Carbon-13 chemical shift tensor measurements for nitrogen-dense compounds.

This paper reports the principal values of the C chemical shift tensors for five nitrogen-dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride).

Do Models beyond Hybrid Density Functionals Increase the Agreement with Experiment for Predicted NMR Chemical Shifts or Electric Field Gradient Tensors in Organic Solids?

predictions of chemical shifts and electric field gradient (EFG) tensor components are frequently used to help interpret solid-state nuclear magnetic resonance (NMR) experiments. Typically, these predictions employ density functional theory (DFT) with generalized gradient approximation (GGA) functionals, though hybrid functionals have been shown to improve accuracy relative to experiment. Here, the performance of a dozen models beyond the GGA approximation are examined for the prediction of solid-state NMR observables, including meta-GGA, hybrid, and double-hybrid density functionals and second-order Møller-Plesset perturbation theory (MP2).

The Scientific Method as a Scaffold to Enhance Communication Skills in Chemistry.

Scientific success in the field of chemistry depends upon the mastery of a wide range of soft skills, most notably scientific writing and speaking. However, training for scientific communication is typically limited at the undergraduate level, where students struggle to express themselves in a clear and logical manner. The underlying issue is deeper than basic technical skills; rather, it is a problem of students' unawareness of a fundamental and strategic framework for writing and speaking with a purpose.

Chemical Shift Tensors of Cimetidine Form A Modeled with Density Functional Theory Calculations: Implications for NMR Crystallography.

The principal components of the C chemical shift tensors for the ten crystallographically distinct carbon atoms of the active pharmaceutical ingredient cimetidine Form A have been measured using the FIREMAT technique. Density functional theory (DFT) calculations of C and N magnetic shielding tensors are used to assign the C and N peaks. DFT calculations were performed on cimetidine and a training set of organic crystals using both plane-wave and cluster-based approaches.

Study of Perfluorophosphonic Acid Surface Modifications on Zinc Oxide Nanoparticles.

In this study, perfluorinated phosphonic acid modifications were utilized to modify zinc oxide (ZnO) nanoparticles because they create a more stable surface due to the electronegativity of the perfluoro head group. Specifically, 12-pentafluorophenoxydodecylphosphonic acid, 2,3,4,5,6-pentafluorobenzylphosphonic acid, and (1H,1H,2H,2H-perfluorododecyl)phosphonic acid have been used to form thin films on the nanoparticle surfaces. The modified nanoparticles were then characterized using infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance spectroscopy.

Moving towards fast characterization of polymorphic drugs by solid-state NMR spectroscopy.

Solid-state nuclear magnetic resonance (SS-NMR) spectroscopy has become a common technique to study polymorphism in pharmaceutical solids at high-resolution. However, high-throughput application of high resolution SS-NMR spectroscopy is severely limited by the long H spin-lattice relaxation (T) that is common to solid phase compounds. Here, we demonstrate the use of paramagnetic relaxation reagents such as chromium (III) acetylacetonate (Cr(acac)) and nickel (II) acetylacetonate (Ni(acac)) for fast data acquisition by significantly reducing the T value for carbamazepine Forms I, II, III, and dihydrate, cimetidine Forms A and B, nabumetone Form I, and acetaminophen Form I polymorphs.

Measuring and Modeling Highly Accurate N Chemical Shift Tensors in a Peptide.

NMR studies measuring chemical shift tensors are increasingly being employed to assign structure in difficult-to-crystallize solids. For small organic molecules, such studies usually focus on C sites, but proteins and peptides are more commonly described using N amide sites. An important and often neglected consideration when measuring shift tensors is the evaluation of their accuracy against benchmark standards, where available.

Evaluating the accuracy of theoretical one-bond C─ C scalar couplings and their ability to predict structure in a natural product.

