Formation of a New BenzeneâEthane Co-Crystalline Structure Under

Article pubs.acs.org/JPCA

Formation of a New Benzene−Ethane Co-Crystalline Structure Under Cryogenic Conditions Tuan Hoang Vu, Morgan L. Cable, Mathieu Choukroun, Robert Hodyss,* and Patricia Beauchamp NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, United States S Supporting Information *

ABSTRACT: We report the first experimental finding of a solid molecular complex between benzene and ethane, two small apolar hydrocarbons, at atmospheric pressure and cryogenic temperatures. Considerable amounts of ethane are found to be incorporated inside the benzene lattice upon the addition of liquid ethane onto solid benzene at 90−150 K, resulting in formation of a distinctive co-crystalline structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are observed in the Raman spectra at 2873 and 1455 cm−1, which are red-shifted by 12 cm−1 from the υ1 (a1g) and υ11 (eg) stretching modes of liquid ethane, respectively. Analysis of benzene and ethane vibrational bands combined with quantum mechanical modeling of isolated molecular dimers reveal an interaction between the aromatic ring of benzene and the hydrogen atoms of ethane in a C−H···π fashion. The most favored configuration for the benzene−ethane dimer is the monodentate-contact structure, with a calculated interaction energy of 9.33 kJ/mol and an equilibrium bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-crystalline complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar organics may act as potential hydrocarbon reservoirs due to this incorporation mechanism.

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INTRODUCTION Titan, the largest moon of Saturn, is the only object in the Solar System aside from Earth with stable standing bodies of liquid on its surface.1,2 Widespread lakes composed primarily of ethane and methane have been discovered in the polar areas of Titan by the Cassini mission in 2006.3,4 Because of the nonpolar nature of hydrocarbons, solutes in these lakes are expected to be quite different than those found in liquid water on Earth. Thermodynamic modeling4,5 has hinted at the presence of a number of organic solutes at or near their saturation levels, including benzene, acetylene, and hydrogen cyanide. As ethane remains marginally volatile at Titan surface conditions (94 K and 1.5 bar), loss of this solvent in the lakes via evaporation or other processes could induce formation of organic precipitates, analogous to evaporite minerals on Earth. Such materials have been tentatively detected2 and may play an important role in the surface chemistry of Titan. The motivation for this work is to understand the fundamental chemical interactions between liquid ethane, a principal component of Titan’s lakes, and likely surface evaporites. In particular, a question that remains is whether these evaporites can preserve hydrocarbons in their lattices in the form of co-crystalline compounds. Co-crystals are a wellknown class of solids that are typically synthesized in the laboratory by co-condensation of the constituent gaseous species in fixed molar ratios followed by multiple heating/ cooling cycles.6 In the present study, formation of co-crystals © 2014 American Chemical Society

will be examined directly from the solid−liquid interface using micro-Raman spectroscopy. The focus is on benzene, a relatively simple organic compound that has been detected at high abundance in Titan’s atmosphere7,8 and tentatively identified at the surface (up to 5% of the surface materials).9,10 Previous work from our laboratory has found that solid benzene is only slightly soluble in liquid ethane (∼18.5 mg/ L),11 indicating a relatively weak interaction between the two species. As a result, benzene would be one of the first evaporites to form as Titan lake levels drop.5 During precipitation, benzene will be exposed to the drying lake fluids, leading to potential formation of benzene−ethane co-crystals. Here the intrinsic molecular-level interactions in these co-crystals will be investigated through vibrational spectra and supplemented by quantum mechanical modeling of isolated complexes. Such knowledge will help determine whether ethane can be stably retained in benzene’s crystalline lattice, thereby constraining in part the processes occurring in and around the lakes on Titan. In addition to planetary science implications, interactions involving aromatic molecules in complexes stabilized by dispersive interactions are of fundamental interest. Benzene has frequently been used as a simple model molecule to examine the nature of these interactions, both theoretically and Received: February 17, 2014 Revised: April 22, 2014 Published: May 8, 2014 4087

