Bent Bonds Research Articles

A Carbon Is a Carbon Is a Carbon: Orbital-Free Simulations of Hydrocarbon Chemistry without Resort to Atom Types.

Semiclassical electrons (aka Lewis dots) have been a mainstay of chemists' thinking about molecular structure, polarizability, and reactivity for over a century. This utility has motivated the development of a corresponding quantitative description. Here we devise pairwise potentials that describe the behavior of valence electron pairs in hydrocarbons, including those in single, double, bridge, and bent bonds of linear, branched, and cyclic compounds, including anionic and cationic states. Beyond predicting structures and energies, the new subatomistic force field, dubbed LEWIS-B, efficiently simulates carbocation addition to a double bond and cation migration to a neighboring carbon. A crucial feature of the semiclassical electrons is variable spread, a fourth degree of freedom in addition to three Cartesian coordinates. In spontaneously adapting to different environments, the spread provides a signature of electron stability, with more contracted clouds where the electron interactions are favorable and expanded clouds where electrons are less tightly held. In addition, the pair potentials provide insight into the subtle trade-offs that govern isomerizations and reactions.

A new mechanism for the thermal and photochemical Diels-Alder cycloaddition based on the bent bond / antiperiplanar hypothesis orbital model

The thermal and photochemical stereochemical reactivity of norbornene derivatives (Diels-Alder, retro-Diels-Alder, Cope rearrangement) can be rationalized through the Bent Bond /Antiperiplanar Hypothesis (BBAH) orbital model at the molecular level. This new orbital model considers unsaturations as Pauling τ bonds and electronic delocalization is expressed through pyramidal radical resonance structures interacting in antiperiplanar fashion with the antibonding orbital of τ bonds.

Stereochemistry of Eʹ and Eʹʹ decarboxylative-dehydration reactions and the bent bond / antiperiplanar hypothesis orbital model

The known concerted decarboxylative-dehydration reaction of unsaturated hydroxy-carboxylic acids 1–18 is analyzed through the Bent Bond/Antiperiplanar Hypothesis (BBAH), which considers double bonds as Pauling τ bonds. These reactions produce various mixtures of dienes or trienes having either the E or Z configuration. The relative formation of these mixtures of dienes or trienes can be rationalized through the BBAH model by the analysis of resonance structures leading to the antiperiplanar (APP) or the synperiplanar (SPP) electronic delocalization modes.

Revisiting the conformational analysis of unsaturated organic compounds using the bent bond / antiperiplanar hypothesis

The bent bond/antiperiplanar hypothesis (BBAH) offers a straightforward qualitative model for interpreting the conformational analysis of simple unsaturated acyclic systems, as well as cyclohexene and methylenecyclohexane derivatives. The torsional profiles of a series of unsaturated compounds were computed by MP2 calculations, and their relative conformational outcomes were interpreted using the BBAH model. • Bent Bond (τ)/Antiperiplanar Hypothesis (BBAH) rationalizing the conformation of unsaturated molecules. • Conformation of simple unsaturated (allylic) acyclic molecules and τ bonds. • Conformation of cyclohexene and methylenecyclohexane molecules and τ bonds. • Torsional profiles of a series of unsaturated compounds computed by MP2 calculation and τ bonds. • Hydrogen Hyperconjugation Effect (HHE: σ.→ τ∗) and Generalized Anomeric Effect (GAE: τ CC → σ∗ CX ) and τ bonds

The thermal rearrangements of naphthalene and azulene. An analysis through the bent bond and antiperiplanar hypothesis orbital model

Abstract The thermal conversion of azulene, naphthalene, norcaradiene, and polycyclic analogues can be rationalized through the Bent Bond/Antiperiplanar Hypothesis (BBAH) orbital model. The BBAH postulates that higher-energy pyramidal singlet diradical intermediates can be used to rationalize molecular rearrangements of these benzenoid aromatics under thermal conditions.

Bent Bond/Antiperiplanar Hypothesis and the Chemical Reactivity of Annulenes.

