11635
2007, 111, 11635-11637 Published on Web 09/20/2007
X-ray Raman Spectroscopic Study of Benzene at High Pressure Michael Pravica,*,† Ognjen Grubor-Urosevic,† Michael Hu,‡ Paul Chow,‡ Brian Yulga,† and Peter Liermann‡ Department of Physics and Astronomy, UniVersity of NeVada Las Vegas and High Pressure Science and Engineering Center (HiPSEC), Las Vegas, NeVada 89154-4002, High Pressure CollaboratiVe Access Team (HP-CAT), Carnegie Institution of Washington, AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: June 4, 2007; In Final Form: September 4, 2007
We have used X-ray Raman spectroscopy (XRS) to study benzene up to ∼20 GPa in a diamond anvil cell at ambient temperature. The experiments were performed at the High-Pressure Collaborative Access Team’s 16 ID-D undulator beamline at the Advanced Photon Source. Scanned monochromatic X-rays near 10 keV were used to probe the carbon X-ray edge near 284 eV via inelastic scattering. The diamond cell axis was oriented perpendicular to the X-ray beam axis to prevent carbon signal contamination from the diamonds. Beryllium gaskets confined the sample because of their high transmission throughput in this geometry. Spectral alterations with pressure indicate bonding changes that occur with pressure because of phase changes (liquid: phase I, II, III, and III′) and possibly due to changes in the hybridization of the bonds. Changes in the XRS spectra were especially evident in the data taken when the sample was in phase III′, which may be related to a rate process observed in earlier shock wave studies.
Introduction. Benzene (C6H6) is an important aromatic hydrocarbon that is a fundamental constituent of petroleum, particularly as a building block for larger molecules, e.g., in asphaltine,1 which is a potpourri of aromatic and aliphatic molecules of varying sizes. Benzene also has a high degree of symmetry (C-C bond lengths are all 1.397 Å and the bond angles are all exactly 120°) that results from resonance behavior.2 These attributes result in benzene’s fascinating chemistry toward formation of more complex molecules (including dyes and pharmaceutical compounds3) and polymers. Recently3-4 and in the past,5-12 there has been interest in understanding the properties of benzene under high pressure and high-temperature, particularly in the spirit of inducing amorphization,4 dimerization,5 and other chemical transformations6 of benzene to develop high-pressure methods for creating novel compounds and polymers.3-4 Shock wave studies of benzene have also been performed13-14 and thus understanding the behavior of this compound under static high pressure would be useful to complement data of the material under dynamic loading conditions. The techniques used to interrogate benzene under pressure have been primarily X-ray diffraction,4-7 Raman6,9,12 and infrared (IR)4,8 spectroscopy, and optical absorption spectroscopy.11 It is in this spirit of better understanding the inter- and intramolecular changes to benzene at high pressure that we performed X-ray Raman spectroscopic measurements using a diamond anvil cell for pressure generation,15 the first such measurement for benzene or any hydrocarbon that we are aware of under extreme conditions. X-ray Raman spectroscopy * Corresponding author. E-mail:
[email protected]. † University of Nevada Las Vegas and High Pressure Science and Engineering Center. ‡ Argonne National Laboratory.
10.1021/jp074321+ CCC: $37.00
(XRS)1,16-17 is a valuable technique for probing core electron bonding where a hard X-ray (2-10 keV1) inelastically scatters from core electrons while exciting them into unoccupied states. At high pressures, X-ray Raman spectroscopy, although more or less equivalent to low-energy X-ray spectroscopies such as X-ray absorption spectroscopy (XAS)1 and extended X-ray absorption fine structure (EXAFS),1,16 is much preferable to these techniques as the probing soft X-rays utilized by these methods will not penetrate the confining diamonds and metallic gasket. These experimental methods also frequently require that the sample be placed in vacuum.1 The only XRS studies on benzene of which we are aware were performed under ambient conditions.1,17 XRS was found to be useful because of the better penetration of X-rays and thus no requirement for the samples to be placed in a vacuum system as well as better interrogating the pristine sample inside its possibly degraded/oxidized surface.1 The technique, although still in its infancy, has become more feasible because of the high photon fluxes available (order of 1012 photons/s) from third-generation synchrotrons. It has, for example, been found useful to qualitatively assess the aromatic fraction of carbons in asphaltenes.1 Though this technique is extremely challenging to use for samples compressed by diamond cells that are typically 10 µg, where the diamonds can contaminate the carbon signal from the sample, it was our aim to observe the behavior of benzene under extreme conditions in the hope of better understanding the bonding chemistry of this interesting molecule with pressure. Experimental. We performed two separate experiments on benzene with two different samples. In both cases, HPLC-grade benzene (fw 78.11 g/mol, mp 5.5 °C, bp 80.1 °C) of purity >99.9%, purchased from Aldrich Chemical, was loaded in the liquid state into a beryllium gasket that had been drilled by © 2007 American Chemical Society
11636 J. Phys. Chem. B, Vol. 111, No. 40, 2007
Letters
Figure 1. . XRS spectra of benzene at ambient temperature as a function of pressure. The dashed line is included as a guide to the eye.
