Si Bond Length Research Articles

Syntheses of Oligometalloles by Catalytic Dehydrocoupling

The dehydrocoupling polycondensation of dihydro(tetraphenyl)metalloles (M = Si or Ge) with 0.2 mol % H2PtCl6·xH2O and excess cyclohexene produces the respective oligometallole in high yield (>80%), where every silicon or germanium atom of the oligomer backbone is part of a silole or germole ring. Slightly less reactive catalyst systems of 1 mol % of Wilkinson's catalyst, (RhCl(PPh3)3), or Pd(PPh3)4 yield the respective oligometallole in good yield (∼60%). With these latter systems, and under less vigorous reflux conditions, the (tetraphenyl)silole dimer may be isolated in 40% yield. X-ray structural characterization of the dimer reveals a Si−Si bond length of 2.363(2) A and an H−Si−Si−H torsion angle of 90(2)°. Using excess cyclohexene as a coreagent with RhCl(PPh3)3 increases the yield of oligomer and also eliminates dimer byproduct. The methyl-terminated dimer forms in the reaction between H2PtCl6·xH2O and methylhydro(tetraphenyl)silole, but not in the similar reaction with the Rh and Pd catalysts. The ...

Structure of Oxygen-Related Defect Centers in Ge1-xSix Alloys Studied by Extended Energy-Loss Fine Structure Analysis

Oxygen atoms dissolved near the surface regions in Ge1-xSix alloys were studied by extended energy-loss fine structure (EXELFS) analysis using electron energy-loss spectroscopy (EELS) and the local structures around oxygen were determined. Oxygen preferentially formed Si–O–Si centers, rather than Si–O–Ge or Ge–O–Ge. The O–Si bond length was increased with decreasing the Si content, in accordance with the increase in the Si–Si, Si–Ge and Ge–Ge bond lengths. The increase in O–Si bond length is much more significant than that expected from the changes in Si–Si, Si–Ge and Ge–Ge bond lengths. The derived local atomic configuration of the Si–O–Si center changed with increasing Si content, which corresponds well to the surface oxidization process of crystalline silicon.

Coordination of phosphanide and trialkylsilylphosphanide ligands at pentacarbonyltungsten fragments: An NMR spectroscopic and structural investigation

AbstractTrialkylsilylphosphanide ligands as pentacarbonyltungsten adducts such as the anions [((CO)5W)2P(H)SiR3]− and [(CO)5W‐PHSiR3]− with R = Me, iPr, and tBu are prepared by the reaction of (thf)W(CO)5 with MP(H)SiR3 or by the metathesis reaction of ((OC)5W)2PHLi2 with ClSiR3 and characterized by NMR spectroscopy with a focus on 183W‐NMR experiments. The solvent‐separated ion pair (tmeda)K+ [((OC)5W)2P(H)SiiPr3]− crystallizes as a coordination polymer with a rather large PSi bond length of 228.1 pm. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:420–425, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20128

Local strain relaxation in Czochralski-grown GeSi bulk alloys

The local atomic structure in GeSi alloys was studied experimentally and theoretically. By extended X-ray absorption fine structure measurements in Czochralski-grown Ge1−xSix bulk alloys, it is found that the Ge–Ge, Ge–Si (Si–Ge) and Si–Si bond lengths maintain distinctly different lengths and vary in a linear fashion as a function of alloy composition across the entire composition range 0<x<1, in good agreement with what is expected from the ab inito electronic structure calculations. It is known that both bond lengths and bond angles are distorted with alloy composition in GeSi.

Role of interface states in electronic properties of (ZnSe)n/(Si2)n (001) superlattices

