halogen - 1-10-2006 at 14:11
How many allotropes of carbon are there, and are there any yet to be discovered? So far:
Diamond
Hexagonal Diamond
Graphite
Nanotubes
Fullerenes
Chaoite (what is that?
)
JohnWW - 1-10-2006 at 18:21
How is hexagonal diamond produced, and has it actually been isolated and tested for optical properties, crystal structure, and hardness etc.? Is it an
high-pressure form of it? If it is hexagonal close packed, it would be the densest possible allotrope, significantly denser than ordinary diamond
which is simple-cubic (as are Si and Ge).
Also, there is more than one fullerene; besides C60, there is an egg-shaped molecule C70, and much larger all-carbon molecules, found in soot, which
approximate in shape to icosahedra with the pentagons at the 12 vertices of an icosahedron. Macro-molecules formed by bonding between fullerene
spheres, with loss of aromatic character at the bonding carbons, are a possibility.
Theoretical possibilities could be cubatetraene, C8, from cubane, C8H8, with the Hs removed, and having 4 double bonds and 8 single bonds which are
interchangeable. However, it would be a very "strained" molecule, and like cyclobutadiene, C4H4, and cyclooctatetraene, it is unlikely to be
stabilized by aromaticity; and dodecahedradecene, C20, with 10 double bonds and 15 single bonds, similarly formed from dodecahedrane, C20H20, and less
strained. Again, in spite of being a conjugated system of interchangeable double and single bonds, it would not have the correct number of carbons, or
at least pi orbitals (2, 6, 10, 14, 18, etc., in neutral molecules) to be aromatic.
Another possibility is an infinite sheet consisting of squares (as in cyclobutane) formed by C atoms, each singly bonded to 4 others. This would be
somewhat strained, but is clearly theoretically possible. It would be rather like graphite physically, but would be transparent and a non-conductor of
electricity. Also an infinite sheet consisting of triangles formed by C atoms, with 1 in 3 bonds being double, like cyclopropene, but this would be
highly strained.
Yet another possibility is poly-yne or poly-cumulene, consisting of an infinite chain of C atoms linked by either alternating single and triple bonds,
or by all double bonds, which arrangements weould be interchangeable. However, it would be very highly strained. In the poly-yne form it would be
conducting and probably black incolor; while the poly-cumulene form would be somewhat like polyethylene.
"Chaotite" is presumably amorphous carbon, with a structure having no repeating cell unit. This would show up in X-ray diffraction as a broad band,
with no sharp peaks corresponding to crystal planes.
halogen - 8-10-2006 at 07:46
Icosidodecahedron carbon. Highly unstable. 20 triangles. Each carbon atom to four neighbors. C30
theres an idea... Also, if one carbon were replaced with a nitrogen, it would form quite the ion.
Chaoite's existence is disputed (some say its polycumulene etc, others that its not really an allotrope) You think of amorphous carbon which is
different. (just found out)
[Edited on 8-10-2006 by halogen]
Ozone - 23-11-2006 at 13:17
Also, pyrolytic graphite! An artificial allotrope prepared in a vacuum furnace, pyrolytic graphite differs from normal graphite by virtue of covalent
bonds between graphene sheets.
This is actually quite cool stuff!
http://en.wikipedia.org/wiki/Pyrolytic_carbon
Happy Thanksgiving,
O3
neutrino - 25-11-2006 at 15:26
Don't forget glassy carbon. We have a thread on it here.
Nerro - 25-11-2006 at 16:07
As I read in a pdf file in the wikipedia entry for glassy carbon it may be considered rather as disorderly graphite interspersed with nanotubes and
fullerenes of all sorts of shapes and sizes. Cool but not really an allotrope as such, rather a mixture of common allotropes that presents us with
properties that exceed those of the added parts alone (much like a wall has quite different properties than loose gravel and quick lime etc).
Nixie - 13-3-2008 at 06:56
http://www.technologyreview.com/Nanotech/19097/
http://www.technologyreview.com/read_article.aspx?ch=special...