Nuclear Magnetic Resonance Chemical Shielding Research Articles

Analytical Second-Order Properties for the Random Phase Approximation: Nuclear Magnetic Resonance Shieldings.

A method for the analytical computation of nuclear magnetic resonance (NMR) shieldings within the direct random phase approximation (RPA) is presented. As a starting point, we use the RPA ground-state energy expression within the resolution-of-the-identity approximation in the atomic-orbital formalism. As has been shown in a recent benchmark study using numerical second derivatives [Glasbrenner, M. J. Chem. Theory Comput. 2022, 18, 192], RPA based on a Hartree-Fock reference shows accuracies comparable to coupled cluster singles and doubles (CCSD) for NMR chemical shieldings. Together with the much lower computational cost of RPA, it has emerged as an accurate method for the computation of NMR shieldings. Therefore, we aim to extend the applicability of RPA NMR to larger systems by introducing analytical second-order derivatives, making it a viable method for the accurate and efficient computation of NMR chemical shieldings.

DFT investigations of AgMC7H10N2 (M = Cl, Br, and I) metal organic molecules: NMR, optoelectronic, and transport properties.

The full-potential linearized augmented plane wave (FP-LAPW) method was used for the calculation of the structural, nuclear magnetic resonance (NMR), optoelectronic, and thermoelectric properties of AgMC7H10N2 (M = Cl, Br, and I) compounds. The calculated wide band gap of AgMC7H10N2 (M = Cl, Br, and I) metal organic molecules with the density of states approach were 3.32, 3.29, and 3.10eV, respectively. The NMR parameters are calculated for the Ag, Cl, Br, I, C, N, O, and H elements. It is found that by decreasing bandgap, the isotropic NMR chemical shielding values of Cl, Br, and I elements increase. The strong hybridization of Ag-4d, Cl-3p, Br-4p, and I-5p states are observed at the top of the valence band. The birefringence and anisotropic properties are observed in the optical spectra with high plasmon energies, and the figure of merit, ZT, of 0.98 for AgCl(C7H10N2) compound is found at 300K. Hence, these compounds are attractive flexible metal organic molecules for optoelectronic and transport applications.

Importance of Nuclear Quantum Effects for NMR Crystallography.

The resolving power of solid-state nuclear magnetic resonance (NMR) crystallography depends heavily on the accuracy of computational predictions of NMR chemical shieldings of candidate structures, which are usually taken to be local minima in the potential energy. To test the limits of this approximation, we systematically study the importance of finite-temperature and quantum nuclear fluctuations for 1H, 13C, and 15N shieldings in polymorphs of three paradigmatic molecular crystals: benzene, glycine, and succinic acid. The effect of quantum fluctuations is comparable to the typical errors of shielding predictions for static nuclei with respect to experiments, and their inclusion improves the agreement with measurements, translating to more reliable assignment of the NMR spectra to the correct candidate structure. The use of integrated machine-learning models, trained on first-principles energies and shieldings, renders rigorous sampling of nuclear fluctuations affordable, setting a new standard for the calculations underlying NMR structure determinations.

Al27 NMR chemical shift of Al(OH)4 - calculated from first principles: Assessment of error cancellation in chemically distinct reference and target systems.

Predicting accurate nuclear magnetic resonance chemical shieldings relies upon cancellation of different types of errors between the theoretically calculated shielding constant of the analyte of interest and the reference. Often, the intrinsic error in computed shieldings due to basis sets, approximations in the Hamiltonian, description of the wave function, and dynamic effects is nearly identical between the analyte and reference, yet if the electronic structure or sensitivity to local environment differs dramatically, this cannot be taken for granted. Detailed prior work has examined the octahedral trivalent cation Al(H2O)6 3+, accounting for ab initio intrinsic errors. However, the use of this species as a reference for the chemically distinct tetrahedral anion Al(OH)4 - requires an understanding of how these errors cancel in order to define the limits of accurately predicting Al27 chemical shielding in Al(OH)4 -. In this work, we estimate the absolute shielding of the Al27 nucleus in Al(OH)4 - at the coupled cluster level (515.1 ± 5.3 ppm). Shielding sensitivity to the choice of method approximation and atomic basis sets used has been evaluated. Solvent and thermal effects are assessed through ensemble averaging techniques using ab initio molecular dynamics. The contribution of each type of intrinsic error is assessed for the Al(H2O)6 3+ and Al(OH)4 - ions, revealing significant differences that fundamentally hamper the ability to accurately calculate the Al27 chemical shift of Al(OH)4 - from first principles.

