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Photolysis of tetramethylsilane near the absorption onset: mechanism and photophysics
Abstract
The excitation of tetramethylsilane (Me4Si) into its lowest excited Rydberg state is followed by two main decomposition channels: a simple SiC bond breaking process with a quantum yield of Φ = 0.45 ± 0.05 and a methane elimination process with the concomitant formation of dimethylsilaethylene (Φ = 0.17 ± 0.04). Other very minor primary processes occur, with quantum yields of the order of Φ ⩽ 5 × 10−3, but their nature could not be identified with certainty. The reactions leading to the stable products are dominated by radical-radical processes and by radical addition reactions to Me2SiCH2. The addition reaction to the SiC double bond occurs preferentially at the Si site. Satisfactory material balance was obtained indicating that the products were mostly recovered. A number of relative rate constants were determined. Reactions in the presence of NO, MeOH, GeH4 and SF6 were also studied. An explanation of the photophysics by a three-state model was attempted. From the experiments, it was concluded that the two decomposition channels occur from different electronic states. The lack of dependence of the CH4 quantum yield on the experimental parameters (liquid or gaseous phase, etc.) suggests a decomposition from a strongly predissociating state, which is identified with the lowest excited state, while the SiC bond breaking process is thought to occur from the triplet state. Molecules which reach the ground state live sufficiently long so that deactivation competes successfully with decomposition.
... Several subsequent reaction pathways exist that could lead to CH 3 • volatilization. 32 Although the photodegradation process is likely more complex, we assume that the CH 3 • groups are irreversibly extracted from the polymer while the Si(CH 3 ) 3 • stays in the film. We also neglected other reaction paths such as crosslinking and recombination reactions. ...
Silicon (Si)-containing block copolymers (BCPs) are promising candidates for directed self-assembly patterning applications and are able to access structures with critical dimensions less than 10 nm. Significant etch contrast between the blocks is required to integrate BCPs for patterning applications and form an initial topographical mask. For Si-containing BCPs, O2 plasma exposure can give high etch contrast between the blocks by forming a thin etch resistant silicon oxide (SiOx) surface layer from the Si-containing block. The authors have also found that H2 and N2/H2 plasmas can form etch resistant barrier layers from organosilicon polymers (OSPs). Photodegradation of the OSPs induced by H2 plasma-generated vacuum ultraviolet (VUV) photons initiates the formation of this etch barrier layer. Fourier transform infrared transmission spectroscopy measurements show enhanced VUV-induced degradation in polymers with higher Si content due to cleavage of the methylsilyl bonds (Si-CH3) and subsequent carbon depletion, leading to formation of an etch resistant Si-enriched surface layer. Furthermore, a dynamic photolysis model based on the dissociation of Si–Si and Si–CH3 bonds shows that higher Si content in the polymer implies deeper photon penetration. The authors conclude that higher VUV fluxes and higher Si content promote the formation of etch resistant surface barriers on the Si-containing block when dry developing Si-containing BCPs with H2-rich plasmas. Finally, plasma dry development of an aligned, Si-containing BCP with sub-10 nm domains is demonstrated using a N2/H2 plasma.
A time-dependent, spatially one-dimensional fluid-Poisson model is applied to analyze the impact of small amounts of tetramethylsilane (TMS) as precursor on the discharge characteristics of an atmospheric-pressure dielectric barrier discharge (DBD) in argon. Based on an established reaction kinetics for argon, it includes a plasma chemistry for TMS, which is validated by measurements of the ignition voltage at the frequency f=86.2kHz for TMS amounts of up to 200 ppm. Details of both a reduced Ar-TMS reaction kinetics scheme and an extended plasma-chemistry model involving about 60 species and 580 reactions related to TMS are given. It is found that good agreement between measured and calculated data can be obtained, when assuming that 25% of the reactions of TMS with excited argon atoms with a rate coefficient of 3.0×10−16m3/s lead to the production of electrons due to Penning ionization. Modeling results for an applied voltage Ua,0=4kV show that TMS is depleted during the residence time of the plasma in the DBD, where the percentage consumption of TMS decreases with increasing TMS fraction because only a finite number of excited argon species is available to dissociate and/or ionize the precursor via energy transfer. Main species resulting from that TMS depletion are presented and discussed. In particular, the analysis clearly indicates that trimethylsilyl cations can be considered to be mainly responsible for the film formation.
Thermal decomposition of tetramethylsilane (TMS) was studied over the temperature range of 298-1450 K by combining flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) and density functional theory (DFT). The initial step of TMS pyrolysis produced methyl radical (Me‧) and Me3Si‧. Me3Si‧ underwent subsequent loss of a hydrogen atom to form Me2Si=CH2 and loss of a methyl radical to form Me2Si:. Isomerizations via 1,2-shift and H2 eliminations were major secondary decomposition reactions of Me2Si=CH2 and Me2Si:. Among the various isomers, silylene species containing Si-H bond, such as :Si(H)CH2CH2CH3, :Si(H)CH2CH=CH2, :Si(H)CH2CH3, and :Si(H)CH=CH2, played an important role in H2 elimination reactions. On the other hand, silene species were insignificant in H2 eliminations. Unlike the silylene species, H2 elimination of :Si=CH2 was energetically unfavorable.
Two primary processes were observed in the Hg-sensitized photolysis of Me 5 Si 2 H: (I) hydrogen abstraction from the Si-H bond with a quantum yield of 0(1) = 0.85, (V) Si-Si bond breaking with 0(V) = 0.04. The hydrogen atoms formed in (/) undergo an H atom abstraction reaction (k(3)), as well as substitution reactions at the Si centers resulting in the formation of dimethylsilane and trimethylsilyl radical (k(4)) or trimethylsilane and dimethylsilyl radical (k(5)). The following branching ratios have been determined:
[xxx]
The ratio of disproportionation (k(2)) to combination (k(1)) for the pentamethyldisilyl radical has been determined with MeOH as the scavenger for 1-methyl-l-trimethylsilylsilene, 0.046 < k(2)/
A: C1) < 0.071. A mechanism with pertinent rate constants has been proposed which accounts for the
results.
