Bent Bonds Research Articles

ChemInform Abstract: Bent Bonds, the Antiperiplanar Hypothesis and the Theory of Resonance. A Simple Model to Understand Reactivity in Organic Chemistry

AbstractReview: 64 refs.

Properties of the Mortar Modified with the Styrene-Acrylic Emulsion

Styrene-acrylic emulsion was selected to modify the cement mortar which was prepared as the crack mending material for the concrete pavement. The results indicated when the percentage of the styrene-acrylic emulsion was from 15% to 20%, the flexural strength and the bent bond strength increased by 39.00%~48.23% and 45.01%~49.60% respectively at 28 days, compared to the reference mortar. The morphology of the cement mortar modified with styrene-acrylic emulsion was observed by SEM. The modification mechanism of the styrene-acrylic emulsion to the mortar was discussed.

Bent bonds, the antiperiplanar hypothesis and the theory of resonance. A simple model to understand reactivity in organic chemistry

By taking into consideration bent bonds (τ-bonds, tau-bonds), the antiperiplanar hypothesis, the classic theory of resonance, and the preference for staggered bonds over eclipsed bonds in tetrahedral systems, a simple qualitative model is presented to rationalize the conformation and reactivity for a wide range of compounds containing double bonds and/or carbonyl groups. Alkenes, carbonyl and carboxyl derivatives, conjugated systems as well as other functional groups are revisited. This also leads to a simple model to understand aromaticity, and electrocyclic reactions. The bent bond model and the antiperiplanar hypothesis provide a qualitative model for better understanding the electron delocalization and the reactivity inherent to unsaturated organic systems by an alternative view of the classic resonance theory.

Hydrogen bonding in Schiff bases – NMR, structural and experimental charge density studies

A series of sixteen Schiff bases (derivatives of salicylaldehydes and aryl amines) was studied to reveal the influence of substituents and the length of the linker on the properties of the H-bonding formed. In theory, two groups of compounds, derivatives of 2-(2-hydroxybenzylidenoamine)phenol) and 2-hydroxy-N-(2-hydroxybenzylideno)benzylamine, can form different types of H-bonds using one or two hydroxyl groups present in the molecules. Two other groups of compounds, derivatives of 4-(2-hydroxybenzylidenoamine)phenol and N-(2-hydroxybenzyideno)benzylamine, can form only one type of H-bond. It was confirmed by (15)N and (13)C NMR experiments, that in all cases only traditional, H-bonded six-membered chelate rings were formed. The positions of the hydrogen atom in the rings depend on the substituent and phase. Generally, the OH H-bond form dominates in solution, with exception of the nitro derivatives, where the NH tautomer is present. In the solid state the tautomeric equilibrium is strongly shifted to the NH form. Only for the 5-Br derivative of one compound was the reverse relationship found. According to the results of experimental charge density investigations, two intramolecular H-bonds in the 5-methoxy derivative of 2-hydroxy-N-(2'-hydroxybenzylideno)benzylamine) differ significantly in terms of charge density properties. The intra- and intermolecular H-bonds formed by the deprotonated oxygen atom from 2-OH group are strong, with significant charge density concentration at the bond critical point and a straight, well-defined bond path, whereas the second intramolecular H-bond formed by the oxygen atom from the 2'-OH group is quite weak, with ca. five times smaller charge density concentration than in the previous case and a bent bond path. In terms of energy densities, the latter H-bond appears to be a non-bonding interaction, with total energy density being slightly positive. In terms of source contributions to the density at the H-bond critical point from the atoms involved, the intermolecular, linear H-bond is very strong and charge-assisted in the source function classification, the N(1)-H(1N)···O(1) H-bond is medium-strength, while the third H-bond is extremely weak.

Network structure and concentration fluctuations in a series of elemental, binary, andtertiary liquids and glasses

Liquids and glasses continue to produce a lively debate about the nature of thedisordered structure in these materials, and whether it is driven by longer rangeconcentration or density fluctuations. One factor often lacking in these studies isan overview of a wide range of structures from which common features of anddifferences between materials can be identified. Here I examine the structure ofa wide range of chain and network, elemental, binary and tertiary liquids andglasses, using available x-ray and neutron diffraction data and combining them withempirical potential structure refinement. Calculation of the Bhatia–Thorntonnumber–number and concentration–concentration structure factors and distributionfunctions highlights common structural motifs that run through many of the series. It isfound that the greatest structural overlap occurs where the nearest-neighbour andsecond-neighbour coordination numbers are similar for different materials. Asthese coordination numbers increase, so the structures undergo a sequence ofcharacteristic changes involving increasingly bent bond angle distributions and increasedpacking fractions. In these regards liquid and amorphous phosphorus appear to bein a structural class of their own, combining both chain-like and network-likecharacteristics.

