Si Br Research Articles

Fourier transform microwave spectroscopy of HSiBr: Exploring the Si–Br bond through quadrupole hyperfine coupling

The 1(01)-0(00) (9-10 GHz) and 2(02)-1(01) (18-19 GHz) rotational transitions of HSi 79Br and HSi 81Br have been measured in a pulsed discharge jet expansion to an experimental uncertainty of approximately 1 kHz using Fourier transform microwave spectroscopy. The data have yielded an effective rotational constant, the centrifugal distortion constant Dj, the bromine nuclear quadrupole coupling constants, and the bromine nuclear spin-molecular rotation interaction parameter for both isotopomers. The derived parameters have been compared to their values calculated ab initio, and the nuclear quadrupole coupling tensor has been used to investigate the Si-Br bond, giving a sigma bond ionic character of 0.60, a pi bond character of 0.22, and a total Si-Br ionic character of 0.38. These bond characteristics have been compared to trends in other halosilylenes, silanes, and the analogous carbenes.

Bond breaking at surfaces: Electrons or phonons?

The desorption of atoms and molecules is one of the most fundamental of all surface processes. A distinction is usually made between ‘‘thermal’’ desorption and desorption induced by energetic sources of excitation, such as incident photons or electrons, that induce electronic transitions. The paper by Trenhaile and co-workers in this issue shows the surprising result that the ‘‘thermal’’ desorption of Br from the Si(001) surface is in fact an electronic process in which the random statistical motion of phonons occasionally converges to excite an electron to an antibonding Si–Br r* state from which desorption of Br occurs [1]. Moreover, it is the entropy associated with the assembly of the phonons that is key to understanding how the desorption occurs. The results are important because they suggest that electronic transitions may have an important, but previously unrecognized, role in ‘‘thermal’’ desorption processes at surfaces. Bond breaking at surfaces, like its gas-phase counterpart, has long been understood based on two distinct mechanisms. In the ‘‘electronic’’ mechanism, some source of energy induces a transition from a bound electronic state to an unbound state; one example would be a transition from the Si–Br r state to a Si–Br r* antibonding state as illustrated in Fig. 1a. The second, ‘‘thermal’’ mechanism involves putting enough energy into a specific vibrational mode, such as an Si–Br stretching vibration, that a chemical bond literally breaks and desorption occurs; this process is illustrated in Fig. 1b. These mechanisms are usually distinct because the time scale for physical motion of an atom is quite long compared with the time needed for electrons to change their spatial distribution; this leads to the Franck–Condon approximation, which states that electronic transitions are usually not accompanied by a change in bond lengths and can usually be depicted as ‘‘vertical’’ transitions like that shown by the solid arrow in Fig. 1a.

Electron-stimulated desorption from an unexpected source: Internal hot electrons for Br–Si(1 0 0)-(2 × 1)

The desorption of Br adatoms from Br-saturated Si(1 0 0)-(2 × 1) was studied with scanning tunneling microscopy as a function of dopant type, dopant concentration, and temperature for 620–775 K. Analysis yields the activation energies and prefactors for desorption, and the former correspond to the energy separation between the Fermi level and Si–Br antibonding states. Thus, electron capture in long-lived states results in Br expulsion via a Franck–Condon transition. Analysis of the prefactors reveals that optical phonons provide the energy needed for the electronic excitation. These results show that desorption induced by an electronic transition can occur in closed system without external stimulus, and they indicate that thermally-excited charge carriers may play a general role in surface reactions.

Formation and reactivity of high quality halogen terminated Si(1 1 1) surfaces

High quality halogenated silicon surfaces have been produced using gas phase reactions of H-terminated Si(1 1 1) with molecular chlorine or bromine at room temperature. STM images show that the resulting surfaces maintain the low defect densities of H/Si(1 1 1). High resolution electron energy loss spectra confirm halogenation of the surface through the observation of the Si–Cl or Si–Br stretch modes and also indicate the absence of significant oxidation or contamination of the surface even after brief air exposure. While chlorine terminated surfaces are found to be surprisingly stable to exposure to water vapour, dipping in water destroys the surface by insertion of oxygen into silicon backbonds and formation of terminal H and O–H groups. In lab air, oxidation is observed to proceed rather slowly and in a patch-like fashion with substantial chlorine still present after >10 h exposure.

Experimental Thermochemistry of the SiCl and SiBr Radicals; Enthalpies of Formation of Species in the Si—Cl and Si—Br Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anti-bonding driving caused by electron emission: halogen desorption from Si surfaces

A new model of the halogen desorption from Si surfaces by electronic excitation is proposed. The Br desorption induced by the electron emission from the Br 4s orbital was found to be due to increase of the anti-bonding components of the Si–Br bonds. This model is an alternative to the traditional model based on the Auger process [Jpn. J. Appl. Phys. 33, 2248 (1994).]. A long lifetime of the Br 4s hole was confirmed by applying a recently developed scheme of the real-time electron ion dynamics for solving the time-dependent Schrödinger equation of the electron wave functions.