Structure Identification of Novel Compounds Using Simple IR, 1H, and 13C NMR Spectroscopy and Computational Tools

A simple method is suggested to identify the structure of novel compounds with basic IR, 1H, and 13C nuclear magnetic resonance (NMR) spectroscopy and computational tools. With the molecular formula obtained from high‐resolution mass spectrometry, all possible isomers are calculated. The absence or presence of particular functional groups which were inferred from IR, 1H, and 13C NMR and DEPT spectra are used to sort all the possible isomers using SMARTS code and RDKit. Then, the final structure(s) are identified by comparing the correlation between Quantum mechanically calculated 13C NMR chemical shielding constants and experimental 13C chemical shifts of molecules in consideration. We have applied this protocol to five natural compounds, the number of heavy atoms ranging from 11 to 15, and correctly identified the structures of all test cases. The limitations and further consideration are discussed.

Comparison between optoelectronic spectra and NMR shielding in tellurium based compounds: a FP-LAPW study

The Nuclear Magnetic Resonance (NMR) of 125Te atom and optoelectronic spectra in the X2TeO3 (X = Na, K, Ag) and PbTeO3 compounds are investigated by the modified Becke-Johnson (mBJ) potential. Obtained magnetic shielding and the experimental chemical shifts are in close agreement. The X ions control polarity of the magnetic shielding around the 125Te nucleus and σiso(X) increases from Na2TeO3 to PbTeO3 compounds. Calculated band gap of X2TeO3 (X = Na, K, Ag) and PbTeO3 compounds are 3.23, 0.24, 3.63 and 3.95 eV, respectively. Density of states results show that the X1,2-p states in the bottom of the valence band have an essential role in the value of 125Te NMR chemical shielding. The magnetic shielding of X atoms governs on the optical properties, such as the static dielectric function, refractive indexes and plasmon energies.

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.

Chemical shielding of doped nitrogen on C20 cage and bowl fullerenes

The C20 (cage), C20 (bowl), C20H10 (bowl) fullerene structures and their nitrogen doped derivatives such as C20NH (cage), C20NH (bowl), C20H10N (bowl), C20H10NH (bowl) are fully optimized at the MPW1PW91/6-31G level of theory. The natural atomic charge comparison shows that in C20H10N (bowl), the nitrogen atom with about–0.58 has a more negative charge with respect to other nitrogen doped structures. The nuclear magnetic resonance chemical shielding is evaluated for nitrogen doped structures and the neighbors connected to nitrogen, C6, and C7 atoms. The nitrogen atom doped on carbon sites of C20H10N (bowl) has the largest shielding isotropic shifts to the upper field (–203.58 ppm). This means that the electron density around nitrogen in the C20H10N (bowl) structure is higher. Interestingly, there is a significant correlation between the charges and σiso values of nitrogen and carbon atoms (C6 and C7). Namely, as the charge becomes more negative, σiso shifts to the upper field. It is predicted that nitrogen doped C20H10N (bowl) with the maximum electron density adopts this structure for electrophilic reactions.

ChemInform Abstract: NMR Studies of the Dynamic Motion of Encapsulated Ions and Clusters in Fullerene Cages: A Wheel Within a Wheel

AbstractReview: 37 refs.

Ab initio calculations of 13C NMR chemical shielding in some N4O2, N4S2 and N6 Schiff base ligands containing piperazine moiety

The calculation of 13 C isotropic shielding constants by means of GIAO and CSGT methods of eight Schiff base ligands containing piperazine moiety at the Hartree-Fock and B3LYP levels of theory are presented. Good linear correlations between the calculated chemical shielding at gas-phase and experimental shift values in CDCl 3 solution were obtained. Density functional theory (DFT) calculations at the B3LYP/6-31G(2d,p) level of theory is used to optimize the geometry of ligands. Calculated nuclear magnetic resonance (NMR) chemical shifts 13 C are reported for the some N 4 O 2 , N 4 S 2 and N 6 Schiff base ligands containing piperazine moiety. In order to establish a convenient and consistent protocol to be employed for confirming the experimental 13 C NMR spectra of Schiff base ligands, different combinations of models and basis sets were considered. The most reliable results were obtained at B3LYP/6-311G++ (d,p) level and CSGT method which can be used to predict 13 C NMR chemical shifts with a very high accuracy for latter compounds. These results show the agreement between theoretical and experimental 13 C NMR chemical shielding of mentioned ligands.