The rate constant for the H atom abstraction of trimethylsilyl radicals from pentamethyldisilane (k (4)) was measured relative to the trimethylsilyl combination reaction k (3). A value for k (4)/√K 2() = (8.53+0.08) · 10-11 cm3/2 s-1/2 was obtained. For the dimethylsilyl radical, a smaller value for the corresponding rate constant ratio (5.9 ± 0.2) · 10-11 cm3/2 s-1/2 was measured, and this was attributed to a disproportionation reaction between the dimethylsilyl and the pentamethyldisilyl radical leading to dimethylsilylene.
Present work reports significantly high levels of ionization, eventually leading to Coulomb explosion of Tetramethyl silane (TMS) clusters, on interaction with laser pulses of intensity ∼109 W/cm2. Tetramethyl silane clusters, prepared by supersonic expansion were photoionized at 266, 355 or 532 nm and the resultant ions were detected using time-of-flight mass spectrometer. It is observed that wavelength of irradiation and the size of the cluster are crucial parameters which drastically affect the nature of charge species generated upon photoionization of cluster. The results show that clusters absorb significantly higher energy from the laser field at longer wavelengths (532 nm) and generate multiply charged silicon and carbon ions which have large kinetic energies. Further, laser-cluster interaction at different wavelengths has been quantified and charge densities at 266, 355 and 532 nm are found to be 4x 1010, 5x 1010 and 5x 1011 charges/cm3 respectively. These unusual results have been rationalized based on dominance of secondary ionization processes at 532 nm ultimately leading to Coulomb explosion of clusters. In another set of experiments, multiply charged ions of Ar (up to +5 state) and Kr (up to +6 state) were observed when TMS doped inert gas clusters were photoionized at 532 and 355 nm. The extent of energy absorption at these two wavelengths is clearly manifested from the charge state of the atomic ions generated upon Coulomb disintegration of the doped cluster. These experiments thus demonstrate a novel method for generation of multiply charged atomic ions of inert gases at laser intensity of ∼ 109 W/cm2. The average size of the cluster exhibiting Coulomb explosion phenomena under giga watt intensity conditions has been estimated to be ∼ 6 nm. Experimental results obtained in the present work agree qualitatively with the model proposed earlier [D. Niu, H. Li, F. Liang, L. Wen, X. Luo, B. Wang, and H. Qu, J. Chem. Phys. 122, 151103(2005)] and point towards interaction of quasi-free electrons, generated during primary multiphoton ionization step, with a given wavelength in the presence of Coulombic field.
Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.
Mercury-sensitized photolysis of Me
2
SiH
2
yields in the primary step an Me
2
HSi radical and an H atom
with a quantum yield of one. The Me
2
HSi radicals undergo a
combination reaction [k(2)] as well as two kinds of
disproportionation reactions leading to dimethylsilylene
[k(3)] and 1-methylsilaethene [k(4)]. The following
branching ratios have been determined:
k(3)/[k(2)+k(3)+k(4)]=0.64
±0.10 and
k(4)/[k(2)+k(3)+k(4)]0.007.
For Me
3
Si radicals the branching ratio for
disproportionation was also determined and a value of 0.063 was
obtained.
Me3Si and C2H5 radicals have been generated by the reaction of H atoms with mixtures of Me3SiH and C2H4 and the subsequent reactions studied by end-product analysis. The primary radicals undergo self- and cross-combination and disproportionation reactions. The Me3Si radical also adds to the C2H4, generating another radical, Me3SiCH2CH2. This radical in turn undergoes a variety of self- and cross-combination as well as disproportionation reactions with all the radicals present, A number of relative rate constants were extracted from the data. In particular the ratio k(H + Me3SiH)/k(H + C2H4) was found to be in good agreement with recent absolute determinations of these two rate constants. For the addition reaction we obtained the relative rate constant k(Me3Si + C2H4)/k(1/2)(2Me(3)Si) = (8.7 +/- 0.2) X 10(-10) cm(3/2) s(-1/2) at room temperature. From studies in the temperature range 300-470 K the following values of the Arrhenius parameters for the addition reaction were obtained: 11.6 less than or equal to E-a/kJ mol(-1) less than or equal to 14.1 and 7.1 x 10(-13) less than or equal to A/cm(3) s(-1) less than or equal to 1.9 X 10(-12). In addition, it was found that alkyl and silyl radicals closely obey the geometric mean rule and that disproportionation reactions between alkyl and silyl radicals are of only minor importance. The reactivity of the silyl radical is much higher than that of the alkyl radical in adding to C2H4, and this can be explained in terms of polar effects.
In the Hg-sensitized photolysis of H-2 in the presence of C2H4 the sensitizer Hg is consumed under formation of a metastable compound. The rate of formation and the steady-state concentration of this compound have been studied as a function of Hg, H-2, and C2H4 concentration, absorbed light intensity, temperature, and in the presence of radical scavengers. All these experiments ask for the participation of HgH and C2H5 in the formation of the Hg compound. We therefore suggest that these two radicals combine under formation of HHgC2H5. Further support comes from two color photolysis experiments where HgH and C2H5 radicals have been generated independently. HHgC2H5 decomposes under 206 nm irradiation and much slower in the dark. It is also very easily attacked by radicals.
