E ﬀ ects of Pressure on Model Compounds of Meteorite Organic Matter

: Extraterrestrial organic matter has been widely studied; however, its response to pressure has not. Primitive organic matter bearing meteorites, such as CI and CM carbonaceous chondrites, have experienced variable pressures, up to 10 GPa. To appreciate the e ﬀ ects of these pressures on the organic content of these bodies, the model compounds isophthalic acid, vanillin, and vanillic acid were subjected to pressures of up to 11.5 GPa and subsequently decompressed. High-resolution synchrotron source Fourier transform infrared spectroscopy was used to determine the e ﬀ ects of di ﬀ erent benzene substituents at high pressure on both the vibrational assignments of the benzene core of the molecules and the ability of the aromatic compounds to form intermolecular hydrogen bonds. The presence of additional peaks at high pressure was found to coincide with molecules that contain carboxyl groups; these features are interpreted as C − H ··· O intermolecular hydrogen bonds. The formation of these hydrogen bonds has implications for the origination of macromolecular organic matter (MOM), owing to the importance of such attractive forces during episodes of cross-linking, such as esteri ﬁ cation. Pressure-induced hydrogen-bond formation is a process by which aromatic MOM precursors could have cross-linked to generate the organic polymers found within extraterrestrial bodies today.


■ INTRODUCTION
Carbonaceous chondrites are a primordial class of meteorites, which formed shortly after the formation of the solar system, <4567.30 ± 0.16 My. 1 These meteorites possess abiotic organic material (>3 wt % in some cases), which was incorporated in their asteroid parent bodies. Approximately 30% of this is free organic matter (FOM), while ∼70% is macromolecular organic matter (MOM). 2 The use of the terms solvent soluble and insoluble to describe meteorite organic matter have been avoided as a result of the ambiguity of solvent choice.
Carbonaceous chondrite parent bodies have experienced a range of pressures. The Murchison meteorite (CM2) is an aggregate of grains that has experienced both lower (<5 GPa) and higher (up to ∼10 GPa) pressures, while Murray and Mighei contain at least one grain indicative of these higher pressures. 3 It is also important to remember that, during the first 100 Ma of the solar system, collisions were commonplace 4,5 and it is unlikely that the parent bodies of carbonaceous chondrites escaped these phenomena; thus, the effect of pressure on their organic cargo represents a new study area with great potential.
Previous work on methyl-substituted polycyclic aromatic hydrocarbons has led to the establishment of the first cosmobarometer. 6 Very few pressure studies have been undertaken on cosmologically relevant organic molecules. Meteorite organic matter is host to a diverse collection of aromatic molecules. Aromatic acids are among the most common FOM compounds and have been found previously within carbonaceous chondrites, such as Orgueil (CI1), Murchison (CM2), and Tagish Lake (C2 ungrouped). 7,8 The MOM fraction of Murchison has also yielded aromatic acids as methyl derivatives after treatment with tetramethylammonium hydroxide (TMAH) and pyrolysis. 9 Meteorite organic matter is complex, and its origin is widely debated; however, it is largely accepted that a portion of the organic matter is presolar in origin (see ref 10 for a detailed review). Studies exposing polar ices to ultraviolet (UV) radiation have shown that it is possible to create both aromatic and aliphatic compounds resembling those found in meteorite organic matter, 11−15 although only small quantities of aromatic compounds were recorded by Nuevo et al. 13 Other experiments using polycyclic aromatic hydrocarbons mixed with polar ices have yielded aromatic structures with functional groups, such as those found previously in meteorites, including carboxyl, methoxy, and aldehyde moieties. 16,17 As a result of the complexity of meteorite organic matter, the current study will focus on model compounds that could represent the aromatic components of the initial precursor compounds but also the aromatic acids or ketones found within the soluble portion and aromatic cores found within the insoluble portion of present day meteorite organic matter. The molecules isophthalic acid, vanillin, and vanillic acid ( Figure 1) are all benzene derivatives but with different substituent groups. Isophthalic acid (C 8 H 6 O 4 ), a benzene dicarboxylic acid, has been found within the FOM extracts 8 and among MOM degradation products 9 of Murchison. Vanillin (C 8 H 8 O 3 ) supports alcohol, methoxy, and aldehyde functionalities, and vanillic acid (C 8 H 8 O 4 ) differs from vanillin only in that it possesses a carboxyl group in place of aldehyde. Both the latter two compounds have been selected because they are representative of side groups that are thought to be present on aromatic units within MOM. 9,18 All three compounds are monoclinic and have a crystal unit cell composed of four molecules, which form dimers connected by a pair of intermolecular hydrogen bonds in the case of isophthalic 19 and vanillic acids 20 and a single intermolecular hydrogen bond in the case of vanillin. 21 Further details concerning the structural parameters of these benzene derivatives can be found in Table 1.
No previous studies have attempted to study the effects of high pressure on our selected benzene derivatives; however, there have been multiple studies concerning benzoic acid, which consists of a benzene ring and carboxyl group, such as vanillic acid, but without the additional alcohol and methoxy functionalities. Horsewill et al. 23 undertook the first study of benzoic acid under pressure using nuclear magnetic resonance (NMR) and concluded that two phase transitions were present, one at 0.1 GPa and the other at 0.4 GPa. Wang et al. 24 used a combination of Raman and photoluminescence spectroscopies to determine three phase transitions (at 0.55, 3.67, and 11.10 GPa), with the third phase transition relating to the amorphization of benzoic acid to its anhydride form. A further Raman study suggested a phase transition between 6 and 8 GPa, resulting from changes to the symmetry of the hydrogen bonding. 25 However, phase transitions up to 18 GPa were disputed by Kang et al. 26 Similarly, Cai and Katrusiak 27 did not record phase transitions in their X-ray diffraction studies of benzoic acid. Instead they described the shortening of hydrogen bonds and the decrease in distance between oxygen atoms in carboxyl groups at pressures above 0.5 GPa.
Previously, Fourier transform infrared (FTIR) spectroscopy has been used to monitor the influence of pressure on organic molecules. FTIR spectroscopy was used to recognize the point at which laser-induced pressure reduction converted benzene to an amorphous solid. 28 Similarly FTIR spectroscopy was used to determine the melting curve of formic acid 29 and assess the stabilities of polycyclic aromatic hydrocarbons under pressure. 30,31 More recently, FTIR spectroscopy aided the determination of hydrogen-bond symmetrization in a study of glycinium oxalate under pressure. 32 In this paper, the effects of different substituent groups on the molecular response of molecular crystals to high pressure and associated hydrogen-bond formation have been determined. The data have implications for the effect of the pressure on the organic matter found within meteorites.

