Chemistry Reference
In-Depth Information
Fig. 1.2
A portion of the diamond lattice seen sideways (
left
) and up-front (
right
)
Fig. 1.3
Hydrogen-depleted
formulas of adamantane (on
the
left
) and iceane (on the
right
)
Recently, two exoplanets supposed to have huge amounts of diamond layers have
been discovered when astronomers observed them transiting across their host star.
The first one gravitates around a millisecond pulsar at a distance of 4,000 light years
from us, and has a radius about ten times larger than that of earth, whereas the second
one is at a distance of only 40 light years, has a radius twice larger than that of earth,
and circles around 55 Cancri. Mass and radius estimates of this “55 Cancri e” planet
suggested that it has a molten iron core surrounded by successive layers of silicon
carbide, silicates, graphite and diamond (Madhusudhan et al.
2012
).
The diamond lattice may be expanded theoretically by replacing each C
−
C bond
by the sequence C
C, affording thereby “polyyne-diamond” (Melnichenko
1982
; Kijima
2005
); this replacement may be carried out in one, two, or three
dimensions, and combines sp
3
with sp hybridization. It was emphasized by Kroto that
a one-dimensional carbon allotrope with sp hybridization (carbyne) has no chance
of existence as a stable substance (Kroto
2010
).
Other theoretical possibilities of carbon nets, combining this time sp
3
and sp
2
hybridizations, were calculated more than 40 years ago (Balaban et al.
1968
) and
continued in later years; the topic has been reviewed by the present author in topic
chapters (Balaban
1989
,
1998
,
2013a
). He collaborated with Roald Hoffmann and
K. M. Merz in order to predict the stability of infinite nets having ordered arrange-
ments of sp
3
and sp
2
hybridized carbon atoms (Merz et al.
1987
). Interconversions
between diamond and graphite must start with local hybridization changes, which
were investigated theoretically (Balaban and Klein
1997
). The near coincidence
between certain interatomic distances in the diamond and graphite nets allowed
some calculations of diamond-graphite hybrids (Balaban et al.
1994
).
−
C
≡
C
−