The photolysis of pentamethyldisilane, Me5Si2H. is characterized by a large number of decomposition processes of the excited molecule. Quantum yield determinations in the presence and absence of various scavengers support the occurrence of Me2Si elimination (Φ = 0.2) and Si-Si bond breaking (Φ = 0.14) as the two major decomposition processes. Other processes include the elimination of various silaethylenes and MeHSi. The quantum yield of these processes sum up to Φ = 0.16. However, the most important pathway of the excited molecule is collisional deactivation (Φ = 0.5). The material balance for the various silaethylenes is poor in the absence of traps but can be improved greatly in the presence of MeOH and is in agreement with computer simulations. Experiments with SF6 suggest that decomposition occurs mainly from the excited states.
The photolysis of Me6Si2 at 206 nm results in two main decomposition processes: simple SiSi bond breaking with a quantum yield of Φ = 0.21 ± 0.03, and Me3SiH elimination with the concomitant formation of Me2SiCH2 with Φ = 0.18 ± 0.01. There is also a minor decomposition channel with a very small quantum yield, Φ = (5.6 ± 0.2) × 10−3, which results in the formation of Me4Si and Me2Si. The main fate of the excited Me6Si2 molecule produced during photolysis is stabilization by collisional deactivation. The end products observed indicate that the reaction pathways followed by the main intermediates, Me3Si and Me2SiCH2, are the same as those found in the photolysis of Me4Si (Ahmed et al., J. Photochem. Photobiol. A: Chem. 86 (1995) 33).
A photochemical process for the modification of polymer surfaces using organosilane compounds has been developed. The process is based upon the UV irradiation of polybutadiene in the presence of liquid ethyldimethylsilane (C2H5)(CH3)2Si–H and gaseous trimethylsilane (CH3)3Si–H. UV irradiation was carried out with a medium pressure Hg lamp and a 193nm ArF* excimer laser. The modified polymer surfaces were investigated by infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), contact angle measurements and atomic force microscopy (AFM). It is found that the photoassisted surface modification with trialkylsilanes leads to the introduction of trialkylsilyl groups onto the surface of the target polymer. From quantitative XPS data the composition of the modified polymer suface (C, O and Si) was determined. The surface modification with trialkylsilanes results in a significant lowering of surface tension γ of polybutadiene. The silane/UV process was found to be very sensitive to small amounts of oxygen in the process gas. Summing up, it is demonstrated that UV irradiation in the presence of gaseous silane compounds is a convenient way to introduce organosilicon groups onto the surface of technical polymers.
The reaction of Me3Si (MeCH3) radicals with N2O has been studied by analysis of the end products of the mercury-sensitized photolysis of N2O with Me3SiH. The main products found were N2, Me3SiSiMe3, Me3SiOSiMe3, and Me3SiOH. The influence on both major and minor products of reactant pressure, different scavenger and quencher molecules, temperature, and the reactor surface permits the derivation of a reaction mechanism and identification of the intermediates involved. The most important steps comprising the mechanism are oxygen abstraction from N2O by Me3Si with formation of Me3SiO and a chain propagation step, the reaction of Me3SiO and Me3SiH with formation of Me3SiOH and Me3Si. Although the precise nature of the last step is unknown, it is a multistep reaction. Me3SiO radicals also undergo combination with Me3Si radicals. Self-combination under formation of a peroxide does not occur. Vibrationally excited species emerge in Si−O bond-forming processes, and a number of minor products stem from unimolecular decomposition of these excited species. The rate constant of O atom abstraction from N2O (reaction 5) relative to Me3Si radical combination (reaction 4) is given by k5/k41/2 = (6.5 ± 1.3) × 10-12 cm3/2 s-1/2 at room temperature. An activation energy of EA(5) = 23 ± 1 kJ mol-1 is obtained from experiments in the temperature range 295−520 K.
The H2 plasma modification of polydimethylsiloxane (PDMS) films of initial thickness up to 600nm was studied. The main plasma component modifying the films is the VUV plasma radiation measured by a VUV monochromator. By variation of the plasma processing parameters, such as H2 pressure, RF forward power and treatment time, the optimum parameters are defined for the plasma chemical modification of liquid PDMS films. The thin films were analysed by Infrared Reflection Absorption Spectroscopy. Gradually changes in the vertical films structures is found with a top methyl-free SiOx layer (60nm), followed by a cross-linked bulk material with a gradually increasing CH3 concentration over the film depth. Additionally, the PDMS plasma modifications results in films shrinkages as measured by Spectroscopic Ellipsometry (SE).
The mercury-sensitized photolysis of Me3SiH was studied as a function of the exposure time, substrate pressure and light intensity, and in the presence of the additives McOH and H2. Two primary processes were observed: hydrogen abstraction from the SiCH bond and, to a minor extent, from the CH bond. The sum of the quantum yields of the two primary processes is only 0.8. The main part of the reaction mechanism, which concerns the reactions of the Me3Si radical, can be quantitatively explained by a previous investigation of the direct photolysis of Me4Si (Ahmed et al., J. Photochem. Photobiol. A: Chem., 86 (1995) 33). With the rate constants given by Ahmed et al., the experimental values can be satisfactorily reproduced by computer simulations. In particular, it is confirmed that silaethylene reacts in an almost collision-controlled manner with radicals and the disproportionation reactions of Si-centred radicals leading to an SiC double bond play only a minor role. The ratio of disproportionation to combination of the Me3Si radical was determined to be 0.07 ± 0.01.
