Bicyclobutonium-type structures

The H NMR spectrum shows resonances for (5.95 ppm), Hg/H g/ exo (4.01 ppm), H /Hg/ endo (3.12 ppm) and Hy (0.30 ppm). The NMR data are in accord with a bridged, puckered bicyclobutonium ion structure that is static on the NMR time scale. The 29Si NMR chemical shift of 43.1 ppm for ion 428 indicates that the silicon is involved in stabilization of the positive charge. The stabilization occurs by shifting electron density from the Cy—Si cr-bond across the bridging bond to the formal carbenium carbon Ca. This y-silyl- type of interaction may be termed silicon homohyperconjugation. 

The experimentally measured coupling constant J(HaHy)exp. = 5.5 Hz is reproduced by calculations for the endo-si y substituted model structure 44 using a finite perturbation level (FPT) 26) (Perdew/IGLO-III) approach J(HaHy)caic. = 5.9 Hz, whereas only 1.2 Hz is calculated for the HaHy-coupling constant in the exo-si y substituted model structure 45. Finally the bonding orbital of the bridging C-C-bond between Cq and Cy in these type of bicyclobutonium ions can be visualized by calculations of the natural bond orbitals (NBO s) (Figure 13). 

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