Theoretical Study and Nbo Analysis of the Kinetics and Mechanism of the Gas Phase Elimination Reactions of 2-Chloroethylsilane and Derivatives

Abstract

The gas phase elimination kinetics of 2-chloroethyltrichlorosilane (1), 2-chloro- ethylethyldichlorosilane (2), 2-chloroethyldiethylchlorosilane (3), and 2-chloro- ethyltriethylsilane (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 C₁-C1₃ bond in the sense C₁ ^δ+ C1^-δ ₃ 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 σc1-c13 bonds decrease for compounds 1-4as 4<3<2<1, while those of ^σ*C1-C13 bonds increase in the opposite order (4>3>2>1). This trend explains the comparatively easier thermal decomposition of the <σ c1-c13 bond in compound 4 compared to compounds 1-3.