The photoassisted modification of LDPE by VUV irradiation in a reactive trimethylsilane/oxygen atmosphere was investigated. The modified LDPE surface was investigated by FT‐IR, XPS, contact‐angle measurements and electron microscopy. Attachment of alkylsilyl and Si–O units to LDPE was observed. Best results were obtained using an equimolar trimethylsilane/oxygen mixture. Low‐energy surfaces were obtained after a few minutes of irradiation at an intensity of 1.7 mW · cm ⁻² . After prolonged irradiation, the thickness of the modified polyethylene layer is approximately 700 nm. Surface modifications of this type may be of interest to obtain protective and barrier layers on polyolefins.
magnified image
A process for the photochemical modification of polystyrene (PS) surfaces employing organosilane compounds has been developed. Polystyrene was irradiated in presence of trialkylsilanes [ethyldimethylsilane (EDMS), trimethylsilane (TrMS)]. UV irradiation was carried out with a medium pressure (MP) Hg lamp and a 193 nm ArF* excimer laser. FT-IR and X-ray photoelectron spectroscopy (XPS) evidenced that after irradiation alkylsilyl groups were covalently bond to the polymer surface. Contact angle measurements proved a significant lowering in surface energy of polystyrene as a result of the photomodification process. When EDMS was used as photoreactive reagent, the introduction of Si-H groups onto the polymer surface was also found. The introduction of Si-H bonds onto polymer surfaces provides new possibilities for further surface functionalization. Atomic force microscopy (AFM) showed that Hg lamp irradiation does not exert significant changes in surface topography, while 193 nm excimer laser irradiation leads to surface corrugation. Summing up, it is demonstrated that organosilanes can be employed in UV reactions to attach silyl and also Si-H groups onto polystyrene surfaces. Possible reaction mechanisms are discussed.
Different hexamethyldisiloxane (HMDSO) dissociation processes are investigated by means of absorption spectroscopy and mass spectrometry. All of these processes are expected to occur in plasma containing Ar-HMDSO gas mixture. We successively study interactions of the HMDSO molecule with electrons (energy ranges from 15 to 70 eV), with Ar((3)P(2)) metastable species (internal energy 11.55 eV) and with VUV photon (7.3 to 10.79 eV). The studies of HMDSO interactions with Ar((3)P(2)) and VUV photon provide new results concerning the dissociation pathways and the collision cross-sections. In the case of Ar((3)P(2)), the dissociation mechanisms result mainly in Si-C or Si-O bond breaking, producing SiMe(2,1) radicals. Less efficient mechanisms involve also Si-C and Si-O bond breaking producing Me, Si(2)Me(5)O, or SiMe(3), on one hand, and, on the other hand, Si-C and C-H bond breaking producing Si(2)Me(4)OH. In the case of photon interaction, the dissociation process is more selective and mainly produces Si(2)OMe(5) pentadisiloxane and methyl radicals due to Si-C bond breaking. Si-O bond breaking produces also SiMe(3) in a lower concentration. Dissociation cross-section values of HMDSO ranging from σ = 45 × 10(-20) m(2) to 180 × 10(-20) m(2) and from σ = 0.7 × 10(-22) m(2) to 18.3 × 10(-22) m(2), correspond to a global dissociation mechanism by Ar((3)P(2)) collision and to a selective dissociation mechanism (producing Si(2)OMe(5) and Me) by VUV photon interaction, respectively. All results are compared and discussed.
Introduction
Compounds of Tetravalent Silicon
Free Radicals and Bond Dissociation Enthalpies
Other Silicon Containing Species
Appendix
Femtosecond laser patterning of octadecylsiloxane monolayers on quartz glass at λ=800 nm , τ≪30 fs , and ambient conditions has been investigated. Selective decomposition of the coating with single laser pulses at subwavelength resolution can be carried out over a wide range of fluences from 4.2 down to 3.1 J / cm 2 . In particular, at a 1/e laser spot diameter of 1.8 μ m , structures with a width down to 250 nm and below were fabricated. This opens up a facile route towards laser fabrication of transparent templates with chemical structures down into the sub- 100- nm -regime.
The review concerns the production of the unsaturated compounds of a doubly bonded silicon via the four-membered silacycles’ 2+2 cycloreversion, a reaction which allowed 35 years ago the evidence of 1,1-dimethylsilene (DMSE), (CH3)2SiCH2, the first compound containing the silicon–carbon double bond, and inspired a renewed interest to the chemistry of the multiply bonded silicon compounds at that time believed to be non-existent. Since then a great many of the doubly bonded silicon compounds have been obtained by various methods. Of them sterically non-hindered compounds are kinetically unstable because of their unprecedented reactivity in the bimolecular reactions, whereas those ones kinetically stabilized by the bulky groups, the ‘crowded’ compounds of the multiply bonded silicon, are relatively stable. Thermal and photochemical 2+2 cycloreversions occurring both in gas and liquid phases are reviewed. A special section is devoted to the gas-phase 2+2 thermocycloreversion of 1,1-dimethyl-1-silacyclobutane as a clean reaction of the transient DMSE production for the study of its chemistry as well as for its characterization by physicochemical and spectroscopic methods. Scope and limitations of 2+2 cycloreversion affected by the numbers and the position of silicon atoms in the four-membered ring, by the presence of Groups 15 and 16 elements, by the substituents and by the reactions conditions are evaluated.
The gas-phase chemical species produced from both the direct decomposition of 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) on a tungsten filament and the secondary reactions in the HWCVD reactor were identified by vacuum ultraviolet laser single-photon ionization coupled with time-of-flight mass spectrometry. TMDSCB decomposes on the filament to methyl and 1,1,3-trimethyl-1,3-disilacyclobutane-1-yl radicals. Subsequent hydrogen abstraction reactions from the parent molecule by methyl radicals and biradical combination reactions are dominant in the reactor. The formation of dimethylsilene (m/z = 72) through the ring Si–C bond cleavage is indirectly confirmed by the observation of the signal from 1,1,3,3,5,5-hexamethyl-1,3,5-trisilacyclohexane at m/z = 216. Tetra- and tri-methylsilane are also found to be formed in the HWCVD reactor.
The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A''<-- X[combining tilde](1)A'' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.
Enthalpies of formation of the methyl-substituted silaethylenes (CH3)nH2-nSi = CH2 (n = 1, 2) and disilenes (CH3)nH2-nSi = Si(CH3)mH2-m (n, m = 0-2) are predicted by using isodesmic reactions which relate the compound of interest to the parent silaethylene (H2Si = CH2) or disilene (H2Si = SiH2), for which accurate enthalpies of formation have recently been determined. In turn, the enthalpies of formation of these methyl-substituted compounds are used in conjunction with homodesmic reactions to reevaluate the enthalpies of formation of the silicon-substituted cyclobutenes CnSi4-nH6 (n = 0-4).
