The Cluster Anion Si94−

Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si₉⁴⁻ (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides.     

Reactions of Selected Bismuth Oxide Cluster Cations with Propene

Bismuth oxide cluster ions are reduced by propene in exactly the same way as solid bismuth oxide (Bi₂O₃), which is used as a catalyst. Even in the case of the smallest bismuth oxide cluster ions, such as the Bi₃O₄⁺ isomer shown in the picture, the supposedly unreactive bridging oxygen atom can participate in the alkene oxidation.     

Hexasupersilyloctaindane (tBu3Si)6In8— A Compound with a Novel In8 Cluster Framework

A doubly capped In ₆ octahedron characterizes the structure of the dark green octaindane (tBu₃Si)₆In₈ (see picture), which is obtained from (C₅Me₅)In and tBu₃SiNa in pentane at −78°C. In1/In1′ are the capping atoms, and In2/In2′ form the apexes of the stretched In₆ octahedron. Thus, in agreement with the results of ab initio calculations, the compound can be classified as a hypoprecloso-indane.     

A Five-Membered Ring with Three Negative Charges and Solvent-Free Lithium Counterions

Remarkably short distances to the ring plane are shown by the η⁵-bound lithium ions in the first compound with a triply negatively charged five-membered ring, 1 , which was obtained by reduction of 2 with lithium. R=CH(SiMe₃)₂, Dur=2,3,5,6-tetramethylphenyl.     

The Parent Hexacarbaborane arachno-C6B6H12 and a Methylated Pentacarbaborane arachno-CH3C5B7H12: Domains of Incipient Hydrocarbon Behavior within Borane Clusters

Increasingly pronounced hydrocarbon character is exhibited by C₆H₆B₁₂, the first unsubstituted hexacarbaborane, and CH₃C₅B₇H₁₂, the first cluster pentacarbaborane. These compounds shed light on the structural dichotomy between open hydrocarbon skeletons and polyhedral borane frameworks for high-carbon carboranes.     

Unusual Five-Center,Four-Electron Bonding in a Rhodium–Bismuth Complex with Pentagonal-Bipyramidal Geometry

A better understanding of the chemical bond in general is gained from the electronic structure of the molecular complex [{RhBi₇}Br₈]. The interactions in the central Bi₅ ring can be interpreted as an unusual five-center, four-electron bond based on ab initio calculations and group theory. Of the linear combinations of five Bi p orbitals two of the molecular orbitals are binding (depicted in the sketch).     

HCSiF and HCSiCl: The First Detection of Molecules with Formal C≡Si Triple Bonds

Neutralization of [C,H,Si,X] radical cations (X=F, Cl) in conjunction with electronic structure calculations provides the first experimental evidence for the formation of the neutral silynes HC≡SiF and HC≡SiCl, which have nonlinear structures (see picture).     

硼化合物的桥原子价及其反应活性的理论研究

根据原子价的量子化学定义,采用3-21G基组的ab initio方法,对B₂H₆及其桥原子取代物H₄B₂X₂(X=F,Cl,OH,NH₂,CH₃)进行几何构型优化,讨论桥原子的成键特征,并计算了B₄H₁₀、B₅H₉和B₅H₁₁等硼氢化合物的原子价,讨论了它们的亲电和亲核取代反应活性。     

Small Potassium Clusters

The molecules K ₂ , K ₃ , and K ₄ (structure shown on the right) have been isolated in krypton matrices at 15 K and characterized by Raman spectroscopy. Comparison of the experimental data with density functional calculations supports the prediction that potassium clusters are not only bonded by the valence electrons, but that there is also a contribution from the core electrons.     

Unimolecular rearrangement of α-silylcarbenium ions. An ab initio study

The unimolecular rearrangements of hydrogen, methyl and phenyl groups at the Si atom in α-silylcarbenium ions have been investigated using an ab initio molecular orbital method. MP2/6–31 + G^*//HF/6–31G^* calculations predict that all three groups migrate from the Si to an adjacent C_α with no energy barrier. Thus, the silicenium ion is the only stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C₆H₅)CH₂−SiH₂⁺, indicates that the phenyl ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si⁺) orbital. In contrast to MP2 results, Hartree-Fuck calculations (both HF/3–21G^* and HF/6–31G^* levels) predict small energy barriers for 1,2-migrations of H and Me (1.4 kcal mol⁻¹ for H migration, and 1.5 kcal mol⁻¹ for Me migration, respectively, at the HF/6–31G^* level). This difference provides convincing evidence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearrangement of α-silylcarbenium ions to silicenium ions. The results of the calculations have also been applied to the possible rearrangement mechanism of α-chlorosilanes to chlorosilanes, assuming that the experimental conditions are favorable toward the generation of ionic species. Various factors which may govern the migratory aptitudes of various R groups, i.e. (1) activation energies, (2) overall reaction energies and (3) the conformational preference of reactants have been investigated. The calculated activation energy obtained, namely the energy for the generation of the silicenium ion and the C⁻¹ ion from an α-chlorosilane, is consistent with the experimental migratory aptitude in the gas phase observed in mass spectrometers.     

