Rosa Becerra

Prototype Si—H insertion reaction of silylene with silane. Absolute rate constants, temperature dependence, RRKM modelling and the potential-energy surface

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH4. The reaction was studied in the gas phase over the pressure range 1–100 Torr, with both Ar and SF6 as bath gases, at six temperatures in the range 298–665 K. The reaction of SiH2 with SiH4 to form disilane, Si2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule–1 s–1)=(–9.91 ± 0.04)+(3.3 ± 0.3 kJ mol–1)/RT In 10. These Arrhenius parameters are consistent with a fast, nearly collision-controlled, process. RRKM modelling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure-dependent curves. The step sizes (energies removed in a down collision) corresponded to collisional efficiencies (βc) of ca. 0.5 for SF6 and ca. 0.2 for Ar.The rate constants for the insertion and reverse decomposition (of Si2H6) have been combined to obtain a precise value of the equilibrium constant Kp at 552 K. Using the third-law method, a value for ΔfH°(SiH2)= 273 ± 2 kJ mol–1 is derived which represents the most precise experimental value for this quantity yet obtained. Ab initio calculations at the correlated level, reveal the presence of two weak complexes (local-energy minima) on the potential-energy surface corresponding to either direct or inverted geometry of the inserting silylene fragment. Surprisingly, the latter is the lower in energy, lying 51.5 kJ mol–1 below the unassociated reactants. These complexes rearrange to disilane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.

ROOM TEMPERATURE OBSERVATION OF GEH2 AND THE FIRST TIME-RESOLVED STUDY OF SOME OF ITS REACTIONS

Abstract Using a laser flash photolysis/laser probe technique, applied to two different gaseous precursor molecules, we report absorption signals in the wavenumber region 17109–17120 cm −1 , attributable to previously unobserved rotational transitions in the vibronic spectrum of GeH 2 . By time-resolved monitoring of GeH 2 , rate constants have been obtained for its reactions with O 2 , C 2 H 2 , i-C 4 H 8 , 1,3-C 4 H 6 , C 3 H 8 and Me 3 SiH. The measurements show that GeH 2 is unreactive towards CH bonds but inserts readily in SiH bonds, as well as reacting rapidly (close to the collision rate), with π-bonds. These results represent the first direct kinetic measurements on GeH 2 . Comparisons show that it is slightly less reactive than SiH 2 .

The UV absorption spectrum of SiH3

Abstract Flash photolysis of CCl 4 /SiH 4 /N 2 mixtures in the far-UV gives rise to strong transient absorptions in the region 205–250 nm which are attributed to the first experimental observation of the (A 2 A 1 ← X 2 A 1 ) transition of the SiH 3 radical.

The gas-phase reaction of silylene with acetaldehyde Part 1. Directrate studies, isotope effects, RRKM modelling and ab initio studiesof the potential energy surface

Time-resolved studies of the title reaction, employing both SiH2 and SiD2, have been carried out over the pressure range 1–100 Torr (with SF6 as bath gas) at five temperatures in the range 297–599 K, using laser flash photolysis to generate and monitor both silylene species. The second order rate constants obtained were pressure dependent indicating that the reaction is a third-body assisted association process. The high pressure rate constants, obtained by extrapolation, gave the following Arrhenius parameters: log(A/cm3 molecule−1 s−1) = − 10.10 ± 0.06, Ea = − 3.91 ± 0.47 kJ mol−1, where the uncertainties are single standard deviations. The parameters are consistent with a fast association process occurring at close to the collision rate. RRKM modelling, based on a transition state appropriate to formation of a three-membered ring product, 3-methylsiloxirane, and employing a weak collisional deactivation model gives reasonable fits to the pressure dependent curves for ΔH°/kJ mol−1 in the range − 215 to − 245. Ab initio calculations at the G2 level indicate the inital formation of a silacarbonyl ylid which can then either form the siloxirane by ring closure, rearrange to form siloxyethene or give ethoxysilylene. Fuller details of the potential surface are given. The energetics are reasonably consistent with siloxirane formation representing the main pathway. The isotope effects are small and close to unity, indicating that secondary isotopic label scrambling, by the reversible ring opening of the siloxirane to ethoxysilylene is not occurring. Differences with the silirane system can be explained by the stabilization of a silylene by an alkoxy substituent.

Absolute rate constant and temperature dependence for the reaction of silylene with nitrous oxide

Abstract Absolute rate constants have been measured for the reaction of SiH 2 with N 2 O at temperatures between 295–747 K. The rate constants are fitted to the Arrhenius equation: log ( k /cm 3 molecule −1 s −1 ) = ( − 12.09 ± 0.04) + (2.02 ± 0.31) kJ mol− / RT ln 10. The data are plausibly consistent with a mechanism involving a short-lived H 2 SiON 2 addition complex leading to formation of H 2 SiO + N 2 . Comparisons are made with other silylene reactions. The fate and stability of H 2 SiO are briefly discussed. The significance of these measurements for the kinetic modeling of the chemical vapour deposition of silicon oxide from silane/N 2 O mixtures is also commented on.