In a semiempirical tight-binding scheme, the detailed electronic structure and optical properties of (ZnSe)n/(Si2)n (001) superlattices (SLs) are studied with a wide range of n=1–20 giving special attention to the role of the interface states at the Zn–Si and Se–Si polar interfaces. The states at the conduction and valence-band edges are confined two dimensionally in the Si layers. Furthermore, we have found two interface bands in the lower and upper regions of the gap. The states of the lower interfaces band are located at the Zn–Si interface, while those of the upper interface band are located at the Se–Si interface. The energies of the interface states depend on the parameters representing the Zn–Si and Se–Si bond lengths and the valence band discontinuity between ZnSe and Si, but the interface states do not disappear from the gap with reasonable choices of the parameters. It is shown that the heterointerface bond relaxation strongly affects interface band in the band gap. In this system, relaxed Si bonds at the heterointerface induce a vacant interface band and a filled interface band in the band gap. By decreasing the SL period n, the energy gap between the confined band-edge states increases (2.07 eV at the Γ point for n=2) due to the quantum confinement effect. A sudden shrinkage in the band gap (Eg=1.76 eV at the M point) is obtained for n=1. The origin of the band-gap shrinkage is related to the fact that the interface states increasingly overlap and combine as band states. Furthermore, the calculated absorption spectra of the superlattices are found to be quite different from those of bulk ZnSe and Si, but fairly close to their average.

Local atomic structure in Czochralski-grown Ge 1− xSi x bulk alloys

We investigated the local atomic structure in GeSi alloy experimentally and theoretically. Extended X-ray absorption fine structure measurements in Czochralski-grown Ge 1− x Si x bulk alloys showed that the GeGe and GeSi bond lengths maintain distinctly different lengths and vary linearly with alloy composition across the whole composition range 0< x<1. This is in good agreement with the expectation derived from ab initio electronic structure calculations. The result indicates that the bond lengths and bond angles are distorted with alloy composition in GeSi.

Oxygen-related radiation-induced defects in SiGe alloys

Electronic properties of the vacancy–oxygen (V–O) complex and the interstitial oxygen and carbon atompair (Ci–Oi) inunstrained Si1−xGex crystals have been studied by means of capacitance transient techniques. TheSi1−xGex crystals were grown by the Czochralski method and were dopedwith either phosphorus or boron during growth. The V–O andCi–Oi centres were introduced into the crystals by room temperature irradiation with 4 MeV electrons or withγ-raysfrom a 60Co source.The enthalpy of electron ionization for the single acceptorlevel of the V–O centre relative to the conduction band edge,ΔHn, was found toincrease with a rate d (ΔHn)/d x = 0.56 eV upon increase in Ge content. The enthalpy of hole ionization for the single donor level of theCi–Oi centre relative to the valence band edge,ΔHp, was found to decreasewith a rate d (ΔHp)/d x = −0.96 eV. For boththe V–O- and Ci–Oi-relatedlevels no significant changes in the values of the electron (hole) capture cross section orentropy of ionization with the changes in Ge content were observed. The levels are notpinned to the conduction band edge or to the valence band edge. It is argued that the valueof the enthalpy of V–O ionization can be correlated with the lattice parameter or the Si–Sibond length: the larger this parameter, the bigger the enthalpy of the ionization.

Local microstructure of Ge layers buried in a silicon crystal studied by extended X-ray absorption fine structure

The local structure of Si-capped Ge quantum dots formed in Stranski–Krastanov growth mode on a (001) Si substrate was probed by X-ray absorption at the Ge K-edge. Analysis by extended X-ray absorption fine structure (EXAFS) was performed under the assumption of a solid solution model of the Si distribution inside the quantum dots (QDs) and taking into account polarization effects. It was found that QDs formed for 8 ML are strained and the Ge–Ge and Ge–Si bond lengths vary with direction. The Si content inside the QDs was found to be at a level of ∼25 at.% for strained (8 ML sample) QDs and ∼12 at.% for unstrained (10 ML sample) QDs. This work demonstrates the usefulness of EXAFS analysis for the determination of the local atomic structure in covered low-dimensional structures.

The Disilane Chromophore: Photoelectron and Electronic Spectra of Hexaalkyldisilanes and 1,(n+2)-Disila[n.n.n]propellanes