Polymere Carbodiimide als Vorstufen für keramische Werkstoffe: Reaktionsmechanismen und Tautomeriebeziehungen

Polymeric Carbodiimides as Precursors for Ceramic Materials: Reaction Mechanisms and Tautomerism The reaction mechanisms for the synthesis of polymeric silicon and titanium carbodiimides via the element chlorides and bis(trimethylsilyl)‐carbodiimid, used as precursors for ceramic materials, have been investigated using quantumchemical methods. The influence of organic bases as catalysts on the transition state and the thermodynamics of the reaction are discussed. The ratio of the tautomers carbodiimid ‐ cyanamid depending on the heteroatom, the solvent and the temperature, has been determined using ab‐initio methods (density functional theory). The results of the theoretical calculations are confirmed by IR and UV spectroscopic data.     

2,3,5-Tri-tert-butyl-1-carba-nido-tetraborane

The missing link in the series of tert-butyl derivatives of the isoelectronic nido clusters B₄H₈, CB₃H₇, and NB₃H₆ with a bicyclobutane-type structure is represented by the title compound CB₃H₄tBu₃ ( 1 ), the clusters B₄H₄tBu₄ and NB₃H₃tBu₃ being well known. Compound 1 may also be considered to be a derivative of the nido-triborane B₃H₇, which has hitherto not been isolated, formed by replacing three H atoms by tBu groups and two H atoms by a CH₂ bridge, as expressed by the formula B₃H₂tBu₃(CH₂).     

Molecular structure of phenylsilane: a study by gas-phase electron diffraction and ab initio molecular orbital calculations

The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at the MP2(f.c.)/6-31G* level. The calculations indicate that the perpendicular conformation of the molecule, with a Si–H bond in a plane orthogonal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si–H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol⁻¹, implying free rotation of the substituent at the temperature of the electron diffraction experiment (301 K). Important bond lengths from electron diffraction are: <r_g(C–C)>=1.403±0.003 Å, r_g(Si–C)=1.870±0.004 Å, and r_g(Si–H)=1.497±0.007 Å. The calculations indicate that the C_ipso–C_ortho bonds are 0.010 Å longer than the other C–C bonds. The internal ring angle at the ipso position is 118.1±0.2° from electron diffraction and 118.0° from calculations. This confirms the more than 40-year old suggestion of a possible angular deformation of the ring in phenylsilane, in an early electron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218.     

Ab initio analysis of the interaction of CO2 with acetylated d-glucopyranose derivatives

Peracetylated d-glucopyranose has a high solubility in CO₂ and can be a promising phase-change physical solvent or absorbent for CO₂, as reported recently. However, peracetylated d-glucopyranose is unstable under acidic atmospheres, especially in sulfur-containing waste gases, and the possibly major decomposition products are 2,3,4,6-tetra-O-acetyl-d-glucopyranose, 1-thiol-d-glucopyranose tetraacetate, and 1-mercaptoethyl-d-glucopyranose tetraacetate. Therefore, it is highly interesting to investigate the interaction between CO₂ and these three compounds using ab initio calculations, including geometry optimizations with HF/3-21G, B3LYP/6-31+G** and single-point energy calibration with MP2/aug-cc-pVDZ. The results indicate that the electrostatic interactions between the substrates and CO₂ are mainly influenced by the interaction distance and the numbers of negative charge donors or the interacting pairs involved in the complex. It is furthermore found that ΔE increases significantly if S and O atoms could interact with CO₂ simultaneously. The binding energy is irrelevant if one considers the chemical environment of the O atom (i.e. O_Ac, O_E or O_S) or the S atom (i.e. SEt or SH), and the electronegativity difference between the S and O atoms. The three substrates studied are still excellent CO₂-philes, although their average ΔE (–20 kJ/mol) is slightly lower than that of peracetylated d-glucose (–22 kJ/mol), which has one more O atom that can interact with CO₂. Therefore, the applications of carbohydrates can be expanded to include adsorbents for CO₂, SO₂ or both, and the functional groups attached to the carbohydrate can vary from those to the acetyl groups.     

