Chalcogen Bonding in the Molecular Dimers of WCh2 (Ch = S, Se, Te): On the Basic Understanding of the Local Interfacial and Interlayer Bonding Environment in 2D Layered Tungsten Dichalcogenides

Abstract

Layered two-dimensional transition metal dichalcogenides and their heterostructures are of current interest, owing to the diversity of their applications in many areas of materials nanoscience and technologies. With this in mind, we have examined the three molecular dimers of the tungsten dichalcogenide series, (WCh2)2 (Ch = S, Se, Te), using density functional theory to provide insight into which interactions, and their specific characteristics, are responsible for the interfacial/interlayer region in the room temperature 2H phase of WCh2 crystals. Our calculations at various levels of theory suggested that the Te···Te chalcogen bonding in (WTe2)2 is weak, whereas the Se···Se and S···S bonding interactions in (WSe2)2 and (WS2)2, respectively, are of the van der Waals type. The presence and character of Ch···Ch chalcogen bonding interactions in the dimers of (WCh2)2 are examined with a number of theoretical approaches and discussed, including charge-density-based approaches, such as the quantum theory of atoms in molecules, interaction region indicator, independent gradient model, and reduced density gradient non-covalent index approaches. The charge-density-based topological features are shown to be concordant with the results that originate from the extrema of potential on the electrostatic surfaces of WCh2 monomers. A natural bond orbital analysis has enabled us to suggest a number of weak hyperconjugative charge transfer interactions between the interacting monomers that are responsible for the geometry of the (WCh2)2 dimers at equilibrium. In addition to other features, we demonstrate that there is no so-called van der Waals gap between the monolayers in two-dimensional layered transition metal tungsten dichalcogenides, which are gapless, and that the (WCh2)2 dimers may be prototypes for a basic understanding of the physical chemistry of the chemical bonding environments associated with the local interfacial/interlayer regions in layered 2H-WCh2 nanoscale systems.

The Pnictogen Bond: The Covalently Bound Arsenic Atom in Molecular Entities in Crystals as a Pnictogen Bond Donor

“…Its strength on As along the Ch–As bond extension is stronger, whereas that along the two C–As bond extensions are equivalent and weaker. Along the chalcogen series, its strength follows the order S–As (16.4 kcal mol −1 ) > Se–As (13.8 kcal mol −1 ) > Te–As (13.8 kcal mol −1 ), indicating that S in the molecule has relatively more of an electron-density-withdrawing capability than Se and Te; this is in accord with the order of their electronegativities (S > Se > Te) [ 102 , 103 ].…”

“…An electrophilic region was identified when V S,min > 0 or V S,max > 0. Similarly, the nucleophilic regions [ 38 , 102 , 103 ] were characterized when V S,min < 0 or V S,max < 0. An attractive interaction between two atomic basins (intermolecular or intramolecular) was recognized when a region of an atom or fragment in a molecular entity with a positive V S,min (or V S,max ) is in close proximity to that with a negative V S,min (or V S,max ).…”

“…Because we were interested in the identification and subsequent characterization of arsenic-centered pnictogen bonding within or between interacting monomer entities, we have used monomer entities in some selected crystals to analyze the pro-molecular charge density based isosurfaces, calculated within the pro-molecular framework of IGM- δ g [ 80 , 81 ]. We did so as to confirm chemical bonding interactions revealed using MESP and inter- and intramolecular geometries since this method has proven to be useful in providing an insightful local description into the source of the atomic or fragmental domains causing the interaction [ 38 , 102 , 103 , 104 , 105 ]. Within the framework of IGM, pro-molecular atomic electron densities are summed up and the associated atomic gradients do not interfere.…”

The Nitrogen Bond, or the Nitrogen-Centered Pnictogen Bond: The Covalently Bound Nitrogen Atom in Molecular Entities and Crystals as a Pnictogen Bond Donor

“…A specific region on an atom in a molecular entity was considered to be electrophilic when the sign of either V S,min or V S,max was positive, i.e., V S,min > 0 or V S,max > 0. When they are negative, V S,min < 0 or V S,max < 0, then the region was considered to be nucleophilic [92][93][94].…”

Adjusting the balance between hydrogen and chalcogen bonds

“…These bonds run a wide gamut of strength, 53 some of which are considerably stronger than a HB. They can accommodate various sorts of electron donors besides the usual bases, even including metal atoms, for example the d z 2 orbital of planar Pd, Pt 54,55 or even W. 56 A particularly interesting facet of ChBs derives from the typical divalent bonding of the central atom which leads to the presence of two separate and distinct σ-holes, each of which can participate in a ChB with a nucleophile. A second aspect is the presence of two lone pairs on each atom, each of which can act as an electron donor.…”