Photoelectron spectra and solution UV absorption and magnetic circular dichroism (MCD) of hexamethyldisilane (1), hexaethyldisilane (2), hexa-tert-butyldisilane (3), and the 1,(n+2)-disila[n.n.n]propellanes [n = 4 (4) and 5 (5)] were measured, as was the linear dichroism (LD) of 3 and 4 partially aligned in stretched polyethylene. The results support the assignment of the lowest energy electronic absorption band of the disilanes 1−5 to a doubly degenerate σSiSi(HOMO) → π*SiC(LUMO) transition and of the next band, observed in the solution spectra of 2−4 and in the gas-phase spectrum of 1, to a σSiSi → σ*SiSi transition. MP2/VTZ optimized geometries of 1−5 and ab initio molecular orbital energies (HF/VTZ//MP2/VTZ) and ionization potentials (ROVGF/VTZ//MP2/VTZ) of these disilanes reproduce the reported geometries and the trends observed in the photoelectron spectra, respectively. B3LYP/6-31G(d,p) calculations of the Kohn−Sham orbital energies and TD B3LYP/6-31G(d,p) calculations of transition energies and intensities of 1 as a function of Si−Si bond length suggest that many of the features of the UV absorption spectrum of 3, including the small energy difference between the two transitions observed and the large extinction coefficient of the band peaking at higher energy (σSiSi → σ*SiSi), are due to its very long Si−Si bond.

HOLOGRAPHIC ANALYSIS OF KIKUCHI ELECTRON DIFFRACTION DATA FROM ${\rm Si}(111)(\sqrt{3}\times\sqrt{3}){\rm R}30^\circ{\rm -Al}$

Holographic analysis was applied to Kikuchi electron diffraction data collected from the [Formula: see text] structure to produce real-space images of the surface structure. Methods for calibrating the initial diffraction data are fully described. The images clearly show features caused by the Al atoms in the T4 absorption site. Measurements of the Al–Si bond length and the height of the Al above the top layer of Si agree with the values known from dynamical LEED studies.

302 雰囲気影響を考慮した強度解析のための原子間ポテンシャル

An interatomic potential applicable to strength analysis taking the effects of atomospheric influence into account is investigated. Properties (bond length, latice constants, and binding energy) of Si, O_2 H_2,H_2O, and SiO_2 are calculated by using the extended Tersoff interatomic potential function. It is found that the calculated values coincide well with the experimental values when the proper potential parameter values are used.

Synthesis, Experimental/Theoretical Characterization, and Thermolysis Chemistry of CpBe(SiMe3), a Molecule Containing an Unprecedented Beryllium−Silicon Bond

The synthesis, characterization, and thermal decomposition of CpBe(SiMe3) are presented as part of an exploratory investigation designed to obtain more effective chemical vapor deposition precursors of metallic beryllium. The title compound provides the first example of a direct bond between beryllium and a non-carbenoid group 14 element. The base-free reaction of LiSiMe3 with CpBeCl in pentane affords the air-sensitive, volatile solid CpBe(SiMe3) (ca. 70% yield based on CpBeCl), which was characterized by single-crystal CCD X-ray diffraction, multinuclear NMR, and mass spectrometric studies, and theoretically by DFT/NBO analysis. The solid-state molecular geometry of CpBe(SiMe3) ideally conforms to C3v symmetry (under assumed cylindrical symmetry for the C5H5 ring); the Be−Si bond length of 2.185(2) Å is markedly longer than the sum of covalent radii (2.01 Å). The DFT-optimized molecular geometry closely conforms to that determined crystallographically. Total fragment charges (based upon atomic charge NBO calculations) of −0.79 e for C5H5, +1.26 e for Be, +0.81 e for Si, and −1.28 e for the three Me groups constitute a polarity pattern consistent with the Be−Cp bonding interaction being mainly ionic and with the Be−Si bonding pair being polarized toward the more electronegative SiMe3 fragment. Beryllium-9 and 29Si NMR spectra exhibit a large J(Be−Si) coupling constant of 51 Hz; the 9Be chemical shift of δ −27.70 ppm, the highest field value recorded to date, is in accordance with the calculated bond-polarity pattern, as well as a bond to Si. Mass spectra (EI) exhibited peaks for the molecular ion and its isotopomers. Thermal decomposition of CpBe(SiMe3) gives rise to trimethylsilane, CpBeMe, and CpBe(SiMe2SiMe3) as the major products, as determined by multinuclear NMR. The latter species is likewise formed by the reaction of CpBeCl with LiSiMe2SiMe3.

