engleski

Ab-initio Study of NMR-chemical Shifts of beta-Silyl Substituted Carbocations

Silyl groups in beta-position to a cationic carbon atom stabilize the positive charge. Electron density id transferred from the Cbeta -Si-sigma bond to the positive center at Calpha (beta-silicon effect) [1, 2]. We present the results of ab-initio calculations (B3LYP/6-31G(d) and MP2/6-31G(d)) of beta-silyl substituted carbocations (1-5). The extend of the beta-silyl-effect in these carbocations is discussed using stabilization energies from isodesmic reactions, geometric parameters, NBO analysis and NMR-chemical shifts [3]. It is demonstrated that the NMR-chemical shifts depend on the electron demand of the cationic center, which is determined by the stabilisation arising from the substituents R1 and R2. References: [1] Siehl, H.-U. ; Müller, T. in The Chemistry of Organosilicon Compounds ; Rappoport, Z., Apeloig, Y., Eds. ; John Wiley and Sons: New York, 1998 ; Vol. 2 ; pp 595 - 701. [2] Siehl, H.-U. ; Müller, B. ; Malkina, O. in Organosilicon Chemistry ; Auner, N., Weis, J., Eds. ; Wiley-VCH ; Weinheim ; 1997 ; Vol III ; p 25. [3] a) Freudenberger, C. ; diploma thesis, Universität Ulm, 1999. b) Freudenberger, C. ; Siehl, H.-U. ; manuscript in preparation.

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