Small Bond Angles Research Articles

Anion photoelectron spectroscopy and chemical bonding of ThS2- and ThSO.

Anion photoelectron spectra of ThSO- and ThS2- were recorded using the third (355nm) harmonic of an Nd-YAG laser; these provided the measured vertical detachment energies of each anion. The experiments are supported by extensive coupled cluster calculations on ThSO, ThSO-, ThS2, and ThS2-, as well as the oxygen congeners ThO2 and ThO2-. The abinitio calculations, which included complete basis set extrapolations, spin-orbit effects using four-component coupled cluster, and higher-order correlation contributions through CCSDT(Q), yielded an adiabatic electron affinity for ThO2 that was within 0.02eV of the previously determined experimental value. The singly occupied molecular orbital (SOMO) in all three anions corresponds primarily to the 7s orbital on Th. Successive substitution of S for each O in ThO2 leads to larger electron affinities and smaller bond angles in the neutral molecules, but larger angles in the anions. As demonstrated by Franck-Condon simulations of the spectra using the CCSD(T) spectroscopic constants, substitution of O by S significantly complicates the resulting detachment spectra due to the lower vibrational frequencies in the sulfur species. Overall the calculated vertical detachment energies are in very good agreement with the experiment. In addition to the adiabatic electron affinities of each species, atomization energies and heats of formation have also been determined via the FPD approach with expected uncertainties of 1-2 kcal/mol.

An N‐Heterocyclic Germylene and Stannylene with a 1,1’‐Ferrocenylene Backbone and Dimesitylboryl N‐Substituents

AbstractThe diaminoferrocene derivative fc[NH(BMes2)]2 (1H2; fc=1,1’‐ferrocenylene, Mes=mesityl) was prepared from fc(NH2)2 and Mes2BCl and used for the preparation of the N‐heterocyclic tetrylenes fc{[N(BMes2)]2E} (1E, E=Ge, Sn) via the sodium amide fc[NNa(BMes2)]2 (1Na2), which was obtained from 1H2 and NaN(SiMe3)2. 1H2 is inert towards LiN(SiMe3)2 or nBuLi. Its reaction with nBuLi in the presence of TMEDA afforded the TMEDA‐coordinated lithium amide 1Li2(TMEDA)2. Crystallisation of 1Na2 from THF furnished the adduct 1Na2(THF)3. The reaction of 1Na2 with Me3SiCl afforded 1(SiMe3)2. Attempts to obtain the thiourea fc{[N(BMes2)]2C=S} (1C=S) from 1Na2 and Cl2C=S were not successful. 1C=S is accessible from fc(NH3)Cl2, which is transformed to fc[(NH)2C=S] (2) by reaction with Cl2C=S and NEt3 (4 equiv.), followed by conversion of 2 to fc{[N(SiMe3)]2C=S} (3) with Me3SiCl and NEt3 (2 equiv.) and subsequent reaction of 3 with Mes2BCl. All new compounds except 1Na2 and 2 were structurally characterised by single‐crystal X‐ray diffraction. The N‐boryl substituents of 1E compete efficiently with the EII atom for nitrogen π‐donation, causing comparatively long EII−N bonds and small EII bond angles. 1C=S contains a four‐membered BNCS ring due to intramolecular adduct formation involving the Lewis basic S atom and a Lewis acidic B atom.

Unraveling the effects of Cr interface segregation on precipitation mechanism and mechanical properties of MC carbides in high carbon chromium bearing steels

This study explored the influence of Cr interfacial segregation on the precipitation mechanism and mechanical properties of MC carbides in high-carbon chromium bearing steels. The precipitation of Cr-doped MC carbides at different concentrations was investigated using microscopic morphological characterization (SEM, EDS and HR-TEM) alongside first principles calculations. The results indicated that both the austenite matrix and MC carbides exhibited, FCC structures, and the following orientation relationships of crystal planes in high-carbon chromium bearing steels were as follows: MC (010)//FCC-Fe (100) and MC (111)//FCC-Fe (111). Initially, Cr atoms adsorbed on the Fe-top site of the MC carbide (010) crystal plane, maintaining a distance of 2.5 Å from the Fe atom. Notably, when the Cr atom doping amount was 0.5, the Cr–Fe metallic bond exhibited a long bond length, large bond angle and low bond strength in the MC transition state (TS), resulting in a reduced, reaction barrier (908.08 kcal/mol), thus promoting the precipitation of MC carbide in bearing steel. However, at a Cr atom doping amount level of 1, the shorter bond length, small bond angle, and higher bonding strength of the Cr–Fe metallic bond in the MC TS, elevated the reaction barrier (49709.72 kcal/mol), inhibiting the precipitation of MC carbide in bearing steel. Additionally, this study quantitatively analyzed the effect of Cr atom content on the brittle plastic properties and hardness of MC carbides. At Cr atom contents of 0.23 and 1, the MC carbide hardness reached a minimum value of 3.0 GPa and a maximum value of 17.1 GPa, respectively. This investigation not only elucidated the atomic scale effects of Cr interfacial segregation on the precipitation mechanism and mechanical properties of MC carbides but also provided new ideas for controlling carbide precipitation in high-carbon chromium bearing steels.