MCSCF Study of Multiple Bonding between Ti and the Main-Group Elements C, Si, N, and P

The nature of the multiple bonding of H2TiEH2 (E = C, Si) and H2TiEH (E = N, P) molecules has been investigated with the multiconfiguration SCF (MCSCF) method. It is shown that the Ti−C and Ti−P bond lengths decrease with bond rotation, whereas the Ti−Si bond length increases. MCSCF geometry optimization shows that Ti−N has triple-bond character with a linear Ti−N−H bond angle that results from strong back-bonding from the N lone pair into the empty Ti d orbitals. The rotation barrier and bond dissociation energy for TiE are estimated with the MCSCF + multireference second-order perturbation theory (MRMP2) method. The singlet rotation barriers in TiC, TiSi, and TiP are estimated to be 15.9, 8.6, and 9.3 kcal/mol, respectively. The TiC and TiSi bond dissociation energies are estimated to be 83.4 and 56.9 kcal/mol, respectively. The Ti⋮N triple-bond energy is about 30 kcal/mol larger than that of the carbon species, whereas the energy of the TiP bond is about 8 kcal/mol smaller than that of the silicon species.

Reactive epitaxy of beryllium on Si(1 1 1)-(7×7)

Scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES) have been used to investigate the nucleation, growth, and structure of beryllium on Si(1 1 1)-(7×7). STM indicates that a chemical reaction occurs at temperatures as low as 120 K, resulting in a nano-clustered morphology, presumed to be composed of a beryllium silicide compound. Upon annealing to higher temperatures, PES data indicate that beryllium diffuses into the selvage region. High temperature annealing (∼1175 K) results in the formation of a universal ring cluster structure suggesting a Be–Si bond length less than 2.5 Å, in agreement with previous calculations regarding hypothetical Be 2Si.

Possible Strategies toward the Elusive Tetraaminodisilene

In this paper we predict, using quantum mechanical calculations, which diaminosilylenes would dimerize to produce strongly bound tetraaminodisilenes, which so far have proven to be elusive. The central idea is that diaminosilylenes with a small singlet-triplet energy difference would dimerize to strongly bonded disilenes. Calculations at the B3LYP/6-311++G(3df,2p)//MP2/6-31G(d) level of theory showed that the energy difference between the singlet and the triplet states (DeltaE(ST)) of diaminosilylenes (R(2)N)(2)Si: (1) strongly depends on (i) the twist angle varphi between the SiN(2) and the R(2)N planes and (ii) the NSiN bond angle alpha at the divalent silicon. DeltaE(ST) decreases with increased twisting (larger varphi) and with widening of alpha. DeltaE(ST) is reduced from 70.7 kcal mol(-1) for planar (H(2)N)(2)Si: (1a) to DeltaE(ST) = 21.7 kcal mol(-1) when varphi is held at 90 degrees. Likewise, the bicyclic diaminosilylenes 1,4-diaza-7-silabicyclo[2.2.1]hepta-7-ylidene and 1,5-diaza-9-silabicyclo[3.3.1]nona-9-ylidene (4a,b), with the nitrogens in the bridgehead positions (varphi = 90 degrees), have DeltaE(ST) values of 45.1 and 38.3 kcal mol(-1), respectively. When dimerized, these silylenes form strongly bonded disilenes 5 (E(dim) = -32.2 kcal mol(-1) (4a) and -41.3 kcal mol(-1) (4b)) with Si=Si bond lengths of 2.239 A (4a) and 2.278 A (4b) (MP2/6-31G(d)//MP2/6-31G(d)). These theoretical predictions pave the way for the synthesis of the first strongly bonded tetraaminodisilene. Due to the steric requirements, also silyl substitution at nitrogen has a significant effect on DeltaE(ST) and [(H(3)Si)(2)N](2)Si: (1d) is predicted to form a stable Si=Si bonded dimer (E(dim)= -24.1 kcal mol(-1)). However, the larger size of the Me(3)Si substituent prevents the formation of a Si=Si bonded dimer of [(Me(3)Si)(2)N](2)Si: (1e).

Raman scattering and x-ray absorption studies of Ge–Si nanocrystallization

We have studied the local structure of GeSi nanocrystals embedded in SiO2 prepared by co-sputtering of Ge, Si, and SiO2 targets onto a Si(100) substrate. From Raman scattering, we conclude that the formation of the isotropic crystalline Ge phase starts at about 800 °C followed by the formation of a GeSi phase at higher temperatures. The formed nanocrystals, whose size depends on the annealing temperature, are randomly oriented. The local structure of the nanocrystals has been studied by x-ray absorption fine structure spectroscopy. They are found to consist of a relaxed Ge core with a typical diameter of ∼4 nm and the Ge–Ge bond length of 2.45 Å and of a GeSi outer shell, the Ge–Si bond length being 2.39 Å. The average composition of the grown nanocrystals is estimated to be Ge0.75Si0.25.

