ABSTRACT
We introduce two open source tools to aid the processing and visualisation of ab-initio computed solid-state NMR parameters. The Magres file format for computed NMR parameters (as implemented in CASTEP v8.0 and QuantumEspresso v5.0.0) is implemented. MagresView is built upon the widely used Jmol crystal viewer, and provides an intuitive environment to display computed NMR parameters. It can provide simple pictorial representation of one- and two-dimensional NMR spectra as well as output a selected spin-system for exact simulations with dedicated spin-dynamics software. MagresPython provides a simple scripting environment to manipulate large numbers of computed NMR parameters to search for structural correlations.
ABSTRACT
Although the electron-mediated spin-spin or J coupling is conventionally viewed as transmitted via covalent bonds, examples of J couplings between atoms that are not formally bonded but are in close proximity (termed "through-space" J couplings) have been reported. In this work, we investigate the observation of homonuclear 31P J couplings in organochalcogen heterocycles, which occur between 31P in two separate molecules, confirming without doubt their through-space nature. The presence of this interaction is even more surprising for one compound, where it occurs between crystallographically equivalent species. Although crystallographically equivalent species need not be magnetically equivalent in the solid state, owing to the presence of anisotropic interactions, we demonstrate that it is not the shielding anisotropy that lifts magnetic equivalence, in this case, but the presence of heteronuclear couplings to 77Se. We support our experimental observations with periodic scalar-relativistic density functional theory calculations and coupling density deformation plots to visualize the mechanism of these interesting interactions.
ABSTRACT
Solid-state NMR spectra of new P-Se heterocycles based on peri-substituted naphthalene motifs show the presence of unusual J couplings between Se and P. These couplings are between atoms in adjacent molecules and occur "through space", rather than through conventional covalent bonds. Experimental measurements are supported by relativistic DFT calculations, which confirm the presence of couplings between nonbonded atoms, and provide information on the pathway of the interaction. This observation improves the understanding of J couplings and offers insight into the factors that affect crystal packing in solids, for future synthetic exploitation.
ABSTRACT
We present a method for the first-principles calculation of nuclear magnetic resonance (NMR) J-coupling in extended systems using state-of-the-art ultrasoft pseudopotentials and including scalar-relativistic effects. The use of ultrasoft pseudopotentials is allowed by extending the projector augmented wave (PAW) method of Joyce et al. [J. Chem. Phys. 127, 204107 (2007)]. We benchmark it against existing local-orbital quantum chemical calculations and experiments for small molecules containing light elements, with good agreement. Scalar-relativistic effects are included at the zeroth-order regular approximation level of theory and benchmarked against existing local-orbital quantum chemical calculations and experiments for a number of small molecules containing the heavy row six elements W, Pt, Hg, Tl, and Pb, with good agreement. Finally, (1)J(P-Ag) and (2)J(P-Ag-P) couplings are calculated in some larger molecular crystals and compared against solid-state NMR experiments. Some remarks are also made as to improving the numerical stability of dipole perturbations using PAW.
