Multicentre Bonds Research Articles

In-Depth Theoretical Investigations of Borazine’s Aromaticity: Tailoring Electron Delocalization through Substituent Effects

The current study investigates the influence of several R substituents (e.g., Me, SiH3, F, Cl, Br, OH, NH2, etc.) on the aromaticity of borazine, also known as the “inorganic benzene”. By performing hybrid DFT methods, blended with several computational techniques, e.g., Natural Bond Orbital (NBO), Quantum Theory of Atoms in Molecules (QTAIM), Gauge-Including Magnetically Induced Current (GIMIC), Nucleus-Independent Chemical Shift (NICS), and following a simultaneous evaluation of four different aromaticity indices (para-delocalization index (PDI), multi-centre bond order (MCBO), ring current strength (RCS), and NICS parameters), it is emphasized that the aromatic character of B-substituted (B3R3N3H3) and N-substituted (B3H3N3R3) borazine derivatives can be tailored by modulating the electronic effects of R groups. It is also highlighted that the position of R substituents on the ring structure is crucial in tuning the aromaticity. Systematic comparisons of calculated aromaticity index values (i.e., via regression analyses and correlation matrices) ensure that the reported trends in aromaticity variation are accurately described, while the influence of different R groups on electron delocalization and related aromaticity phenomena is quantitatively assessed based on NBO analyses. The most relevant interactions impacting the aromatic character of investigated systems are (i) the electron conjugations occurring between the p lone pair electrons (LP) on the F, Cl, Br, O or N atoms, of R groups, and the π*(B=N) orbitals on the borazine ring (i.e., LP(R)→π*(B=N) donations), and (ii) the steric-exchange (Pauli) interactions between the same LP and the π(B=N) bonds (i.e., LP(R)↔π(B=N) repulsions), while inductive/field effects influence the aromaticity of the investigated trisubstituted borazine systems to a much lesser extent. This work highlights that although the aromatic character of borazine can be enhanced by grafting electron-donor substituents (F, OH, NH2, O−, NH−) on the N atoms, the stabilization due to aromaticity has only a moderate impact on these systems. By replacing the H substituents on the B atoms with similar R groups, the aromatic character of borazine is decreased due to strong exocyclic LP(R)→π*(B=N) donations affecting the delocalization of π-electrons on the borazine ring.

Weak Bonding and Anharmonicity in Thermoelectric ZnSb

AbstractThe Zn─Sb binary system contains two high‐performing thermoelectric materials, namely the ordered ZnSb and the disordered Zn13Sb10. Both systems exhibit low thermal conductivity, which is speculated to originate from multicentre bonding within Zn2Sb2‐rhombi. Here, the electron density of ZnSb is reported based on multipole modelling of accurate X‐ray diffraction data measured at 20 K. Topological analysis reveals that the bond paths in the rhombus are endocyclically strained and that electron density is concentrated within the rhombus rather than along its geometric bonds consistent with a multicentre bond description. However, the electron density is not equally shared between the geometric bonds of the rhombus. Electron density analysis and modelling of low‐temperature anharmonicity reveal that one Zn–Sb interaction is weaker than the other. Taken together with the orientation of bonds external to the rhombus structure, an alternative description emerges wherein the multicentre bond is more partially localised along one set of the opposite legs of the rhombus. In this description, the stronger bond can be considered a traditional 2‐centre‐2‐electron bond, while the weaker interaction is coordinative to the covalent bond. The anharmonicity and low thermal conductivity may consequently be understood as Zn rattling along the coordinative interaction.

Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding

AbstractAmong the list of planar tetracoordinate atoms, the smallest element hydrogen is missing. No experimental and theoretical evidence have ever been put forwarded. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of In4H+ cluster. Bonding analysis indicates that the central hydrogen atom is acting like a proton and significant charge transfer from the surrounding In4 framework results in a negative charge of the central hydrogen atom. The proposed global minimum geometry possesses σ‐aromaticity and the central hydrogen atom forms unusual multicentre bond with more than three centres.

Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding.

Among the list of planar tetracoordinate atoms, the smallest element hydrogen is missing. No experimental and theoretical evidence have ever been put forwarded. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of In4 H+ cluster. Bonding analysis indicates that the central hydrogen atom is acting like a proton and significant charge transfer from the surrounding In4 framework results in a negative charge of the central hydrogen atom. The proposed global minimum geometry possesses σ-aromaticity and the central hydrogen atom forms unusual multicentre bond with more than three centres.

