Gas phase electron diffraction study of SiH3N3 and (SiH3)2 NCN

The molecular structures of SiH₃N₃ and (SiH₃)₂ NCN have been determined by electron diffraction : SiH₃N₃ has a non-linear SiN₃ skeleton, and (SiH₃)₂ NCN is a carbodiimide with SiNCN Si probably linear. A new model is proposed which rationalises the observed geometry of a number of molecular pseudohalides.     

The isomeric structures of SiH2LiF

The structures of four isomeric forms of the model “silylenoid” SiH₂LiF have been investigated by ab initio molecular orbital theory. The two most stable forms are suggested to be the SiH₂Li⁺F^? ion pair and the H₂Si : FLi complex, analogous to the similar structures previously found for carbenoids. Two further species, a H₂Si : LiF complex and the “classical” (tetrahedral) SiH₂LiF, are also local minima on the potential energy surface, but are higher in energy.     

The time-resolved LMR method as used to measure elementary reaction rates of CI atoms and SiH3 radicals in pulse photolysis of S2Cl2 in the presence of SiH4

The time-resolved laser magnetic resonance (LMR) method has been applied to kinetic measurements for the first time. An intracavity spectrometer based on a CO₂ laser with resonant modulation of the magnetic field and with phase-sensitive detection of the signal has been used. Kinetic curves of generation and disappearance of CI atoms and SiH₃ radicals were obtained in the pulse photolysis of a mixture of S₂Cl₂ + SiH₄ under the fourth harmonic of a Nd laser (265 nm, 0.5 mJ, 12.5 Hz) at a total pressure of 520–980 Pa (he as diluent) and a temperature of 326 K. The reagent concentrations were: [S₂Cl₂ = (2.0?10.2)×10¹⁴ cm^?3, [SiH₄ = (2.4?17.4)×10¹³ cm^?3. To remove the transition saturation, 5.3×10¹⁵ cm^?3 CCl₄ was introduced into the reactor. The fraction of dissociated S₂Cl₂ was 1‰ Rate constants of the reactions (I) Cl+S₂Cl₂ → products, (II) Cl+SiH₄ → HCl+SiH₃ and a preliminary rate constant of the reaction (III) SiH₃ + S₂Cl₂ → products were obtained: k₁ ≤ (4.3±1.2)×10^?12 cm³/s, k₂ = (2.3±0.5)×10^?10 cm³/s, k₃ = (2.4±0.5)×10^?11 cm³/s. At a signal-to-noise ratio of 1:1, 1000 pulses and a 12 cm long detection zone the sensitivity to Cl atoms and to SiH₃ radicals was 4×10¹⁰ cm^?3 and = 10¹¹ cm^?3, respectively. The time resolution of the method was 4 μs. The method is shown to be promising for kinetic investigations and experiments on fast processes.     

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

The first band in the vacuum ultraviolet photoelectron spectrum of the silyl radical, corresponding to the process SiH₃ (X ¹A′₁) ← SiH₃(X ²A₁), has been observed with HeI radiation. Extensive vibrational fine structure associated with the SiH₃⁺ deformation vibration was observed in this band and analysis of the structure gave a value of ω = 820 = 40 cm^-1 for the out-of-plane deformation mode in the ion. The vertical and adiabatic ionization energies were measured as 8.74 = 0.01 eV and 8.14 ± 0.01 eV respectively and use of the latter value together with the established heat of formation of the silyl radical allows an improved heat of formation of SiH₃^-. ΔH₂₉₈⁰ (SiH₃^-) to be derived as 980 ± 7 kJ mol^?1.     

SiH2自由基与HNCO反应机理的理论研究

采用密度泛函理论B3LYP方法研究了SiH₂自由基与HNCO的反应机理, 并在B3LYP/6-311＋＋G**水平上对反应物、中间体、过渡态进行了全几何参数优化, 通过频率分析和内禀反应坐标(IRC)确定了中间体和过渡态. 为了得到更精确的能量值, 又用QCISD(T)/6-311＋＋G**方法计算了在B3LYP/6-311＋＋G**水平优化后的各个驻点的相对能量. 计算结果表明SiH₂自由基与HNCO的反应有五条反应通道, 其中顺式反应通道SiH₂＋HNCO→IM3→ TS4→IM5→TS5→IM6→SiH₂NH＋CO反应能垒最低, 为主反应通道.     