Vinyl dichlorosilane and vinyl dibromosilane [formula omitted] conformational structure and vibrational properties determined by gas-phase electron diffraction, ab initio molecular orbital calculations, and variable-temperature Raman spectroscopy

The molecular structures, conformations, vibrational spectra, and torsional potentials of vinyl dichlorosilane (VDC) H2CCH–SiHCl2, and vinyl dibromosilane (VDB) H2CCH–SiHBr2, have been studied using gas-phase electron diffraction (GED) data at 23–25°C and variable-temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were handled by a dynamic theoretical model using a cosine Fourier potential function in describing the torsional coordinate. According to the GED refinements, these molecules exist in the gas phase at room temperature as a mixture of two minimum energy conformers, syn (torsional angle φ(CCSiH)=0°) and gauche (torsional angle φ(CCSiH)≈120°). Relevant structural parameters for syn-VDC are as follows: Bond lengths (rg): r(Si–C)=1.847(5)Å,r(Si–Cl)=2.042(2)Å,r(CC)=1.357(7)Å. Bond angles (∠α): ∠CSiCl=110.3(6)°, ∠CCSi=121.8° (calc.). Relevant structural parameters for syn-VDB are as follows: bond lengths (rg): r(Si–C)=1.827(9)Å,r(Si–Br)=2.206(2)Å,r(CC)=1.366(10)Å. Bond angles (∠α): ∠CSiBr=110.1(8)°, ∠CCSi=121.7° (calc.). Uncertainties are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). From the variable-temperature Raman investigation in the liquid phase, the energy differences are: VDC, ΔE°S−G=+0.11±0.06kcalmol−1; VDB, ΔE°S−G=+0.23±0.07kcalmol−1. The Raman energies are average values obtained from two separate line doublets for each molecule, and they have been used in the GED least-squares refinements as valuable constraints.

Analysis of the sidewall films formed during Si trench etching with photoresist mask in Cl 2/HBr-based plasma

The sidewall passivation films produced during Si trench etching with photoresist masks were examined in two etching systems, helicon and inductively coupled plasma (ICP). The trench profiles obtained from the ICP system were more tapered with thicker sidewall films compared to those from the helicon system. The trench etch profiles with Cl 2/HBr/O 2 plasma in the ICP system appeared to be double-sloped. X-ray photoelectron spectroscopy (XPS) analysis indicated that N 2 addition to Cl 2/HBr/O 2 plasma induced the formation of SiN bonds in the sidewall films, in addition to the SiO and SiBr bonds observed with Cl 2/HBr/O 2. The sidewall films formed in Cl 2/HBr/O 2 plasma showed higher oxygen intensities and chemical binding energies compared to those formed in Cl 2/HBr/N 2/O 2 plasma.

Trisilylamines, a13C,15N and29Si NMR study in solution—evidence for Si—Br—Si bridging

Trisilylamines, a13C,15N and29Si NMR study in solution—evidence for Si—Br—Si bridging

High-Resolution Infrared Spectrum of D3Si79Br in the ν1/ν4Region: The Structure of Silyl Bromide

FTIR spectra of monoisotopic D3Si79Br covering the bands ν1(a1, 1580.637 cm−1) and ν4(e, 1615.085 cm−1) have been recorded with a resolution of 3 × 10−3cm−1. The rovibrational analysis revealed severe perturbations of the −5 ≤KΔK≤ 4 series of ν4while ν1and the high-Ksubbands of ν4are almost unperturbed and served to determine the parameters of thev1= 1 andv4= 1 states, with σ(Fit) ca. 5 × 10−4cm−1. Equilibrium rotational constants of H3Si79Br and D3Si79Br were deduced with the help of the vibrational corrections αi. Ther0,rs,rmρ, andrestructures of silyl bromide have been determined. The experimental values of the HSiH angle and of the Si–H distance are found in excellent agreement with theirab initiopredictions. Therestructure isre(Si–H) = 1.470(2) Å,re(Si–Br) = 2.207(1) Å, and αe(HSiH) = 110.5(2)°.