■ EXPERIMENTAL SECTION
Isophthalic acid, vanillin, and vanillic acid were loaded separately into a membrane-driven diamond anvil cell (DAC) ( Figure 2) and subjected to pressures up to 11.5 GPa. In situ high-pressure synchrotron source FTIR spectroscopy was used to observe changes to the bonding of the compounds. A 0.15 mm thick pre-indented Inconel gasket, containing a sample chamber hole of 0.25 mm in diameter, was placed between two type-II diamonds with 0.5 mm cutlets. Within the sample chamber, a cesium iodide (CsI) window was created. The

ACS Earth and Space Chemistry
Article sample crystal was placed onto the CsI window along with a ruby (0.05 mm sphere) for measuring pressure.
The pressure was observed throughout the duration of the experiment using the ruby fluorescence technique. 33 The error in the pressure calculation relating to the precision of the spectrometer unit was less than 0.01 GPa, and the uncertainty of the pressure in the DAC resulting from non-hydrostatic conditions was estimated to be ±0.1 GPa. 30 The pressure was increased incrementally by 0.5 to 3 GPa and then by 1 GPa to above 10 GPa, where the pressure was decreased by 2.5 GPa until the minimum pressure was reached. The samples were held for between 15 and 60 min at each pressure. Further details regarding the apparatus and methodology can be accessed elsewhere. 6,30 Transmission FTIR microspectrometry was undertaken using synchrotron source light at the SMIS beamline, SOLEIL Synchrotron, France. The beamline is host to a custom-made horizontal infrared (IR) microscope, with two Schwarzschild objectives (47 mm working distance, NA 0.5) yielding a 22 μm (full width at half maximum) IR spot and 4 cm −1 resolution. Each spectra recorded consisted of 128 scans. The system spectrometer was a Nexus 6700, 34 operated concurrently with the liquid N 2 -cooled MCT/A detector.
Background spectra were taken at every measured pressure in a blank area of the sample chamber before the acquisition of sample spectra. The corresponding background signal for a given pressure was removed from the sample spectra. The resulting spectra were truncated to yield two files using Fityk, 35 a low-wavenumber region (650−1800 cm −1 ) and a highwavenumber region (2600−3700 cm −1 ), omitting the region of interference generated by the diamond. The high-wavenumber region experiences fringing, resulting from reflections between internal and external diamond faces, which were removed by a custom program. The peaks were fitted as Gaussian peaks using Fityk, to demonstrate peak center shifts.

