Tori and Dumbbells

Bill wrote:
>
> > Making nanotubes requires atoms that can take three neighbors to
> form
> > graphitic-like sheets. C, B, and N can happily bond with only three
> > neighbors, but Si cannot. Si only forms covalent bonds in a
> tetrahedral
> > geometry. Therefor, Si nanotubes cannot be one atom thick as C, N,
> and B
> > nanotubes can.
>
> But SiO2, can for three covalent bonds, so SiO2 nanotubes should be
> possible. Also SiCl3H2 has the same property. The sp2 orbital is the
> dominant structure in SiN so that should be able to form nanotubes.
>

This is correct. I'm not sure if any have been made or not.

> >
> Now is it absolutely necessary to have an sp2 bond structure for
> nanotubes?

For single atom thickness I think it is.

>
> How small a radius does the helix need before it is called a nanotube?
> This is not a acedemic question since most protiens form helices one
> molecule thick, but the helices is hundreds of molecules in diameter.
>

Artificial protein tubes have been made one amino acid thick at Scripps
(I can't remember the PIs name -- Ghaddiri I think). Proteins are very
capable molecules but they are too weak to use in most aerospace
applications (although perhaps not all).