Chiral Bicyclic Imidazole-Catalyzed Direct Enantioselective C -Acetylation of Indolones

Chiral Bicyclic Imidazole-Catalyzed Direct Enantioselective <i>C</i> -Acetylation of Indolones

Which triatomic monohalosilylenes, monohalogermylenes, and monohalostannylenes (HMX) fluoresce or phosphoresce and why? An ab initio investigation.

The possibilities of emission from the Ã1A″ and ã3A″ excited states of the triatomic halosilylenes, halogermylenes, and halostannylenes (HMX, M = Si, Ge, Sn; X = F, Cl, Br, I) have been explored in a series of extensive ab initio calculations. The triplet states are found to have deep bonding wells supporting an extensive manifold of vibrational levels, which could give rise to observable triplet-singlet phosphorescence. The ã-X̃ band systems of the halosilylenes are calculated to occur at the red edge of the visible and are likely to be very weak. In contrast, the HGeX and HSnX triplet-singlet spectra are shifted 1000-2000cm-1 to the higher energy and are expected to be significantly stronger due to increased spin-orbit coupling, making the spectra viable targets for experimental investigations. The Ã-X̃ fluorescence is found to be limited by the possibility of HMX (Ã1A″) → H (2S) + MX (2Π) dissociation in the excited state, leading to the expectation that HGeF is unlikely to be detectable by laser-induced fluorescence (LIF) spectroscopy. The HSiX and HGeX species with known LIF spectra are found to have deeper à state bonding wells and minimal or no calculated barriers to dissociation. It is generally found that the intensity in their LIF spectra tails off due to a diminution of vibrational overlap rather than the abrupt opening of a dissociation channel. Few of the HSnX species are known experimentally. HSnF and DSnF are found to dissociate very low down in the à state vibrational manifold and are predicted to be unobservable by LIF spectroscopy. The LIF spectrum of HSnCl is expected to consist of only one or two bands, with slightly more activity for DSnCl, precisely as has recently been found experimentally. HSnBr and DSnBr have deeper à state bonding wells, and their LIF spectra are thus likely to be more extensive. Although HSnI and DSnI are calculated to have deep bonding wells with respect to the H + MX dissociation, predictions are complicated by the existence of a global small bond angle minimum and the opening of a second SnH + I dissociation channel.

Acetylenic Carbon-Containing Stable Five-Membered Metallacycles

Due to the linear property around an acetylenic carbon, the introduction of such an atom to a small cycle would result in high ring strain. Currently, the smallest isolated rings are five-membered, including metallacycloalkynes and metallapentalynes. Both types contain at least one unusual small bond angle around the acetylenic carbon, thus exhibiting abnormal reactivities. This feature article gives a comprehensive overview on these two kind complexes. The synthesis and reactivities are extensively described, the source of stability is presented, and the future prospect is discussed. The article aims to provide a better development for the chemical diversity of five-membered metallacycloalkynes and metallapentalynes.

Recent Advances in the Reactions of Cyclic Carbynes.