The properties and stereochemical reactivity of cyclobutadiene, benzene, cyclooctatetraene, and the [10]- to [14]annulenes can be uniformly rationalized through the bent bond/antiperiplanar hypothesis (BBAH). This new orbital model considers electronic delocalization between pyramidal diradical resonance structures and associated bent bonds, as it applies to aromatic, nonaromatic, and antiaromatic molecules.

Bent Bond/Antiperiplanar Hypothesis: Modulating the Reactivity and the Selectivity in the Glycosylation of Bicyclic Pyranoside Models.

Glycosylation reactions were performed on a series of bicyclic C2-substituted pyranoside models to isolate and analyze factors that control the glycosylation stereoselectivities observed in carbohydrates. The bent bond/antiperiplanar hypothesis (BBAH) orbital model rationalizes all of these results by considering hyperconjugation interactions between groups at C2 and the two τ bonds (bent bonds) of oxocarbenium ion intermediates formed under the glycosylation conditions. According to the BBAH, nucleophiles add to oxocarbenium intermediates by SN2-like antiperiplanar displacement of the weaker of their two τ bonds.

Probing the bent bonds in cyclopropane systems for gas storage and separation process: A computational study.

The hydrogen, carbon dioxide, and carbon monoxide gas adsorption and storage capacity of lithium-decorated cyclopropane ring systems were examined with quantum chemical calculations at density functional theory, DFT M06-2X functional using 6-31G(d) and cc-pVDZ basis sets. To examine the reliability of M06-2X DFT functional, a few representative systems are also examined with complete basis set CBS-QB3 method and CCSD-aug-cc-pVTZ level of theory. The cyclopropane systems can bind to one Li+ ion; however, the corresponding the methylated systems can bind with two Li+ ions. The cyclopropane systems can adsorb six hydrogen molecules with an average binding energy of 3.8 kcal/mol. The binding free energy (ΔG) values suggest that the hydrogen adsorption process is feasible at 273.15 K. The calculation of desorption energies indicates the recyclable property of gas adsorbed complexes. The same number of CO2 and CO gas molecules can also be adsorbed with an average binding energy of -14.4 kcal/mol and -10.7 kcal/mol, respectively. The carbon dioxide showed ~3-4 kcal/mol better binding energy as compared to carbon monoxide and hence such designed systems can function as a potential candidate for the separation of these flue gas molecules. The nature of interactions in complexes was examined with atoms in molecules analysis revealed the electrostatic nature for the interaction of Li+ ion with cyclopropane rings. The chemical hardness and electrophilicity calculations showed that the gas adsorbed complexes are rigid and therefore robust as gas storage materials.

Electronic Wavefunction Tiles

We review the pre-quantum theories of electronic structure of Lewis and Langmuir, and how this relates to the post-quantum double-quartet theory of Linnett. Linnett’s ideas are put on a firm theoretical footing through the emergence of the wavefunction tile: The 3N-dimensional repeating structure of the N-electron wavefunction. Wavefunction tiles calculated by the dynamic Voronoi Metropolis sampling method are reviewed, and new results are presented for bent bonds of cyclopropane, and electron correlation in Be-O-Be.

Applying the Bent Bond/Antiperiplanar Hypothesis to the Stereoselective Glycosylation of Bicyclic Furanosides.

The glycosylation stereoselectivities for a series of bicyclic furanoside models have been carried out in the presence of weak nucleophiles. These results were analyzed through the bent bond/antiperiplanar hypothesis (BBAH) orbital model to test its validity. According to the BBAH, incoming nucleophiles displace one of the two bent bonds of bicyclic oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation stereoselectivity is then governed by the displacement of the weaker bent bond as determined by the presence of electron-withdrawing or -donating substituents at C2. Overall, the BBAH analysis expands Woerpel's "inside/outside attack" glycosylation model by considering the stereoelectronic influence of neighboring electron-withdrawing and -donating groups on the nucleophilic addition to oxocarbenium ion intermediates.