dx.doi.org/10.1021/jp501698j | J. Phys. Chem. A 2014, 118, 4087−4094

The Journal of Physical Chemistry A

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350, Linkam Scientific Instruments, Ltd.). The internal temperature of the cryostage is held at 90 K, while liquid ethane is pipetted onto the frozen benzene powder. Formation of benzene−ethane co-crystals is then initiated by warming and holding the sample at 105 K for 10 min followed by recooling to 90 K. While not essential, this temperature cycle helps accelerate the incorporation of ethane to within a reasonable time frame for the experiment. The cryostage is then mounted onto an XYZ translation stage underneath the objective lens of a high-resolution confocal dispersive micro-Raman spectrometer (Horiba Jobin Yvon LabRam HR), which allows for continuous microscopic observation of the sample during the experiment. The stark difference in the optical images before and after co-crystal formation (Figure 2) shows an unambiguous morphological change and serves as a clear indication that ethane has been incorporated into the benzene lattice. Raman spectra are typically obtained at 0.4 cm−1 resolution using an 1800 grooves/mm grating, unless specified otherwise. All samples are excited by an external frequency-doubled Nd:YAG 532 nm laser operating at 50 mW output. A silicon chip is used for frequency calibration, exhibiting a well-defined sharp peak at 520.7 cm−1. To improve the signal-to-noise ratio, spectra are accumulated in triplicate with a typical acquisition time of 1.5 min per spectrum. At cryogenic conditions, the benzene−ethane complex is found to be sufficiently stable in the strong laser beam to give reproducible Raman spectra. Spectral deconvolution is performed using both Gaussian and Voigt functions; both methods yield good agreements. Theoretical calculations are also implemented to support interpretation of the experimental spectra. These are carried out at the MP2 level of theory to account for electron correlation using the Spartan ′14 suite of programs (Wave Function, Inc.).18 The 6-31G* basis set is used for all complexes, and geometries are optimized fully without imposing any constraints. Vibrational frequencies are calculated at the same level to verify whether the predicted structures have reached true energy minima and to determine frequency shifts following complex formation. As computational methods are known to yield an overestimate of the harmonic vibrational frequencies,19 the obtained values are scaled by a factor of 0.93 in order to match experimental data.

experimentally. The slightly higher electronegativity of carbon relative to hydrogen and the overall D6h symmetry of benzene lead to a permanent negative quadrupole moment that can act as a weak hydrogen acceptor.12 The π electron cloud of benzene is especially versatile in its binding abilities as it usually accepts one donor on each side to form a linear aromatic arrangement.13 For example, the unusual O−H···π interaction between benzene and water has received recent attention due to its potential role in binding interactions at biological surfaces containing aromatic amino acids.14 Benzene clusters with CO, CO2, and N2 have also been investigated using jet expansion mass spectrometry,15 and the formation of charge transfer complexes between benzene and halogen molecules have been reported.16 These various binding motifs have since prompted a substantial rethinking of bimolecular interactions involving aromatic rings and produced a new view of benzene as a polar molecule.17 The work reported here will further contribute to this emerging outlook by presenting another example of a C− H···π interaction between benzene and a saturated aliphatic hydrocarbon. The remainder of this article is organized as follows. The next section describes the experimental apparatus and sample preparation procedures. Raman spectra along with their interpretation and theoretical modeling comprise the results and discussion section. Finally, we conclude with a summary and remark on implications of this work for Titan.

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EXPERIMENTAL METHODS Liquid ethane is produced from direct condensation of gaseous ethane (Matheson Tri-Gas, Ultra High Purity grade 99.95%) into a custom-built liquid nitrogen cryostat maintained at 90 K (Figure 1). The cryostat is kept under nitrogen atmosphere to

