Multicenter Bonds Research Articles

Structure and bonding of new boron and carbon superpolyhedra

Using the DFT methods, we computationally predict the stability of cage compounds E4nRn (E = B, C; R = H, F; n = 4, 8, 12, 24) based on Platonic bodies and Archimedean polyhedrons in which all vertices are replaced by tetrahedral E4R fragments. Cage compounds B60R12 and C60 with pyramidal units B5R or C5 are also examined and it is shown that only boron compounds are stable. The nature of chemical bonding in the discussed compounds is analyzed using the AdNDP and NBO methods. The hydrocarbons have classical 2c-2e C-C σ-bonds, while the boron compounds are formed by the polyhedral units with the delocalized multicenter bonds which connected three and more boron atoms. The new example of spherical aromaticity according to the 2(N+1)2 rule in the case of B16F4 with multicenter 16c-2e bonds are revealed. Stable compound B60H12 contains 12 5c-2e B-B bonds.

Reversible Solution π-Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical.

Reversible solution π-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for π-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the π-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related to dispersion interactions between the flanking nearly parallel quinolyl rings.

Metal-Organic Frameworks Based on Multicenter-Bonded [MI ]8 (M=Mn, Zn) Clusters with Cubic Aromaticity.

A new concept for constructing metal-organic frameworks (MOFs) based on multicenter-bonded [MI ]8 (M=Mn, Zn) clusters with cubic aromaticity is discussed. In principle, intermolecular/intramolecular hydrogen-bonding, π-π stacking, coordinated covalent bonding and ionic bonding usually account for the structures of MOFs, and multicentered bonds generally exist in cation, anion, neutral and zwitterionic radicals, while rarely appear in MOFs. Meanwhile, aromaticity became one of the most vexing yet fascinating key concepts in chemistry since the proposition of the structure of benzene molecule in 1865. In this context, this concept article exhibits the preparation, structures, characterization methods, theoretical analysis of current research for MOFs which feature with multicentered bonding [MI ]8 (M=Mn, Zn) cluster with cubic aromaticity, followed by their fluorescence probe and magnetic properties are also summarized. Finally, prospective outlook in this field is given.

Conjugation in multi-tetrazole derivatives: a new design direction for energetic materials.

Multi-tetrazole derivatives with conjugated structures were designed and investigated in this study. Using quantum chemistry methods, the crystal structures, electrostatic potentials (ESPs), multicenter bond orders, HOMO-LUMO energy gaps, and detonation properties of the derivatives were calculated. As expected, these molecules with conjugated structures showed low energies of their crystal structures, molecular layering in their crystals, high average ESPs, high multicenter bond order values, and enhanced detonation properties. The derivative 1,2-di(1H-tetrazol-5-yl)diazene (N2) was predicted to have the best density (1.87g/cm3), detonation velocity (9006m/s), and detonation pressure (36.8GPa) of the designed molecules, while its total crystal energy was low, suggesting that it is relatively stable. Its sensitivity was also low, as the molecular stacking that occurs in its crystal allows external forces to be dissipated into movements of crystal layers. Finally, its multicenter bond order was high, indicating a highly conjugated structure.

Multicenter Bonding Effects in Oxygen Vacancy in the Bulk and on the Surface of MgO

We have studied the electronic structure of F-centers in the bulk and on the (100) surface of the MgO crystal. Chemical bonding analysis was performed using the natural bond orbital (NBO) analysis with periodic boundary conditions and the solid state adaptive natural density partitioning (SSAdNDP) method. The bonding in the pristine MgO was found to be very ionic, i.e., there are four lone pairs (s-, px-, py-, and pz-type) on every oxygen atom with the occupation numbers (ONs) in the range of 1.76–1.86 |e|. Within the O vacancy (Ov) located in the bulk of MgO we found a pair of electrons in the place of the missing oxygen atom that is revealed by SSAdNDP as a six-center two-electron (6c–2e) σ bond delocalized over six Mg atoms with ON = 1.87 |e|. In the other defected MgO system featuring the Ov on the (100) surface we also found a pair of electrons in the place of the missing oxygen atom that is revealed as a 5c–2e σ bond with ON = 1.84 |e|. Inclusion of the four closest Mg atoms of the second layer incr...

