Surprising Stability of Cubane under Extreme Pressure

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Surprising Stability of Cubane Under Extreme Pressure Haw-Tyng Huang, Li Zhu, Matthew D. Ward, Brian L Chaloux, Rostislav Hrubiak, Albert Epshteyn, John V. Badding, and Timothy A. Strobel J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00395 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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The Journal of Physical Chemistry Letters

Surprising Stability of Cubane under Extreme Pressure Haw-Tyng Huang†,*, Li Zhu┴, Matthew D. Ward┴, Brian L. Chalouxα, Rostislav Hrubiakβ, Albert Epshteynα, John V. Badding†,‡,ǁ,§ and Timothy A. Strobel┴,* †

Department of Materials Science and Engineering, ‡Department of Chemistry, §Materials Research Institute, and Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, United States ┴ Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road Northwest, Washington, D.C. 20015, United States α Chemistry Division, US Naval Research Laboratory, Washington, D.C. 20375, United States β High Pressure Collaborative Access Team (HPCAT), Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, United States High-Pressure, Cage-like Carbon, Equation of State ǁ

ABSTRACT: The chemical stability of solid cubane under high-pressure was examined with in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) up to 60 GPa. The Raman modes associated with solid cubane were assigned by comparing experimental data with calculations based on density functional perturbation theory, and lowfrequency lattice modes are reported for the first time. The equation of state of solid cubane derived from the PXRD measurements taken during compression gives a bulk modulus of 14.5(2) GPa. PXRD and Raman data indicate that solid cubane is highly compressible and exhibits anomalously large chemically stability under extreme pressure, in contrast with previous work and chemical intuition, due to its immensely strained 90º C-C-C bond angles.

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Cage-like carbons such as diamondoids have attracted considerable interest from both the experimental and theoretical communities as they are anticipated to share the advanced properties of their bulk diamond counterpart.1 As diamondoids are hydrogen-terminated diamond lattice fragments, they are now being considered as promising molecular building blocks for novel materials with superlative properties.2,3 For instance, zero-dimensional (0-D) adamantane, the smallest diamondoid molecule, has been demonstrated to possess excellent structural stability and is able to sustain large volume deformation without degradation after tens of pressure loading cycles. 4 Platonic hydrocarbons are another relevant group of cage-like carbons, which are the analogs of platonic solids at the molecular level. Only two members in the group have been synthesized: cubane (C8H8), a cube-shaped molecule,5 and dodecahedrane (C20H20), with the shape of pentagonal dodecahedron.6 Amongst the platonic hydrocarbons, cubane is unique, exhibiting interesting chemical properties due to its extremely strained chemical structure. In contrast to diamondoids that only contain normal sp3 hybridized carbon atoms with unstrained 109.5o tetrahedral bond angles, cubane has highly strained 90o C-C-C bond angles. Such large C-C-C bond bending is unfavorable energetically. Surprisingly, the molecule is found to be air- and light-stable, in spite of a strain energy as high as 657-695 kJ/mol.7,8,9 This metastability is understood in terms of symmetry-forbidden reactions that allow for robust kinetic persistence.10 Moreover, cubane has a high density (1.29 g/cm3) compared to other cage-like hydrocarbons, which implies that much strain energy is stored within a small volume. Therefore, cubane (and cubane derivatives) has been investigated for utility as a high-energy propellant or explosive ever since its synthesis by Eaton and Cole in 1964.5,11 At, or near, ambient pressure, the effect of temperature on cubane’s chemical reactivity,12 phase transformations,13 and decomposition/isomerization14,15,16 has been established. These studies are important for determining triggering dynamics and reaction mechanisms, which could assist in the design and synthesis of cubane-based energetic materials.12 Yildirim et al. reported that cubane undergoes a phase transition at 394 K from an orientationally ordered phase to an orientationally disordered phase before it melts at 405 K.13 Pyrolysis studies also show that cubane possesses anomalously high chemical stability at elevated temperature (no decomposition below 473 K).14,15,16 Although the study of energetic materials under pressure has received significant attention recently,17,18,19,20 as the pressures for reactions involving energetics often far exceed ambient conditions, the effect of pressure on the chemical stability of solid cubane has largely not been investigated. The only report is that of Piermarini et al.21, who examined the high-pressure reactivity of cubane and 1,4-dinitrocubane using vibrational spectroscopy, and found that cubane exploded spontaneously at 3 GPa and ambient temperature. In this work we reinvestigate the barochemistry of cubane. The previous report of explosive decomposition under pressure suggests that kinetically-controlled reaction pathways might be accessible under controlled compression and could provide valuable insights into reaction mechanisms, which has not been discussed in detail in the literature.17 In addition, the decomposition of cubane under pressure could produce interesting reaction products such as polymerized hydrocarbons that have been demonstrated to form extended carbon-rich networks.22,23,24 Moreover, the experimental bulk modulus of