■ RESULTS
The vibrational assignments of key functionalities are reported at ambient pressure and summarized in Table 2. Observed peaks are referred to by their ambient wavelength center at the lowest pressure and highest amplitude. Spectra are displayed for each pressure for a given molecule where data are simple. Pressure versus peak center plots are provided where the data are complex.
Vibrational Assignments. Vibrations can refer to a variety of motions; stretching vibrations are the major focus of this study. However, the bending modes of OH and 18b in isophthalic acid are discussed. The character "ν" is used to denote internal stretching vibrations of a substituent, while "β" is used to represent the bending vibrations where the atoms retain a well-defined plane and "γ" where they do not. The numbers and letters "8a", "8b", "19a", and "19b" describe different tangential C−C stretching vibrations found within the benzene ring. The other stretching vibration of importance in this study is "13", which describes one of the radial C−X stretching vibrations of the benzene ring. The vibration 18b is unlike the other normal vibrations of benzene mentioned thus far and represents an in-plane bending vibration. More specifically, the 18b vibration is a translational mode resulting from the displacement of a carbon atom as a result of the angular momentum of its bonded hydrogen. The assignments of 8a, 8b, 19a, 19b, and 13 were taken from Varsańyi. 36 Isophthalic Acid (C 8 H 6 O 4 ). Peak assignments have not been made previously for isophthalic acid. Here, we assign peaks to isophthalic acid, guided by those published for phthalic acid: 36 βOH (1417 cm −1 ), νCO (1685 cm −1 ), νOH (3081 cm −1 ), 8a (1608 cm −1 ), 8b (1581 cm −1 ), 13 (1078 cm −1 ), 18b (1102 cm −1 ), 19a (1457 cm −1 ), and 19b (1487 cm −1 ).

■ DISCUSSION
At lower wavenumbers, the behavior of isophthalic acid is quite complex (Figure 3); the 18b and 13 peaks converge and finally coalesce by 6 GPa, and the νCO response moves toward the 8a peak before diverging and then finally merging after 8 GPa. The convergence of the 18b and 13 peaks suggests a limitation on their translational bending and radial C−X stretching movements, respectively, with increasing pressure. The C−C stretching vibrations associated with the 8a, 8b, 19a, and 19b peaks appear to be relatively unaffected by increasing pressure, only showing the typical blue shift during pressurization. 30,31 The νCO response adds an extra layer of complexity; it appears that the stretching vibrations associated with this peak both increase and decrease with pressurization. Although it is not clear why this complex behavior may occur, the formation of a new peak (3139 cm −1 ) coincides with the divergence of the νCO response around 4 GPa (Figure 4).
At lower wavenumbers, vanillin shows only minor variation ( Figure 5). The major change is the splitting of the νCO peak into a doublet at pressures above ambient, followed by the convergence of this doublet by 4.3 GPa, which may result from a change in the symmetry of the CO bond. Other changes include the decrease in intensity of the peaks in the 800−860 cm −1 region, relating to vibrational suppression of the γCH (aldehyde), γOH (alcohol), and βC−CHO/νCO moieties with increasing pressure.
Vanillic acid varies only slightly with pressure in the lower wavenumber region (Figure 6). The notable changes include the splitting of the lower wavenumber νCO peak and the trend of decreasing νCO peak amplitude with increasing pressure.
The higher wavenumber region of isophthalic acid depicts the νOH peak (3081 cm −1 ) (Figure 7a), but no red shift is present, suggesting that the O−H bond remains stable during the pressures experienced in this study. More significant is the appearance of a separate peak (3139 cm −1 ) at 4 GPa, which persists to 9.5 GPa, the maximum pressure reached during this Figure 3. Diagram depicting the low-wavenumber FTIR peak centers of isophthalic acid during pressurization up to 9.5 GPa and subsequent decompression to 1.3 GPa. Note the merging of the 18b peak (green) with the 13 peak (orange) and the undulating νCO peak center (red) and its final coalescence with the 8a peak (blue).