The structure and energy of SiH+5. comparisons with CH+5 and BH5

Differences between SiH⁺₅ and CH⁺₅ are more significant than the similarities. The proton affinity of SiH₄ exceeds than of CH₄ by ≈25 kcal/mol. but the heat of hydrogenation of SiH⁺₃ is smaller than that of CH⁺₃ by nearly the same amount. Like CH⁺₅ the C₅ structures of SiH⁺₅ are preferred, but SiH⁺₅ is best regarded as a weaker SiH⁺₃—H₂ complex. D_3h, C_2v, and C_4v forms are much higher in energy and SiH⁺₅ should not undergo hydrogen scrambling (pseudorotation) readily, as does CH⁺₅ The neutral BH₅ is only weakly bound toward loss H₂, and the D_3h. C_2v, and C_4v forms are also high in energy. The contral-atom electronegativities, C⁺ > B > Si⁺, control this behavior. The electronegativities also determine the ability to bear positive charges. Thermodynamically. SiH⁺₅ and SiH⁺₃ are more stable than CH⁺₅ and CH⁺₃, respectively; hydride transfer occurs from SiH₄ to CH⁺₃ and proton transfer from CH⁺₅ to SiH₄.     

Ci calculations of the low lying electronic states of the radical SiH3, SiH+3 and SiH−3

SCF and CI calculations for the silicon-hydrogen compounds SiH₃, SiH⁺₃ and SiH^? with D_3h and C_3v geometries are carried out f     

Mean amplitudes and shrinkages for SiH3NCO and SiH3NCS

Mean amplitudes and shrinkages are calculated from spectroscopic data for SiH₃NCO, SiD₃NCO, SiH₃NCS and SiD₃NCS. The lowest frequency (ν₁₀) in each of the molecules is uncertain, and its influence on the calculated amplitude parameters is investigated. The results are compared with electron diffraction studies.     

Reaction rate studies of atomic germanium (3P0,1) and silicon (3PJ) with various oxidizers

The gas phase reactions of Ge(³P_0,1) and Si(³P_J) with O₂, NO and N₂O have been studied in a flow tube system at 350 K. Atomic Ge and Si were produced by flowing GeH₄ and SiH₄ through a hollow cathode discharge. The subsequent disappearance of the Ge and Si atoms was followed with atomic absorption spectroscopy. Rate constants were determined for the reactions at 4 and 5 torr pressures.     

Laser-induced fluorescence of the SiH2 radical

The laser-induced fluorescence of the SiH₂ radical was observed in the photolysis of phenylsilane by an ArF excimer laser in the 550–650 nm range. The apparent fluorescence lifetime is 60 ± 5 ns at 580.1 nm. The bending vibrational spacing in the ground state was found to be 990 ± 20 cm^?1 from the dispersed fluorescence spectrum.     

Theoretical study of the molecular ions SiH−5 and SiH−3

Ab initio HF and Cl calculations were performed to determine the equilibrium geometry of SiH^?₅ and SiH^?₃, the barrier for internal rotation (SiH^?₅) and inversion (SiH^?₃) and the stability of SiH^?₅ and further to study the effect of electron correlation on reaction energies. The gaussian-type basis included d and f functions on Si and a p set on II. The D_3h structures of SiH^?₅ is lower in energy than the C_4v structure by 2.9(3.2) kcal/mol (corresponding HF results in parentheses). SiH^?₃ has C_3v structure, the inner-ion barrier computed is 26.2 (27.3) kcal/mol. SiH^?₅ turns out to be stable with respect to SiH₄ + H^? by 20.3 (13.8) kcal/mol, but it is unstable with respect to SiH^?₃ ← H₂ by 6.3 (5.6) kcal/mol. These results show that electron correlation has a small effect on barriers of inversion (SiH^?₃) or pseudorotation (SiH^?₅), but may have a pronounced effect on reaction energies even if all systems involved have closed shells. The correlation energy contributions are analyzed in terms of intrapair and interpair terms in order to get a better understanding of the influence of correlation on reaction and activation energies.     

