupolar effects dominate the (25)Mg spectra of magnesium cations in non-cubic environments. The chemical shift anisotropy appears to be rather small and only in a few cases could the contribution of the CSA be detected reliably. A good correspondence between the calculated shielding constants and experimental chemical shifts was obtained, demonstrating the good potential of computational methods in spectroscopic assignments of solid state (25)Mg NMR spectroscopy.

13C CP MAS NMR and GIAO-CHF/DFT calculations of flavonoids: Morin, kaempferol, tricin, genistein, formononetin and 3,7-dihydroxyflavone

13C CP MAS NMR spectra of the flavonoids: morin, kaempferol, 3,7-dihydroxyflavone, tricin and isoflavones: genistein and formononetin were recorded to characterize solid-state conformations. Intramolecular hydrogen bonds forming five-, six- and seven-membered rings are present in the two morin molecules in the crystals – their 13C resonances have been assigned with the aid of the calculated shielding constants. Linear relationships between the calculated shielding constants σ DFT (ppm) and chemical shifts ( δ CPMAS, ppm) were obtained for all studied compounds. Higher correlation coefficients suggest that the conformation with “clockwise” orientation of both OH groups is more probable in the solid 3,7-dihydroxyflavone, whereas in the solid formononetin the OH and OCH 3 substituents are directed “anticlockwise”. The barrier to the rotation of phenyl ring B decreases in the order: morin (2′-OH, 3-OH) > kaempferol (3-OH) > tricin.

A conformational study of aromatic imide compounds. part 1. Compounds containing diphenyl ether and benzophenone moieties

An attempt was made to estimate the dihedral angles φ, ψ, ω 1, and ω 2, of bis(4-hydroxyphthalimide)s (BHPI) and bis(phenylphthalimides)s (BPI) having diphenyl ether or benzophenone linkages at the center of molecules using solid-state 13C CP/MAS NMR and ab initio nuclear shielding calculations. The TOSS and TOSS & DD pulse sequences were performed in the NMR measurements to obtain exact chemical shifts of each carbon. Total energies were calculated using the B3LYP/6-31G(d) level of theory, and shielding constants were calculated using the RHF/6-31G(d) level of theory for diphenyl ether and benzophenone by varying the angles of φ, ψ from 0 to 180° at intervals of 10°. The comparison of nuclear shielding calculations between RHF/6-31G(d) and B3LYP/6-311+G(2d,p) levels of theory indicates that RHF/6-31G(d) has sufficient reliability for calculating shielding constants of diphenyl structures. Since the nuclear shieldings are displaced depending on the dihedral angles, the dihedral angles of BHPIs and BPIs can be estimated by comparing the experimental chemical shifts with the calculated shielding constants. The BHPIs, BPIs and a related polyimide ODPA/ODA were estimated to have symmetric conformations having identical φ and ψ and identical ω 1 and ω 2.

Calculation of nuclear magnetic shieldings. XI. Vibrational motion effects

Nuclear magnetic shieldings in first- and second-row hydrides were calculated with electron correlation taken into account through third order. The calculation was performed using London’s gauge-invariant atomic orbitals (GIAOs) and finite-field Mo/ller–Plesset perturbation theory (FF-MPPT). Furthermore, the vibrational motion corrections to the magnetic shieldings were evaluated. It was shown that the calculated isotropic shielding constants at the experimental geometries are higher than the experimental values, but that vibrational corrections are generally negative and improve the calculated shielding constants.

Theoretical Study of 13C and 17O NMR Shielding Tensors in Transition Metal Carbonyls Based on Density Functional Theory and Gauge-Including Atomic Orbitals

A theoretical study has been carried out on 13C and 17O NMR shielding tensors in M(CO)6 (M = Cr, Mo, and W), Fe(CO)5, and Ni(CO)4. The study was based on modern density functional theory (DFT) and gauge-including atomic orbitals (GIAO). The calculated shielding constants and tensors are in good agreement with the available experimental data. The difference in the isotropic shielding Δδ = σCO − σM(CO)n between free CO, σCO, and CO as a ligand, σM(CO)n was analyzed in detail. It was shown that the stretch of CO on coordination as well as the back-donation from dπ on the metal to π*CO of the CO ligand has a positive contribution to the coordination shift Δδ whereas the donation from the σCO HOMO of CO to the metal orbitals has a negative contribution to Δδ.

Non‐empirical calculations of the nuclear magnetic resonance spectra of imidazole and hydrated imidazole

AbstractThe magnetic shielding constants of the 1H, 13C and 15N nuclei of imidazole are calculated for the isolated and hydrated molecules. The results show that the hydrogen bonds produce not only large variations of the chemical shifts for the nitrogen nuclei and the NH proton which are directly involved in the intermolecular bonding, but also measurable shifts for the carbon nuclei. The calculated shielding constants and their variation with hydration are discussed in relation to experimental results concerning imidazole, the 5‐membered ring of the purine bases and the imidazole ring of histidine. The calculated values of the spin‐spin coupling constants confirm that it is possible to study the tautomeric equilibrium of the imidazole ring from the measurement of these coupling constants and that spin‐spin coupling constants are not very sensitive to solvent effects.

13C NMR studies on M(CN)5Xn− complexes

The 13C chemical shifts for a variety of M(CN)5Xn− complexes are reported. These chemical shifts correlate extremely well with the MC and CN stretching frequencies and Mössbauer isomer shifts and quadrupole splitting values for the iron complexes. However, the calculated shielding constants do not correlate well with the observed shifts.

Proton magnetic shielding in H2O and (H2O)2

The proton magnetic shielding constants in the water molecule and its linear perpendicular dimer are computed from SCF-MO-LCGO wave functions by using the uncoupled Hartree-Fock variation-perturbation procedure due to Karplus and Kolker. The convergence of the calculated shielding constants as well as their gauge dependence is studied. The final results for 17-term polynomial variation function indicate that the best choice for the gauge origin corresponds to the molecular electronic centroid.