Tug-of-war between classical and multicenter bonds in H-(Be)n-H species

Quantum chemical calculations were performed for beryllium homocatenated compounds [H-(Be)n-H]. Global minimum structures were found using machine searches (Coalescence Kick method) with density functional theory. Chemical bonding analysis was performed with the Adaptive Natural Density Partitioning method. It was found that H-(Be)2-H and H-(Be)3-H clusters are linear with classical two-center two-electron bonds, while for n > 3, three-dimensional structures are more stable with multicenter bonding. Thus, at n = 4, multicenter bonding wins the tug-of-war vs. the classical bonding.

FALDI-based decomposition of an atomic interaction line leads to 3D representation of the multicenter nature of interactions.

Atomic interaction lines (AILs) and the QTAIM's molecular graphs provide a predominantly two-center viewpoint of interatomic interactions. While such a bicentric interpretation is sufficient for most covalent bonds, it fails to adequately describe both formal multicenter bonds as well as many non-covalent interactions with some multicenter character. We present an extension to our Fragment, Atomic, Localized, Delocalized and Interatomic (FALDI) electron density (ED) decomposition scheme, with which we can measure how any atom-pair's delocalized density concentrates, depletes or reduces the electron density in the vicinity of a bond critical point. We apply our method on five classical bonds/interactions, ranging from formal either two- or three-center bonds, a non-covalent interaction (an intramolecular hydrogen bond) to organometallic bonds with partial multicenter character. By use of 3D representation of specific atom-pairs contributions to the delocalized density we (i) fully recover previous notion of multicenter bonding in diborane and predominant bicentric character of a single covalent CC bond, (ii) reveal a multicenter character of an intramolecular H-bond and (iii) illustrate, relative to a Schrock carbene, a larger degree of multicenter MC interaction in a Fischer carbene (due to a presence of a heteroatom), whilst revealing the holistic nature of AILs from multicenter ED decomposition. © 2018 Wiley Periodicals, Inc.

An electron compensation mechanism for the polymorphism of boron monolayers.

Boron monolayers have been increasingly attractive, while it is still a challenge to understand their structural stabilities, due to electron deficiency and multi-center bonds. In this work, we propose the average electron compensation (AEC) mechanism for boron monolayers based on high-throughput first-principles calculations. It is found that the AEC parameter (λ) tends to be zero for the stable free-standing boron monolayers. In addition, this mechanism can quantitatively describe the stability of boron monolayers on various metal substrates, providing direct suggestions for experimentalists to synthesize various boron monolayers for practical applications.

Global minimum beryllium hydride sheet with novel negative Poisson's ratio: first-principles calculations.

As one of the most prominent metal-hydrides, beryllium hydride has received much attention over the past several decades, since 1978, and is considered as an important hydrogen storage material. By reducing the dimensionality from 3 to 2, the beryllium hydride monolayer is isoelectronic with graphene; thus the existence of its two-dimensional (2D) form is theoretically feasible and experimentally expected. However, little is known about its 2D form. In this work, by a global minimum search with the particle swarm optimization method via density functional theory computations, we predicted two new stable structures for the beryllium hydride sheets, named α–BeH2 and β–BeH2 monolayers. Both structures have more favorable thermodynamic stability than the recently reported planar square form (Nanoscale, 2017, 9, 8740), due to the forming of multicenter delocalized Be–H bonds. Utilizing the recently developed SSAdNDP method, we revealed that three-center-two-electron (3c–2e) delocalized Be–H bonds are formed in the α–BeH2 monolayer, while for the β–BeH2 monolayer, novel four-center-two-electron (4c–2e) delocalized bonds are observed in the 2D system for the first time. These unique multicenter chemical bonds endow both α– and β–BeH2 with high structural stabilities, which are further confirmed by the absence of imaginary modes in their phonon spectra, the favorable formation energies comparable to bulk and cluster beryllium hydride, and the high mechanical strength. These results indicate the potential for experimental synthesis. Furthermore, both α– and β–BeH2 are wide-bandgap semiconductors, in which the α–BeH2 has unusual mechanical properties with a negative Poisson's ratio of −0.19. If synthesized, it would attract interest both in experiment and theory, and be a new member of the 2D family isoelectronic with graphene.

Topological Aspects of Boron Nanotubes

The degree‐based topological indices are used to correlate the physical and chemical properties of a molecule with its chemical structure. Boron nanotubular structures are high‐interest materials due to the presence of multicenter bonds and have novel electronic properties. These materials have some important issues in nanodevice applications like mechanical and thermal stability. Therefore, they require theoretical studies on the other properties. In this paper, we present certain degree‐based topological indices such as ABC, the fourth ABC, GA, and the fifth GA indices for boron triangular and boron‐α nanotubes.