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cubane remains unknown, with theoretical predicted values ranging from 7.2 to 14.8 GPa.25 The possibility for pressureinduced structural phase transitions also remains unexplored. We loaded molecular cubane in a diamond anvil cell (DAC) under an inert gas environment (Ar) and compressed it to 60 GPa. A detailed analysis of the vibrational spectra and structural evolution under pressure by in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) measurements indicates a remarkable chemical stability and structural integrity of solid cubane under extreme pressure. Cubane was previously reported to crystallize with one formula unit in the rhombohedral space group of R3.13 In the conventional hexagonal setting, the lattice parameters are a = 6.164 Å and c = 11.374 Å. Within this structure, there are three cubane molecules per unit cell (Z = 3) that are centered on the -3. sites (0,0,0) of the hexagonal lattice (Figure 1). Here, this indexing is confirmed from the observed powder XRD pattern of cubane at 1 GPa. Full-profile refinements were performed using the Le Bail analysis, which gives lattice parameters a = 6.1110(4) Å, c = 11.2714(16) Å at 1 GPa, in good agreement with the literature result accounting for the small degree of compression.13,26

Figure 1. (a) The crystal structure of solid cubane in the hexagonal setting. (b) Le Bail analysis of the XRD data of molecular cubane at 1 GPa. The inset image plate shows the 2D diffraction pattern with incomplete powder averaging statistics.

As cubane is compressed, the Bragg peaks of the XRD patterns shift toward higher 2θ (Figure 2). Other than a slight broadening of the diffraction peaks during compression, which can be attributed to a slight deviatoric stress arising from the lack of a pressure medium, the XRD patterns remain largely the same up to 60 GPa. This lack of change in the patterns indicates that no reaction or phase transition occurs during compression. This conclusion is further supported by in situ Raman spectroscopy presented in the following section. To better understand the structural evolution of cubane under pressure, lattice parameters and unit cell volumes at each pressure point were obtained using full-profile refinement in GSAS.27 As shown in Figure 3a, as pressure increases from 1

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The Journal of Physical Chemistry Letters function of pressure and the fitted equation of state. The inset shows the hexagonal unit cell and the definition of a and c.

GPa to 60 GPa, a and c decrease monotonically and the structure remains hexagonal. The lattice is slightly more compressible along the c-axis than the a-axis. This is attributed to the cubane molecules being more loosely packed along the c-axis.

On the basis of the experimentally-derived equation of state, we further examine the structural evolution of solid cubane by comparison with theory. Good agreement is found when comparing the vdW-DF predicted bulk modulus (B0=14.8 GPa) by Berland et al. to the presented fitted bulk modulus (B0=14.5 GPa).22 Given that Berland’s prediction is based on the assumption that the internal atomic coordinates remain frozen because of the rigidity of the C8 cage, it is suggested that the cubane molecules maintain the cubic geometry under high-pressure. Consequently, no significant changes in the atomic positions of carbon should result because of the cubic geometry constraint. Cubane possesses eighteen fundamental modes of vibration of which eight are Raman active. For such centrosymmetric molecules, the IR and Raman spectra are mutually exclusive. The cubic symmetry (Oh) restricts infrared activity thus only three modes among all of the fundamental modes are IR active. In the solid state, the rhombohedral distortion of the molecules by their nearest neighbors lowers the symmetry from the molecular group (Oh) to the factor group (C3i), which activates the remaining modes and splits some of the degenerate modes. The assignments for the observed Raman and IR modes at 1 atm are listed in Table 1 as shown below.