The acyclic organic alkynes and carbyne bonds exhibit linear shapes. Metallabenzynes and metallapentalynes are six- or five-membered metallacycles containing carbynes, whose carbine-carbon bond angles are less than 180°. Such distortion results in considerable ring strain, resulting in the unprecedented reactivity compared with acyclic carbynes. Meanwhile, the aromaticity of these metallacycles would stabilize the ring system. The fascinating combination of ring strain and aromaticity would lead to interesting reactivities. This mini review summarized recent findings on the reactivity of the metal–carbon triple bonds and the aromatic ring system. In the case of metallabenzynes, aromaticity would prevail over ring strain. The reactions are similar to those of organic aromatics, especially in electrophilic reactions. Meanwhile, fragmentation of metallacarbynes might be observed via migratory insertion if the aromaticity of metallacarbynes is strongly affected. In the case of metallapentalynes, the extremely small bond angle would result in high reactivity of the carbyne moiety, which would undergo typical reactions for organic alkynes, including interaction with coinage metal complexes, electrophilic reactions, nucleophilic reactions and cycloaddition reactions, whereas the strong aromaticity ensured the integrity of the bicyclic framework of metallapentalynes throughout all reported reaction conditions.

UV-Initiated Si–S, Si–Se, and Si–Te Bond Formation on Si(111): Coverage, Mechanism, and Electronics

Diaryl and dialkyl chalcogenide molecules serve as convenient precursors to silicon–chalcogenide bonds, ≡Si–E–R groups, on silicon surfaces, where E = S, Se, and Te. The 254 nm light, coupled with gentle heating to melt and liquefy the chalcogenide precursors for 15 min, enables formation of the resulting silicon–chalcogenide bonds. R groups analyzed comprise a long alkyl chain, octadecyl, and a phenyl group. Quantification of substitution levels of the silicon-hydride on the starting ≡Si(111)–H surface by an organochalcogen was determined by XPS, using the chalcogenide linker atom as the atomic label, where average substitution levels of ∼15% were found for all ≡Si–E–Ph groups. These measured substitution levels were found to agree with 2-dimensional stochastic simulations assuming kinetically irreversible silicon–chalcogen bond formation. Due to the small bond angle about the chalcogen atom, the phenyl rings in the case of ≡Si–E–Ph effectively block otherwise reactive Si–H bonds, leading to the observed...

Correlation between superconductivity and bond angle of CrAs chain in non-centrosymmetric compounds A2Cr3As3 (A = K, Rb).

Non-centrosymmetric superconductors, whose crystal structure is absent of inversion symmetry, have recently received special attentions due to the expectation of unconventional pairings and exotic physics associated with such pairings. The newly discovered superconductors A2Cr3As3 (A = K, Rb), featured by the quasi-one dimensional structure with conducting CrAs chains, belongs to such kind of superconductor. In this study, we are the first to report the finding that superconductivity of A2Cr3As3 (A = K, Rb) has a positive correlation with the extent of non-centrosymmetry. Our in-situ high pressure ac susceptibility and synchrotron x-ray diffraction measurements reveal that the larger bond angle of As-Cr-As (defined as α) in the CrAs chains can be taken as a key factor controlling superconductivity. While the smaller bond angle (defined as β) and the distance between the CrAs chains also affect the superconductivity due to their structural connections with the α angle. We find that the larger value of α-β, which is associated with the extent of the non-centrosymmetry of the lattice structure, is in favor of superconductivity. These results are expected to shed a new light on the underlying mechanism of the superconductivity in these Q1D superconductors and also to provide new perspective in understanding other non-centrosymmetric superconductors.

Studies on the σ–hole bonds (halogen, chalcogen, pnicogen and carbon bonds) based on the orientation of crystal structure

ABSTRACTThe 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole (DDBT) crystal has two polymorphic forms [1], where the close contacts between two electronegative atoms are identified and studied through Hirshfeld surface analysis. Br---Br and O---O cut-off distances are addressed and analysed. The σ–hole of bromine, sulphur, oxygen atoms and π-hole of carbon and nitrogen atoms were subjected to study using molecular electrostatic potential map and 3D-deformation density map. Sixteen types of dimers from the two forms of crystal structure (6 for form I and 10 for form II) were studied using the charge transfer properties and interaction energies and made detailed analysis of halogen bond (Br---N), dihalogen bond (Br---Br), chalcogen bond (O---Br and S---Br), dichalcogen bond (S---O, O---S and O---O), pnicogen bond (N---O) and carbon bond (C---O and C---Br) interactions. The impact of orientations is discussed to define the type of interaction and its strength through charge transfer mechanism. The contribution of bond angle values for the σ-hole and π–hole bonds are discussed. Utilisation of σ–hole in smaller bond angle values (above 30°) of |θ1 − θ2| existing in type II halogen–halogen bond have been examined in the two forms.

