Theoretical study and NBO analysis of the kinetics and mechanism of the gas phase elimination reactions of 2-chloroethylsilane and derivatives

The gas phase elimination kinetics of 2-chloroethyltrichlorosilane (1), 2-chloroethylethyldichlorosilane (2), 2-chloroethyldiethylchlorosilane (3), and 2-chloroethyltriethylsilane (4), have been studied at the ab initio B3LYP/6-311 + G**, B3PW91/6-311 + G** and MPW1PW91/6-311 + G** levels of theory. The B3LYP/6-311 + G** method is in good agreement with the experimental data for the activation parameters. The calculated data demonstrate that, in the elimination mechanism of 2-chloroethylsilane and derivatives, the polarization of the C-1-Cl-3 bond in the sense C-1(delta+)-Cl-3(delta-) is a determining factor. Based on the optimized ground state geometries using the B3LYP/6-311 + G** method, the natural bond orbital analysis (NBO) of donor-acceptor (bonding-antibonding) interactions revealed that in accordance with the increase of activation energy, the HOMO-LUMO energy-gaps in the ground state structures increase. Moreover, the order of energy barriers obtained could be explained by the number of electron-releasing chlorine atoms substituted at the silicon atom. NBO analysis shows that the occupancies of sigma(C1-Cl3) bonds decrease for compounds 1-4 as 4 < 3 < 2 < 1, while those of sigma*(C1-Cl3) bonds increase in the opposite order (4 > 3 > 2 > 1). This trend explains the comparatively easier thermal decomposition of the sigma(C1-Cl3) bond in compound 4 compared to compounds 1-3.