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Article experiment. The peak (3127 cm −1 ) then disappears by 5 GPa during decompression. This peak could arise from the formation of further intermolecular hydrogen bonds between the carboxyl functionalities of isophthalic acid or between carboxyl oxygen and aromatic ring or methoxy hydrogen (Figure 1).
The higher wavenumber region of vanillin exhibits the typical red shift of the νOH peak with increasing pressure (Figure 7b). However, unlike isophthalic acid, there is no evidence of a separate peak forming. This indicates a lack of further hydrogen-bond formation within vanillin at higher pressures, likely as a result of the lack of a carboxyl group.
The higher wavenumber region of vanillic acid (Figure 7c) shows a red shift of the νOH peak (3485 cm −1 ), similar to that observed for vanillin. A separate peak is not superposed on the νOH peak, as was observed in the case of isophthalic acid. However, there are three peaks found within the area associated with C−H n bonding. The peak found at 3098 cm −1 is due to the C−H ring stretch, but the other two have not been previously reported. At ambient pressure, only one of these peaks is present (3139 cm −1 ), and this peak increases in amplitude with increasing pressure up to 6.1 GPa, after which it remains constant, before decreasing upon depressurization. The other peak is first seen at 5.1 GPa and has begun to merge with the νOH peak by 10.1 GPa. Tao et al. 25 reported two peaks in a similar region in their Raman study of benzoic acid (3154 and 3163 cm −1 ), which form between 6 and 8 GPa. It is not clear what modes these relate to and, thus, whether they are IRactive, but there is a possibility that these may relate to the Figure 6. Diagram depicting the low-wavenumber FTIR spectra of vanillic acid during pressurization up to 11.5 GPa and subsequent decompression to 1.0 GPa. Figure 7. Diagram depicting the high-wavenumber FTIR spectra of (a) isophthalic acid, (b) vanillin, and (c) vanillic acid during pressurization and subsequent decompression. In the spectra of isophthalic acid, the red dot highlights the appearance of a new peak at around 4 GPa, which persists to the highest pressure used in this experiment. The vanillic acid spectra show three peaks in the high-wavenumber region apart from the νOH peak: C−H ring stretch (3098 cm −1 ) and two peaks that potentially relate to hydrogen bonds. The first peak is present at ambient (3139 cm −1 ), and the second peak (red dot) is first seen at 5.1 GPa. Vanillin does not produce any new peaks at high pressure but does demonstrate a red shift with increasing pressure.

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Article formation of intermolecular hydrogen bonds between carboxyl oxygen and aromatic ring hydrogen of different vanillic acid molecules (Figure 1). Similarly, Chang et al. 39 reports the appearance of two peaks (2891 and 2994 cm −1 ) associated with the formation of C−H···O hydrogen bonds at around 1.9 GPa in solutions of 1,4-dioxane and D 2 O.
The distances between nearest neighbors can be affected by pressure, 40 and this may help to explain the appearance of new peaks in isophthalic and vanillic acids. Figure 1 demonstrates that there is geometric potential for numerous C−H···O hydrogen bonds in both isophthalic and vanillic acids. With increasing pressure, the distances between C−H on both the aromatic rings and methoxy functionalities and oxygen atoms becomes shorter, and this may result in the formation of C− H···O hydrogen bonds. 39 It is also the case that the distance between oxygen atoms and hydrogens will decrease, and this may result in an increase in the strength of repulsions, although these can be overcome by the increased attraction of C−H···O and O−H···O hydrogen bonds with increasing pressure. 41,42 In the case of isophthalic and vanillic acids, the reason for the appearance of peaks in the νO−H and νC−H n regions may be the result of the C−H···O bond forming between either the aromatic or methoxy C−H and the C−O−H of their carboxyl groups, respectively. Vanillin does not possess a carboxyl functionality and contains an aldehyde group instead; therefore, peak formation at a high pressure is not expected. Peak formation could also arise from symmetric changes to the crystals of isophthalic or vanillic acid arising from C−H···O bond formation or changes to the lengths of both of these and classic hydrogen bonds (O−H···O). Indeed, the increase in amplitude of the peak at 3139 cm −1 (Figure 7c) in vanillic acid at higher pressures may result from the lengthening of a hydrogen bond, caused by reorganization of the molecules in the crystal structure. Symmetric changes in the carboxyl groups have been described as the cause of hydrogen-bond lengthening in benzoic acid. 27 An alternative explanation might be a change in the nature of interactions between the dimerized carboxyl groups of isophthalic and vanillic acids. An increase in hydrostatic pressure can reduce the distance between oxygen atoms and hydrogen-bond lengths in carboxyl groups, forcing these functionalities closer together and potentially facilitating the formation of different intramolecular hydrogen bonds, C···H− O and CO···H, through H hopping. 27,40 This change in hydrogen-bond dynamics may introduce new peaks as a direct result of these new hydrogen bonds or as a result of crystalwide symmetry changes resulting from different bond parameters.
The formation of hydrogen bonds with increasing pressure is of potential interest to the formation of MOM from simpler precursors because of the role of pressure in cosmic environments. Hydrogen-bond interactions may have allowed for the assembling of some macromolecular organic matter from individual aromatic units, such as those observed in the interstellar medium and meteorite FOM, via cross-linking of oxygen-containing moieties, such as ether or ester bonds. Indeed, it was demonstrated by Cai and Katrusiak 27 that esterification can occur between benzoic acid units, in methanol and ethanol, at 1.48 GPa and 483 K.