1,1,1,3,3,3-Hexabromotrisilane: structure and conformation determined by gas-phase electron diffraction, ab initio molecular orbital and molecular mechanics calculations, and vibrational spectroscopy

The molecular structure of 1,1,1,3,3,3-hexabromotrisilane at 140°C has been studied using gas-phase electron diffraction. The two SiBr 3 groups are both staggered relative to the central SiH 2 group, but a twist of about 13° of the SiBr 3 groups relative to the exactly staggered position reduces the symmetry to C 2. From the vibrational spectra a distinction between C 2 or C 2v symmetry is possible, and point group C 2v can be ruled out by arguments based on the selection rules. Bond lengths ( r g) and valence angles (∠ a) are: r(Si Si) = 2.344(18) Å, r(Si Br) = 2.205(4) Å ∠SiSiSi = 112.9(19)°, ∠〈BrSiBr〉 = 109.6(6)° (average BrSiBr angle), and ∠〈SiSiBr angle). Error limits are given as 2 σ where σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The infrared and Raman vibrational spectra have been assigned using normal coordinate calculations as well as ab initio calculations, and harmonic force constants are reported. In addition molecular mechanics and ab initio (RHF/SBK and RHF/STO-3G) calculations were performed to assist the structural analysis and to compare with the diffraction results.

1,1,1,3,3,3-Hexabromotrisilane: structure and conformation determined by gas-phase electron diffraction, ab initio molecular orbital and molecular mechanics calculations, and vibrational spectroscopy

The molecular structure of 1,1,1,3,3,3-hexabromotrisilane at 140°C has been studied using gas-phase electron diffraction. The two SiBr 3 groups are both staggered relative to the central SiH 2 group, but a twist of about 13° of the SiBr 3 groups relative to the exactly staggered position reduces the symmetry to C 2. From the vibrational spectra a distinction between C 2 or C 2v symmetry is possible, and point group C 2v can be ruled out by arguments based on the selection rules. Bond lengths ( r g) and valence angles ( o ̌ α ) are: r( Si Si) = 2.344(18) A ̊ , r( Si Br) = 2.205(4) A ̊ o ̌ SiSiSi = 112.9(19)° , o ̌ 〈 BrSiBr〉 = 109.6(6)° (average BrSiBr angle), and o ̌ 〈 SiSiBr〉 = 109.3(6)° (average SiSiBr angle). Error limits are given as 2σ where σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The infrared and Raman vibrational spectra have been assigned using normal coordinate calculations as well as ab initio calculations, and harmonic force constants are reported. In addition molecular mechanics and ab initio (RHF/SBK and RHF/STO-3G calculations were performed to assist the structural analysis and to compare with the diffraction results.

Microwave spectroscopic study on silicon monobromide cation

The microwave spectrum of the SiBr + cation has been observed. The reactive species was generated in a free-space cell by a discharge through pure SiBr 4 gas. In addition to the spectra of both 28Si 79Br + and 28Si 81Br + species, those of the less abundant isotopic species of silicon, 29Si and 30Si, are also observed. The spectrum was analyzed using a Dunham expansion for the rotational energy levels. The SiBr equilibrium bond length is determined to be 2.095456(5) Å. From the nuclear quadrupole coupling constant the ionic character of the SiBr bond is calculated to be 86%.

Closely Approaching the Silylium Ion (R 3 Si + )

The crystal structure of the tri-isopropyl silyl species, i-Pr(3)Si(Br(6)-CB(11)H(6)), where the brominated carborane Br(6)-CB(11) H(6)(-) is perhaps the least nucleophilic anion presently known, has revealed the highest degree of silylium cation character (R(3)Si(+)) yet observed. The average C-Si-C angle is 117 degrees , only 3 degrees short of the planarity expected of a pure silylium ion(120 degrees ). This value compares to 114 degrees recently reported for a toluene-solvated silyl cation, [Et(3)Si(toluene)](+) by Lambert and co-workers. The greater silylium ion character of i-Pr(3)Si(Br(6)-CB(11)H(6)) versus [i-Pr(3)Si(toluene)](+) is also reflected in the larger downfield shift of the silicon-29 nuclear magnetic resonance, 109.8 versus </=94.0 parts per million. The unusual bonding of toluene to R(3)Si(+) in Lambert's compound is reinterpreted as a significant covalent pi interaction.

4-Phenyl-1,2,3,5-dithiadiazolyl: a novel coupling reagent for the formation of E–E bonds (E = C, P, Si)

[Ph[graphic omittted]]2 reacts under mild conditions with molecules containing bonds of the types P–Cl (Ph2PCl and PhPCl2), Si–Br (Me3SiBr) and activated C–Cl or C–Br (ortho-chloranil or MeCOBr) with the formation of [Ph[graphic omittted]]X (X = Cl or Br) and the corresponding E–E (E = P, Si or C) bonded compounds; the application of dithiadiazolyl radicals as coupling reagents is discussed in the context of the strength of the E–Hal bond.