Electron density distribution of β-rhombohedral boron constructed from icosahedral cluster

Boron forms unusual structures and chemical bonds. Pure boron and many boron-rich borides have local icosahedra in their crystalline structures. The electron density distribution (EDD) in β-rhombohedral boron (β-B105) is visualized by the maximum entropy method (MEM)/Rietveld analysis of powder X-ray diffraction (XRD) data in order to discuss the peculiarity of the chemical bonds. In α-rhombohedral boron (α-B12) and its derivative crystals, peculiar covalent bonds, i.e., intracluster three-center bonds and intercluster bent bonds, have been observed. In β-B105, the intracluster three-center bonds are observed, but the intercluster bent bonds are not. The origin of the bent bond can be explained by the mismatch in the directions of the bond and the lattice vector.

Addition of nucleophile to bent bonds of the carbonyl group

Addition of nucleophile to bent bonds of the carbonyl group

Theoretical study of bifurcated bent blue-shifted hydrogen bonds CH2···Y

Ab initio quantum chemistry methods were applied to study the bifurcated bent hydrogen bonds Y··· H2CZ (Z = O, S, Se) and Y···H2CZ2 (Z = F, Cl, Br) (Y = Cl−, Br−) at the MP2/6-311++G(d,p) and MP2/6-311++G(2df,2p) levels. The results show that in each complex there are two equivalent blue-shifted H-bonds Y···H-C, and that the interaction energies and blue shifts are large, the energy of each Y···H-C H-bond is 15–27 kJ/mol, and Δr(CH) = −0.1 − −0.5 pm and Δv(CH) = 30 − 80 cm−1. The natural bond orbital analysis shows that these blue-shifted H-bonds are caused by three factors: large rehybridization; small direct intermolecular hyperconjugation and larger indirect intermolecular hyperconjugation; large decrease of intramolecular hyperconjugation. The topological analysis of electron density shows that in each complex there are three intermolecular critical points: there is one bond critical point between the acceptor atom Y and each hydrogen, and there is a ring critical point inside the tetragon YHCH, so these interactions are exactly H-bonding.

DFT Study on the SnII‐Catalyzed Diastereoselective Synthesis of Tetrahydrofuran from D–A Cyclopropane and Benzaldehyde

AbstractBy means of density functional theory (DFT), the regio‐ and diastereoselective mechanism for the synthesis of 2,5‐disubstituted tetrahydrofuran from a donor–acceptor (D–A) cyclopropane cycloaddition reaction with benzaldehyde catalyzed by Sn(OTf)2 was investigated. As demonstrated, the overall reaction includes the activation of the D–A cyclopropane (S)‐1, an unusual SN2 attack on the activated cyclopropane, the formation of the tetrahydrofuran coordination complex and the regeneration of the catalyst. The D–A cyclopropane is activated by its combination with Sn(OTf)2, which leads to a decrease in the natural bond orbital (NBO) energy ofσ*(C1–C2) and an increase in the dipole moment and angular deviation of the bent bond σ(C1–C2). Two nucleophilic reactions generates two chiral carbon centres, and the first one is rate controlling. In the SN2 reaction, one molecule of trifluoromethane sulfonate forms a hydrogen‐bond H(CHO)···O(OTf), which can control the stretching direction of the benzaldehyde. On the whole, the solvation energies of the species increase with increases in the molecular dipole moments, and the solvent effects increase the reaction barriers. The theoretically predicted dominant product is cis‐(2R,5R)‐tetrahydrofuran. So these can satisfactorily account for the experimental observations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Conformational and stereoelectronic investigation in 1,2-difluoropropane: The gauche effect

The effect of attaching an additional fluorine atom at C-2 in 1-fluoropropane (FP), giving 1,2-difluoropropane (DFP), on its conformational equilibrium, is theoretically evaluated. This substitution causes critical implications on the conformer stabilities of DFP (TG, GT and GG conformations) and the steric and electrostatic interactions should favor the conformer with fluorine atoms trans. However, the gauche effect plays a major role in describing the energies balance in DFP, shifting the equilibrium towards the conformation in which the two fluorine atoms are gauche. The origin of this effect is discussed through an NBO analysis, which allows the evaluation of both classical and non-classical (hyperconjugation and bent bonds) interactions as the prevailing factors governing the conformational equilibrium of molecules containing the 1,2-difluoroethane fragment.