■ CONCLUSION
Three organic model compounds, with chemical relevance to the organic matter within meteorites, were investigated for their responses to pressure. Synchrotron FTIR data reveal that the lower wavenumber regions of the three benzene derivatives are significantly different. Isophthalic acid demonstrates the highest degree of complexity; translational and radial skeletal stretching movements are reduced with increasing pressure, while C−C stretching vibrations are unaffected. Meanwhile, the νCO stretching vibrations appear to both increase and decrease during pressurization. Vanillin displays the least complex response to increasing pressure, with only the splitting of the νCO peak and the vibrational suppression of the peaks relating to γCH (aldehyde), γOH (alcohol), and βC−CHO/ νCO above ambient pressure notable. With increasing pressure, vanillic acid displays splitting of the lower wavenumber νCO peak and a decrease in the amplitude of the νCO peak.
At higher wavenumbers, the spectra of both isophthalic and vanillic acids record peaks that may relate to C−H···O hydrogen-bond formation. However, the peak at 3139 cm −1 in vanillic acid, which increases in amplitude with higher pressures, may represent hydrogen-bond lengthening, potentially resulting from changes in the symmetries of the carboxyl groups present.
Oxygen-containing substituent groups facilitate the formation of intermolecular hydrogen bonding during the application of pressure. It appears that in isophthalic and vanillic acids the presence of a carboxyl side chain gives rise to greater hydrogen bridging than in vanillin. The role of the methoxy group may lead to hydrogen-bond formation between methoxy hydrogen and alcohol oxygen as well as between methoxy oxygen and aldehyde hydrogen at increased pressure in vanillin and give rise to bond lengthening and further formation of intermolecular hydrogen bonds in vanillic acid at increased pressure. The migration of the νCO peak in isophthalic acid is not fully understood, but the formation of C−H···O intermolecular hydrogen bonds between different molecules may be responsible. This may occur through either the interaction of different substituent groups or H hopping between two carboxyl groups.
The presence of certain functional groups seems to promote the formation of hydrogen bonds under pressure. Hydrogenbond formation may be a precursor to esterification and, thus, the cross-linking of aromatic molecules to form organic polymers. The findings reported here may help to constrain the origin of some meteorite MOM units. MOM is thought to contain numerous ester and ether linkages. The recognition of pressure-induced intermolecular hydrogen-bond formation that promotes the assembly of organic networks from oxygencontaining aromatic precursors will help the identification of records of such processes in meteorite organic matter. Future work, such as the undertaking of high-pressure neutron diffraction studies, will allow for a better understanding of the formation of hydrogen bonds in these molecules under pressure.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
The authors acknowledge SOLEIL synchrotron for the allocation of synchrotron radiation facilities (Proposal ID "20131014" and "20141394"). The authors thank Dr. P. Dumas and Dr. Ferenc Borondics for their assistance in using the SMIS beamline. The authors also thank Dr. J. P. Itie for allowing us use of the PSICHÈhigh-pressure facilities.