Homoaromatic Heterocations

The Si NMR resonances of the homocyclotrisilylenium ion 241 show a highly deshielded central silicon atom [8 Si(2) 315.7 vs. 8 Si(l) and Si(3) 77.3,8 Si(4) 15.7]. The molecular structure in the solid state shows that the four-membered ring is folded with a completely planar cationic part. These features and the observation that the most deshielded Si is the central tricoordinated Si(2) indicate homoaromatic character with charge delocalization at Si(2). 

Sekiguchi and coworkers have prepared and characterized the bishomocy-clopropenylium ion 242 and the trishomoaromatic cation cluster 243 Cation 242 shows the characteristic deshielded Si NMR resonances (S Si 

The value of drawing comparisons between hypercarbon systems and analogous isoelectronic boranes has been illustrated in earlier chapters in this book. 

NMR chemical shift values, helped researchers to deduce the likely structures of many carbocationic systems. 

In Equation (5.35), 8 B is the chemical shift of the B nucleus in parts per million relative to the BF3 OEt2 absorption, while 8 C is the chemical shift of the cationic carbon of the corresponding carbocation relative to tetramethylsUane. 

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