A new equation of state for associating Lennard–Jones fluids with two sites: small bond angle

ABSTRACTWe developed a new equation of state for Lennard–Jones spheres with two short-ranged, directional association sites. The theory is novel in that association is dependent on bond angle between the two sites, and formation of self-assembled linear and cyclic clusters is predicted in the model. The competition between ring and chain formations at various densities and temperatures is predicted by the theory and verified by Monte Carlo simulations. It was found that closed-loop structures become important at low temperatures and densities. Also, at fixed association energy, intensifying the Lennard–Jones energy reduces the extent of association in the fluid. The theory and simulation are in excellent agreement.

Trans Fatty Acids, does Exist Safety Dosage?

Recent data regarding Trans Fatty Acids (TFAs) have implicated this lipid as being particularly deleterious to human health. TFAs are unsaturated fatty acids that contain at least one non conjugated double bond in the trans configuration, resulting in a more linear shape. The presence of a trans double bond in a fatty acid chain results in a smaller bond angle, or kink, than in a cis double bond, resulting in a fatty acid chain conformation that is more similar to a saturated fatty acid than to an unsaturated fatty acid. Several studies have identified an association between trans-fat intake and a risk of neurodiseases, cardiometabolic disease and pro-inflammatory effects. It focuses on a series of recent studies that have supporting the pathogenic role through human or animal experiments in vitro or in vivo. Overall, the findings of this short communication suggest that fat composition is more important than the quantity of fat consumed in terms of dietary cis and trans fatty acids.

Naphthodithiophene-Based Conjugated Polymer with Linear, Planar Backbone Conformation and Strong Intermolecular Packing for Efficient Organic Solar Cells.

Two donor-acceptor copolymers, PBDT and PNDT, containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b']dithiophene (BDT) and 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b']dithiophene (NDT), respectively, as an electron-rich unit and 5,6-difluoro-2,1,3-benzothiadiazole (2FBT) as an electron-deficient unit, were synthesized and compared. The introduction of the NDT core into the conjugated backbone was found to effectively improve both light harvesting and the charge carrier mobility by enhancing chain planarity and backbone linearity; the NDT copolymer has stronger noncovalent interactions and smaller bond angles than those of the BDT-based polymer. Moreover, the introduction of the NDT core brings about a drastic change in the molecular orientation into the face-on motif and results in polymer:PCBM blend films with well-mixed interpenetrating nanofibrillar bulk-heterojunction networks with small-scale phase separation, which produce solar cells with higher short-circuit current density and fill factor values. A conventional optimized device structure containing PNDT:PC71BM was found to exhibit a maximum solar efficiency of 6.35%, an open-circuit voltage of 0.84 V, a short-circuit current density of 11.92 mA cm(-2), and a fill factor of 63.5% with thermal annealing, which demonstrates that the NDT and DT2FBT moieties are a promising electron-donor/acceptor combination for high-performance photovoltaics.

A simulation approach to study of entropic elasticity properties of polymer chain molecules using atomic scale Monte-Carlo

This paper describes atomic scale Monte–Carlo studies of entropic elasticity properties of individual polymer chain molecules. An efficient numerical Monte –Carlo sampling approach is outlined and used to evaluate the entropic contribution to the total elastic force.Theoretic predictions of mechanical properties of polymer molecules, particularly complex bio-molecules (proteins, lipids, etc.), are difficult due to effects of entropic elasticity. Entropic elastic force can be a significant contributor to the free energy F of the polymer chain and can even exceed interatomic potential energy U under external mechanical load. Monte-Carlo based approach allows to achieve atomic resolution for molecular structure in contrast to analytical methods. Specific load-extension curves are obtained numerically for a group of molecules with degenerate potential energy profiles. Results of the atomistic modeling are compared with the limiting continuum model of the same type of polymers. The extent of the linear and nonlinear elastic regimes and dependence on the molecular weight and geometric parameters of the molecules are discussed. A significant divergence with the continuummodel behavior is observed at smaller bond angles for all elongationsof the molecule. Linearity of the entropic force exists in a wide range ofthe elongations, however, molecules with low gyration radii(densely packedpolymers) are linear mostly in extensionor unfolding, while very sparsely packed molecules are linear mostly in thecontraction mode. The achieved result cannot be reproduced withinthe settings of the continuum model and required an application of atomic scale Monte-Carlo approach developed by our group.

