Published:
April 27, 2011
r 2011 American Chemical Society
9023
dx.doi.org/10.1021/ja201786y
|
J. Am. Chem. Soc. 2011, 133, 9023–9035
ARTICLE
pubs.acs.org/JACS
Benzene under High Pressure: a Story of Molecular Crystals
Transforming to Saturated Networks, with a Possible
Intermediate Metallic Phase
Xiao-Dong Wen,
†
Roald Ho
ffmann,* and N. W. Ashcroft
Baker Laboratory, Department of Chemistry and Chemical Biology, and Laboratory of Atomic and Solid State Physics
and Cornell Center for Materials Research, Clark Hall, Cornell University, Ithaca, New York 14853, United States
b
S
Supporting Information
T
he stable phase of carbon under high pressure (up to 500
GPa) is diamond, a wide band gap insulator.
1
Hydrogen,
which has been metalized,
2
does not become metallic under static
conditions until P > 350 GPa.
3
Could an
“alloy” of C and H be
di
fferent and metalize at a lower pressure? This is the impetus
behind the work reported in what follows.
’ THE BENZENE PHASE DIAGRAM, AND PREVIOUS
STUDIES
Consider an equal carbon
Àhydrogen ratio, a 1:1 assembly.
Among CH systems, benzene (C
6
H
6
), the emblem of organic
chemistry, comes
first to mind as a realization. In fact, solid
benzene under pressure has been extensively investigated, from
P. W. Bridgman
’s classic pioneering study
4
to the present.
There are two main views of the phase diagram of solid
benzene. The
first phase diagram of benzene, shown in Figure 1a,
was constructed by Thi
ery and Leger from Raman and X-ray
studies
5
under high pressure. Liquid benzene crystallizes, at room
temperature and about 700 bar, in an orthorhombic phase I (Pbca).
An intermediate phase I
0
was suggested from discontinuities in
the cell constants of phase I. Phase II (P4
3
2
1
2) was said to exist
between 1.4 and 4 GPa, and phase III is stable between 4 and 11
GPa. The structure of phase III is monoclinic P2
1
/c, with two
molecules per unit cell. Both the I
ÀII and IIÀIII phase transi-
tions are extremely sluggish. Upon increase of pressure, two
more phases were suggested by Thi
ery and Leger: benzene III
0
,
stable between 11 and 24 GPa, and benzene IV, stable at even
higher pressure.
When benzene is compressed above 24 GPa at 25
°C, a solid
compound is recovered after the pressure is released. The structure
of that solid is not known. Benzene III
0
is supposed to be only a
modi
fication of benzene III, and benzene IV is thought to be
polymer-like. Phases I
0
, III
0
, and IV are not well-characterized,
and there is still some debate on whether they are established
phases or not.
Another phase diagram was developed by Ciabini et al.
6,7
in
2005 from infrared spectroscopy and X-ray analysis under high
pressure, shown in Figure 1b. Phase I is orthorhombic Pbca.
Phase II, crystallizing in space group P2
1
/c, is the same as phase
III assigned by Thi
ery and Leger. The P2
1
/c phase is stable up to
Received:
March 8, 2011
ABSTRACT:
In a theoretical study, benzene is compressed up
to 300 GPa. The transformations found between molecular
phases generally match the experimental
findings in the mod-
erate pressure regime (<20 GPa): phase I (Pbca) is found to be
stable up to 4 GPa, while phase II (P4
3
2
1
2) is preferred in a
narrow pressure range of 4
À7 GPa. Phase III (P2
1
/c) is at
lowest enthalpy at higher pressures. Above 50 GPa, phase V
(P2
1
at 0 GPa; P2
1
/c at high pressure) comes into play, slightly
more stable than phase III in the range of 50
À80 GP, but unstable
to rearrangement to a saturated, four-coordinate (at C), one-dimensional polymer. Actually, throughout the entire pressure range,
crystals of graphane possess lower enthalpy than molecular benzene structures; a simple thermochemical argument is given for why this
is so. In several of the benzene phases there nevertheless are substantial barriers to rearranging the molecules to a saturated polymer,
especially at low temperatures. Even at room temperature these barriers should allow one to study the e
ffect of pressure on the
metastable molecular phases. Molecular phase III (P2
1
/c) is one such; it remains metastable to higher pressures up to
∼200 GPa, at
which point it too rearranges spontaneously to a saturated, tetracoordinate CH polymer. At 300 K the isomerization transition occurs at
a lower pressure. Nevertheless, there may be a narrow region of pressure, between P = 180 and 200 GPa, where one could
find a
metallic, molecular benzene state. We explore several lower dimensional models for such a metallic benzene. We also probe the possible
first steps in a localized, nucleated benzene polymerization by studying the dimerization of benzene molecules. Several new (C
6
H
6
)
2
dimers are predicted.