Structural and electronic properties of medium-sized beryllium doped magnesium BeMgn clusters and their anions

Bimetallic clusters have attracted much attention because of the structural and property changes that occur: cluster size and doping. Here, we performed a structural search of the global minimum for bimetallic BeMgn0/− (n = 10–20) clusters by utilizing efficient CALYPSO structural searching program with subsequent DFT calculations. A large number of low energetic isomers converge and the most stable structures are confirmed by comparing the total energies for different cluster sizes. Satisfactory agreement between theoretical and experimental PES spectra demonstrates the validity of our predicted global minimum structures. It is found that the most stable structures of BeMgn0/− clusters are filled cage-like frameworks at n = 10–20. The localized position of Be atoms changes from completely encapsulated sites to surface sites, after which the position reverts to the caged Mg motif. In all BeMgn0/− clusters, the charge transfers from the Mgn motif to Be atoms. Increasing occupations of p orbitals manifest their increasing metallic behaviors. A stability analysis revealed that the D4d symmetric BeMg16 caged structure with one centred Be atom has robust stability, which can be because BeMg16 possesses a closed electronic shell of 1S21P61D101F42S21F10 filled with 34 valence electrons and strong Be-Mg bonds due to s-p hybridization. This finding is supported by multi-centre bonds and Mayer bond order analyses.

Are Multicentre Bond Indices and Related Quantities Reliable Predictors of Excited-State Aromaticity?

Systematic scrutiny is carried out of the ability of multicentre bond indices and the NOEL-based similarity index dAB to serve as excited-state aromaticity criteria. These indices were calculated using state-optimized complete active-space self-consistent field wavefunctions for several low-lying singlet and triplet states of the paradigmatic molecules of benzene and square cyclobutadiene and the inorganic ring S2N2. The comparison of the excited-state indices with aromaticity trends for individual excited states suggested by the values of magnetic aromaticity criteria show that whereas the indices work well for aromaticity reversals between the ground singlet and first triplet electronic states, addressed by Baird’s rule, there are no straightforward parallels between the two sets of data for singlet excited states. The problems experienced while applying multicentre bond indices and dAB to singlet excited states are explained by the loss of the information inherently present in wavefunctions and/or pair densities when calculating the first-order density matrix.

Analysis of physicochemical properties in covalent network chalcogenide glasses (ChGs): critical review of theoretical modeling of chemical bond approach

The Group 16 elements are frequently named as chalcogens (‘ore builders’). From the geographical point of view, they are known as chalcophiles which are the soft elements (keeping in mind the terminology of the hard/soft acids and bases concept). In the geographical sense, chalcophiles preferably form minerals with sulfur (S), selenium (Se) and tellurium (Te). However, oxygen (O) tends to bind preferably to lithophiles, which are hard elements such as the alkali metals, early transition metals, and silicon. The special position of oxygen amongst the chalcogen elements is its association with its very high electronegativity, which in turn correlates with its high ionization energy and the small atomic radius; which makes it difficult for oxygen to act as a central atom in electron-rich multicentre bonds. In the present script, we have reviewed the models for theoretical prediction of some significant physical parameters i.e. coordination number, constraints, density, molar volume, cohesive energy and heat of atomization etc. for the three generations of Se rich multi-component glasses are presented. The present theoretical study of the multi-component chalcogenide glasses reveals that there is a significant change in the structural environment of the three generations (binary, ternary and quaternary systems) due to rigidity percolation.

Manifestation of exo-cyclic aromaticity in triangular heterocyclic $$\hbox {B}_{2}\hbox {F}_{2}$$ B 2 F 2 X systems (X $$=$$ = O, S, Se, NH)

Aromaticity is an important concept in chemistry which extends over a wide range of molecular systems and imparts unique features to the molecules possessing it. In the present work, novel heteroatomic molecular systems are proposed which demonstrate non-conventional aromaticity where the molecules accomplish the aromatic sextet and hence stabilization through the conjugation of $$\uppi $$ -electrons from exo-cyclic substituents. A considerable $$\upsigma $$ -aromaticity is also observed which does not involve the exo-cyclic atoms. At first, the stability of these molecular systems is theoretically ascertained through various density functional theory and ab-initio calculations along with the energy decomposition analysis, T1 diagnostic run, estimation of ring strain energy and highest occupied molecular orbital–lowest unoccupied molecular orbital gap which indicate towards the viability of these molecular systems. Then, a detailed study of aromaticity with the aid of different computational probes such as nucleus-independent chemical shift (NICS), dissected canonical molecular orbital-NICS analysis, multi-centre bond index (MCI), adaptive natural density partitioning and theoretical tools such as aromatic stabilization energy based on a fully ab-initio approach are performed which establish unique exo-cyclic aromaticity in these systems.

Pancake Bonding in π-Stacked Trimers in a Salt of Tetrachloroquinone Anion.

The crystal structure of [4-damp])2 [Cl4 Q]3 (4-damp=4-dimethylamino-N-methylpyridinium, Cl4 Q=tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl4 Qs in the trimer. The strong interactions within a trimer (Cl4 Q)32- have a partially covalent character with two-electron/multicentered bonding, that is extended over three rings, plausibly termed as "pancake bonding". The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.

Two-dimensional materials: Polyphony in B flat.

Early theories suggested the possibility of atomically thin boron layers, but electron-deficient boron favours multicentre bonds and assembles into various polymorphs, making the synthesis of such layers challenging. Now, in two independent experiments, the deposition of atomic boron has offered this long-sought material on a silver platter.