Molecular structures of (SiCl3)2CH2 and (SiCl3)2CCl2 as studied by electron diffraction

An electron diffraction analysis of the molecular structures of 1,1,1,3,3,3-hexachloro-1,3-disilapropane and octachloro-1,3-disilapropane has been carried out. Deviations from the staggered conformation are indicated. The data may be approximated by models with C₂ symmetry and a small tilt of the SiCl₃ groups. The main bond lengths (r_g) and bond angles obtained for (SiCl₃)₂ CH₂ are: SiCl, 202.7(4); SiC, 186.6(6); CH, 109.8(24) pm, ClSiCl, 107.9(1); SiCSi, 118.3(7)°; and for (SiCl₃)₂CCl₂: SiCl, 202.0(4); SiC, 190.2(9); CCl, 179.6(9) pm; ClSiCl, 109.5(1); SiCSi, 120.6(9); ClCCl, 110.9(16); SiCCl, 106.3(3)°.     

An iets study of alumina and magnesia supported —sihx and their use as substrates for inorganic vibrational spectroscopy

Tunneling spectra of Al₂O₃/—SiH_x, MgO/—SiH_x, Al₂O₃/—SiD_x, and Al₂O₃/—SiH_x + NCS^? are reported. Analysis of the vibrational spectra observed from isotopic substitution of the barriers obtained by deposition of a thin film of SiO onto alumina and magnesia indicate that the supported species is —SiH. The Al₂O₃/—SiH barrier can be used as a support for studying inorganic ions by IETS.     

Theoretical Study of Interaction Between S2 and SiHx (x=1, 2, 3) in Porous Silicon

The interaction between S₂ molecule and SiH_x (x=1, 2, 3) in porous silicon is investigated using the B3LYP method of density functional theory with the lanl2dz basis set. The model of porous silicon doped with CH₃,Si-O-Si and OH species is built. By analyzing the binding energy and electronic transfer, we conclude that the interaction of S₂ molecule with SiH_x (x=1, 2, 3) is much stronger than the interaction of S₂ molecule with CH₃ and OH, as S₂ molecule is located in different sites of the model. Using the transition state theory, we study the Si₂H₆+S2→H₃SiH₂SiS+HS reaction, and the reaction energy barrier is 50.2 kJ/mol, which indicates that the reaction is easy to occur.     

The molybdenum-molybdenum triple bond—XVII. Syntheses,spectroscopic and structural characterizations of Mo4F4(O—t-Bu)8 and Mo2F2(O—t-Bu)4(PMe3)2 (MoMo)

Hydrocarbon solutions of Mo₂(O—t-Bu)₆ and PF₃ (2 equiv) yield Mo₄F₄(O—t-Bu)₈, I, and PF₂(O—t-Bu). Compound I contains a bisphenoid of molybdenum atoms with two short MoMo distances, 2.26 Å, and four long MoMo distances, 3.75 Å, corresponding to localized MoMo triple bonding and non-bonding distances, respectively. The tetranuclear compound may be viewed as a dimer, [Mo₂(μ₂-F)₂(O-t-Bu)₄]₂, and addition of PMe₃ to hydrocarbon solutions of I yields Mo₂F₂(O—t-Bu)₄(PMe₃)₂, II, which contains an unbridged MoMo triple bond of distance 2.27 Å. Each molybdenum atom is coordinated to two oxygen atoms, one fluorine atom and the phosphorus atom of the PMe₃ ligand in a roughly square planar manner. The overall central Mo₂O₄F₂P₂ skeleton has C₂ symmetry and NMR studies (¹H, ¹⁹F and ³¹P) are consistent with the maintenance of this type of structure in solution. Infrared and electronic absorption spectral data are reported. These are the first compounds containing fluorine ligands attached to the (MoMo)⁶⁺ unit.     