Figure 2. (a) XRD patterns of cubane at selected pressures. (b) dspacings of the main Bragg peaks of solid cubane as a function of pressure.

Figure 3b plots the pressure dependence of the volume per unit cell and is fitted with a third-order Birch-Murnaghan equation of state (EOS),28

3 B 2

Table 1. Experimental Raman and IR Modes of Solid Cubane at 1 atm

Mode

3 1 4 1 4

υ1

, where P is the pressure, V is the volume at P, and Vo is the volume at 1 atm. The zero-pressure bulk modulus, B0, was determined to be 14.5(2) GPa, while B0’, the first pressure derivative of B0, was determined to be 6.2(2). These values agree well with the theoretically predicted value of B0 (14.8 GPa) based on the van der Waals density functional (vdW-DF) approach.25 The EOS analysis shows that molecular cubane is highly compressible, but stiffens rapidly with increasing pressure.

Sym.a ag

Exp.(cm-1) 2998 d

Lit.b (cm-1)

Activityc (cm-1)

2995

R, f

2978

I, f

υ10

au

~2980

υ3

ag

2981

2978

R, s

υ13

eg

2972

2970

R, f

1230

IR, f

1182

R, f

υ11

au

~1228

υ14

eg

1181

d

υ7

eu

1151

1151

IR, s

υ9

ag

1131

1130

R, s

υ5

eg

1083

1083

R, f

υ2

ag

1004

1002

R, f

υ6

eg

913

912

R, f

υ12

eu

~853d

853

IR, f

υ18

ag

828

829

R, s

υ15

eg

816

821

R, f

υ16

ag

665

665

R, f

lattice 2

ag

90

-

R, s

lattice 1

eg

67

-

R, s

a

Symmetry is assigned by DFPT calculations, which were conc ducted at a pressure of 1 GPa. bReference 29. IR = infrared active, R= Raman active, f= fundamental modes, s= modes exhibited only in solid phase. dThe position of υ10, υ11, and υ12 fundamentals can’t be precisely determined due to saturation in the IR absorption experiments.

The assignments of the symmetry of the vibrational modes listed in Table 1 are based on DFPT calculations. We note that the observed Raman and IR modes are in good agreement with the report of Della et al.29 All the peaks in the

Figure 3. (a) Variation of the unit cell parameters a and c as a function of pressure. (b) Volume per unit cell of solid cubane as a