A theoretical study of a ZnO graphene analogue: adsorption on Ag(111) and hydrogentransport

A single sheet of zinc oxide (ZnO) based on the same flat two-dimensional (2D) hexagonaltopology as graphene, but with alternating neighbouring Zn and O atoms in place ofcarbon atoms, is studied theoretically. Following experimental studies, the adsorption of2D-ZnO with the Ag(111) surface is investigated using density functional theory, withand without a semi-empirical correction for dispersive interactions, and withclassical interatomic potentials. The interaction of H atoms with the hexagonalZn3O3 rings of 2D-ZnO is given special attention where multi-centre bond formation isobserved to significantly assist the transport of H atoms through the 2D-ZnO sheet.

Hydrogen in polycrystalline ZnO thin films

The influence and the states of hydrogen in polycrystalline ZnO thin films were investigated by preparing films with different amounts of oxygen vacancies at various hydrogen potentials. The most notable effect of hydrogen addition was passivation of grain boundaries. The majority of hydrogen incorporation in polycrystalline ZnO films was attributed to hydrogen interstitials, and a substantially smaller number of multicentre bonds at oxygen vacancies were formed even at high hydrogen potentials. The major source of free carriers in polycrystalline ZnO films deposited without intentional hydrogen addition was oxygen vacancies with 2+ charge state with large atomic relaxation.

Multi-centre, hydrogen and dihydrogen bonds in lithium closo-hydroborate obtained from liquid ammonia

Charge density analysis reveals multi-centre bonds in the cluster, hydrogen bridges in the ammonia network, and the dihydrogen bond between H(delta-) and H(delta+); all observed in one compound, [Li(NH3)4]2B6H6.2NH3.

Hydrogen multicentre bonds

The concept of a chemical bond stands out as a major development in the process of understanding how atoms are held together in molecules and solids. Lewis' classical picture of chemical bonds as shared-electron pairs evolved to the quantum-mechanical valence-bond and molecular-orbital theories, and the classification of molecules and solids in terms of their bonding type: covalent, ionic, van der Waals and metallic. Along with the more complex hydrogen bonds and three-centre bonds, they form a paradigm within which the structure of almost all molecules and solids can be understood. Here, we present evidence for hydrogen multicentre bonds-a generalization of three-centre bonds-in which a hydrogen atom equally bonds to four or more other atoms. When substituting for oxygen in metal oxides, hydrogen bonds equally to all the surrounding metal atoms, becoming fourfold coordinated in ZnO, and sixfold coordinated in MgO. These multicentre bonds are remarkably strong despite their large hydrogen-metal distances. The calculated local vibration mode frequency in MgO agrees with infrared spectroscopy measurements. Multicoordinated hydrogen also explains the dependence of electrical conductivity on oxygen partial pressure, resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity of ZnO (refs 8-10).

Ab initio theoretical study of three-centre bonding on the basis of bond index

Various methods of derivation of a multi-centre bond index and its physical significance are briefly discussed. The usefulness of the bond index model for the study of three-centre (3c) bonding is illustrated by taking a number of examples chosen from earlier calculations. The origin of the sign of 3c bond indices (IABC) predicted for three-centre two-electron (3c-2e) and three-centre four-electron (3c-4e) bonds is investigated using simple models with different bonding topologies. Some examples are given highlighting the role of IABC in elucidating the nature of bonding in controversial molecules. The bond index and the effective pair population models are compared and shown to be equivalent for an RHF wave function and even-centre bond indices. It is concluded that despite some inherent limitations such as the basis set sensitivity the bond index model provides a quite satisfactory description of 3c bonding.

Synthesis and crystal structure of the tris(amide) cations [M{N(SiMe3)2}3]+ (M = Zr or Hf ): evidence for M–Si–C interactions

Reaction of the strong Lewis acid B(C6F5)3 with [M{N(SiMe3)2}3Me] (M = Zr or Hf) gave the compounds [M{N(SiMe3)2}3][MeB(C6F5)3]. Crystallographic analyses of these compounds revealed the formation of multicentre bonds between the SiCH3 units of the amide ligands and the otherwise electron-deficient metal centre. The new tris(amide) complexes [M{N(Ph)SiMe3}3Cl] and [M{N(Ph)SiMe3}3Me] (M = Zr or Hf) have also been synthesized.

Theoretical study of multi-centre bonding using a delocalised MO approach

The nature of the bonding in about thirty electron-deficient molecules has been studied on the basis of two-centre and multi-centre (mainly three-centre) bond indices and atomic valencies calculated from ab initio SCF density matrices using 4-31G (or 6-31G) and 6-31G ∗ (or 6-31G ∗∗) basis sets. For comparison, the density matrices corresponding to non-orthogonal and symmetrically orthogonalised basis functions have been employed. The nature of the bonding deduced on the basis of bond indices and valencies has been confirmed by localised MO calculations. It has been observed that a three-centre bond is invariably associated with a positive and significantly high value (⩾0.1) of the bond index. For a four-centre bonding interaction the corresponding bond index is generally positive, but rather small.