The calculation of the molecular orbital parameters for the square coplanar Cu(NH3)2+4 cation in Cu(NH3)4PtCl4

The calculation of the molecular orbital parameters of the σ-bonding square-planar Cu(NH₃)²⁺₄ cation in Cu(NH₃)₄PtCl₄ is reported, involving metal—ligand and ligand—ligand overlap integrals. In order to obtain a reasonable correlation of the magnetic g- and A-values and the electronic transition energies it is necessary to use a Cu⁺ wavefunction to represent the radially expanded Cu²⁺ wavefunction in this complex.     

Crystal and molecular structure of (η3-allyl)carbonylchlorobis(dimethylphenylphosphine)- iridium(III) hexafluorophosphate, [Ir(η3-C3H5)Cl(CO)(P(CH3)2(C6H5))2][PF6

The structure of (η³-allyl)carbonylchlorobis(dimethylphenylphosphine)-iridium(III) hexafluorophosphate, [Ir(η³-C₃H₅)Cl(CO)(P(CH₃)₂(C₆H₅))₂][PF₆], has been determined from three-dimensional X-ray data to add support for a proposed mechanism of the oxidative addition of allyl halides to IrX(CO)(PR₃)₂ (X = halide). The compound crystallizes in space group C⁵_2h-P2₁/c with four formula units in a cell of dimensions a = 11.027(1), b = 12.230(2), c = 19.447(5) Å, and β = 103.16(2)⁰. Least-squares refinement of the structure has led to a value of the conventional R index (on F) of 0.066 for the 3018 independent reflections having F²₀>3—(F²₀). The crystal structure consists of discrete, monomericions. The hexafluorophosphate anion is disordered. The coordination geometry around the iridium atom may be described as octahedral, with the chloro ligand trans to the carbonyl group and each phosphorus atom trans to a terminal carbon of the allyl group. Structural parameters: Ir—P = 2.366(4), 2.347(3);Ir—Cl = 2.389(3); Ir—C(allyl) = 2.28(1), 2.24(1),2.25(1); Ir—C (carbonyl) = 1.85(1) Å; P—Ir—P = 105.7(1); C(terminal)—Ir—C(terminal) = 66.2(8); C—C—C = 125(2)^o. The allyl group makes an angle of 126^o with the P—Ir—P plane. Correlations between geometric structure and number of d electrons are noted among several M—C₃H₅-complexes, and are interpreted in the light of theoretical models of the M—C₃H₅- bond.     

The microwave spectrum of trimethyl silyl isocyanate (CH3)3SiNCO

The microwave spectrum of trimethyl silyl isocyanate has been investigated in the region 26.5–40 GHz. The spectrum belonging to the ground vibrational state is characteristic of a symmetric top indicating that the equilibrium configuration of the SiNCO chain is either linear or very nearly so. The ground state B₀ value is 1203.83 MHz which is consistent with the structure observed for SiH₃NCO. The ground state transitions are accompanied by many vibrational satellites belonging to the lowest bending mode whose frequency was estimated to be 64 ± 15 cm⁻¹. These results are consistent with electron diffraction results from which the SiNC angle is deduced to be ≈ 150°.     

Infrared multiple photon dissociation of dichlorosilane: The production of electronically excited SiCl2

The infrared multiple photon excitation of SiH₂Cl₂, under collision-free and collisional conditions, via its v₂(a₁) SiH₂ bending fundamental resulted in dissociation and in an ultraviolet luminescence. The photodissociative products were found to be H₂ and SiCl₂ while the luminescence was shown to arise from a spontaneous one-photon radiative decay from electronically excited SiCl₂(¹B₁ → ¹A₁). The radiative lifetime of this transition has been measured as 4.5 μs and the collisional quenching rate of SiCl₂(¹B₁ → ¹A₁) by SiH₂Cl₂ as 2.6 × 10⁶ s^?1 Torr^?1. The reaction mechanism leading to product formation as well as models for populating the electronic state of the fragment are proposed and discussed in conjunction with present multiple photon dissociation theory.