4 research outputs found
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A dramatic isotope effect in the reaction of ClSiH with trimethylsilane-1-d: experimental evidence for intermediate complexes in silylene Si-H(D) insertion reactions
A kinetic isotope effect (kD/kH) of 7.4 has been found for the reaction of chlorosilylene with trimethysilane (Me3SiD vs Me3SiH). Such a value can be accounted for by theoretical modelling, but only if an internal rearrangement of the initially form complex is included in the mechanism. This provides the first concrete evidence for such complexes
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Unusual isotope effect in the reaction of chlorosilylene with trimethylsilane-1-d: Absolute rate studies and quantum chemical and Rice–Ramsperger–Kassel–Marcus calculations provide strong evidence for the involvement of an intermediate complex
Time-resolved studies of chlorosilylene, ClSiH,
generated by the 193 nm laser flash photolysis of 1-chloro-1-
silacyclopent-3-ene, have been carried out to obtain rate
constants for its bimolecular reaction with trimethylsilane-1-d,
Me3SiD, in the gas phase. The reaction was studied at total
pressures up to 100 Torr (with and without added SF6) over
the temperature range of 295−407 K. The rate constants were
found to be pressure independent and gave the following
Arrhenius equation: log[(k/(cm3 molecule−1 s−1)] = (−13.22
± 0.15) + [(13.20 ± 1.00) kJ mol−1]/(RT ln 10). When
compared with previously published kinetic data for the
reaction of ClSiH with Me3SiH, kinetic isotope effects, kD/kH, in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These
far exceed values of 0.4−0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level)
confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization
pathway which can scramble the D and H atom labels. By means of Rice−Ramsperger−Kassel−Marcus modeling and a necessary
(but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope
effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete
evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes
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Gas-phase kinetics of chlorosilylene reactions II. ClSiH + C2H4: absolute rate measurements and quantum chemical and RRKM calculations for the prototype pi addition reaction
Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, are carried out to obtain rate constants for its bimolecular reaction with ethene, C2H4, in the gas-phase. The reaction is studied over the pressure range 0.13-13.3 kPa (with added SF6) at five temperatures in the range 296-562 K. The second order rate constants, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equation: log(k(infinity)/cm(3) molecule(-1) s(-1))=(-10.55 +/- 0.10) + (3.86 +/- 0.70) kJ mol(-1)/RT ln10. The Arrhenius parameters correspond to a loose transition state and the rate constant at room temperature is 43% of that for SiH2 + C2H4, showing that the deactivating effect of Cl-for-H substitution in the silylene is not large. Quantum chemical calculations of the potential energy surface for this reaction at the G3MP2//B3LYP level show that, as well as 1-chlorosilirane, ethylchlorosilylene is a viable product. The calculations reveal how the added effect of the Cl atom on the divalent state stabilisation of ClSiH influences the course of this reaction. RRKM calculations of the reaction pressure dependence suggest that ethylchlorosilylene should be the main product. The results are compared and contrasted with those of SiH2 and SiCl2 with C2H4
Unusual Isotope Effect in the Reaction of Chlorosilylene with Trimethylsilane-<i>1</i>-<i>d</i>. Absolute Rate Studies and Quantum Chemical and Rice–Ramsperger–Kassel–Marcus Calculations Provide Strong Evidence for the Involvement of an Intermediate Complex
Time-resolved studies of chlorosilylene, ClSiH, generated
by the
193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have
been carried out to obtain rate constants for its bimolecular reaction
with trimethylsilane-<i>1</i>-<i>d</i>, Me<sub>3</sub>SiD, in the gas phase. The reaction was studied at total pressures
up to 100 Torr (with and without added SF<sub>6</sub>) over the temperature
range of 295–407 K. The rate constants were found to be pressure
independent and gave the following Arrhenius equation: logÂ[(<i>k</i>/(cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>)] = (−13.22 ± 0.15) + [(13.20 ± 1.00) kJ mol<sup>–1</sup>]/(<i>RT</i> ln 10). When compared with
previously published kinetic data for the reaction of ClSiH with Me<sub>3</sub>SiH, kinetic isotope effects, <i>k</i><sub>D</sub>/<i>k</i><sub>H</sub>, in the range from 7.4 (297 K) to
6.4 (407 K) were obtained. These far exceed values of 0.4–0.5
estimated for a single-step insertion process. Quantum chemical calculations
(G3MP2B3 level) confirm not only the involvement of an intermediate
complex, but also the existence of a low-energy internal isomerization
pathway which can scramble the D and H atom labels. By means of Rice–Ramsperger–Kassel–Marcus
modeling and a necessary (but small) refinement of the energy surface,
we have shown that this mechanism can reproduce closely the experimental
isotope effects. These findings provide the first experimental evidence
for the isomerization pathway and thereby offer the most concrete
evidence to date for the existence of intermediate complexes in the
insertion reactions of silylenes