Dependence of NMR chemical shifts upon CH bond lengths of a methyl group involved in a tetrel bond

Methyl-π Interactions. Nature of Bonding and Limits of Strength

Both methyl groups and benzene rings are exceedingly common, and they lie near one another in many chemical situations. DFT calculations are used to gauge the strength of the attractive forces between them, and to better understand the phenomena that underlie this attraction. Methane and benzene are taken as the starting point, and substituents of both electron-withdrawing and donating types are added to each. The interaction energy varies between 1.4 and 5.0 kcal/mol, depending upon the substituents placed on the two groups. The nature of the binding is analyzed via Atoms in Molecules (AIM), Natural Bond Orbital (NBO), Symmetry-Adapted Perturbation Theory (SAPT), nuclear magnetic resonance (NMR) chemical shifts, and electron density shift diagrams. While there is a sizable electrostatic component, it is dispersion that dominates these interactions, particularly the weaker ones. As such, these interactions cannot be categorized unambiguously as either H-bonds or tetrel bonds.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

Origins and properties of the tetrel bond

The tetrel bond (TB) recruits an element drawn from the C, Si, Ge, Sn, Pb family as electron acceptor in an interaction with a partner Lewis base. The underlying principles that explain this attractive interaction are described in terms of occupied and vacant orbitals, total electron density, and electrostatic potential. These principles facilitate a delineation of the factors that feed into a strong TB. The geometric deformation that occurs within the tetrel-bearing Lewis acid monomer is a particularly important issue, with both primary and secondary effects. As a first-row atom of low polarizability, C is a reluctant participant in TBs, but its preponderance in organic and biochemistry make it extremely important that its potential in this regard be thoroughly understood. The IR and NMR manifestations of tetrel bonding are explored as spectroscopy offers a bridge to experimental examination of this phenomenon. In addition to the most common σ-hole type TBs, discussion is provided of π-hole interactions which are a result of a common alternate covalent bonding pattern of tetrel atoms.

Engineering Crystals Using sp3 -C Centred Tetrel Bonding Interactions

1,1,2,2-Tetracyanocyclopropane derivatives 1 and 2 were designed and synthesized to probe the utility of sp3 -C centred tetrel bonding interactions in crystal engineering. The crystal packing of 1 and 2 and their 1,4-dioxane cocrystals is dominated by sp3 -C(CN)2 ⋅⋅⋅O interactions, has significant C⋅⋅⋅O van der Waals overlap (≤0.266 Å) and DFT calculations indicate interaction energies of up to -11.0 kcal mol-1 . A cocrystal of 2 with 1,4-thioxane reveals that the cyclopropane synthon prefers interacting with O over S. Computational analyses revealed that the electropositive C2 (CN)4 pocket in 1 and 2 can be seen as a strongly directional 'tetrel-bond donor', similar to halogen bond or hydrogen bond donors. This disclosure is expected to have implications for the utility of such 'tetrel bond donors' in molecular disciplines such as crystal engineering, supramolecular chemistry, molecular recognition and medicinal chemistry.

Nature of the Interaction of Pyridines with OCS. A Theoretical Investigation

Ab initio calculations were carried out to investigate the interaction between para-substituted pyridines (X-C₅H₄N, X=NH₂, CH₃, H, CN, NO₂) and OCS. Three stable structures of pyridine.OCS complexes were detected at the MP2=full/aug-cc-pVDZ level. The A structure is characterized by N…S chalcogen bonds and has binding energies between −9.58 and −12.24 kJ/mol. The B structure is bonded by N…C tetrel bond and has binding energies between −10.78 and −11.81 kJ/mol. The C structure is characterized by π-interaction and has binding energies between −10.76 and −13.33 kJ/mol. The properties of the systems were analyzed by AIM, NBO, and SAPT calculations. The role of the electrostatic potential of the pyridines on the properties of the systems is outlined. The frequency shift of relevant vibrational modes is analyzed.

On the capability of metal–halogen groups to participate in halogen bonds

Halogens in a M–X bond are inhibited from forming a halogen bond but can do so in certain circumstances, with or without a σ-hole.