Photoelectron spectra of cyclopolysilanes

Summary He (I) PE spectra of cyclopentasilane and cyclohexasilane show excellent agreement with STO-3 G + ab initio calculations. The HOMO in both compounds is of (SiSi) 3 p character. First IP's appear at 9.4 and 9.6 eV respectively.

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Photoelektronenspektren von Cyclopolysilanen (Kurze Mitt.)

Zusammenfassung Die He (I) PE-Spektren der Cyclosilane Si₅H₁₀ und Si₆H₁₂ zeigen ausgezeichnete Übereinstimmung mit STO-3 G + ab initio Rechnungen. Beide Ringe besitzen HOMO's mit (SiSi) 3 p Charakter. Die ersten Ionisierungspotentiale liegen bei 9.4 bzw. 9.6 eV.

     

The Haloboration of Tri‐tert‐butylazadiboriridine NB2R3: Ring Opening versus NB3 Cluster Formation

The azadiboriridine [–BR–NR–BR–] ( 1 ; R = tBu) is bromoborated at the B–B bond by alkyldibromoboranes R′BBr₂ to give the products Br–BR–NR=BR–BR′–Br ( 8 a – g : R′ = Me, Bu, iBu, Bzl, CH₂CHEt₂, CH₂Cy, CH₂(4‐C₆H₄tBu)). Two isomers of each of the products 8 a – g are formed and attributed to a cis/trans isomerism at the BN double bond; the isomerization is followed thermodynamically and kinetically by NMR methods with 8 a – d . The analogous chloroboration of 1 with BCl₃ yields Cl–BR–NR=BR–BCl₂ ( 8 h ), which at ambient temperature undergoes a degenerate exchange of the ligands Cl and BCl₂ along the B–N–B skeleton. At room temperature, the isomer Cl–BR–NR=BCl–BR–Cl ( 8 h ′) is slowly formed by an irreversible exchange of R and Cl along the B–B bond of 8 h . Different from BCl₃, the chloroborane BH₂Cl is simply added to the B–B bond of 1 under formation of the aza‐nido‐tetraborane NB₃R₃H₂Cl ( 2 b ). The chloroborane BHCl₂ gives a mixture of 8 h ′ and 2 b upon addition to 1 , apparently according to a preceding dismutation into BCl₃ and BH₂Cl. The configuration at the B3 atom of the nido‐clusters NB₃R₃H₂X (X = H, Cl) is discussed on the basis of the corresponding model molecules NB₃Me₃H₂X, whose structure and NMR signals are computed by the B3LYP method. The boranes 8 b – g can be debrominated with Li in the presence of tmen on applying ultrasound. The products are found to be the B‐borylated azadiboriridines [–BR–NR–B(BRR′)–] ( 9 b – g ). The 2‐borylazadiboriridines NB₃H₄ ( 9 h ) and NB₃Me₄ ( 9 i ) were found as local minima on the energy hyperface by the B3LYP method, but minima for structural isomers with lower energy were also found; the tetrahedral clusters NB₃R₄ give high‐energy minima with triplet ground states. Computations of the ¹¹B NMR shifts of 9 h and 9 i support the proposed structures of 9 b – g .     

封闭型硼烷B15H15^2—（D3h）的结构及稳定性的从头算分析

利用量子化学从头算方法在ＨＦ／６－３１Ｇ水平上，优化计算了带２个负电荷的封闭型硼烷Ｂ１５Ｈ１５＾２－（Ｄ３ｈ）的结构、能量和正则振动频率。该硼烷的简正振动频率计算表明，它是ＨＦ／６－３１Ｇ势能超曲面上的真实稳定点。理论预测它在化学上和动力学上都是稳定的。     

Triphenylphosphonioacetylide: A Species Isoelectronic with Isocyanides

Stabilized only by triphenylphosphane , the title compound 1 was synthesized and investigated theoretically (the HOMO of 1 is depicted on the right). Both calculations and experimental NMR spectra show that no Lewis structure can describe the geometry and charge distribution accurately. Compound 1 undergoes an equivalent of the Passerini reaction. Ph₃P⁺−C≡C⁻: 1     

Electron and Geometry Structure of Hydrogen-Bonded Complexes of Guanine with One Molecule Methanol. A DFT Level Study

Summary. The geometries, harmonic vibrational frequencies, and energies of eight hydrogen-bonded complexes of guanine with one molecule methanol are computed using the DFT (B3LYP) method together with the 6-31+G* basis functions. In the investigation two stable tautomers of guanine (oxo-amino N9H and oxo-amino N7H) were chosen. They were included in a variety of H-bonded complexes with one molecule methanol. In order to investigate the nature of the intermolecular bonds, the bonding energies and thermodynamic properties of the complexes were calculated.