Theoretical study on the mechanisms and kinetics of the β-elimination of 2,2-dihaloethyltrihalosilanes (X = F, Cl, Br) compounds: a DFT study along with a natural bond orbital analysis

Abstract

The β-elimination kinetics of 2,2-dihaloethyltrihalosilanes in the gas phase has been studied computationally using density functional theory (DFT) along with the M06-2x exchange–correlation functional and the aug-cc-pVTZ basis set. The calculated energy profiles have been supplemented with calculations of rate constants under atmospheric pressure and in the fall-off regime, by means of transition state theory (TST), variational transition state theory (VTST), and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the M06-2x/aug-cc-pVTZ approaches are in good agreement with the available experimental data. Analysis of bond order, natural bond orbitals, and synchronicity parameters suggests that the β-elimination of the studied compounds can be described as concerted and slightly asynchronous. The transition states of these reactions correspond to four-membered cyclic structures. Based on the optimized ground state geometries, a natural bond orbital (NBO) analysis of donor–acceptor interactions also show that the resonance energies related to the electronic delocalization from $\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}$ bonding orbitals to $\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$ antibonding orbitals, increase from 2,2-difluoroethyltrifluorosilane to 2,2-dichloroethyltrichlorosilane and then to 2,2-dibromoethyltriboromosilane. The decrease of $\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}$ bonding orbitals occupancies and increase of the $\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$ antibonding orbitals occupancies through $\sigma_{{{\text{C}}_{ 1} - {\text{C}}_{ 2} }} \to \sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$ delocalizations could facilitate the β-elimination of the 2,2-difluoroethyltrifluorosilane compound, compared to 2,2-dichloroethyltrichlorosilane and 2,2-dibromoethyltriboromosilane.