[Enthalpy]

Hello dear friends!

Bathyscaphes able to reach the deepest Ocean points are very scarce (two presently) because a float that resists 114MPa and still buoys is difficult. Syntactic foam (microballoons in epoxy) and thin alumina spheres are used; I've already suggested a graphite fibre tank aided by gas pressure, and even lithium with a liner (I knew you'd like that one  )
http://www.scienceforums.net/topic/85991-bathyscaphe-float/
and now I dream up compounds lighter than water, strong, solid or liquid.

What I've seen on the web are diamondoids that organize by weak bonds, typically hydrogen bonds, example
http://www.electronicsandbooks.com/eab1/manual/Magazine/J/Journal%20of%20the%20American%20Chemical%20Society%20US/Journal%20of%20the%20American%20Chemical%20Society%201879-1995/113/12/ja00012a057.pdf
more in "Supramolecular Chemistry" by Jonathan W. Steed, Jerry L. Atwood,
but these solids must be weak, supposedly worse than polyamide. Silicagel also must be crushed by 1000bar (is it?) and aerogel for sure.

A bathyscaphe (and certainly more uses!) demands the good strength, it also favours smaller cavities: easier to keep clean, and more chemical bonds would improve the strength. It doesn't need the extreme density of aerogels: 300kg/m³ buoy nearly as much as 1kg/m³. And my guess is that the compound needs covalent (or ionic) bonds, not van der Waals.

Where I am up to now (pictures attached):

• Tetravalent carbon, silicon... as the vertices of an FCC arrangement have proven strong in diamond and silicon.
• I'd like to insert linear edges covalently between the vertices, to keep a good strength - if possible.
  
• Phenyl edges between carbon vertices are too short (899kg/m³), but diphenyl (246kg/m³), phenanthrene or pyrene (321kg/m³) suffice already.
  
• Other compounds are linear: barrelene, bicyclo(1,1,1)pentane... but look less useable. Maybe bicyclopentane's size helps the connection to the vertices.
• Tetraphenylsilane isn't pyrophoric nor very flammable, so silicon vertices should also be fine.
  

Quite obscure to me is how to synthesize such compounds. Attaching a fourth phenyl to a carbon doesn't look trivial, billion times in a small crystal less so. Some naive ideas:

• Make tetrahalo-tetraphenylmethane (or silane) in a first step, purify then. Couple to diphenyl edges by some metal.
• Silicon vertices, being bigger and more reactive than carbon, might help.
• Would dihalopyrene, or a straight dihalothing, react with silane? The leaving HX could diffuse through the compound. Or would it make SiX₄ instead?
  

Maybe you can shed your lights? Thanks!
Marc Schaefer, aka Enthalpy