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kW/cm2) in the Raman measurement. In the course of our experiments, we found that a power density of ~100 kW/cm2 (10 mW with 5 µm spot) is required to give an observable signal. While information regarding the laser power density used by Della et al. is unavailable, we presume that a lower power density was used. Moreover, in the previous study, a Spex Ramalog instrument with a double monochromator was used to remove scattering near the laser emission line. The low throughput of this instrument can hinder the observation of weak lattice modes.29,31 We note that pressure may play a role in activating the lattice modes, as well, as the intensities of these modes seems to increase upon compression. The compression may enhance the coupling between neighboring molecules and therefore facilitate the rhombohedral distortion of cubane in the lattice.32 The amount of irradiation solid cubane can withstand under high pressure is impressive. No sign of irreversible reaction was observed in any samples over the course of measuring Raman spectra. We next consider the high-pressure vibrational behavior of solid cubane. Upon compression, continuous changes were observed in all the distinct vibrational regions (C-H stretching, C-C bending, C-C-H and C-C-C bending). Peak merging was minimal and peak splitting was not observed up to 50 GPa. In Figure 5a, all of the Raman peaks are relatively strong and sharp all the way up to ~50 GPa. We note that the Raman shifts of the C-H stretching modes (υ1, υ3, and υ13) as a function of pressure can be explained by a parabolic relationship which is attributed to the gradual increase of C-H bond force constant under compression. The C-C stretching modes (υ15, υ18, υ6, and υ2) exhibit a similar blue shift with compression. Above 20.8 GPa, the υ3 and υ13 modes start to overlap and lead to a seemingly “merged” peak in the range of 3100cm-1 to 3200 cm-1. However, this overlapping was observed to be completely reversible upon compression and subsequent decompression. Therefore, chemical reaction can be ruled out. Given that the shoulder on the low-frequency side of the peak still appears and can be easily deconvoluted up to 50 GPa, the shoulder peak is assigned to the υ13 mode. Most importantly, we find that the frequency of the two lattice modes of the rhombohedral R3 phase shows a systematic shift toward higher wavenumbers as the pressure increases, which indicates no structural phase transition. (Figure 5c) These results agree with the powder X-ray diffraction data, which show that the rhombohedral phase is stable up to 60 GPa. Yildirim et al. reported that the R3 phase of solid cubane is the most stable phase from 77 to 394 K.13 On the basis of spectroscopic and PXRD observations, we determined that the R3 phase is also the most stable phase up to 60 GPa. Combining the previous temperature-dependent study and the pressure-dependent study presented here, it is suggested that the R3 phase is likely to be the most densely packed crystal structure exhibited by cubane. Therefore, it would be sensible to observe no phase transformation either at low-temperature or under high-pressure. However, dense polymeric phases are expected to form under extreme pressure as pressure sharply increases intermolecular interactions, which may trigger polymerization when intermolecular distances approach a critical value.33 Compression of molecular crystals simultaneously strains intermolecular and intramolecular bonds, increasing their potential energy, and decreasing the transition state energy by forcing a smaller transition state volume. With a sufficiently high pressure, the activation barrier to the nearest tran-

Raman spectra were fit using mixed Gaussian-Lorentzian peak shapes. The highest-frequency Raman modes observed around the range of 2972 cm-1-2998 cm-1 are assigned to the C-H stretching modes (υ1, υ3, and υ13). The modes observed in the range of 1084 cm-1 – 1182 cm-1 are assigned to the C-C-H bending modes (υ14, υ9, and υ5). Furthermore, the modes observed from 816 cm-1 to 1004 cm-1 are assigned to the C-C stretching modes (υ15, υ18, υ6, and υ2) and the mode observed at 668 cm-1 is assigned to the C-C-C bending mode (υ16). Table 1 also presents a comparison between the experimentally observed modes at 1 atm with the previous study.29 In contrast to the literature, we found that there are two weak, rotational lattice modes located at 67 cm-1 and 90 cm-1, respectively, but no translational lattice modes. Figure 4 shows measured Raman spectrum at 0.9 GPa and the calculated spectrum by density functional perturbation theory (DFPT). The DFPT calculation is done at 1 GPa to minimize the effects of van der Waals interactions, which play an important role at ambient pressure. The calculation result matches with the experimental data, although small discrepancies may result from the PBE functional, which can cause errors in the estimated bond lengths in small organic molecules30. Given the good agreement between the simulation and experiments, the symmetry of the two observed lattice modes of solid cubane are therefore assigned to be eg and ag of the C3i factor group respectively as shown in Table 1. The observation of these two lattice modes indicates the presence of the R3 phase cubane at low pressure.

Figure 4. Experimental and theoretically calculated Raman spectra. The DFT calculation was conducted at 1 GPa and the calculated spectrum is shown with an arbitrary Gaussian peak width. The experimental Raman spectrum was collected at 0.9 GPa.

