H. Monty Frey

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.

Absolute rate constants for the gas-phase reactions of silylene with silane, disilane and the methylsilanes

Absolute rate constants for reactions of silylene have been determined by time-resolved measurements of its decay at room temperature, following formation by pulsed-laser photolysis of phenylsilane in the presence of various added silanes. For SiH4 and Si2H6 the rate coefficients are pressure dependent and the former reaction is successfully modelled using RRKM theory. High-pressure (or pressure-independent) rate constants (in 10–10 cm3 molecule–1 s–1) are: SiH4, ca. 4.0; Si2H6, ca. 6.5; MeSiH3, 3.66 ± 0.22; Me2SiH2, 3.31 ± 0.26; and Me3SiH, 2.47 ± 0.14. These results are compared with other determinations and the rate constants for the analogous reactions of SiMe2. A model for the insertion reaction is proposed in which the nucleophilic stage of the process plays an important role.

Absolute rate measurements for some gas-phase addition reactions of dimethylsilylene

Time-resolved studies of the room-temperature reactions of dimethylsilylene, SiMe2, with a series of olefins, acetylenes and buta-1,3-diene are reported. SiMe2 was generated by 193 nm laser flash photolysis of pentamethyldisilane or octamethyltrisilane and was monitored in real time by c.w. laser absorption techniques. Bimolecular rate constants ranging from 0.072 to 17.0 × 10–11 cm3 molecule–1 s–1 were obtained for the reactions of SiMe2 with the thirteen unsaturated compounds studied. The trends with substrate class and substituent are consistent with a mechanism in which the electrophilic character of SiMe2 is important in the rate-controlling stage of the reaction and fit in well with theoretical models of the corresponding carbene addition reactions developed from Holfmanns original calculations. Although it is probable that the three-membered ring siliranes and silirenes are the primary products of these addition reactions, they appear to be too reactive to be characterised by gas chromatography.

Studies of methylene chemistry by pulsed laser-induced decomposition of ketene. Part 2.—Ketene in the presence of ethylene and acetylene

Gaseous mixtures of ketene with C2H4, and with C2H2, diluted in added argon to a total pressure of 400 Torr have been photodecomposed at room temperature by 308 nm u.v. radiation from a pulsed exciplex laser. Conversions were limited to ca. 2% and a reasonably complete hydrocarbon product analysis achieved (H2 was also monitored). In the C2H4 system products up to C5 were detected whilst in the C2H2 system products up to C6 were found. The product distributions suggest the intermediacy of ethyl and vinyl radicals (C2H4 system) and vinyl and propynyl (C3H3) radicals (C2H2 system) as well as the involvement of methylene (CH2, both 1A1 and 3B1 states), methyl, methyne and H atoms in both systems. Most of the products arising from the C3H3 radicals are reported for the first time in this system.Kinetic modelling based on the Gear algorithm has been used to predict the product distribution and its dependence on either C2H4 or C2H2 pressures. The modelling suggests the following. (a) The added gases scavenge CH2 predominantly, if not exclusively, in its 1A1 state with the following rate constants (in cm3 molecule–1 s–1)1CH2+ C2H4→ C3H6 2.1 × 10–10, 1CH2+ C2H2→ C3H4 2.5 × 10–10. (b) The CH2(3B1) analogues of these processes are probably not occurring and upper limits to the rate constants (same units) are 3CH2+ C2H4→ C3H6 <5 × 10–16, 3CH2+ C2H2→ C3H4 <2.6 × 10–16. (c) C3H3 radicals are formed via the decomposition of vibrationally excited C3H4, with a rate constant, k= 4.5 × 108 s–1, consistent with an R.R.K.M. theoretical estimate. (d) Most of the remaining products in both systems are formed via radical–radical reactions with plausibly high collision efficiencies. These findings are compared and contrasted with previous flash-photolysis and other studies of ketene photodecomposition.

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).

Vinylidene: probable intemediacy in methylenecyclopropane decomposition and heat of formation

Abstract The high-temperature (=400°C) decomposition of methylenecyclopropane and its methyl derivatives proceeds, in part, with formation of an olefin and acetylene. The latter probably arises via a vinylidene intermediate. From the measure of activation energy a value of Δ H f 0 (CH 2 C:) ⩽ 409 ± 10 kJ mol -1 is obtained. This figure is in good agreement with theoretical calculations and also with the results of chemical activation experiments involving the reaction of methylene ( 1 A 1 ) with allene.

TIME-RESOLVED GAS-PHASE KINETIC STUDIES OF THE REACTIONS OF SILYLENE WITH DISILANE AND TRISILANE

Abstract Absolute rate constants for the reactions of SiH2 with Si2H6 and Si3H8 have been determined over the temperature range 295–595 K by means of laser flash photolysis. The 193 nm UV photolyses of di- and trisilane were themselves used as the sources of SiH2. Rate constants were independent of total pressure (1–30 Torr) and inert buffer gas (C3H8 or SF6). Arrhenius plots of the rate constants were slightly curved but gave the following average parameters: Si2H6, log(A/cm3 molecule−1 s−1) = −9.51 ± 0.04, Ea = −1.9 ± 0.3 kJ mol−1; Si3H8, log(A/cm3 molecule−1 s−1) = −9.43 ± 0.06, Ea = −2.0 ± 0.4 kJ mol−1. These values show both reactions to be rapid, collisional association processes. The results are consistent with SiH bond insertion processes led by the electrophilic interaction between the bonding electron pair and the SiH2 empty p orbital.

Thermal unimolecular decomposition of β-propiolactone (oxetan-2-one)

The thermal decomposition of oxetan-2-one has been investigated in the gas phase in the temperature range 262–322 °C. The reaction, which yields ethylene and carbon dioxide, is homogeneous and obeys first-order kinetics. There is a minor heterogeneous isomerization to yield acrylic acid. The decomposition is almost certainly a unimolecular process, which in the pressure range studied, ⩽ 6 Torr, is in the fall-off region. High-pressure rate constants were determined by extrapolation using three procedures that are discussed. The preferred method using plots of k–1 against p–0.61 yielded the Arrhenius equation logk/s–1= 14.86±0.30 – 180.46±3.20 kJ mol–1/RT ln10. Theoretical (RRKM) calculations are close to the experimental fall-off curves on the basis of the strong-collision assumption or with 〈ΔE〉d(stepladder model) 24 kJ mol–1, from which it is hardly distinguishable. Certainly it is necessary to assume very efficient intermolecular energy transfer. Comparison with the result from other studies suggests a concerted decomposition via an activated complex with zwitterionic character rather similar to those involved in cyclobutanone decompositions.

Kinetics of the reaction of silylene with hydrogen and a possible resolution of discrepancies over ΔHf⊖(SiH2)

RRKM (Rice–Ramsperger–Kassel–Marcus) calculations, carried out to fit some new experimental data, are found to reconcile apparent differences in two recent studies of the fast reaction between SiH2 and H2: the calculations lead to ΔHf⊖(SiH2)= 65.3 ± 1.5 kcal mol–1.

The insertion reaction of 1CH2 into the silicon–carbon bond

Singlet methylene reacts with 1,1-dimethylsilacyclobutane to yield as well as the expected products formed by C–H insertion some 1,1-dimethylsilacyclopentane which strongly suggests that it can insert in the silicon–carbon bond.