The kinetics and mechanism of the mercury‐photosensitized decomposition of hexamethyldisilane have been studied. It has been confirmed that the initially‐formed radical, Me 3 SiSi(Me 2 )CH 2 , undergoes a unimolecular rearrangement; Arrhenius parameters have been measured for this rearrangement and for subsequent reactions of the rearranged radical:
magnified image
The significance of these Arrhenius parameters in silicon chemistry is discussed by means of thermochemical calculations.
The syn elimination of HX from CH3CH2X produces a C2H4‐HX complex, which is stabilized relative to ethylene and HX. The primary kinetic H/D isotope effects in the complex‐forming steps of such reactions have been examined ab initio for X = H, BH2, CH2, NH2, NH3+, OH, OH2+, F, Cl, and Br, using 3–21G, 3–21G(d), 6–31G(d), and MP2/6–31G(d)optimized structures and vibration frequencies. Four‐centered transition structures are found for all but X = H, BH2, and CH3; with BH2, no transition structure exists at MP2/6–31G(d); with H and CH3, the transition structures are three‐centered. In addition, although the syn mechanism has lower energy for X=H than the four‐centered Woodward‐Hoffman allowed anti mechanism, C‐H, C‐C, and H‐H bond breaking would be favored over either pathway. The semiclassical primary kinetic isotope effect increases systematically as the central atom of a neutral X is varied from left to right along a row, or down a column, of the periodic table. Concurrently, the X‐H‐C angle in the transition state increases. The increase in kH/kD as the X‐H‐C angle becomes more linear is the direction predicted by E.S. Lewis for hydride (deuteride) transfer, and by More O'Ferrall for hydron transfer. One‐dimensional corrections to these isotope effects predict significant quantum mechanical tunneling only in the cases of F and OH, for which the Bell and Eckart barriers are narrower than those obtained from intrinsic reaction coordinate (IRC) calculations. In contrast, for NH2, where tunneling seems unimportant, the Eckart barrier is wider than the IRC. A quantitative measure of barrier width is the imaginary frequency of the transition state. Tunneling occurs when this is large, when the barrier is large, and, most importantly, when the transition structure is symmetrical.
Ab initio molecular orbital calculations on the transition states and
barrier heights for the addition of atomic hydrogen to silaethylene are
carried out. The activation energy for the addition to the silicon site
is lower than that to the carbon site, while the exothermicity is
smaller.
Analysis of the endproducts of the photolysis of hexamethyldisilane at 206 nm reveals two major decomposition channels: SiSi bond rupture yielding trimethylsilyl radicals (71%), and the molecular elimination of dimethylsilene (27%). A mechanism is proposed which accounts for the products in the presence and absence of scavengers. Laser pulsed photolysis experiments were carried out to measure the absorption cross-section and the rate constant for combination of the trimethylsilyl radical. The values obtained are σ(260 nm, base e) = (3.7 ± 0.9) × 10−17 cm2 molecule−1 and k = (3 ± 1) × 10−11 cm3 s−1.
The photoelectron spectra of CF4, SiF4, GeF4, C(CH3)4, Si(CH3)4, Ge(CH3)4, Sn(CH3)4, and Pb(CH3)4 h
The relative energetics for eight competing decomposition reactions of methylsilane are evaluated using full fourth-order (MP4) perturbation theory and the combined McLean-Chandler/6-311G basis set, augmented by polarization functions on all atoms. The lowest energy processes are predicted to be the 1, 1-elimination of molecular hydrogen to form methylsilylene and the extrusion of silylene to form methane. The 1,2-H2 elimination is predicted to be a much higher energy process; however, unlike disilane, because of the high barrier to the methylsilylene-silaethene isomerization, the lowest energy route to the latter isomer is still likely to be the direct 1,2-elimination.
The thermal decomposition of 1,1-dimethyl-1-silacyclobutane has been investigated between 400 and 460°. In this range the reaction is first order and homogeneous. Ethylene and 1,1,3,3-tetramethyl-1,3-disilacyclobutane are the only products. The rate equation is given by k1(sec.–1)= 1015·80 ± 0·20 exp(–63,800 ± 500/RT). The mechanism of the reaction is interpreted in terms of a reversible initial reaction to give an unstable intermediate containing a silicon–carbon double bond. Some reactions of this intermediate are investigated.
An experimental study of the photolysis of trimethylsilane–diazomethane–oxygen mixtures and tetramethylsilane–trimethylsilane–diazomethane–oxygen mixtures at 4358 and 3660 Å is reported. Experimental rate constants for the decomposition of chemically activated ethyltrimethylsilane, ethyldimethylsilane, and tetramethylsilane are determined. These specific rates range from 3 × 104 sec−1 to 6.5 × 105 sec−1. A comparison of the 4358- and 3660-Å rate constants for tetramethylsilane decomposition with RRKM theory calculations suggest that a thermal A factor of 1015.0±0.5 sec−1 is correct for primary decomposition by Si&sngbnd;C bond rupture. The uncertainty in this A factor reflects the uncertainties in the E* and E0 values for chemically activated tetramethylsilane. A discussion of this A factor relative to that found previously for neopentane decomposition is given.
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The combination and disproportionation of ethyl radicals has been studied over a range of pressures and under various conditions designed to separate the various reaction mechanisms which can occur. It is shown that in addition to the normal two-body reaction, a three-body reaction is important at high pressures, while at low pressures wall reactions become prominent. An interesting feature is the effect on the combination/disproportionation ratio of radicals formed with excess energy which retain that excess long enough so that the subsequent reactions are modified. It is suggested that these alternative reaction mechanisms are responsible for the variety of results reported in the literature for the products of reaction of ethyl radicals.