High-Temperature Isomerization of Benzenoid Polycyclic Aromatic Hydrocarbons. Analysis through the Bent Bond and Antiperiplanar Hypothesis Orbital Model.

L. T. Scott has discovered the 1,2-swapping of carbon and hydrogen atoms which is known to take place on benzenoid aromatics (up to ∼1000 °C range). For example, 13C-1-naphthalene is specifically converted to 13C-2-naphthalene, and there is evidence that this occurs through the formation of benzofulvene and a naphthalene-carbene intermediate. Application of the bent bond/antiperiplanar hypothesis leads to the postulate that higher in energy pyramidal singlet diradical intermediates can be used to propose a mechanism that rationalizes various atom rearrangements on benzenoid aromatics and related isomeric compounds.

The Bent Bonds of Cubane

The carbon–carbon bonding in cubane is shown not to be along the C–C vectors between carbon atoms at the corners of a cube. Instead, the bonding is “bent” as seen in a new single‐crystal X‐ray structure of cubane at 93 K. The bent bond angles and lengths are compared with the geometric angles and with predictions. The distortions from the tetrahedral angle of the sp3‐hybridized carbon atoms are shown to be relatively small in cubane, unlike those in cyclopropane, which are large. Despite this and other differences, cubane and cyclopropane have similar reactivity, a fact that has not been widely recognized. It is suggested that cubane might mimic the chemistry of the cyclopropane ring in the pharmacology of some cyclopropane‐containing drugs.

Hydrothermal Synthesis, Crystal Structure, and Superconductivity of a Double-Perovskite Bi Oxide

Double-perovskite Bi oxides are a new series of superconducting materials, and their crystal structure and superconducting properties are under investigation. In this paper, we describe the synthesis and characterization of a new double-perovskite material that has an increased superconductive transition temperature of 31.5 K. The structure of the material was examined using powder neutron diffraction (ND), synchrotron X-ray diffraction (SXRD), and transmission electron microscopy (TEM). Rietveld refinement of the sample based on ND and SXRD data confirmed an A-site-ordered (K1.00)(Ba1.00)3(Bi0.89Na0.11)4O12 double-perovskite-type structure with the space group Im3m (No. 229). This structural analysis revealed the incorporation of Na with Bi in the structure and a bent bond between (Na, Bi)–O–(Na, Bi). TEM analyses also confirmed a cubic double-perovskite structure. This hydrothermally synthesized compound exhibited a large shielding volume fraction, exceeding 100%, with onset of superconductivity at ∼31...

Bent bonds (τ) and the antiperiplanar hypothesis, and the reactivity at the anomeric center in pyranosides.

The stereoselectivity of nucleophilic addition on oxocarbenium ions derived from the bicyclic pyranoside model with or without a C2-OR group can be understood through the use of the bent-bond and the antiperiplanar hypothesis in conjunction with the concept of hyperconjugation as an alternative interpretive model of structure and reactivity.

Bent bonds and the antiperiplanar hypothesis - a simple model to rationalize [1,3]-sigmatropic alkyl shifts.

The bent bond/antiperiplanar (BBA) hypothesis has been applied to the analysis of [1,3]-sigmatropic alkyl shifts. These thermal rearrangements, for which there is evidence that they proceed through diradical intermediates, can be interpreted by considering their transient allyl radical structures. For the thermolysis of cyclic molecules, the preferred generation of pyramidal allyl radicals in staggered conformations is postulated on the basis of the BBA hypothesis. This accounts for the preference of suprafacial rearrangement pathways as well as the extent of inversion or retention of configuration at the migrating carbons.

Unique bonding pattern and resulting bond stretch isomerism in Be32−

The bond-stretch isomers are characterized by a principal change in the bond-length with the rest of the molecule being unaltered. The electronic structure regulates the bond stretch isomerism phenomenon in which has been investigated with density functional theory, ab initio CASSCF, highly efficient n-electron valence state perturbation theory and multireference configuration interaction calculations. Two isomers are distinguished on different potential energy surfaces and the corresponding avoided crossing is also studied in details. The bonding pattern in two isomers are analysed through adaptive natural density partitioning analysis and quantum theory of atoms in molecules analysis. The bonds in both the isomers primarily involve the 2p orbitals, which overlap face-to-face in long-bond isomer. Whereas, in-plane π-bonding occurs at the short-bond isomer leading to unusual bent bond. © 2015 Wiley Periodicals, Inc.