Cover Feature: Multi‐Center vs. Two‐Center Bonding within the Hetero‐Polyanion in Eu2GaPt2 and its Prototype Ca2SiIr2 (Z. Anorg. Allg. Chem. 21/2017)

Cover Feature: Multi‐Center vs. Two‐Center Bonding within the Hetero‐Polyanion in Eu₂GaPt₂ and its Prototype Ca₂SiIr₂ (Z. Anorg. Allg. Chem. 21/2017)

Investigation of pyramidal inversion of nitrogen atom in carbamate and thiocarbamate ions formed at the reaction of СО2, СOS, CS2 with 2-aminoethanol

Dynamic NMR was applied to measuring the value ΔG≠ characterizing the height of the barrier to the pyramidal inversion of the nitrogen atom in carbamate and thiocarbamate anions formed at the reaction of 2-aminoethanol with CO2 and COS. The refinement was introduced in formerly suggested cyclic structures of anions containing an intramolecular hydrogen bond NH···O(S), which contradicted the found large values of the barrier of inversion (ΔG ≠ ∼ 70 kJ mol–1). The hydrogen bond in the cyclic anions of carbamates and thiocarbamates is two-electron and three-center. Analogous cyclic structure with a multicenter hydrogen bond does not form in the case of dithiocarbamate anion that is the product of 2-aminoethanol reaction with CS2.

Boron Chemistry – A Rapidly Expanding Research Field

Guest Editors Vladimir Bregadze and Zuowei Xie give a short introduction to the field and history of boron chemistry. This thematic issue is devoted to boron chemistry. This field of chemistry quickly started to develop when scientists tried to investigate boron hydrides in the middle of the 20th century after Alfred Stock's discovery. At that time, the leading world countries started to invest a lot of money in research into this field, in view of the good prospects for the application of boron hydrides as rocket fuel. However the toxicity and flammability of these compounds prevented the investigations in this field. So, attempts were directed to synthesize less explosive and less toxic boron hydride derivatives. It was found that polyhedral boron hydrides are less toxic and not flammable. The first polyhedral boron hydrides were described in 1959 and 1960 by Hawthorne and Pitochelli1 with the examples of decahydro-closo-decaborate [B10H10]2– and dodecahydro-closo-dodecaborate [B12H12]2–. In these clusters, the whole polyhedron consist of boron atoms. At the end of 1963 the first communications describing the synthesis of icosahedral carboranes, a new unusual class of polyhedral compounds whose cage consisted of ten boron atoms and two carbon atoms,2, 3 were simultaneously published in USSR and USA. Although papers on derivatives known as lower and medium carboranes had already been published,4 the publications on higher icosahedral carboranes made a great impression on the scientific community of that time. It became clear that a new, important page in the development of chemistry was thus opened. The subsequent vigorous development of this field demonstrated that the discovery of carboranes is as significant as the discovery of metallocenes and fullerenes for the development of modern chemistry or even science as a whole. The boron hydrides and polyhedral boron compounds proved to be interesting from both practical and theoretical points of view. The studies of their structure and reactivity formed the basis for important chemical concepts such as three-center two-electron bonds, multicenter bonds, and three-dimensional aromaticity. The unusual structures and properties of polyhedral borates and carboranes, and metallaboranes and metallacarboranes derived from them, caught the attention of scientists to this field and led to the preparation of a great number of new compounds. A comprehensive monograph on carboranes was recently presented by Grimes.5 In addition to boranes and carboranes, other types of boron compounds also received an enormous amount of attention. As shown in this thematic issue, a total of 41 articles highlight the different aspects of current boron chemistry and encompass a broad spectrum of compounds bearing one to ten boron atoms. This field continues to remain important and popular as can be seen by the contributions from all over the world. We hope that readers will find this timely cluster issue interesting and stimulating. We wish to thank all contributors, reviewers, and the EurJIC editorial team for their high-quality and professional work. Vladimir Bregadze graduated from the Chemistry Department of M.V. Lomonosov Moscow State University in 1960. He received his Ph.D. and D.Sc. degree, Professor of Chemistry in USSR Academy of Sciences. He is now Head of Laboratory of Organoaluminum and Boron Compounds of the A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. In 1976 V.I.Bregadze received the USSR State Prize for the Development and Application of New Materials in Industry, and in 1996 he received the Russian Federation State Prize in Science and Technology for the “Chemistry of Carboranes and Polyhedral Boranes”. His fields of interest are organic and inorganic derivatives of boron and main group metals, the chemistry of polyhedral boranes and carboranes, the study of their reactivity and application in medicine (antitumour activity, boron neutron capture therapy) and for the design of materials. He is Adjunct Professor of the Beijing University of Chemical Technology. He has published over 400 research papers, reviews and chapters in books. Zuowei Xie obtained a B.Sc. from Hangzhou University in 1983, a M.Sc. from Shanghai Institute of Organic Chemistry (SIOC), the Chinese Academy of Sciences in 1986, and his Ph.D. in 1990, working in a joint program between SIOC and Technische Universität Berlin. After serving as a Research Associate in SIOC and a Postdoctoral Fellow at University of Southern California, he joined the chemistry faculty of The Chinese University of Hong Kong in 1995, where he is now a Choh-Ming Li Professor of Chemistry. He has co-authored over 270 papers and received several prestigious awards including the State Natural Science Prize in 2008 and the Chinese Chemical Society Yao-Zeng Huang Award in Organometallic Chemistry in 2010.

