THE RING INVERSION OF SILACYCLOHEPTATRIENE AND CYCLOHEPTATRIENE - COMPARISON OF THE AROMATICITY OF PLANAR AND BOAT CONFORMERS ESTIMATED BY NUCLEUS-INDEPENDENT CHEMICAL-SHIFT

Nucleus-independent chemical shifts (NICS) of boat and planar structur e of silepins (1) and cycloheptatriene (2) were calculated at the HF/6 -31+G* level for the Becke3LYP/6-31G* optimized geometries in order to investigate the correlation between the inversion barrier and aromati c nature in the boat and planar forms of 1 and 2. The NICS of unsubsti tuted silepin la indicated that there is almost no stabilization in th e boat form (NICS, -0.5) and that the planar form (NICS, 3.5) is relat ively destabilized in comparison with the boat form. On the other hand , the MCS of 2 showed that there is considerable homoconjugative stabi lization in the boat form (NICS, -4.2) and destabilization in the plan ar form (NICS, 8.2). In addition to the geometrical effects resulting from angle strain, these electronic effects are considered to affect t he energy barriers for ring inversion, which have been calculated to b e 0.9 kcal mol(-1) for 1a and 5.2 kcal mol(-1) for 2. (C) 1998 John Wi ley & Sons, Ltd.