Investigation of the stereoelectronic properties in 1,2-difluorocyclopropane and 1,2-difluorocyclohexane: Evaluation of the gauche effect

The gauche effect, the phenomenon of approximating (dihedral angle about 60°) electronegative atoms or groups in a 1,2-disubstituted ethane fragment, is theoretically evaluated in this report, taking 1,2-difluorocyclopropane and 1,2-difluorocyclohexane ( cis and trans) as models. NBO analysis was used in order to give insight into the nature of the gauche effect in the studied compounds, i.e. its origin has been evaluated as due to hyperconjugation or to bent bonds.

Synthesis, structure and spectroscopic study of Rh III polypyridine complexes with phenylcyanamide derivative ligands

Several new Rh III complexes, [Rh(tpy)(bpy)L](PF 6) 2 (tpy=2,2′:6′,2″-terpyridine, bpy=2,2′-bipyridine, and L=monoanions of phenylcyanamide(pcyd)), 4-methylphenylcyanamide (4-MePcyd), 2,4-dimethylphenylcyanamide (2,4-Me 2pcyd), 4-methoxyphenylcyanamide (4-MeOPcyd), 2-chlorophenylcyanamide (2-Clpcyd) and 2,5-dichlorophenylcyanamide (2,5-Cl 2pcyd) have been synthesized and characterized by elemental analysis, IR, 1H NMR and electronic absorption spectroscopies. ORTEP drawing of [Rh(tpy)(bpy)(2,5-Cl 2pcyd)](PF 6) 2·1/2CH 3CN shows three pyridyl rings of the tpy ligand that are nearly coplanar, as are the two rings of bpy. The anionic cyanamide group is coordinated end-on by the nitrile nitrogen to the Rh III. The Rh III–NCN bond is bent, having an angle of 125.4°. This bent bond is largely determined by the σ-bonding interaction of a cyanamide non-bonding electron pair in a sp 2 hybrid orbital.

“Bent Bonds” between Bismuth and Carbon Atoms as a Result of CH Activation in MoBi Complexes

The reaction of molybdocenedihydride with two equivalents of [Bi(OtBu)(3)] proceeds via alcohol elimination and provides the compound [Cp(2)Mo{Bi(OtBu)(2)}(2)] (1), which contains two Mo--Bi metal bonds, in good yields. If the two reagents are employed in a 1:1 ratio continuative condensation reactions occur. These initially lead to [{Cp(2)Mo}(2){mu-Bi(OtBu)}(2)] (2), which, however, is very unstable in solution and decomposes via additional alcohol elimination: Complex-induced proximity effects facilitate the cleavage of C--H bonds within the cyclopentadienyl ligands by the residual alkoxide ligands, so that spontaneously two further equivalents of alcohol are released, thereby yielding two isomeric compounds 3 and 4 with Cp ligands bridging Mo--Bi metal bonds: The first isomer (3) contains two mu(2)-eta(5):eta(1)-C(5)H(4) ligands, the second isomer (4) contains one bridging mu(3)-eta(5):eta(1):eta(1)-C(5)H(3) ligand. The binding of these ligands to molybdenum and bismuth atoms at the same time is made possible through "bent bonds" between the bismuth and certain carbon centres. These unusual bonding situations were analysed by means of calculations based on density functional theory (DFT), the atoms in molecules (AIM) theory, natural bond order (NBO) considerations and the electron localisation function (ELF). According to the results the bonds can be understood in terms of carbanionic centres interacting with bismuth cations (i.e. closed-shell interactions). The formation of these bonds and the thermodynamics/kinetics involved on going from 2 to 3 and 4 were also studied by theoretical methods, so that the product formation is rationalised. The crystal structures of all four new compounds were determined. These structures but also the properties and mechanisms of formation are discussed against the background of the corresponding results obtained while studying the system [(Me)Cp(2)MoH(2)]/[Bi(OtBu)(3)].

A Classical Approach to Aromaticity and a New Representation of Benzene

AbstractFor Abstract see ChemInform Abstract in Full Text.

3-Silaoxetane and 3-silathietane: structure and reactivity studies on the basis of spectroscopic data and theoretical calculations

The ab initio and DFT calculations (structural parameters, electron localization function (ELF)) on 3-silaoxetane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-oxa-3-silacyclobutane ( 1) and 3-silathietane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-sila-3-thiacyclobutane ( 2) show the cyclobutane ring in 2 as being non-planar with a C–Si–C angle of 89.2° and a C–S–C angle of 93.3°, whereas the cyclobutane ring in 1 is planar with an unusual small bond angle at the silicon atom of 74.7°, which can only be explained by bent bonds. Since the synthesis was performed in water, small bent angles cannot be indicative for high reactivity. The Raman spectra of 1 and 2 were then recorded and analyzed in the 1800–200 cm −1 spectral region at various temperatures (300–10 K) with the help of the DFT calculation results (harmonic vibrational wavenumbers, Raman scattering activities). Although the wavenumber shifts are quite small, the subtle changes in the spectral features of the 3-silaoxetane and phenyl rings vibrational modes may indicate a loss of symmetry in 1 (between 200 and 150 K) and a possible phase transition in 2 (at about 200 K). Furthermore, the Raman spectra of 1 and 2 confirmed the ELF calculation results, excluding any bond interaction between the silicon and the oxygen or sulfur atom.