Della et al. investigated the relationship between crystal structure and vibrational modes for cubane.29 The molecule is found to be no longer truly cubic in the solid phase due to the rhombohedral distortion arising from the van der Waals interactions between neighbors. If the molecule were still cubic in the solid state, rotation would not change the polarizability and the rotational lattice modes would be Raman-inactive. It was therefore predicted that there should be no translational lattice modes and only two rotational lattice modes, although Della et al. observed no evidence of such lattice modes down to 30 cm1 in their study.29 However, in the present study, we clearly observed two lattice modes for solid cubane located at 67 cm-1 and 90 cm-1 under ambient conditions. We attribute observation of these modes to a higher laser power density used (~100


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sition state becomes small enough to be overcome by thermal energy.34 In cubane, C-C bond cleavage is expected as the C8 cage structure weakens the C-C bonds significantly due to ring strain. The C-C bonds in cubane have a mean bond length of 1.551 Å,26 similar to the C-C bond length in 4-membered cyclobutane (1.549 Å),35 but significantly larger than a C-C bond in typical alkanes having ideal tetrahedral bond angles such as adamantane (1.54 Å).36 Increased bond length is a result of poor atomic orbital overlap and weaker bonding. Compression might be expected to result in easier bond cleavage for cubane when compared with unstrained hydrocarbons, as already poor orbital overlap is exacerbated by further displacement of atomic positions. The previous experimental result indicating the spontaneous explosion of cubane at 3 GPa is consistent with this expectation. However, a pressure-induced reaction was not observed in the present study. Even at a pressure of 60 GPa, where the intermolecular C···C distance is estimated to be 2.89 Å, cubane remains kinetically stable. This indicates that even a ~20% decrease in the C···C van der Waals contact (3.608 Å) is insufficient to trigger unimolecular decomposition or intermolecular reaction between two cubane molecules, despite its immensely strained chemical structure.

librational motions of molecules are much less effective in modulating the intermolecular distance. In addition to the distance between nearest-neighbor reactive carbon atoms, the lack of double bonds in cubane, in contrast to benzene, is considered to be a more important factor when discussing the kinetic stability at high pressure. From a thermodynamic standpoint, this phenomenon can be explained by the volume of activation ∆‡V (volume change from reactant to transition state). The expression for the Gibbs free energy change from reactant to transition state can be written as ∆‡G= ∆‡U+P∆‡V-T∆‡S, where G is Gibbs free energy, U is internal energy, P is pressure, V is volume, T is temperature, and S is entropy. Le Noble et al. assigned ∆‡V= +10 cm3·mol-1 to a general bond cleavage event and ∆‡V= -10 cm3·mol-1 to a general bond formation event. The Diels-Alder reaction has a negative activation volume in the range of -25 to -40 cm3·mol-1. Therefore, it’s natural for benzene to undergo a [4+2] Diels-Alder reaction under high-pressure as the negative activation volume would result in a value of ∆‡G < 0.37,38 In the case of cubane, however, any polymerization reaction would require a bond cleavage and a bond formation event because of its saturated sp3 hybridization. Therefore, the net activation volume is approximately zero. Isomerization is also not favored under high pressure. Despite its thermodynamic instability, cubane’s kinetic stability is explained by an unusually high-energy transition state required for decomposition. For instance, cyclooctatetraene (COT) is one of the major C8H8 isomers in the cubane pyrolysis product.14 The formation of COT is symmetry-forbidden and therefore the isomerization reaction must overcome the activation energy, resulting in a high onset temperature of cubane decomposition (~473K).16 Pressure may lower the kinetic barrier only if the process has a negative activation volume. In the present case, the activation volume of isomerization is positive. Although the immensely strained molecule started out in a high-energy reactant state, pressure tends to destabilize the transition state and leads to a higher kinetic barrier. This is simply due to the fact that the reactant occupies smaller volume than the transition state does. Therefore, isomerization reactions are not facilitated but rather hindered under high-pressure. 34 Consequently, cubane is “trapped” in its highly strained chemical structure since neither polymerization nor isomerization reactions are favored under high pressure. All the work performed on the system is converted into elastic potential energy. This concept can also apply to other sp3 hybridized cage-like carbons such as the diamondoids, which have been demonstrated to exhibit high chemical stability under high pressure.4 Note that cubane should not be considered as unbreakable under pressure. In principle, a diradical polymerization reaction can still occur since the C-C bonds in cubane are much weaker than in diamondoid molecules. However, the present results suggest that pressures higher than 60 GPa are required in order to strain the molecules enough for bond cleavage. The discrepancies between the present study and the previous report of a “spontaneous explosion” of cubane at 3 GPa is attributed to the lack of oxygen as a reagent since the present samples were kept in an inert gas environment throughout the sample loading and entire compression process. While information regarding the previous sample loadings is unavailable, we presume that samples were loaded in air.21 In summary, we investigated the stability and phase behavior of solid cubane, an energetic material, with Raman spectroscopy and synchrotron X-ray diffraction in a DAC in