Investigation on Tri-hexagonal Boron Nanotube by Exploiting the Certain Topological Indices and Their <i>M</i><i>-</i>polynomials

Due to the presence of multicenter bonds and their novel electronic properties, boron nanotubes are attractive. The tri-hexagonal boron nanotubes are build up from triangles and hexagons. It is useful to the QSPR/QSAR studies. Topological indices are classified in different forms such as, degree based topological indices, distance based topological indices and counting related topological indices etc. Here, we concentrated the reckoning of topological indices such as first zagreb, second zagreb, modified second zagreb index, generalized randic index, symmetric division degree index for the tri-hexagonal boron nanotube. Also, established their M-polynomials and using maple software plotted the 3D structure.

Crystal Structure, Stability, and Physical Properties of Metastable Electron-Poor Narrow-Gap AlGe Semiconductor.

We report for the first time the full crystal structure, the electronic structure, the lattice dynamics, and the elastic constants of metastable monoclinic AlGe. In addition to ultrarapid cooling techniques such as melt spinning, we show the possibility of obtaining monoclinic AlGe by water-quenching in a quartz tube. Monoclinic AlGe and rhombohedral Al6Ge5 are competing phases with similar stability since they both begin to decompose above 230 °C. The crystal structure and electronic bonding of monoclinic AlGe are similar to those of ZnSb and comply with its 3.5 valence electrons per atom: besides classical two electron-two center Al-Ge and Ge-Ge covalent bonds, Al2Ge2 parallelogram rings are formed by uncommon multicenter bonds. Monoclinic AlGe could be used in various applications since it is found theoretically to be an electron-poor semiconductor with a narrow indirect energy bandgap of about 0.5 eV. The lattice dynamics calculations show the presence of low energy optical phonons, which should lead to a low thermal conductivity.

All‐Metal Aromatic Sandwich Binuclear Complexes: Electronic Structures, Aromaticity and Interactions with Hydrogen via Multicenter Bonds

AbstractAccording to the first‐principle calculations, the aromatic Al42− unit can sandwich the two transition metal (TM) atoms forming a stable all‐metal binuclear compound [Al4TM2Al4]q‐ (q=‐2,0,2; TM=Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta). The stability analysis shows that these all‐metal binuclear compounds Al4TM2Al4 exist as stable complexes forming aluminum rings which can maintain their two‐fold aromaticity of the molecule, which is either the or the aromaticity. Moreover the interaction between Al4Sc2Al4 and hydrogen atom has been also studied using density functional theory (DFT). It has been found that one Al4Sc2Al4 molecule can adsorb 12 hydrogen atoms, whose hydrogen storage mass density is 3.77 wt%. Furthermore, the average binding energy which is in the range between 0.2‐0.7 eV/H2, can be either adsorbed or desorbed reversibly under near standard conditions. We have also found that a multicenter bond is formed between the H atoms and the Al or Sc atoms during the process of hydrogen adsorption, which is characterized by specific vibration frequency. Therefore, in order to store hydrogen reversibly and effectively, we can select the specific mode in the infrared band which is able to induce the hydrogen desorption from that which is adsorbed by Al8Sc2.

