An ab initio study of the electronic structure of SiH4, SiH3F and SiH2F2

Changes in molecular properties and in the electronic charge distribution of the molecules SiH₄, SiH₃F and SiH₂F₂ are studied within the framework of the ab initio Hartree—Fock SCF—LCAO—MO method. The ionisation potentials, calculated with the use of Koopmans' theorem, correlate well with the experimental vertical ionisation potentials. The combined s and p electronic populations of the Si atom are not substantially altered when adding Si 3d functions to the basis set.     

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.     

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.     

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     

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.     

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.     

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.     

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.     

Microwave transitions of CD2=NCN

In continuation of our studies of the pyrolysis (1000°C) of (CH₃)₂NCN to alleged CH₂=NCN (I), N-cyanoformimine, (CD₃)₂NCN has been prepared and pyrolyzed (1000°C) to CD₂=NCN (II) as seen by its observed rotational transitions (18.6–40.0 GHz). The published rigid structural model of I and a corresponding structure of II are practically identical.     

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.     

Vibrational spectra and molecular symmetry of tetrasilylhydrazine and tetrasilylhydrazine-d12

The Raman spectra of tetrasilylhydrazine and tetrasilylhydrazine-d₁₂ have been recorded for the gas, liquid and solid phases from 25 to 2500 cm^?1. The infrared spectra of N₂(SiH₃)₄ and N₂(SiD₃)₄ have been recorded for the gas and solid phases from 40 to 2500 cm^?1. The vibrational data have been interpreted on the basis of a twisted (D_2d) molecular configuration for both the fluid and solid states.     

Preparation and electrochemical behaviour of dinuclear platinum complexes containing NCN ligands (NCN=[C6H3(Me2NCH2)2-2,6]). The crystal structure of [C6H3(Me2NCH2)2-1,3-(CC)-5]2

A series of homodinuclear Pt compounds containing the anionic, potentially terdentate NCN ligand (NCN=[C₆H₃(Me₂NCH₂)₂-2,6]⁻) or its 4-ethynyl derivative were prepared. The two platinum centres are linked together in two different fashions: (i) directly linked by an ethynyl or diethynylphenyl group (head-to-head) and (ii) indirectly bonded by a ethynyl- or butadiynyl-linked bis-NCN ligand (tail-to-tail). The reaction of the head-to-head σ,σ′-ethynylide complex {Pt}CC{Pt} ({Pt}=[Pt(C₆H₃{CH₂NMe₂}₂-2,6)]⁺) with [CuCl]_n yields {Pt}Cl and [Cu₂C₂]_n, while with [Cu(NCMe)₄][BF₄] a Cu(I) bridged complex was formed: [(η²-{Pt}CC{Pt})₂Cu][BF₄]. The results of cyclic voltammetry experiments reveal that both connection modes of the two platinum centres lead to electrochemically independent Pt–NCN units. The X-ray crystal structure analysis of the neutral, tail-to-tail bridging butadiyne bis-NCNH ligand [C₆H₃(CH₂NMe₂)-1,3-(CC)-5]₂ is reported.     

Energy analysis of metal-metal bonding in [RM-MR] (M = Zn, Cd, Hg; R = CH3, SiH3, GeH3, C5H5, C5Me5)

The metal-metal bonds of the title compounds have been investigated with the help of energy decomposition analysis at the DFT/TZ2P level. In good agreement with experiment, computations yield Hg-Hg bond distance in [H₃SiHg-HgSiH₃] of 2.706 Å and Zn-Zn bond distance in [(η⁵-C₅Me₅)Zn-Zn(η⁵-C₅Me₅)] of 2.281 Å. The Cd-Cd bond distances are longer than the Hg-Hg bond distances. Bond dissociation energies (-BDE) for Zn-Zn bonds in zincocene −70.6 kcal/mol in [(η⁵-C₅H₅)₂Zn₂] and −70.3 kcal/mol in [(η⁵-C₅Me₅)₂Zn₂] are greater amongst the compounds under study. In addition, [(η⁵-C₅H₅)₂M₂] is found to have a binding energy slightly larger than those in [(η⁵-C₅Me₅)₂M₂]. The trend of the M-M bond dissociation energy for the substituents R shows for metals the order GeH₃ < SiH₃ < CH₃ < C₅Me₅ < C₅H₅. Electrostatic forces between the metals are always attractive and they are strong (−75.8 to −110.5 kcal/mol). The results demonstrate clearly that the atomic partial charges cannot be taken as a measure of the electrostatic interactions between the atoms. The orbital interaction (covalent bonding) ΔE_orb is always smaller than the electrostatic attraction ΔE_elstat. The M-M bonding in [RM-M-R] (R = CH₃, SiH₃, GeH₃, C₅H₅, C₅Me₅; M = Zn, Cd, Hg) has more than half ionic character (56-64%). The values of Pauli repulsions, ΔE_Pauli, electrostatic interactions, ΔE_elstat, and orbital interactions, ΔE_elstat are larger for mercury compounds as compared to zinc and cadmium.     