Figure 5. (a) Selected Raman spectra of cubane with pressure showing that (b) the fundamental vibrational Raman modes and (c) the low-wavenumber rotatory lattice modes systematically shift with increasing pressure.

The surprising chemical stability of cubane under high pressure can be explained by examining nearest-neighbor distances between atoms with potential for reaction. In crystalline benzene, the nearest-neighbor reactive carbon atoms in neighboring molecules are separated by approximately 2.6 Å once the translatory lattice phonon vibrations are taken into account.33 In solid cubane, however, there are no translatory lattice phonons, but only rotatory lattice phonons, which do not provide a thermal translational contribution to the atomic positions. The maximum instantaneous linear displacement from the equilibrium position of carbon atoms is small because the



order to understand the effect of pressure on the chemical stability, potential phase transformations and decomposition or isomerization. The structure and vibrational properties of cubane measured at 1 atm are in excellent agreement with previous studies. Contrary to the previous report, we observed excellent chemical and structural stability of solid cubane under compression up to 60 GPa, defying chemical intuition associated with cubane’s highly-strained 90 degree C-C-C bond angles. Furthermore, we observed that solid cubane is highly compressible with zero-pressure bulk modulus of 14.5(2) GPa.

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pressure of ~1 GPa or less within the inert argon atmosphere of a high-purity glovebox. X-ray Diffraction. X-ray diffraction patterns were collected at the High Pressure Collaborative Access Team (HPCAT), beamline 16-IDB and 16-BMD, of the Advanced Photon Source (APS), Argonne National Laboratory. A monochromatic beam of λ= 0.4066 Å was focused on the sample and diffraction patterns were recorded on a MAR image plate. 2-D Diffraction patterns were processed and converted into 1D patterns with intensity versus 2θ or intensity versus momentum transfer using the data analysis program DIOPTAS.42 The whole pattern refinement was completed in the program GSAS with EXPGUI.27 Le Bail analysis was implemented in order to obtain the lattice parameters.43 Rietveld refinement was attempted, but determined unacceptable due to incomplete powder averaging statistics and the slight deviatoric stress in the molecular crystal. Raman Spectroscopy. Cubane samples were excited by a 532 nm diode laser. The laser was focused through a 20× long working distance objective (NA=0.40) onto the samples in a symmetrical diamond anvil cell. The backscattered Raman signals were detected using a Princeton Instrument spectrograph SP2750. A 50 µm confocal pinhole and two narrowband notch filters (Ondax) were used to detect the lowwavenumber Raman modes down to 20 cm-1 and a 1800 groove/mm grating was used to disperse the Raman light onto a liquid nitrogen cooled CCD, which enabled a spectral resolution of 97%) in a manner analogous to previously described methodologies.39,40 1,4-Cubanedicarboxylic acid (1.0167 g, 5.291 mmol) was suspended in a round bottom flask with 25 mL CHCl3. A catalytic amount of DMF (~5 drops) and 1.09 mL (12.5 mmol) oxalyl chloride were added to the flask and the mixture heated and stirred in an oil bath to 65 °C for 2 hours, after which all solids had dissolved and bubbling had ceased. Separately, sodium pyrithione (2.0618 g, 13.824 mmol) and catalytic DMAP (0.1338 g, 1.089 mmol) were added to 25 mL THF. The solution of cubane-1,4-bis(acid chloride) in CHCl3 was transferred to the sodium pyrithione suspension and the mixture irradiated with UV light (~350 nm, 35 W) for 16 hours under a N2 blanket. Volatiles