A metallic carbon consisting of helical carbon triangle chains

Carbon is the basis of life on Earth and many technological applications. We computationally report an sp3-hybridization-assembled carbon allotrope constructed by helical triangle chains through the evolutionary structure prediction method. Different from the previous metallic carbon K4, this carbon, called Tri-carbon, is mechanically and dynamically stable at ambient pressure. High ring strain in the carbon triangle blocks forces the C–C bond in Tri-carbon to be a ‘bent bond’, rather than the common single bond in diamond or the π bond in graphite. Unlike the unstrained sp3-hybridization in semiconductive diamond, valence electrons in the ‘bent bond’ are recombined to form extremely anisotropic sp3-hybridized bonds, thus conferring metallicity to Tri-carbon. Under nonhydrostatic conditions, Tri-carbon shows significantly anisotropic ideal tensile and compressive strength. Tri-carbon is expected to be achieved through chemical methods, such as the synthesis of cyclopropane derivatives (e.g. triangulane and tetrahedrane). These methods eliminate the restriction of ultra-high pressure to obtain metallic carbons.

DFT study of N–H···O hydrogen bond between model dehydropeptides and water molecule

The strength of the hydrogen bond formed between a water molecule and two α,β-dehydroalanine derivatives including Ac-ΔAla-NMe2 (1) and Ac-ΔAla-NHMe (2) in comparison with standard amino acid Ac-Ala-NMe2 (3) is studied by density functional theory (with M06-2X and B3LYP functionals). Calculations were conducted for two different conformations of the peptides: extended (C5) and bent (β) with polyproline II backbone dihedral angles. The obtained results show that both dehydro and standard peptides in bent conformation form stronger hydrogen bonds with water than in the extended ones. Moreover, due to higher polarity of the N–H group of α,β-dehydroalanine residues, the H-bond in their complexes with water are stronger than for standard alanine.

ChemInform Abstract: Bent Bonds and the Antiperiplanar Hypothesis as a Simple Model to Predict Diels—Alder Reactivity: Retrospective or Perspective?

AbstractReview: 123 refs.

New iron nitrosyl complexes containing strong electron donating carbaporphyrins

A serious of new iron nitrosyl complexes containing carbaporphyrins as the macrocyclic ligand has been synthesized. Two methods have been developed to introduce the axial nitric oxide, either through nitrite reduction or through direct reaction with nitric oxide gas. In the case of nitrite reduction, the formation of the reduced {Fe (NO)}7 complex or the oxidized {Fe (NO)}6 can be controlled through the stoichiometry of the nitrite anion used as the reactant. Both oxidized and reduced carbaporphyrin iron nitrosyl complexes exhibit a 3N1C coordination environment for iron metal center to bind with the carbaporphyrin with which containing an inverted pyrrole ring to give an inner carbon atom in the coordination sphere. The linear and bent bond angles for {Fe (NO)}6 and {Fe (NO)}7, respectively, as well as the NO stretching frequencies in IR spectroscopy resemble regular iron porphyrin nitrosyl complexes. On the other hand, unique chemistry has been observed using the inner carbon methylated carbaporphyrin as the ligand. It is found that the iron oxidation and the nitric oxide coordination synchronized a C–H activation on the inner methyl group to give an unusual {Fe (NO)}6 iron nitrosyl complex. The conformation confirmed by the single crystal X-ray structure suggests a non-planar porphyrinic core with a strong pi-donating from the inner alkene to the iron center. The strong backbonding also resulted a small Fe–N–O angle at 148 degree. The unusual spectroscopies, electronic structures, and reactions of these carbaporphyrin complexes will be discussed.