Conformations of silicon-containing rings. Part 4. Gas-phase structure of 1,3,5-trisilacyclohexane and comparison with cyclohexane and cyclohexasilane

The gas-phase structure of 1,3,5-trisilacyclohexane 1 has been determined by gas electron diffraction (GED). The experimental structure is compared to the result of quantum chemical (QC) calculations (HF, MP2 and B3LYP) using the 6-31G∗ basis set. It is found that all three QC methods give similar results. The Si–C–Si bond angle is found by experiment/B3LYP calculation to be 113.0(4)/113.5°. whereas the C–Si–C bond angle is 110.7(14)/110.1° The large Si–C–Si bond angle makes 1 rather flat with an endocyclic dihedral angle of 53.7(4)/53.8° compared with 55.1(7)/54.7 and 57.9(9)/56.0° for cyclohexane and cyclohexasilane, respectively. The Si–C bond length is found to be 1.872(1)/1.893Å. It is shown that HF, MP2 and B3LYP calculations using the 6-31G∗ basis set commonly predict Si–C and Si–Si bond lengths in small molecules to be about 0.02Å longer than experimental values. The energy difference between the chair and twisted boat conformers of 1 is calculated to be 2.2kcalmol−1 compared to 6.5 and 1.9kcalmol−1 for cyclohexane and cyclohexasilane, respectively.

Ab initio studies of the Si 1− xGe x alloy and its intrinsic defects

We have performed systematical ab initio studies of the electronic and structural properties of the disordered phase of the Si 1− x Ge x alloy. For the defect free alloy, we find that the Si–Si bond length has a tendency to maintain its natural value ( x=0) all the way up to x>0.5, similarly to what is found experimentally. We also analyze the vacancy, and note, as expected for a structurally as well as chemically disordered solid, that there is no local symmetry around the vacancy site. Finally, we perform some studies of the alloy under pressure all the way up to 30 GPa, and find that the structure is highly distorted in some cases, indicating that an amorphous phase may be found in the pressure region of the diamond–β-tin phase transition.

Dissociative adsorption of NF 3 on Si(001)-(2×1)

Using the first-principles pseudopotential–generalized gradient approximation (GGA) approach, we have performed an accurate determination of the atomic and electronic structure of the dissociative adsorption for nitrogen trifluoride on Si(001)-(2×1). The chemisorbed system is energetically favourable and occurs in a very similar way to that found for the ammonia molecule: the NF 3 molecule dissociates with the formation of SiNF 2 and SiF bonds. We find that the system is characterized by elongated (around 6%) and symmetric silicon dimers (the tilt angle is just 1.5°), and by the fact that the SiNF 2 group retains the pyramidal geometry of the nitrogen trifluoride molecule. We also observe that the SiN bond is inclined at almost 8.6° with respect to the surface normal. The NSi bond length is 1.85 Å and the NF bond length is 1.47 Å. Finally, our electronic structure calculations suggest that the adsorption of NF 3 molecules almost completely passivates the silicon surface.

Synthesis and structures of a series of GeM (M=C, Si, and Sn) compounds derived from germyllithium containing three 2-(dimethylamino)phenyl groups on germanium

Reactions of {tris[2-(dimethylamino)phenyl]germyl}lithium ( 1 ) with Group 14 electrophiles afford the corresponding carbogermane, silylgermane, and stannylgermane compounds containing three 2-(dimethylamino)phenyl groups on the germanium atom. X-ray structure analysis of these structures was performed and revealed that: (1) the steric repulsion between the amino groups and the substituents is mainly reflected by the torsion angle MGeC ipso C ortho ; (2) the N⋯M atomic distances are within the sum of the van der Waals radii; (3) in a series of the silylgermanes, the GeSi bond length increases as the size of the silyl group increases: SiH 3 ≤SiMe 2 H≤SiMe 3 ≤SiMe 2 t -Bu.