Abstract The gas-phase thermal decomposition kinetics of silacyclobutane (
1), 1-methyl- silacyclobutane (
2), and 1,1-dimethyl-1-silacyclobutane (
3) has been theoretically studied at the B3LYP/6-311G**, B3PW91/6-311G**, and MPW1PW91/6-311G** levels. The B3LYP/6-311G** method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds
1–
3 yields ethylene and the corresponding silene. Based on the optimized ground state geometries using B3LYP/6-311G** method, the natural bond orbital (NBO) analysis of donor-acceptor (bonding–antibonding) interactions revealed that the perturbation energies (E
2) associated with the electronic delocalization from σ
Si1–C2 to σ*
C4–Si1 orbitals increase from compounds
1 to
3. The σ
Si1–C2→σ*
C4–Si1 resonance energies for compounds
1–
3 are 1.17, 1.26, and 1.43 kcal/mol, respectively. Also, the decomposition process in these compounds is controlled by σ→σ* resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Si
sp2 atom. NBO analysis shows that the occupancies of σ
Si1–C2 bonds decrease for compounds
1–
3 as
3 < 2 < 1, and the occupancies of σ*
Si1–C2 bonds increase in the opposite order (
3 > 2 > 1). Moreover, these results can fairly explain the decrease of the energy barriers (ΔE
o) of the decomposition reaction of compounds
1 to
3. The calculated data demonstrate that in the decomposition process of the studied compounds, the polarization of the C
3–C
4 bond is the rate determining factor. Analysis of bond orders, NBO charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.
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