Effect of Alkali Metal Atoms Doping on Structural and Nonlinear Optical Properties of the Gold-Germanium Bimetallic Clusters

A new series of alkali-based complexes, AM@GenAu (AM = Li, Na, and K), have been theoretically designed and investigated by means of the density functional theory calculations. The geometric structures and electronic properties of the species are systematically analyzed. The adsorption of alkali metals maintains the structural framework of the gold-germanium bimetallic clusters, and the alkali metals prefer energetically to be attached on clusters’ surfaces or edges. The high chemical stability of Li@Ge12Au is revealed by the spherical aromaticity, the hybridization between the Ge atoms and Au-4d states, and delocalized multi-center bonds, as well as large binding energies. The static first hyperpolarizability (βtot) is related to the cluster size and geometric structure, and the AM@GenAu (AM = Na and K) clusters exhibit the much larger βtot values up to 13050 a.u., which are considerable to establish their strong nonlinear optical (NLO) behaviors. We hope that this study will promote further application of alkali metals-adsorbed germanium-based semiconductor materials, serving for the design of remarkable and tunable NLO materials.

MnBx monolayers with quasi-planar hypercoordinate Mn atoms and unique magnetic and mechanical properties

Searching and designing two-dimensional (2D) materials with unique topological structures and physical and chemical properties is extremely significant in the field of materials science. Here we report the design of 2D MnBx (x=1, 2, 3, 6) monolayers containing quasi-planar hypercoordinate motifs by means of density functional theory (DFT) computations and particle swarm optimization technique. These systems exhibit high cohesive energy as well as good kinetic and thermal stability, indicating that the MnBx monolayers can be prepared in experiments. Bond order analyses suggest that the abundance of multicenter bonds contributes to the stability of MnBx monolayers. Among them, MnB3 monolayer has nonmagnetic ground state while others exhibit ferromagnetic metallic properties. Moreover, MnB6 monolayer exhibits high spin polarization and negative Poisson’s ratio, which endows MnB6 with many special applications to cushioning and nanoelectronics.

Covalent versus Ionic Bonding in Al-C Clusters.

The low-energy structures of AlnCm (n = 4, 6; m = 1-4) are determined by using the genetic algorithm combined with density functional theory and the QCISD models. The electronic structures and bonding features are analyzed through the density of states (DOS), valence molecular orbitals (MOs), and electron localization function (ELF). The results show that the carbon atoms tend to aggregate and sit at the center of the clusters. The C-C bond lengths in most cases agree with the double C═C bond. Because of the large difference between the electronegativities of carbon and aluminum atoms, almost all of the 3p electrons of Al transfer to C atoms. The 3s orbitals of Al and the 2s2p orbitals of C form bonding and antibonding orbitals; the bonding orbitals correspond to the covalent C-Al bonds, and the antibonding orbitals form lone pair electrons on the outer side of Al atoms. The lone pair electrons form large local dipole moments and enhance the electrostatic interactions between C and Al atoms. Planar geometry and multiconnection are prominent structural patterns in small AlnCm clusters. However, the multiconnection does not correspond to multicenter chemical bonding. There are multicenter bonds, but they are much weaker than the σ C-Al bonds.

The chemical bond as an emergent phenomenon.

We first argue that the covalent bond and the various closed-shell interactions can be thought of as symmetry broken versions of one and the same interaction, viz., the multi-center bond. We use specially chosen molecular units to show that the symmetry breaking is controlled by density and electronegativity variation. We show that the bond order changes with bond deformation but in a step-like fashion, regions of near constancy separated by electronic localization transitions. These will often cause displacive transitions as well so that the bond strength, order, and length are established self-consistently. We further argue on the inherent relation of the covalent, closed-shell, and multi-center interactions with ionic and metallic bonding. All of these interactions can be viewed as distinct sectors on a phase diagram with density and electronegativity variation as control variables; the ionic and covalent/secondary sectors are associated with on-site and bond-order charge density wave, respectively, the metallic sector with an electronic fluid. While displaying a contiguity at low densities, the metallic and ionic interactions represent distinct phases separated by discontinuous transitions at sufficiently high densities. Multi-center interactions emerge as a hybrid of the metallic and ionic bond that results from spatial coexistence of delocalized and localized electrons. In the present description, the issue of the stability of a compound is that of the mutual miscibility of electronic fluids with distinct degrees of electron localization, supra-atomic ordering in complex inorganic compounds coming about naturally. The notions of electronic localization advanced hereby suggest a high throughput, automated procedure for screening candidate compounds and structures with regard to stability, without the need for computationally costly geometric optimization.