The reaction CH3+NO (+M) → CH3NO (+M) was initiated by pulse radiolysis of acetone/nitric oxide mixtures and the kinetics of methyl radicals was studied by time-resolved infrared absorption spectroscopy. The rate constant was found to be strongly pressure dependent in the range of p(M) = 6.5−150 mbar at 298 K with M = acetone as the third body. The experimental results are represented in terms of a fall-off curve centered at 37 mbar with limiting high- and low-pressure rate constants of krec,∞=(6.6 ± 0.9) × 109 × (T/300)0.6 M−1 s−1 and krec,0/[M] = (4.4 ± 0.4) × 1012 × (T/300)−3.5 M−2s−1, respectively.
The thermolysis of tetramethylsilane has been studied in a pulsed stirred-flow system between 840 and 1055 K, yielding significantly different Arrhenius parameters above and below 950 K. The high temperature results relate to a non-chain mechanism, whence D(Me3Si–Me)= 355 ± 6 kJ mol–1 while the low temperature results relate to a short chain sequence. A mechanism for the latter is shown by computer-aided numerical integration to be consistent with experimental results, as is a similar chain mechanism for the thermolysis of trimethylsilane.
The reaction of 1,1-dimethylsilaethene with Me3SiOMe provides unequivocal evidence for regiospecific addition of Me2SiCH2 across the silicon-oxygen bond.
The standard molar enthalpy of combustion of tetramethylsilane has been measured by oxygen aneroid rotating-bomb calorimetry using vinylidene fluoride polymer to promote the combustion and form a well defined final solution of fluorosilicic acid in excess hydrofluoric acid. A standard enthalpy of formation ΔfHmo(Me4Si, l) = − (257.9±3.2) kJ·mol−1 was determined. The corresponding gas-phase standard molar enthalpy of formation ΔfHmo(Me4Si, g) = − (233.2±3.2) kJ·mol−1 has been combined with kinetic data from the literature to obtain bond-dissociation enthalpies and the standard molar enthalpy of formation of the trimethylsilyl radical.
The UV absorption spectrum of trimethylsilyl radical was observed at 256 nm for the first time by photolysing allyltrimethylsilane and hexamethyldisilane with an ArF excimer laser. A bimolecular rate constant for recombination of trimethylsilyl radical of (2.5±0.5×10−11 molecule−1 cm3 s−1 was measured.
1CH2 produced from the photolysis of ketene inserts into the C-H bond of methylfluorosilanes (MexSiF4-x, = 1-4), the relative rates being in the order Me4Si > Me3SiF > Me2SiF2 > MeSiF3. In one case, that of Me2SiF2, the observed products are also consistent with insertion into the Si-F bond. 3CH2 undergoes reactions leading to hydrocarbon products and a radical exchange process is proposed in its reaction with the silanes.
The reaction between CH3OH or C2H5OH and 1,1-dimethylsilaethene, the disproportionation product of the trimethylsilyl radical, has been studied to establish its efficiency for quantitative measurement of the disproportionation reaction. It has been found that standing in a Pyrex vessel, in mixtures of (CH3)3SiH - the usual precursor of the (CH3)3Si radical - and CH3OH or C2H5OH, a dark surface reaction takes place giving H2, (CH3)3SiSi(CH3)3 and (CH3)3SiOR. Using the Hg(3P1) photosensitization of (CH3)3 SiH as a source of (CH3)3Si radicals in the presence of 10–20 Torr CH3OH or 10 Torr C2H5OH, the (CH3)3SiOR product yield increased linearly with increasing photolysis time but did not vanish when extrapolated to zero time. After minimizing this interfering dark reaction and applying a correction for its contribution we determined the value kd/kc = 0.10±0.01 for the disproportionation-combination reactions of the trimethylsilyl radical.
When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (〈ETr 〉=0.82Eavl ) and angular distribution (β=−0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration.
The HeI photoelectron and vacuum uv absorption spectra of a number of methyl, ethyl, methylfluoro, and ethylfluoro derivatives of silane are presented. The spectra can be interpreted on the basis of their similarity to those of the related methane or ethane derivatives. The lowest lying excited states are related to transitions to 4s, 4p, and 3d orbitals having mixed Rydberg–valence shell character. The spectra can be divided into two categories: methane (’’round field’’) or ethane (’’long field’’) types. Fluorine substitution influences these spectra in a characteristic way.
Spectroscopic evidence as to the primary processes caused by absorption of 1849‐Å radiation by HI is evaluated and clarified as a basis for consideration of chemical reactions of the 3.6 and 2.6‐eV H atoms which may be formed at this wavelength. The energetics of possible hot H reactions with HI which might raise ϕH2 above the value of unity observed at 2537 Å are evaluated. Experimental results are presented on the photolysis of HI by 1849‐Å radiation at —78° and 25°C, showing that little or no enhancement of the yield by such hot processes occurs. Quantum yields of HI decomposition at 1849 Å of 0.5 or greater are found in HI☒I2 systems where 90% of the radiation is absorbed by I2, indicating a high probability of photosensitized decomposition of HI by excited I2 molecules or of absorption of I2 fluorescence radiation by HI, or both.
The kinetics and products of pyrolysis of trimethylchlorosilane, dimethylchlorosilane, and methyldichlorosilane have been studied by low-pressure pyrolysis. It was concluded that all three pyrolyses proceeded mainly by radical chain mechanisms, with little involvement of silylenes or silenes. Nonchain conditions were achieved for the pyrolysis of trimethylchlorosilane, leading to a value of 366.5 ± 7 kJ mol-1 for the silicon-methyl bond dissociation energy in trimethylchlorosilane. Sulfur hexafluoride was found to be a useful trap for silyl radicals.