electric discharge machining. The gasket hole had preloading dimensions of ∼200 µm inner diameter and ∼90 µm preindented thickness in both experiments. A few small (∼10 µm in size) ruby crystals were placed inside the sample hole for pressure measurement,18 and then the diamonds were brought into contact with the gasket to seal the sample. No pressure medium was used in these studies, and all measurements were conducted at room temperature. The cylindrical diamond cell used for sample pressurization has been specially designed with a wide aperture solid angle perpendicular to the diamond-diamond axis for high collection efficiency of inelastically scattered X-rays (“side-in, side-out” geometry19). Our experiment was performed at the High-Pressure Collaborative Access Team’s (HP-CAT’s) 16 ID-D beamline at the Advanced Photon Source of Argonne National Laboratory. The diamond cell was installed into the beamline in such a way that the polarized X-ray beam axis traversed the sample through the beryllium gasket. A diamond monochromator filtered X-rays of energy 9.97 keV, which were then focused down to 35 µm in the vertical direction and 50 µm in the horizontal direction at the sample using a Kirkpatrick-Baez mirror. The inelastically scattered X-rays were collected by six curved Si (660) analyzers, which were vertically mounted on a Rowland circle and set at an angle of 30° relative to the incident X-ray beam axis. The analyzers refocused the inelastically scattered X-rays onto a Si detector (Amptek) for data collection. In a typical data set, the analyzer is fixed in energy and the incident beam energy is scanned with the monochromator. As is typical with these experiments, data collection took roughly 8-12 h/spectrum. The overall resolution of the analyzer was ∼0.7 eV. Great care was taken to avoid interference from the diamonds by repeatedly checking the sample thickness by scanning the diamond cell in the direction perpendicular to the beam (along the diamond-diamond axis) and measuring the transmitted X-ray beam on the other side of
the cell. For all of our measurements, the beam focus was always smaller than the sample thicknesses. Results. Our results are presented in Figure 1. In the first experiment, we took two scans, each at 14.65 and 19.2 GPa, respectively. The ambient pressure scan was taken using a glass capillary loaded with liquid benzene and looks qualitatively similar with the ambient pressure spectrum displayed in ref 17. In the second experiment, we took scans at 2.98 and 7.8 GPa. In both cases, the experiments terminated when the brittle beryllium gasket failed, cracking and suddenly releasing sample. We were unable to recover the sample in either case. Pressure changes were slowly varied to allow the system to come to equilibrium and to reduce the chances for failure of the brittle beryllium gasket. Pressure was measured before and after taking a spectrum to ascertain any significant pressure change due to possible chemical reactions (e.g., release of hydrogen). We saw very little change in pressure with time. All of our spectra were taken in compression (pressure ever increasing) for both experiments. Benzene undergoes four phase transitions in the 0-20 GPa pressure range: liquid f solid-phase I f phase II f phase III f phase III′.7 Thus, for the 19.2 and 14.65 GPa spectra, the sample was likely in phase III′. For the 7.8 GPa scan, the sample was in phase III, and the 2.98 GPa spectrum was probably in phase II based upon the phase diagram of ref 7. Because of time and DAC-related constraints, we were unable to check the phases at various pressures with other spectroscopic methods. The first peak in the liquid ambient pressure spectrum near 284 eV is the 1s f π* electronic transition (labeled I in Figure 1),1,17 which rapidly diminishes at higher pressures relative to the other spectral features so that it is not visible above 7.8 GPa. This may be due to rehybridization of π orbitals, resulting in concomitant polymerization with pressure. The 1s f σ* transitions are more easily seen as the series of peaks at high energies (II, III, IV, V, and beyond in Figure 1), which vary in
