Combining insights from solid-state NMR and first principles calculation: applications to the 19F NMR of octafluoronaphthalene

Abstract

Advances in solid-state NMR methodology and computational chemistry are applied to the (19)F NMR of solid octafluoronaphthalene. It is demonstrated experimentally, and confirmed by density functional theory (DFT) calculations, that the spectral resolution in the magic-angle spinning spectrum is limited by the anisotropy of the bulk magnetic susceptibility (ABMS). This leads to the unusual observation that the resolution improves as the sample is diluted. DFT calculations provide assignments of each of the peaks in the (19)F spectrum, but the predictions are close to the limits of accuracy and correlation information from 2-D NMR is invaluable in confirming the assignments. The effects of non-Gaussian lineshapes on the use of 2-D NMR for mapping correlations of spectral frequencies (e.g. due to the ABMS) are also discussed.

On the orientational dependence of resolution in1H solid-state NMR, and its role in MAS, CRAMPS and delayed-acquisition experiments

“…NMR frequencies in solids are always subject to some degree of inhomogeneous variation, e.g. inhomogeneous magnetic fields, ABMS broadenings, 24,28 etc. But in general these inhomogeneous shifts will not be correlated to the orientational dependence of d rss that are responsible for the differential dephasing.…”

Chemical shift computations on a crystallographic basis: some reflections and comments

“…It is worth noting that the article on octafluoronaphthalene 15 also discusses the use of NMR-CASTEP to provide other NMR-related physical parameters such as the anisotropy of the bulk magnetic susceptibility (ABMS), which can be related to solid-state MAS NMR linewidths. Application of first principles calculation to quadrupolar nuclei also provides additional information in the form of predicted quadrupole parameters.…”

“…The generalised gradient approximation (GGA) due to Perdew, Burke and Ernzerhof (PBE) 14 has been widely used to date for the computation of shielding in crystalline materials, with good results. However, the KT3 functional has also been used, 15 and the results have been compared to those obtained using PBE.…”

“…As an example of computation on a different nuclide, we mention here recent work which used NMR-CASTEP calculations to fully assign the 19 F spectrum of octafluoronaphthalene (IV) to the Chemical shift computations on a crystallographic basis S179 crystallographically distinct sites of its ambient-temperature structure. 15 The problem was demanding because the chemical shift range is very narrow (about 7 ppm) and fluorine is one of the more challenging atoms for first principles calculations; a high cut-off energy of 1110 eV was required to ensure convergence. Despite optimisation of all the atomic positions and the use of different DFT functionals, agreement with experimental results was only partial.…”

Solid-state NMR and computational investigation of solvent molecule arrangement and dynamics in isostructural solvates of droperidol

“…This difference in linewidths can not be explained by the presence of two slightly different droperidol molecules in the unit cell, as the linewidths of the organic solvates was identical to that of NSH, where there is a single droperidol molecule in the asymmetric unit. The "line-broadening factors" associated with the anisotropy of the bulk magnetic susceptibility (ABMS) for powder samples [57] were determined using the magnetic susceptibility tensor calculated by CASTEP-NMR. This was found to be ~3.6 ppm for the isostructural solvates and ~1.7 ppm for both form II and dihydrate, suggesting that the width of the lines in the solvate spectra is associated with a large ABMS, making it intrinsically difficult to resolve overlapped resonances.…”