NMR Studies of the Dynamic Motion of Encapsulated Ions and Clusters in Fullerene Cages: A Wheel Within a Wheel

Nuclear magnetic resonance (NMR) has provided an important approach to investigate the structure and dynamics of encapsulated metal ions and clusters in endohedral metallofullerenes (EMFs). In this paper, we review NMR studies of the environment and dynamics of dimetallic, trimetallic nitride, metal carbide and metal cyanide clusters encapsulated in EMFs. The NMR chemical shielding parameter is a sensitive probe for monitoring the carbon cage (13C) and encapsulated clusters (13C, 14N, 139La, 45Sc, and 89Y). Future NMR EMF studies could be very fruitful and help elucidate coupled motion between the carbon cage and encapsulated cluster or other “wheel within a wheel” motional processes.

Theoretically describing the 17O magnetic shielding constant of biomolecular systems: uracil and 5-fluorouracil in water environment

The nuclear magnetic resonance chemical shielding of 17O is of great importance for biomolecular characterization in water environment. In these systems, oxygen atoms occupy important positions and are involved in hydrogen bonds with the water environment. In this work, different solvation models are used for the theoretical determination of the 17O chemical shielding of the nucleobase uracil and the substituted 5-fluorouracil in aqueous environment. Continuum, discrete and explicit solvent models are used, and an analysis is made of the role played by the solute polarization due the solvent. The best results are obtained using the sequential quantum mechanics/molecular mechanics methodology using an iterative procedure for the solute polarization, but a good compromise is obtained by using the electronic polarization provided by the polarizable continuum model. Quantum mechanical calculations of the chemical shieldings are made using density-functional theory in two different exchange–correlation approximations. Using an iterative procedure for the solute polarization and the mPW1PW91/aug-pcS-2 model in the electrostatic approximation, we obtained magnetic shielding constants for the two O atoms of uracil within 2 ppm of the experimental results. For 5-fluorouracil, the theoretical results, with the same model, are again in good agreement with the experimental values. An analysis of the influence of the solute–solvent hydrogen bonds in the chemical shielding of uracil case is also made, and it is concluded that the most important contribution to the calculated shielding derives from the electrostatic contribution to the solute–solvent interaction.

Theoretical investigation of azo dyes adsorbed on cellulose fibers: 2. Spectroscopic study

Electronic absorption spectra and the frontier orbitals of 1-arylazo-2-naphtol dyes are computed and analyzed in four models, namely in the gas phase (model I), in a solvent (model I + CPCM), adsorbed on the cellulose surface (model II), and model II in the presence of solvent (model II + CPCM) via time-dependent density functional theory (TD-DFT) and conductor-like polarizable continuum model (CPCM) at the B3LYP/6-31G** level of theory. A bathochromic shift is observed for the λmax peak due to both short-range and long-range interactions of the non-ionic dyes with cellulose, while the ionic dyes exhibit hypsochromic shift in their λmax peak. The results predict that the studied dyes should be nearly yellow after being adsorbed on cellulose with excellent color strength. Furthermore, the ionic dyes are suitable for the dyeing of cellulose fibers. The nuclear magnetic resonance (NMR) chemical shieldings calculated for the azo dyes in the gas phase and adsorbed states and for their tautomeric equilibrium mixtures show that the NMR technique can be used successfully to follow the dyeing process.