Letters relative intensity with pressure. In particular, there is a peak that grows in intensity around 305 eV. This is likely a 1s f σ* (C-C) transition,17 which is a likely indicator for an increasing fraction of C-C bonds as opposed to the reduction of CdC bonds (peak I vanishes) with pressure. We suggest that these spectral fine structure alterations with pressure (particularly in phase III′ at 19.2 GPa) are likely due to significant chemical changes in the molecule and possibly polymerization and/or dimerization. This supports experimental observations of Dick14 of a the beginning and end of a rate process in the range of 13.3 and 19.4 GPa backed up by a theoretical calculation, suggesting that this might be due to dimerization and other extended intermolecular bonding10 may be occurring in this pressure range. Although the shock experiments of Dick14 would have likely been at much higher temperature (estimated at 2300 K10) than our room temperature measurements, we argue that the large photon flux provided by the synchrotron would provide ample energy to aid in facilitating bond rearrangements. Conclusion. We have performed an X-ray Raman spectroscopic study of benzene, the first such study of any hydrocarbon at high pressure and ambient temperature. We have demonstrated that this technique can be useful to interrogate bonding changes in benzene at extreme conditions and have probably observed significant changes in the inter- and intramolecular bonding of solid benzene at elevated pressures. Further work will endeavor to explain these bonding changes and will extend the pressure range above 19.4 GPa to interrogate what happens beyond the rate process window. Acknowledgment. We thank Prof. Malcolm Nicol and Dr. David S. Moore for critical reviews of the manuscript. We gratefully acknowledge the support from the DOE Cooperative
J. Phys. Chem. B, Vol. 111, No. 40, 2007 11637 Agreement no. DE-FC08-01NV14049 with the University of Nevada, Las Vegas. Use of the Advanced Photon Source is supported by the DOE Office of Science, Office of Basic Energy Sciences, under contract no. W-31-109-Eng-38. HPCAT is supported by DOE-BES and DOE-NNSA. References and Notes (1) Bermann, U.; Groenzin, H.; Mullins, O. C.; Glatzel, P.; Fetzer, J.; Cramer, S. P. Pet. Sci. Technol. 2004, 22, 863-875. (2) Wade, L.G., Jr. Organic Chemistry, 5th Ed.; Prentice Hall: New York, 2003. (3) McMillan, P. F. Nat. Mater. 2007, 6, 7-8. (4) Ciabini, L.; Santoro, M.; Gorelli, F. A.; Bini, R.; Schettino, V.; Raugei, S. Nat. Mater. 2007, 6, 39-43. (5) Piermarini, G. J.; Mighell, A. D.; Weir, C. E., Block, S. Science 1969, 163, 1250-1255. (6) Thiery, M. M.; Leger, J. M. J. Chem. Phys. 1988, 89, 4255-4271. (7) Budzianowski, Armand; Katrusiak, A. Acta. Crystallogr., Sect. B 2006, 62, 94-101. (8) Besson, J. M.; Thiery, M. M.; Pruzan, P. In Molecular Systems under High Pressure; Elsevier: Amsterdam, 1991, pp 341-357. (9) Cansell, F.; Fabre, D.; Petitet, J.-P. J. Chem. Phys. 1993, 99, 73007304. (10) Engelke, R.; Hay, P. J.; Kleier, D. A.; Wadt, W. R. J. Chem. Phys. 1983, 79, 4367-4375. (11) Gaidai, S. I.; Meletov, K. P. Chem. Phys. 1992, 166, 241-247. (12) Ellenson, W. D.; Nicol, M. J. Chem. Phys. 1974, 61, 1380-1389. (13) Schmidt, S. C.; Moore, D. S.; Schiferl, D.; Shaner, J. W. Phys. ReV. Lett. 1983, 50, 661-664 and references therein. (14) Dick, R. D. J. Chem. Phys. 1979, 71, 3203. (15) Jayaraman, A. ReV. Mod. Phys. 1983, 55, 65-108. (16) Tohji, K.; Udagawa, Y. Phys. ReV. B 1989, 39, 7590-7594. (17) Gordon, M. L.; tulumello, D.; Cooper, G.; Hitchcock, A. P.; Glatzel, P.; Mullins, O. C.; Cramer, S. P.; Bergmann, U. J. Phys. Chem. A 2003, 107, 8512-8520. (18) Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. J. Appl. Phys. 1978, 49, 3276-3283. (19) Mao, H. K.; Kao, C.; Hemley, R. J. J. Phys.: Condens. Matter 2001, 13, 7847-7858.