This study explores the feasibility of using a combination of experimental and theoretical 1-bond C─ C scalar couplings ( J ) to establish structure in organic compounds, including unknowns. Historically, J and J studies have emphasized 2 and 3-bond couplings, yet J couplings exhibit significantly larger variations. Moreover, recent improvements in experimental measurement and data processing methods have made J data more available.

Calculations of solid-state Ca NMR parameters: A comparison of periodic and cluster approaches and an evaluation of DFT functionals.

We present a computational study of magnetic-shielding and quadrupolar-coupling tensors of Ca sites in crystalline solids. A comparison between periodic and cluster-based approaches for modeling solid-state interactions demonstrates that cluster-based approaches are suitable for predicting Ca NMR parameters. Several model chemistries, including Hartree-Fock theory and 17 DFT approximations (SVWN, CA-PZ, PBE, PBE0, PW91, B3PW91, rPBE, PBEsol, WC, PKZB, BMK, M06-L, M06, M06-2X, M06-HF, TPSS, and TPSSh), are evaluated for the prediction of Ca NMR parameters.

Semi-empirical refinements of crystal structures using O quadrupolar-coupling tensors.

We demonstrate a modification of Grimme's two-parameter empirical dispersion force field (referred to as the PW91-D2* method), in which the damping function has been optimized to yield geometries that result in predictions of the principal values of O quadrupolar-coupling tensors that are systematically in close agreement with experiment. The predictions of O quadrupolar-coupling tensors using PW91-D2*-refined structures yield a root-mean-square deviation (RMSD) (0.28 MHz) for twenty-two crystalline systems that is smaller than the RMSD for predictions based on X-ray diffraction structures (0.

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

Periodic-boundary and cluster calculations of the magnetic-shielding tensors of (119)Sn sites in various co-ordination and stereochemical environments are reported. The results indicate a significant difference between the predicted NMR chemical shifts for tin(ii) sites that exhibit stereochemically-active lone pairs and tin(iv) sites that do not have stereochemically-active lone pairs. The predicted magnetic shieldings determined either with the cluster model treated with the ZORA/Scalar Hamiltonian or with the GIPAW formalism are dependent on the oxidation state and the co-ordination geometry of the tin atom.

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

(29) Si and (31) P magnetic-shielding tensors in covalent network solids have been evaluated using periodic and cluster-based calculations. The cluster-based computational methodology employs pseudoatoms to reduce the net charge (resulting from missing co-ordination on the terminal atoms) through valence modification of terminal atoms using bond-valence theory (VMTA/BV). The magnetic-shielding tensors computed with the VMTA/BV method are compared to magnetic-shielding tensors determined with the periodic GIPAW approach.

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

Calculations of the principal components of magnetic-shielding tensors in crystalline solids require the inclusion of the effects of lattice structure on the local electronic environment to obtain significant agreement with experimental NMR measurements. We assess periodic (GIPAW) and GIAO/symmetry-adapted cluster (SAC) models for computing magnetic-shielding tensors by calculations on a test set containing 72 insulating molecular solids, with a total of 393 principal components of chemical-shift tensors from 13C, 15N, 19F, and 31P sites. When clusters are carefully designed to represent the local solid-state environment and when periodic calculations include sufficient variability, both methods predict magnetic-shielding tensors that agree well with experimental chemical-shift values, demonstrating the correspondence of the two computational techniques.

Monitoring the refinement of crystal structures with (15)N solid-state NMR shift tensor data.

The (15)N chemical shift tensor is shown to be extremely sensitive to lattice structure and a powerful metric for monitoring density functional theory refinements of crystal structures. These refinements include lattice effects and are applied here to five crystal structures. All structures improve based on a better agreement between experimental and calculated (15)N tensors, with an average improvement of 47.

Density functional investigation of intermolecular effects on 13C NMR chemical-shielding tensors modeled with molecular clusters.