The thermal decomposition kinetics of disilane and a number of methylated disilanes have been analyzed with the aid of transition-state theory and their back-reaction rate constants to obtain heats of formation of the reactant disilanes and heats of formation of their product silylenes. On the basis of comparisons of the values so obtained with semiempirical and ab initio molecular orbital calculations and with existing literature values, recommendations for the heats of formation of the methylated disilanes and for silylene and the methylated silylenes are made (e.g., to an estimated accuracy of ±2 kcal/mol, heats of formation for Me6Si2, SiH2, MeSiH, and Me2Si are -78, 64, 48, and 32 kcal/mol, respectively).
Beweise für eine Disproportionierung von photolytisch aus Bis-[trimethylsilyll-Hg- Dampf erzeugten Trimethylsilyl-Radikalen zu Tiimethylsilan und Me2Si=CH2 werden mitgeteilt.
In the presence of excess tert-butyl alcohol, the product ratio of MeâCOSiMeâ/MeâSiSiMeâ = 0.19 +- 0.05 remained constant with photolytic irradiation time. That 2-methyl-2-silapropene was being trapped was demonstrated by use of MeâCOD either generated in situ from MeâSiD or added in excess. MeâCOD + CHâ = SiMeâ ..-->.. MeâCOSiMeâCHâD. Formation of this monodeuterated tert-butoxytrimethylsilane was established by ¹H NMR and mass spectroscopy. It is believed that the disproportionation of trimethylsilyl radicals in solution is established by these trapping experiments with deuterated tert-butyl alcohol. 13 references.
Untersuchungen zeigen, daß die bei der Hg-Photosensiibilisierung von Trimethylsilan gebildeten Trimethylsilyl-Radikale (III) sich nicht nur zu Hexamethyldisilan (IV) vereinigen, sondern auch zum Trimethylsilan (I) und Dimethylmethylensilan (II) disproportionieren.
The photoelectron-photoion coincidence (PEPICO) technique has been used to investigate the unimolecular decomposition of the (CH//3)//6Si//2** plus molecular ion. The absolute rates of C//3H//9Si** plus and C//5H//1//5Si//2** plus ion formation were measured as a function of the internal energy by analyzing the ion time-of-flight distribution. The results are compared to the rates predicted by the statistical theory (RRKM/QET). The two dissociation channels are in competition with each other, and their observed onsets are subject to a considerable kinetic shift which is taken into account in evaluating the thermochemical dissociation limits. The rate data show that methyl loss is associated with a tighter transition state than the complex producing C//3H//9Si** plus ions.
The kinetics of the reactions of CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, s-C{sub 4}H{sub 9}, and t-C{sub 4}H{sub 9} with HI were studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (typically between 295 and 648 K) to determine Arrhenius parameters. These results were combined with determinations of the rate constants of the reverse reactions (I + hydrocarbon) determined previously by others to obtain equilibrium constants for the following reaction: R + HI {leftrightarrow} R-H + I. Second and Third Law based analyses using these equilibrium constants yielded heats of formation for the five alkyl radicals whose R + HI reactions were studied. The Third Law heats of formation (obtained using calculated entropies) are extremely accurate, within {plus minus} 2 kJ mol{sup {minus}1} of the current best values. The cause of the long-standing disparity that has existed between the heats of formation of the alkyl radicals derived from studies of R + HI {leftrightarrow} R-H + I equilibria and those obtained from investigation of dissociation-recombination equilibria has been identified. It is the difference between the assumed generic activation energy of R + HI rate constants (4 kJmore » mol{sup {minus}1}) that had been used in all prior thermochemical calculations and the actual values of these activation energies. A complex mechanism for R + HI reactions that is consistent with the observed kinetic behavior of these reactions is discussed.« less
The kinetics of the elementary reactions of Cl and Br with HSi(CH3)3 (1 and 2) have been measured by flash-photolysis/time-resolved atomic resonance spectroscopy over the approximate temperature range 300-460 K. The results are k1 = (1.24 +/- 0.35) X 10(-10) exp(1.3 +/- 0.8 kJ mol-1/RT) cm3 s-1 and k2 = (7.6 +/- 3.3) x 10(-10) exp(-28.4 +/- 1.3 kJ mol-1/RT) cm3 s-1, with confidence limits of about +/-20%. The activation energy of 2, combined with an estimated activation energy for the reverse reaction, yields a bond dissociation enthalpy of D298(H-Si(CH3)3) = 398 +/- 6 kJ mol-1, which is about 14 kJ mol-1 larger than D298(H-SiH3). This difference is supported by ab initio calculations. Implications for DELTA-H(f)(Si(CH3)3) and the Si-Si bond strength in disilanes are discussed in the text.
The 147-nm gas-phase photolysis of tetramethylsilane yielded ten measurable and several trace products along with a solid deposit. From the effect of pressure, exposure time, deuterium labeling, and added nitric oxide on the quantum yields of individual products, the following primary steps were postulated: CH3 + Si(CH3)3 (φ = 0.43); 2CH3 + Si(CH3)2 (φ = 0.24); CH4 + CH2Si(CH3)2 (φ = 0.17); CH3 + H + CH2Si(CH3)2 (φ = 0.10); H2 + CHSi(CH3)3 (φ = 0.02); and CH2 + (CH3)3SiH (φ = 0.04). Fluorescence could not be observed and the upper limit for φf is 10-5. The secondary reactions of the principal silicon radicals Si(CH3)3, CH2Si(CH3)2, and Si(CH3)2 and the mechanism of the nitric oxide inhibited reaction are discussed and it is shown that the siloxy radical (CH3)3SiO can displace CH3 from the substrate to give hexamethyldisiloxane.
