[jennyjo]

Please help us!! I have a question about monosubstituted cyclohexane.  We calculated the % equatorial VS % axial for trimethylsilylcyclohexane and tert-butylcyclohexane.  We found that  trimethylsilylcyclohexane  had a 97% equatorial (which we would have expected to be 100%) and tert-butylcyclohexane was 100% equatorial(which we expected to have a % equatorial less then that of trimethylsilylcyclohexane). We know that Silicon is 150% larger then Carbon , so why isn't Silicon 100% equatorial and Carbon is?  If you can help it would be much appreciated! Thanks,
jenny

PS we made the calulations with Spartan

[movies]

Think about the effect of introducing a silicon in that position: the bond from the carbon in the ring to the silicon will be a lot longer than the bond to the quaternary carbon in the t-butyl case!  Think about what the high energy interactions in the axial case are and how they would be affected by the longer C-Si bond.

For reference, C-C bonds are ~154 pm long, which C-Si bonds are ~185 pm.