A classical approach to aromaticity and a new representation of benzene

The classical chemical structure theory (CST) provides “localised” qualitative models for the structure and reactivity of organic molecules, while, in addition to quantitative methods, the modern molecular orbitals (MO) theory provides “delocalised” qualitative models. In particular, these two theories provide two different descriptions of the ethylenic double bond, bent bonds and σ/π orbitals, respectively. In the framework of CST, a graphical model of aromaticity and antiaromaticity, related to a classical model of Woodward–Hoffmann rules, is presented. By application of Linnett's double-quartet theory, a new representation for benzene is proposed. It can be easily visualised when a new split representation of Linnett-type formulae is used.

Superoxo, mu-peroxo, and mu-oxo complexes from heme/O2 and heme-Cu/O2 reactivity: copper ligand influences in cytochrome c oxidase models.

The O(2)-reaction chemistry of 1:1 mixtures of (F(8))Fe(II) (1; F(8) = tetrakis(2,6-diflurorophenyl)porphyrinate) and [(L(Me(2))N)Cu(I)](+) (2; L(Me(2))N = N,N-bis(2-[2-(N',N'-4-dimethylamino)pyridyl]ethyl)methylamine) is described, to model aspects of the chemistry occurring in cytochrome c oxidase. Spectroscopic investigations, along with stopped-flow kinetics, reveal that low-temperature oxygenation of 1/2 leads to rapid formation of a heme-superoxo species (F(8))Fe(III)-(O(2)(-)) (3), whether or not 2 is present. Complex 3 subsequently reacts with 2 to form [(F(8))Fe(III)-(O(2)(2-))-Cu(II)(L(Me(2))N)](+) (4), which thermally converts to [(F(8))Fe(III)-(O)-Cu(II)(L(Me(2))N)](+) (5), which has an unusually bent (Fe-O-Cu) bond moiety. Tridentate chelation, compared with tetradentate, is shown to dramatically lower the nu(O-O) values observed in 4 and give rise to the novel structural features in 5.

Studies of Pulsed Laser Deposition Mechanism of WO3 Thin Films

The growth dynamics of WO3 thin films by laser ablation of a WO3 monoclinic phase target was studied. Space and time-resolved optical spectra indicated the presence of neutral (WI), ionized (WII) and electronically excited molecule (WO) species in the plume. The reaction between W and O2 was manifested in the laser deposition. Raman spectra showed the films deposited over a target-substrate distance longer than 5 cm or (/and) under an ambient oxygen pressure lower than 200 mTorr deviated from the monoclinic phase of the target with shortened and bent bonds. This was due to the insufficient surface or (/and) gas oxidation. The oxygen pressure and the target-substrate distance were found to be important features in the stoichiometric and monoclinic phase formation. Both gas and surface reactions were responsible for the ultimate monoclinic phase formation in the pulse laser deposition. Parameters were optimised for the oxygen pressure and target-substrate distance.

The geometrical effect on the chemical reactivity of nucleophilic carbenes. A theoretical study

The reaction mechanism for the insertion of the nucleophilic carbenes into the C–H bond of methane has been investigated using the B3LYP/6-31G ∗ level of theory. It is found that the singlet–triplet splitting of a carbene can be used as a diagnostic tool for the prediction of its reactivity. Our theoretical findings also suggest that stronger p(π) donor atoms bonded the carbene atom, and a more bent bond angle at the carbene center should enhance stability of carbenes. Theoretical data for 5- and 4-membered ring carbenes are compared.

Peculiar Covalent Bonds inα-Rhombohedral Boron

A peculiar bonding feature of $\ensuremath{\alpha}$-rhombohedral boron is found in the electron density distribution of the icosahedral ${\mathrm{B}}_{12}$ cluster. The electron density distribution is obtained by using the maximum entropy method with synchrotron radiation powder data. It clearly shows the characteristic two- and three-center covalent bond network which threads through atoms on the cluster surface. Besides, two kinds of covalent bonds in intercluster space are also shown in detail: a three-center trilaterally formed bond among the three clusters and a two-center bond with a remarkable feature of a bent bond between the clusters. The obtained covalent bonding features indicate the cluster-based nature of this material.