13C and 29Si chemical shifts and coupling constants involving tris[(trimethylsilyl)methyl] silicon compounds

Silicon-29(δ 29Si) NMR chemical shifts are reported for the first time of tris[(trimethylsilyl)methyl] silicon compounds (disilylated derivatives) (Me 3Si A) 3 C αL, where L = Si BR 1R 2R 3 and where R varies widely in electronegativity. 29Si chemical shifts exhibit good correlation with the electronegativities of the groups bonded to the silicon atom. The 13C NMR spectra of these compounds have been recorded and assigned. δ 13C α is shown to depend on the type of substitutent on Si B. The variation of 29SiH coupling constants with electronegativity of R is studied.

29Si and 13C chemical shifts and coupling constants of tris(phenyldimethylsilyl)methyl silicon compounds

Silicon-29 (δ 29Si) NMR chemical shifts are reported for the first time for a number of tris(phenyldimethylsilyl)methyl silicon compounds (disilylated derivatives), (PhMe 2Si A) 3C αL, where LSi BR 1R 2R 3, where R varies widely in electronegativity. δ 29Si B for these series exhibited to some extent good correlations with the electronegativities of the groups bonded to silicon (particularly, δ 29Si BMe 2X, X H, OH, OCH 3, COCH 3 and Cl). Substitution with electronegative atoms shifts the chemical shift of silicon to high field. The 13C NMR spectra of these compounds have been recorded and assigned. The chemical shifts of the α-carbon (C α) resonances are shown to depend on the type of substituent on the silicon-B, thus 13C α exhibit downfield shifts when X=oxygen ligand. The 13C phenyl resonances have been measured and show the same order of o, m and p signals, viz. δ ortho (downfield)>δ para>δ meta. The variation of 29Si- 1H coupling constants with the electronegativity of X was studied.

First principles investigation of chemisorption of moderately heavy atoms on semiconductor surfaces—bromine on silicon

Self‐consistent field Hartree–Fock cluster investigations have been carried out for bromine atoms chemisorbed on Si (111) surface using 5, 14, and 27 atom clusters. For the first two clusters, both all‐electron and pseudopotential procedures have been used and the total energy curves as a function of adatom position have been found to compare very well. The third cluster has been studied by the pseudopotential procedure alone. For the five‐atom cluster which corresponds to the SiH3Br molecule, the Si–Br bond distance and 79Br quadrupole coupling constant (e2qQ) are obtained as 2.20 A and 355.4 MHz, respectively, in very good agreement with the experimental values from microwave measurements of 2.21 A and 346 MHz. For the surface chemisorbed system, we have studied the location of the bromine atom, the local density of states (LDOS), the frequencies and amplitudes of the Si–Br vibrations and the 79Br quadrupole coupling constant. The calculated Si–Br bond distance of 2.23 A agrees very well with two experi...

1,1,2,2-tetrabromodisilane: gas-phase molecular structure and conformational composition as determined by electron diffraction

The molecular structure of 1,1,2,2-tetrabromodisilane has been investigated using gas-phase electron diffraction data obtained at 110°C. At this temperature the molecules exist as a mixture of about equal parts ( X = 0.5 ±0.2) of the two conformers with the HSiSiH torsion angle equal to 180° ( anti) or 60° ( gauche). Assuming that the two conformers differ in their geometries only in the torsion angle φ, some of the important distance ( r a) and angle (∠ α) parameters are: r(SiSi) = 2.349(19) Å, r(SiBr) = 2.205(5) Å, r(SiH) = 1.485 Å (assumed), ∠BrSiBr = 110.1(1.6)°, ∠SiSiBr = 107.1(1.2)° ∠SiSiH = 108.6° (assumed). The error limits are 2σ. The observed conformational composition ( X anti = 0.5(0.2)) corresponds to an energy difference between the conformers of Δ E = E( gauche) — E( anti) = 0.5 ± 0.6 kcal mol −1, assuming Δ S = Rln2.

ChemInform Abstract: MICROWAVE SPECTRUM, STRUCTURE, AND CHEMICAL BONDING OF TRIMETHYLBROMOSILANE

Abstract The microwave spectrum of trimethylbromosilane (trimethylsilylbromide) has been investigated in the 26.5–40.0-GHz region. Spectra of several isotopic species have been observed, including the previously unreported spectrum of the 1H12C30Si81Br species, which yielded a B rotational constant of 1440.92 ± 0.02 MHz. From the experimental data it has been possible to derive the following structural parameters: SiBr = 2.235 ± 0.002 A, SiC = 1.856 ± 0.010 A, and ∠CSiBr = 107.5 ± 0.1°. The results have been compared to other relevant data and discussed in terms of prevailing chemical bonding principles.