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.     

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.     

Studies on the reaction N + N3 → N2(B 3Πg) + N2(X 1Σ+g)

Strongly enhanced N₂ first positive emission N₂(B ³Π_g → A ³Σ⁺_u) has been observed on addition of N atoms into a flowing mixture of Cl and HN₃. The dependence of the emission intensity on N atom concentration gave a rate constant for the reaction N + N₃ → N₂(B ³Π_g) + N₂(X ¹Σ⁺_g) of _i(1.6 ± 1.1) × 10^?11 cm³ molecule^?1 s^?1. That for the reaction Cl + HN₃ → HCl + N₃ is (8.9 ± 1.0) × 10^?13 cm³ molecule^?1 s^?1 from the decay of the emission. Comparison of the emission intensity in ClHN₃ with that in ClHN₃N gave the rate constant of the reaction N₃ + N₃ → N₂(B ³Π_g) + 2N₂(X ¹Σ⁺_g) as 1.4 × 10^?12 cm³ molecule^?1 s^?1 on the assumption that N + N₃ yields only N₂(B ³Π_g) + N₂(X ¹Σ⁺_g).     

Perfluoralkyljod-verbindungen: das trifluormethyljoddinitrat CF3J(NO3)2

CF₃I(NO₃)₂ is formed from the reactions of CF₃IF₂ or CF₃IO with N₂O₅ as well as CF₃I with ClNO₃. During the reactions of CF₃IF₂ with N₂O₅ or CF₃I with ClNO₃ the intermediate products CF₃IF(NO₃) or CF₃ICl(NO₃) can be identified. The preparations, properties, ¹⁹F-nmr spectra and the thermal decomposition of CF₃I(NO₃)₂ are described.     

One-step synthesis of pentavalent triphenylantimony derivatives Ph3Sb(OSiR3)2, Ph3Sb(OCH2CH2)2NH and Ph3Sb(OCH2CH2NMe2)2: X-ray molecular structure of Ph3Sb(OSiMe3)2

Pentavalent bis(triorganosiloxy)triphenylantimony derivatives, Ph₃Sb(OSiR₃)₂ (R = Me, Ph), were synthesized by reaction of triphenylantimony with trimethyl- or triphenylsilanol in the presence of tert-butylhydroperoxide by the mild reaction conditions (0-5 °C, 2 h). The reaction of triphenylantimony with diethanolamine in the presence of tert-butylhydroperoxide gave the cyclic compound Ph₃Sb(OCH₂CH₂)₂NH. The mixture of Ph₃SbO and Ph₃Sb(OCH₂CH₂NMe₂)₂ was obtained by the reaction of triphenylantimony with 2-(N,N-dimethylamino)ethanol in the presence of tert-butylhydroperoxide.     

Three-coordinate RhX[P(C6H11)3]2, their reactions with N2 and O2, and the trans-influence in RhX[P(C6H11)3]2L (X = anionic,L = neutral ligand)

Three-coordinate RhX(PCy₃)₂ complexes (X = F, Cl, Br, I; Cy = cyclohexyl) have been prepared from rhodium(I) cyclooctene compounds. RhCl(PCy₃)₂ is in equilibrium with its dimer. The complexes RhX(PCy₃)₂ (X = Cl, Br, I) form the adducts RhX(PCy₃)₂(N₂) with dinitrogen, the times for N₂-fixation being 4 days, 3 hours and 15 minutes respectively. The three-coordinate complexes form four-coordinate dioxygen adducts RhX(PCy₃)₂(O₂) which have unusually high ν(OO) at about 990 cm^?1. This high frequency is attributed to the four-coordination, which is exceptional for dioxygen complexes. From RhF(PCy₃)₂ carbonyl, ethene, and diphenylacetylene complexes RhX(PCy₃)₂L (X = F, Cl, Br, I, N₃, NCO, NCS; L = CO, C₂H₄, C₂Ph₂) (X = CN, NO₃, acetate; L = CO) have been prepared. The trans-influence of the anionic ligands on the infrared frequencies of the neutral ligands is discussed in terms of the different π-bonding properties of the X- and L-ligands.