29 Si NMR Chemical Shifts and Reactivity Effects of the Series (CH 3 ) 3 – n Cl n SiX An ab initio Study

Abstract

The ²⁹Si NMR chemical shifts of the methyl chlorine silanes (CH₃)_{3 – n}Cl_nSiX (n = 0...3) with X = H, CH₃, C₂H₅, CH₂Cl, CHCl₂, NH₂, N(H)CH₃, N(CH₃)₂, OH, OCH₃, OC₂H₅, OSiH₃, F, SiH₃, and Cl have been calculated using the IGLO (Individual Gauge for Localized Orbitals) method. The theoretical results are in good agreement with available experimental values. The partitioning of the theoretical shielding constants into contributions from individual bonds, the lone pairs, and the inner shells is used to discuss their respective influences on the silicon chemical shifts. We investigate the correlation of the silicon chemical shifts of the mentioned series for different n. In some cases, this correlation shows a distinct separation between groups of substituents with and without lone pairs. This effect is well verified by the theoretical results. It can be understood as a consequence of the mutual influences of the four σ bonds surrounding the silicon atom and their contributions to the shielding of the silicon nucleus. The excited states are substantially involved in this effect. The silicon net charges do not contribute. Based on the phenomenological similarity with the behaviour of silicon chemical shifts, we suggest that some peculiarities of the reaction rates known from the synthesis of silicones might be attributed to similar electronic influences. In particular, it appears that the four σ bonds surrounding the silicon reaction center play a much more predominant role than p_π – d_π interactions and back donation effects between the oxygen lone pairs and the silicon reaction center. For describing the reactivities, the excitation energies have to be taken into account.