Hexamethyldisilane vapor was photolyzed at 147 nm and various additives were used to intercept suspected reactive intermediates. The following primary products (quantum yields) were deduced from the product distribution of the collected experiments: (CH3)3Si (0.99), CH3 (0.51), (CH3)2SiCH2 (0.41), (CH3)3SiH (0.26), H (0.20), CH4 (0.08), and H2 (0.06). The cross disproportionation to combination ratio of CH3 and (CH3)3Si is determined to be 0.22 ± 0.06 with no observable dependence on pressure or diluent (N2) concentration. (CH3)3Si abstraction of hydrogen competitive with radical recombination rates is also consistent with the results. The rate of reaction of (CH3)SiCH2 with methanol is found to be similar to that of (CH3)SiCH2 produced by photolyzing (CH3)4Si and 1,1-dimethyl-1-silacyclobutane; however, the pressure dependence found for this reaction in the last two systems is absent for (CH3)2SiCH2 produced in the photolysis of hexamethyldisilane.
A study of the photolysis of 1,1-dimethylsilacyclobutane at 147-214 nm shows that of the four primary processes identified the predominant mode of decomposition is to C{sub 2}H{sub 4} and dimethylsilaethene. Evidence from experiments in the presence of SF{sub 6} suggests that the dimethylsilaethene is formed initially in a vibrationally excited state. Laser pulsed photolysis experiments at 193 nm have been carried out to measure the absorption spectrum of Me{sub 2}SiCH{sub 2}, its absorption cross section, and the rate constant for Me{sub 2}SCH{sub 2} combination. The values obtained are {sigma} (240 nm, base e) = (1.0 {plus minus} 0.2) {times} 10{sup {minus}17} cm{sup 2} and k{sub 7} = (3.3 {plus minus} 0.8) {times} 10{sup {minus}11} cm{sup 3} s{sup {minus}1}. The kinetics of the pyrolysis of Me{sub 2}SiCH{sub 2}CH{sub 2}CH{sub 2} have also been reexamined.
The 147-nm photolyses of 2,2-dimethylbutane, 2,2,3-trimethylbutane, 2,2,3-trimethyl-2-silabutane (isopropyltrimethylsilane), and 2,2,3,3-tetramethyl-2-silabutane (tert-butyltrimethylsilane) are reported. In addition, the mercury-sensitized photolyses of i-C4H10, trimethylsilane, and mixtures of i-C4H10 and trimethylsilane are reported which give disproportionation to combination (D/C) ratios of 2.1 ± 0.2 and 0.28 ± 0.05 for (CH3)3C + (CH3)3C and (CH3)3Si + (CH3)3Si, respectively, and D/C ratios of 1.86 ± 0.15 and 0.55 ± 0.08 for (CH3)3C + (CH3)3Si to form 2-methyl-2-silapropene and i-C4H8, respectively. With the completion of this work, several trends and generalizations can be drawn concerning the importance of various processes in linear vs. branched alkanes and alkylsilanes. These conclusions are summarized in this report.
The photolysis of tetramethylsilane at the long wavelength side of the absorption spectrum gives rise to two main primary photochemical processes: A simple SiC bond-breaking process with a quantum yield of 0.55 ± 0.17 and a methane elimination reaction with a quantum yield of 0.22 ± 0.07. The radicals disappear by recombination and addition reactions to dimethylsilaethylene. Only an upper limit for the ratio of disproportionation to recombination of 0.13 for the trimethylsilyl radicals can be given. Beside the radical addition reaction dimethylsilaethylene also dimerizes. A semiquantitative evaluation of the data shows that the radical addition reaction as well as the dimerisation reaction proceed with a negligible activation energy.Durch Einstrahlung in die langwellige Seite des Absorptionsspektrums zerfällt Tetramethylsilan im wesentlichen nach zwei Prozessen: Einem einfachen SiC-Bindungsbruch mit einer Quantenausbeute von 0,55 ± 0,17 und einer Methan-Eliminierungsreaktion mit einer Quantenausbeute von 0,22 ± 0,07. Die Radikale verschwinden durch Rekombinationsreaktionen und Addition an Dimethylsilaäthylen. Für das Verhältnis von Disproportionierung zu Rekombination der Trimethylsilylradikale kann nur eine obere Grenze von 0,13 angegeben werden. Dimethylsilaäthylen verschwindet neben Radikaladditionsreaktionen auch durch Dimerisation. Eine semiquantitative Auswertung der Daten zeigt, daß sowohl die Radikaladditionsreaktionen an Dimethylsilaäthylen als auch dessen Dimerisierung mit einer sehr kleinen Aktivierungsenergie ablaufen.
The helium-(I) photoelectron spectra of the species (CH3)4A, where A = C, Si, Ge, Sn and Pb, are reported. The observed bands are assigned using a simple molecular orbital model which employs localized bond orbitals as the basis set.
Data on the kinetics of the decomposition of methane, ethane, propane, and isobutane and the reverse radical combination processes have been examined. From room to combustion temperatures, the limiting high-pressure rate expressions have been found to be , , , , , , .Falloff effects have been treated in the context of RRKM theory and collision efficiencies and step sizes down determined for a number of collision partners. With argon the step size down appear to increase from a very low value of about 100 cm−1 at room temperature to a 600-cm−1 plateau at combustion temperatures. With a large polyatomic, the step size down is in the 1000–2000 cm−1 range for temperatures from 300 to 1100 K. These results provide a basis for the prediction of rate constants over a wide range of conditions. The extension of these results to related systems is considered.
A program package is provided for analysis of kinetic mechanisms on personal computers. KINAL consists of four programs called DIFF, SENS, PROC and YRED. These require similar input data and use common subroutines. DIFF solves stiff differential equations and SENS computes the local concentration sensitivity matrix. PROC generates the rate sensitivity matrix or the quasi-stationary sensitivity matrix from concentration data or uses a matrix computed by SENS and extracts the kinetic information inherent in sensitivity matrices by principal component analysis. Finally, YRED provides suggestions for the elimination of species from the reaction mechanism.