Group IV tetrel elements may act as tetrel bond donors, whereby a region of positive electrostatic potential (σ-hole) interacts with a Lewis base. The results of calculations of NMR parameters are reported for a series of model compounds exhibiting tetrel bonding from a methyl carbon to the oxygen or nitrogen atoms in various functional groups. The (13)C chemical shift (δiso) and the (1c)J((13)C,Y) coupling (Y = (17)O, (15)N) across the tetrel bond are recorded as a function of geometry. The sensitivity of the NMR parameters to the noncovalent interaction is demonstrated via an increase in δiso and in |(1c)J((13)C,Y)| as the tetrel bond shortens. Gauge-including projector-augmented wave density functional theory (DFT) calculations of δiso are reported for crystals that exhibit tetrel bonding in the solid state. Experimental δiso values for solid sarcosine and its tetrel-bonded salts corroborate the computational findings. This work offers new insights into tetrel bonding and facilitates the incorporation of tetrel bonds as restraints in NMR crystallographic structure refinement.

Nepheline (Na 6 K 2 Al 8 Si 8 O 32 ) is a rock-forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline-based glass-ceramics proceeds through several polymorphic transformationsmainly orthorhombic, hexagonal, cubicdepending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline-based glass-ceramics. Accordingly, two different sets of glasses (meta-aluminous and per-alkaline) have been designed in the system Na 2 O-CaO-Al 2 O 3 -SiO 2 in the crystallization field of nepheline and synthesized by the melt-quench technique. The detailed structural analysis of glasses has been performed by 29 Si, 27 Al, and 23 Na magic-angle spinningnuclear magnetic resonance (MAS NMR), and multiple-quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non-isothermal conditions using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass-ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na 2 O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO 2 -poor glass-ceramics with (Na 2 O+CaO)/Al 2 O 3 > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.

This Highlight article discusses the role of solid-state NMR spectroscopy in crystal engineering with the aid of several examples from the literature.

The feasibility and value of Pb solid-state NMR experiments on compounds featuring lead tetrel bonds is explored. Although the definition remains to be formalized, lead tetrel bonds may be qualitatively described as existing when there is evidence of a net attractive interaction between an electrophilic region associated with lead in a molecular entity and a nucleophilic region in another, or the same, molecular entity. Unambiguous identification of lead tetrel bonds can be challenging due to the hypervalent tendency of lead. We report here a series ofPb solid-state NMR experiments on five metal-organic frameworks featuring lead coordinated to hydrazone-based ligands. Such frameworks may be held together in part by lead tetrel bonds. The acquisition of Pb solid-state NMR spectra for such materials is feasible and is readily accomplished using a combination of magic-angle spinning and Carr-Purcell-Meiboom-Gill methods in moderate to low applied magnetic fields. The lead centres are characterized byPb isotropic chemical shifts ranging from -426 to -2591 ppm and chemical shift tensor spans ranging from 910 to 2681 ppm. Careful inspection of the structures of the compounds and the literature Pb NMR data may suggest that a tetrel bond to lead results in chemical shift parameters which are intermediate between those which are characteristic of holodirected and hemidirected lead coordination geometries. Challenges associated with DFT computations of thePb NMR parameters are discussed. In summary, the Pb data for the compounds studied herein show a marked response to the presence of non-coordinating electron-rich moieties in close contact with the electrophilic surface of formally hemidirectionally coordinated lead compounds.