were removed from the resulting dark red suspension at ambient temperature under vacuum, the remaining solids taken up in dilute HCl, and organics extracted with n-pentane in a separatory funnel. The pale yellow pentane fraction was dried over MgSO4, drying agent removed by suction filtration, and volatiles distilled into a liquid nitrogen-cooled trap under static vacuum (owing to the volatility of cubane: vapor pressure at 25°C ≈ 1.3 mm Hg)41. The resulting crude, yellow oil assayed for 4.6 : 1 trichloromethyl-2-pyridylthioether (C5H4N-S-CCl3: δ 8.73 (m, 1H), 7.83 (m, 2H), 7.41 (m, 1H)) to cubane (C8H8: δ 4.02 (s, 8H)) by 1H NMR (CDCl3, 300 MHz). Methanol was added to the oil and the solution chilled to -78 °C in a dry ice/acetone bath. The precipitated fluffy, white solids were filtered off and weighed at 31.6 mg (6% yield). Prior to use, cubane was sublimed under static vacuum at 50 °C onto a cold finger to consolidate the material. Sample Loading. Cubane samples, along with ruby and gold particles, were loaded into DACs with culet sizes ranging between 300 and 400 µm. The DAC was prepared by indenting a rhenium gasket to a thickness between 40 to 50 µm. The sample chamber was prepared by drilling a ~100-150 µm hole into the center of indentation. Pressure was mainly calibrated by measuring the florescence of ruby. The equation of state of Au served as a secondary pressure calibration. A pressure transmitting medium was not used because molecular crystals have low shear strength. All samples were sealed to a starting

AUTHOR INFORMATION Corresponding Authors *H.-T. Huang. E-mail: [email protected] *T. A. Strobel. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank V. Prakapenka and E. Greenberg for help with the in situ XRD measurements. This work was supported by DARPA under ARO Contract No.. Portions of this work were performed at HPCAT (Sector 16) and GSECARS (The University of Chicago, sector 13), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974, with partial instrumentation funding by NSF. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466).


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The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Figure 1. (a) The crystal structure of solid cubane in the hexagonal setting. (b) Le Bail analysis of the XRD data of molecular cubane at 1 GPa. The inset image plate shows the 2D diffraction pattern with incomplete powder averaging statistics. 268x281mm (300 x 300 DPI)


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Figure 2. (a) XRD patterns of cubane at selected pressures. (b) d-spacings of the main Bragg peaks of solid cubane as a function of pressure. 191x131mm (300 x 300 DPI)



Figure 3. (a) Variation of the unit cell parameters a and c as a function of pressure. (b) Volume per unit cell of solid cubane as a function of pressure and the fitted equation of state. The inset shows the hexagonal unit cell and the definition of a and c. 170x124mm (300 x 300 DPI)


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Figure 4. Experimental and theoretically calculated Raman spectra. The DFT calculation was conducted at 1 GPa and the calculated spectrum is shown with an arbitrary Gaussian peak width. The experimental Raman spectrum was collected at 0.9 GPa. 156x115mm (300 x 300 DPI)



Figure 5. (a) Selected Raman spectra of cubane with pressure showing that (b) the fundamental vibrational Raman modes and (c) the low-wavenumber rotatory lattice modes systematically shift with increasing pressure. 203x219mm (300 x 300 DPI)


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TOC graphic 50x50mm (600 x 600 DPI)


Surprising Stability of Cubane under Extreme Pressure

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