DFT Calculations of the Elimination Kinetics of Silacyclobutanes and its Methyl Derivatives in the Gas-Phase

Abstract

The gas-phase thermal decomposition kinetics of silacyclobutane (1), 1-methylsilacyclobutane (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 ( E o ) of the decomposition reaction of compounds 1 to 3. The calculated data demonstrate that in the decomposition process 619 Downloaded by [McMaster University] at 07:15 17 March 2015 620 A. SHIROUDI AND E. ZAHEDI 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.

New Orbital Symmetry-Allowed Route for Cycloreversion of Silacyclobutane and Its Methyl Derivatives

“…Note here that the terms “allowed” and “forbidden” refer to orbital symmetry-allowed and -forbidden processes. The geometry for TS-CR-C1-SCB agrees well with that reported by Gordon et al, and the one for TS-CR-C1-MSCB is in agreement with that obtained recently by Shiroudi et al using the density functional theory (DFT) methods (shown in parentheses in Figure ). The imaginary frequencies for transition states TS-CR-C1 correspond to synchronized vibration between the two olefins.…”

“…Optimized structures of the transition states involved in the concerted [2+2] cycloreversion of (a,d) SCB, (b,e) MSCB, and (c,f) DMSCB at the MP2/6-311++G­(d,p) level of theory. The values in parentheses for TS-CR-C1-SCB are obtained from ref at the CASSCF­(8,8)/6-31G­(d) level of theory, and those for TS-CR-C1-MSCB are from ref at the B3LYP/6-311G** level of theory. All numbers shown are for bond distances between the heavy atoms.…”

“…In addition to the extensively studied cyclobutane molecule, cycloreversion of silacyclobutanes (SCBs), a group of heterocyclobutane molecules containing a silicon atom, has also been studied. ,,− Also, the ring opening of the silacyclobutene ring either as a substituent or fused to an annulene ring has been theoretically studied . In their work on nonpolar versus polar cycloadditions, Bernardi et al reported that the concerted cycloaddition can be stabilized in polar π-bonds containing CSi and CO .…”