Carbon-13 chemical-shift tensors in indigo: A two-dimensional NMR-ROCSA and DFT Study

Holmes, Sean T.; Dybowski, Cecil

doi:10.1016/j.ssnmr.2015.08.004

Cited by 9 publications

(8 citation statements)

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“…In the solid state, the values of δ iso cover the range between 92.8 and 171.3 ppm, which is similar to other nitrogen‐dense heterocyclic compounds 16,88–90 . The chemical shift spans (Ω = δ 11 − δ 33 ) vary between 132.1 and 221.9 ppm, which is also representative of the 13 C chemical shift tensors of carbon atoms in nitrogen‐dense heterocyclic compounds 16,88–90 . Finally, we note that the 13 C chemical shift tensors for uracil have been previously measured at 11.7 T 48 ; the values presented in the previous study are different from those herein (i.e., differences as large as 4.5 ppm are observed for different principal values, whereas the uncertainty in our experimental values are within ±1.3 ppm), possibly reflecting the importance of high magnetic fields suppress the effects of 14 N‐ 13 C residual dipolar coupling.…”

Section: Resultssupporting

confidence: 70%

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

Holmes,

Boley,

Dewicki

et al. 2024

Magnetic Reson in Chemistry

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This paper reports the principal values of the 13C chemical shift tensors for five nitrogen‐dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have 13C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from 1H→13C cross‐polarization magic‐angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of 14N‐13C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the 13C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double‐hybrid functional PBE0‐DH, along with the triple‐zeta basis sets TZ2P or pc‐3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate 13C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2‐week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies.

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Section: Resultssupporting

confidence: 70%

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

Holmes,

Boley,

Dewicki

et al. 2024

Magnetic Reson in Chemistry

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“…The indirect contribution arises from changes in the electron density in the vicinity of the NMR-active nuclei due to interactions with nearby molecules in the lattice that alter the structure of the molecule or the electronic wave function (or both) from its gas-phase structure. In molecular solids, many of the largest deviations in chemical shifts between solid-state and solution-state measurements result from intermolecular hydrogen bonding. − In addition, lattice forces often distort the structures of molecules in a solid and lower their effective symmetry, resulting in spectra of solid materials that are more complex than those of the same molecule in a dilute gas. Many materials exhibit structural polymorphism, with the material crystallizing in one of several possible space groups, which leads to a variety of possible local geometries readily distinguishable by NMR spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

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

Holmes

Iuliucci

Mueller

et al. 2015

J. Chem. Theory Comput.

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138

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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. At the basis-set limit, we find that the small differences in the computed values have no statistical significance for three of the four nuclides considered. Subsequently, we explore the effects of additional DFT methods available only with the GIAO/cluster approach, particularly the use of hybrid-GGA functionals, meta-GGA functionals, and hybrid meta-GGA functionals that demonstrate improved agreement in calculations on symmetry-adapted clusters. We demonstrate that meta-GGA functionals improve computed NMR parameters over those obtained by GGA functionals in all cases, and that hybrid functionals improve computed results over the respective pure DFT functional for all nuclides except 15N.

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“…The prediction of magnetic-shielding tensors from structure by computational chemistry provides the link between electronic structure and experimental NMR parameters. Benchmark calculations of magnetic-shielding tensors of first- and second-period nuclides such as 13 C, 15 N, 29 Si, and 31 P have been presented for a variety of model chemistries, and it is typically possible to obtain excellent agreement between structure-based theoretical calculations and experimental results, provided a suitable model chemistry is employed. − …”

Section: Introductionmentioning

confidence: 99%

Role of Exact Exchange and Relativistic Approximations in Calculating ¹⁹F Magnetic Shielding in Solids Using a Cluster Ansatz

Alkan

Holmes

Dybowski

2017

J. Chem. Theory Comput.

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Calculations of F magnetic shielding in various materials are presented. In calculations on gas-phase molecules, the variation of magnetic shielding with the amount of Hartree-Fock exchange (HFX) in the functional demonstrates that excellent agreement with experiment is obtained with an admixture of 50%, here denoted PBE0 (50%). Calculations at the PBE, PBE0 (25%), and PBE0 (50%) levels on 10 crystalline organofluorines and 15 crystalline inorganic fluorides, in which a cluster ansatz is used to model the lattice environment, were performed. For fluorine-containing aromatics, increasing the admixture of HFX results in the prediction of larger magnetic-shielding spans, whereas increasing the admixture of HFX in calculations for CFCl decreases the span. In calculations of F magnetic shielding of the inorganic fluorides, the use of sufficiently large clusters of inorganic fluorides results in accuracies similar to those calculated for the organofluorines. Relativistic effects on the magnetic shielding of inorganic fluorides, modeled with ZORA at both the scalar and spin-orbit levels, are dominated by the scalar terms that increase the shielding of mostF sites over the non-relativistic results. These effects appear to scale with the atomic number of the cation. For most elements of the sixth row (Cs, Ba, La, and Pb), the scalar relativistic contribution to the magnetic shielding is in the range of 20-77 ppm. For elements of group XII (Zn, Cd, and Hg) bonded to fluorine, the scalar relativistic contribution results in deshielding of the F site.

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Carbon-13 chemical-shift tensors in indigo: A two-dimensional NMR-ROCSA and DFT Study

Cited by 9 publications

References 50 publications

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

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

Role of Exact Exchange and Relativistic Approximations in Calculating ¹⁹F Magnetic Shielding in Solids Using a Cluster Ansatz

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Carbon-13 chemical-shift tensors in indigo: A two-dimensional NMR-ROCSA and DFT Study

Cited by 9 publications

References 50 publications

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

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

Role of Exact Exchange and Relativistic Approximations in Calculating 19F Magnetic Shielding in Solids Using a Cluster Ansatz

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Role of Exact Exchange and Relativistic Approximations in Calculating ¹⁹F Magnetic Shielding in Solids Using a Cluster Ansatz