SI-29 NMR CHEMICAL-SHIFTS AND REACTIVITY EFFECTS OF THE SERIES (CH3)(3-N)CLNSIX - AN AB-INITIO STUDY

The Si-29 NMR chemical shifts of the methyl chlorine silanes (CH3)(3-n )ClnSiX (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 calculat ed using the IGLO (Individual Gauge for Localized Orbitals) method. Th e theoretical results are in good agreement with available experimenta l values. The partitioning of the theoretical shielding constants into contributions from individual bonds, the lone pairs, and the inner sh ells is used to discuss their respective influences on the silicon che mical shifts. We investigate the correlation of the silicon chemical s hifts of the mentioned series for different n. In some cases, this cor relation shows a distinct separation between groups of substituents wi th and without lone pairs. This effect is well verified by the theoret ical results. It can be understood as a consequence of the mutual infl uences of the four a bonds surrounding the silicon atom and their cont ributions 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 beha viour of silicon chemical shifts, we suggest that some peculiarities o f the reaction rates known from the synthesis of silicones might be at tributed to similar electronic influences. In particular, it appears t hat the four sigma bonds surrounding the silicon reaction center play a much more predominant role than p(pi)-d(pi) interactions and back do nation effects between the oxygen lone pairs and the silicon reaction center. For describing the reactivities, the excitation energies have to be taken into account.