An investigation of the germylene addition reaction, GeH2 + C2H2: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Time resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with ethylene, C2H4. The reaction was studied in the gas-phase over the pressure range 1–100 Torr, with SF6 as bath gas, at 5 temperatures in the range 293–555 K. The reaction shows the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule−1 s−1) = (−10.61 ± 0.08) + (5.37 ± 0.56 kJ mol−1)/RT ln10. These Arrhenius parameters are consistent with a fast reaction occurring at approximately half the collision rate at 298 K. RRKM modelling based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves for a choice of the critical energy, E0 = 130 kJ mol−1. Quantum chemical calculations (both DFT and ab initio G2//QCISD) of the GeC2H6 potential energy surface (PES), show that GeH2 + C2H4 initially form a π-complex, which can either collapse to germirane or isomerise by a 1,2 H-shift to ethylgermylene with a relatively low barrier. This indicates that the observed pressure dependence must correspond the formation of two products, of which ethylgermylene is the more stable. It also shows that germiranes with 1-H substituents will thermally rearrange to ethylgermylenes with very low barriers. A detailed examination of the PES shows that other potential reaction products are unlikely to be formed. Thermochemical considerations show that germirane is less strained than silirane, and that divalent state stabilisation energies (DSSE) for germylenes are hardly greater than those for silylenes.

Absolute rate constants for the reactions of germylene and dimethylgermylene with dimethylgermane: the deactivating effect of methyl groups in heavy carbenes

Abstract Gas-phase rate constants for the title reactions have been obtained by laser flash photolysis at 297 K, by use of photoprecursors, 3,4-dimethyl-1-germacyclopent-3-ene for GeH 2 and pentamethyldigermane for GeMe 2 . The values obtained were ( k ( cm 3 molecule −1 s −1 ) ): (2.38±0.11)×10 −10 for GeH 2 , (2.26±0.10)×10 −13 for GeMe 2 . These results show that the insertion reaction of GeMe 2 is 1050 times slower than that of GeH 2 into the GeH bonds of Me 2 GeH 2 . This is explained in terms of a general mechanism involving an intermediate H-bridged complex, applicable to both silylene and germylene insertions. For the GeMe 2 insertion, reactants are in equilibrium with the complex, which rearranges to the product in the rate controlling step.

A gas-phase kinetic study of the reaction of silylene with germane: absolute rate constants, temperature dependence and mechanism

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monogermane, GeH4. The reaction was studied in the gas-phase at 10 Torr total pressure in SF6 bath gas, at five temperatures in the range 295–553 K. The second order rate constants fitted the Arrhenius equation: log(k3/cm3 molecule−1 s−1) = (−9.88 ± 0.02)+(2.13 ± 0.17 kJ mol−1)/RTln10. Experiments at other pressures showed that these rate constants were unaffected by pressure. The data are consistent with a fast association process occuring (at 298 K) close to the collision rate. Although the probable initial product is silylgermane, H3SiGeH3, thermochemical considerations show that this will decompose to GeH2 + SiH4 under experimental conditions. This silylene insertion process is compared to others as well as to the insertion processes of methylene, CH2 and germylene, GeH2.

Time-resolved studies of the temperature dependence of the gas-phase reactions of methylsilylene with silane and the methylsilanes

The 193 nm laser flash photolysis of gas-phase 1,2-dimethyldisilane has been found to give a broad-band absorption in the wavelength range 454–515 nm, which is plausibly shown to be due to the transient species methylsilylene, MeSiH. By carrying out the title studies, the first direct kinetic studies of MeSiH, second-order rate constants have been obtained for reactions of MeSiH with SiH4, MeSiH3, Me2SiH2 and Me3SiH, in the temperature range 360–580 K. The reactions are fast but show negative activation energies, increasing from –7.5 kJ mol–1 for SiH4 to –18.4 kJ mol–1 for Me3SiH. The data are interpreted as proceeding via an intermediate complex, whose rearrangement becomes rate-determining at higher temperatures. Comparisons of reactivity of MeSiH with those of other silylenes reveal the general pattern of methyl substituent effects of these complexes. In conjunction with ab initio theory (for the reaction of SiH2 with SiH4) these show that the electrophilic interaction probably precedes the nucleophilic interaction, although the latter is important in the rate-determining (second) step for the insertion reactions of both MeSiH and SiMe2. Combination of MeSiH insertion rate constants with the reverse unimolecular decomposition rate constants of the product disilanes enable the calculation of an improved value of 202 ± 6 kJ mol–1 for ΔfH⊖(MeSiH).

The insertion reaction of germylene into the Si–H bond of silane: absolute rate constants, temperature dependence, RRKM modelling, and quantum chemical (ab initio and DFT) calculations

Time resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH4. The reaction was studied in the gas-phase over the pressure range 1–100 Torr, with SF6 as bath gas, at 5 temperatures in the range 295–554 K. The reaction shows the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: These Arrhenius parameters are consistent with a moderately fast reaction occurring at approximately one thirtieth of the collision rate. Rice–Ramsperger–Kassel–Marcus (RRKM) modelling based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves for a choice of the critical energy, E0 = 138 kJ mol−1, for the reverse decomposition of H3SiGeH3 , the reaction product. There is no previous experimental determination of this quantity. From it we derive ΔHf0(GeH2) = 233 ± 12 kJ mol−1, in reasonable agreement with earlier estimates. Ab initio and DFT calculations reveal the presence of two weak complexes (local energy minima) on the potential energy surface corresponding to either direct or inverted geometry of the inserting germylene fragment. As found earlier for the GeH2 + GeH4 reaction, the latter is lower in energy and has left and right handed forms. These complexes rearrange to H3SiGeH3 with low barriers. The implications of these findings and the nature of the insertion process are discussed.