A quantum-chemical method for modeling solid-state nuclear magnetic resonance chemical-shift tensors by calculations on large symmetry-adapted clusters of molecules is demonstrated. Four hundred sixty five principal components of the (13)C chemical-shielding tensors of 24 organic materials are analyzed. The comparison of calculations on isolated molecules with molecules in clusters demonstrates that intermolecular effects can be successfully modeled using a cluster that represents a local portion of the lattice structure, without the need to use periodic-boundary conditions (PBCs).

13C chemical-shift anisotropy of alkyl-substituted aromatic carbon in anthracene derivatives.

The (13)C chemical-shift anisotropy in anthracene derivatives (9,10-dimethylanthracene, 9,10-dihydroanthracene, dianthracene, and triptycene) has been measured by the 2D FIREMAT timed pulse sequence and the corresponding set of principal values has been determined by the TIGER processing method. These molecules expand the data base of (13)C CSA measurements of fused aromatic rings some bridged by sp(3) carbon resulting in an unusual bonding configuration, which leads to distinctive aromatic (13)C CSA values. Crystal lattice distortions to the CSA were observed to change the isotropic shift by 2.

Intermolecular shielding contributions studied by modeling the (13)C chemical-shift tensors of organic single crystals with plane waves.

In order to predict accurately the chemical shift of NMR-active nuclei in solid phase systems, magnetic shielding calculations must be capable of considering the complete lattice structure. Here we assess the accuracy of the density functional theory gauge-including projector augmented wave method, which uses pseudopotentials to approximate the nodal structure of the core electrons, to determine the magnetic properties of crystals by predicting the full chemical-shift tensors of all (13)C nuclides in 14 organic single crystals from which experimental tensors have previously been reported. Plane-wave methods use periodic boundary conditions to incorporate the lattice structure, providing a substantial improvement for modeling the chemical shifts in hydrogen-bonded systems.

Redetermination of 1,4-dimethoxy-benzene.

The structure of the centrosymmetric title compound, C(8)H(10)O(2), originally determined by Goodwin et al. [Acta Cryst.(1950), 3, 279-284], has been redetermined to modern standards of precision to aid in its use as a model compound for (13)C chemical-shift tensor measurements in single-crystal NMR studies.

Modeling NMR chemical shift: A survey of density functional theory approaches for calculating tensor properties.

The NMR chemical shift, a six-parameter tensor property, is highly sensitive to the position of the atoms in a molecule. To extract structural parameters from chemical shifts, one must rely on theoretical models. Therefore, a high quality group of shift tensors that serve as benchmarks to test the validity of these models is warranted and necessary to highlight existing computational limitations.

(1)H/(29)Si cross-polarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers: probing the polymer-filler interface.

Polydimethylsiloxane (PDMS) elastomers reinforced with fumed silica exhibit unusual strength characteristics that are necessary for their designed applications. The microscopic details of the surface interaction between the polymer and silica are not well characterized. (1)H/(29)Si cross-polarization (cp) experiments are used to characterize cured and uncured samples of Dow Corning silastic 745.

Modeling the (13)C chemical-shift tensor in organic single crystals by quantum mechanical methods: finite basis set effects.

The influence of using finite basis sets to calculate (13)C magnetic shieldings were explored using the Hartree-Fock and the B3LYP hybrid density functional methods. The shielding values were compared in a linear least-squares fashion for a test group of 102 (13)C complete chemical-shift tensors determined from 14 organic single crystals. Pople's basis sets allow for the addition of polarization and diffuse functions in a straightforward way, allowing the examination of 81 combinations at the double and triple zeta level.

Ring-chain tautomerism in solid-phase erythromycin A: evidence by solid-state NMR.

Chemical shift modeling, utilizing the DFT B3LYP/D95** method, provides the spectral assignment of the 35 visible 13C resonances from the solid-phase erythromycin A dihydrate. A new resonance at 110.8ppm is observed in the high-resolution 13C CP/MAS spectrum upon the application of heat or sample desiccation.