Study of iron dimers reveals angular dependence of valence-to-core X-ray emission spectra.

Transition-metal Kβ X-ray emission spectroscopy (XES) is a developing technique that probes the occupied molecular orbitals of a metal complex. As an element-specific probe of metal centers, Kβ XES is finding increasing applications in catalytic and, in particular, bioinorganic systems. For the continued development of XES as a probe of these complex systems, however, the full range of factors which contribute to XES spectral modulations must be explored. In this report, an investigation of a series of oxo-bridged iron dimers reveals that the intensity of valence-to-core features is sensitive to the Fe–O–Fe bond angle. The intensity of these features has a well-known dependence on metal–ligand bond distance, but a dependence upon bond angle has not previously been documented. Herein, we explore the angular dependence of valence-to-core XES features both experimentally and computationally. Taken together, these results show that, as the Fe–O–Fe angle decreases, the intensity of the Kβ″ feature increases and that this effect is modulated by increasing amounts of Fe np mixing into the O 2s orbital at smaller bond angles. The relevance of these findings to the identification of oxygenated intermediates in bioinorganic systems is highlighted, with special emphasis given to the case of soluble methane monooxygenase.

Resummed thermodynamic perturbation theory for bond cooperativity in associating fluids with small bond angles: effects of steric hindrance and ring formation.

In this paper we develop a thermodynamic perturbation theory for two site associating fluids which exhibit bond cooperativity (system energy is non-pairwise additive). We include both steric hindrance and ring formation such that the equation of state is bond angle dependent. Here, the bond angle is the angle separating the centers of the two association sites. As a test, new Monte Carlo simulations are performed, and the theory is found to accurately predict the internal energy as well as the distribution of associated clusters as a function of bond angle.

Densification of a continuous random network model of amorphous SiO2glass

We have investigated the mechanism of densification of a nearly perfect continuous random network (CRN) model of amorphous SiO2 (a-SiO2) glass with 1296 atoms and periodic boundary conditions. The model has no under- or over-coordinated atoms and small bond length and bond angle distributions. This near-perfect model is systematically densified up to a pressure of 80 GPa using ab initio constant-pressure technique. By assessing a full spectrum of properties including atomic structure, bonding characteristics, effective charges, bond order values, electron density of states, localization of wave functions, elastic and mechanical properties, and interband optical absorption at each pressure, we reveal the pertinent details on the structural, mechanical and optical characteristics of the glass model under pressure. They all confirm the central theme that amorphous to amorphous phase transformation (AAPT) from a low-density state to a high-density state is at a pressure between 20 and 35 GPa in this nearly ideal a-SiO2 network. This pressure range represents an upper limit for such a transition in vitreous silica. The phase transformation roots from the change of Si-O bonding from a mixture of ionic and covalent nature at low pressure to a highly covalent bonding under high pressure. In addition, the calculated theoretical refractive index of the glass model as a function of the pressure is reported for the first time and in good agreement with the available experimental data.

BaRh2Si9– a new clathrate with a rhodium–silicon framework

The semiconducting compound BaRh2Si9 is a new kind of intermetallic clathrate. It was obtained by reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a = 6.2221(5) Å, b = 21.399(2) Å, c = 6.2272(5) Å, β = 90.306(7)°) displays a covalently bonded Rh-Si framework, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of 0.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba(+1.3)) and negatively charged Rh atoms (Rh(-1)). Si atoms with neighboring Rh atoms are positively charged, while those connected only with Si atoms are negatively charged.

Erratum: “Molecular theory for the phase equilibria and cluster distribution of associating fluids with small bond angles” [J. Chem. Phys. 139, 054902 (2013)

Erratum: “Molecular theory for the phase equilibria and cluster distribution of associating fluids with small bond angles” [J. Chem. Phys. 139, 054902 (2013)

Molecular theory for the phase equilibria and cluster distribution of associating fluids with small bond angles

We develop a new theory for associating fluids with multiple association sites. The theory accounts for small bond angle effects such as steric hindrance, ring formation, and double bonding. The theory is validated against Monte Carlo simulations for the case of a fluid of patchy colloid particles with three patches and is found to be very accurate. Once validated, the theory is applied to study the phase diagram of a fluid composed of three patch colloids. It is found that bond angle has a significant effect on the phase diagram and the very existence of a liquid-vapor transition.