A linear- and sublinear-scaling method for calculating NMR shieldings in atomic orbital-based second-order Møller-Plesset perturbation theory

An atomic-orbital (AO) based formulation for calculating nuclear magnetic resonance chemical shieldings at the second-order Møller-Plesset perturbation theory level is introduced, which provides a basis for reducing the scaling of the computational effort with the molecular size from the fifth power to linear and for a specific nucleus to sublinear. The latter sublinear scaling in the rate-determining steps becomes possible by avoiding global perturbations with respect to the magnetic field and by solving for quantities that involve the local nuclear magnetic spin perturbation instead. For avoiding the calculation of the second-order perturbed density matrix, we extend our AO-based reformulation of the Z-vector method within a density matrix-based scheme. Our pilot implementation illustrates the fast convergence with respect to the required number of Laplace points and the asymptotic scaling behavior in the rate-determining steps.

First-principles calculations of 17O nuclear magnetic resonance chemical shielding in Pb(Zr1/2Ti1/2)O3 and Pb(Mg1/3Nb2/3)O3: Linear dependence on transition-metal/oxygen bond lengths

First-principles density functional theory oxygen chemical shift tensors were calculated for A(B,B′)O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). Quantum chemistry methods for embedded clusters and the gauge including projector augmented waves (GIPAW) method [C. J. Pickard and F. Mauri, Phys. Rev. B 63, 245101 (2001)]10.1103/PhysRevB.63.245101 for periodic boundary conditions were used. Results from both methods are in good agreement for PZT and prototypical perovskites. PMN results were obtained using only GIPAW. Both isotropic δiso and axial δax chemical shifts were found to vary approximately linearly as a function of the nearest-distance transition-metal/oxygen bond length, rs. Using these results, we argue against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements. Our findings indicate that 17O NMR measurements, coupled with first-principles calculations, can be an important probe of local structure in complex perovskite solid solutions.

Solid-state 207Pb nuclear magnetic resonance studies of adducts of 1,10-phenanthroline with lead(II) halides

Experimental and predicted 207Pb nuclear magnetic resonance (NMR) parameters of solid-state coordination complexes of lead(II) bromide, lead(II) iodide, and lead(II) chloride with 1,10-phenanthroline are reported. Structural and electronic differences between the 1:1 and 1:2 adducts are reflected by the NMR parameters. The NMR chemical shieldings of the isostructural complexes indicate that the electronic state of the lead(II) center depends on the number of ligands and the halogen anion. The NMR results suggest that the 1:2 adduct may be more stereochemically active than the 1:1 adduct.

On the choice of optimal protocol for calculation of 13C and 15N NMR isotropic chemical shifts in nitramine systems

Calculated nuclear magnetic resonance (NMR) chemical shifts ( 13C and 15N) are reported for the RDX conformers and additional nine cyclic and acyclic nitramines. In order to establish a convenient and consistent protocol to be employed for confirming the experimental 13C and 15N NMR spectra of nitramine compounds, different combinations of models and basis sets were considered. The most reliable results were obtained at B3LYP/6-311+G(2d,p) level and CSGT model and can be used to calculate 13C and 15N NMR chemical shifts with a very high accuracy for nitramine compounds. These results show that the agreement between theoretical and experimental 15N NMR chemical shieldings of nitramines could be used for the evaluation of the intrinsic relationship between structure and explosive properties.

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.

Dependence of 29Si NMR chemical shielding properties of precursor silicate species, Q 0 on its local structure at the pre-nucleation stages of zeolite synthesis – A DFT based computational correlation

The exploration for new zeolite structures with tailored framework architectures for enhanced catalytic applications requires the knowledge about their nucleation and crystallization at molecular level. Nuclear magnetic resonance (NMR) is one of the most widely tried techniques to understand this. However, by NMR, it is difficult to accurately assign the molecular level precursor silicate structures at the pre-nucleation stages of zeolite synthesis. Hence, understanding the chemical shielding of such precursor molecules using quantum mechanical (QM) computations is extremely useful. Alkali is a fundamental component in the alkali based hydrothermal zeolite synthesis and its nature plays a major role. In the present report, we attempt to understand the differences in the local structure of the primary building block such as Si(OH) 4 (Q 0 silicate species) due to the associated alkali and their influence on NMR chemical shielding properties. Present work reports the calculation of 29Si NMR isotropic chemical shifts of Q 0 species with different cations such as Na, K and Ca using density functional theory (DFT). Results of natural bonding orbital (NBO) analysis, Perturbation theory energy analysis and electron density iso-surfaces were employed to obtain a deeper insight about their influence on the chemical shielding and on zeolite synthesis.