Campus Units

Chemistry

Document Type

Article

Publication Version

Published Version

Publication Date

10-1998

Journal or Book Title

Journal of Physical Chemistry A

Volume

102

Issue

44

First Page

8564

Last Page

8572

DOI

10.1021/jp981957f

Abstract

Time-resolved studies of the title reactions have been carried out over the pressure range 1−100 Torr (in SF6 bath gas) and at temperatures in the range 293−600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH2 and SiMe2. All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressure-dependent rate constants gave the following Arrhenius parameters:  SiH2 + C3H6, log(A/cm3 molecule-1 s-1) = −9.79 ± 0.03, Ea (kJ mol-1) = −1.9 ± 0.3; SiH2 + C4H8, log(A/cm3 molecule-1 s-1) = −9.91 ± 0.04, Ea (kJ mol-1) = −2.5 ± 0.3; SiMe2 + C4H8, log(A/cm3 molecule-1 s-1) = −12.12 ± 0.02, Ea(kJ mol-1) = −8.5 ± 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH2 but a tighter transition state for SiMe2. Rice, Ramsperger, Kassel, Marcus theory (RRKM) modeling, based on consistent transition states for silirane decomposition, and employing a weak collisional deactivation model, gave good fits to the pressure-dependent curves for each system, provided an appropriate value of Eo (fitting parameter) was used for each reaction. The kinetic results are consistent with an electrophilically led addition mechanism, although methyl substitution in the alkene hardly affects the rate constants. The RRKM-derived Eo values have been used to derive reaction enthalpies which are in reasonable agreement with values obtained by ab initio calculations at the G2 (MP2,SVP) level. The experimental ΔH° values yield strain energies of 190, 196, and 216 kJ mol-1 for 2-methyl-, 2,2-dimethyl-, and 1,1-dimethylsilirane, respectively. Compared to the strain enthalpy of 167 kJ mol-1 for silirane itself, this shows that methyl substituents in the silirane products substantially increase the strain energies. Theory supports this.

Comments

Reprinted (adapted) with permission from Journal of Physical Chemistry A 102 (1998): 8564, doi:10.1021/jp981957f. Copyright 1998 American Chemical Society.

Copyright Owner

American Chemical Society

Language

en

File Format

application/pdf

Included in

Chemistry Commons

Share

COinS