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RESULTS AND DISCUSSION The present study investigates the incorporation of ethane into solid benzene at conditions similar to those at the surface of Saturn’s moon Titan. The objectives are to examine the formation and stability of the benzene−ethane co-crystals and to unravel the fundamental molecular interactions between the two species. These issues are accordingly addressed in the subsequent subsections. 1. Experimental Evidence for Formation of Solid Binary Benzene−Ethane Complex. Figure 3 shows the Raman spectrum of the solid benzene−liquid ethane mixture at 90 K before (black curve) and after (blue curve) incorporation of the guest alkane. To facilitate comparison, a control spectrum of pure solid benzene at this temperature is also enclosed (red curve). In the 2800−3100 cm−1 spectral region, the prominent features of solid benzene include the υ1 (a1g) symmetric stretch at 3063 cm−1 and the υ7 (e2g) C−H stretch between 3040 and 3050 cm−1 (the latter fundamental is split into a doublet with components at 3044 and 3049 cm−1 due to the crystal field). There are also two weaker resonances at 2925

Figure 1. Schematic of the cryostat used for condensation of liquid ethane.

avoid condensation of water vapor onto the sample. The temperature is regulated to ±0.1 °C via a Pt100 temperature sensor and a wire heater coupled to a temperature controller (Model 321, Lake Shore Cryostronics, Inc.). Benzene (HPLC grade, 99.9%) is obtained from SigmaAldrich and used without further purification. Benzene crystals are synthesized by freezing liquid benzene in liquid nitrogen followed by grinding the solids with a prechilled spatula. An aliquot of the benzene powder is quickly deposited onto a glass slide stationed inside a liquid nitrogen-cooled cryostage (LTS 4088



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Figure 2. Microscope images taken at 10× magnification of solid benzene at 90 K (top) and benzene−ethane co-crystals (bottom) at different regions on the surface. Note the recrystallization associated with the incorporation of ethane into the benzene lattice.

and 2945 cm−1, which have been assigned to a combination band and an overtone, respectively.20 Upon the addition of liquid ethane (black curve), the spectrum initially shows a sharp peak at 2885 cm−1 due to the υ1 (a1g) stretch of ethane.21 As the sample is warmed incrementally to 105 K and recooled to 90 K, a new feature emerges at 2873 cm−1 (Figure 3, blue). The significant red shift of this band (12 cm−1) clearly signifies a new molecular environment for ethane that is different from the liquid phase. A control spectrum of solid ethane at 80 K (yellow curve) shows a maximum at 2877 cm−1 for the υ1 vibration, consistent with earlier reports.22,23 As a result, the shoulder at 2873 cm−1 is not due to ethane in the solid state and is ascribed to ethane molecules incorporated inside the crystalline structure of benzene. This assignment is based on a similar observation in ethane clathrate hydrates, where the vibrational frequency of the caged ethane molecules is redshifted by 9−14 cm−1 from its gas phase value.24,25 The occurrence of this resonance thereby indicates incorporation of ethane in the benzene lattice and points to the formation of a co-crystalline complex between these two compounds at atmospheric pressure. In the presence of such a molecular complex, the fundamental vibrations of the host benzene molecules should also be sensitive to interactions with the ethane guests. In Figure 3, the spectral region between 2900 and 3000 cm−1 is

composed of several broad and overlapping peaks from both benzene and ethane and is too convoluted to separate specific modes that may help characterize co-crystal formation. However, much information can be deduced from the υ7 (e2g) stretch of benzene, a particularly efficient probe for changes in its molecular environment. A summary of the experimental Raman frequencies for materials measured in this study is given in Table 1. For the υ7 mode of benzene, in particular, considerable red shifts are observed upon ethane incorporation: approximately 3 cm−1 for the 3044 cm−1 peak and 2 cm−1 for the 3049 cm−1 peak. Since the resolution of our spectrometer here is better than 0.4 cm−1, these shifts represent significant modification of the crystalline lattice of benzene to accommodate the incorporated ethane molecules. In contrast, the υ1 (a1g) symmetric stretching mode of pure solid benzene at 3063 cm−1, in which all six C−H bonds vibrate in-sync in the plane of the aromatic ring, is essentially unchanged compared to that of benzene containing ethane (Table 1). As the polarizability of this vibration lies entirely in the molecular plane, any interaction of the sp2 hydrogen atoms would cause this peak to shift significantly. The absence of such a shift in the benzene−ethane co-crystal, together with the presence of a red shift in the υ7 mode, indicate an interaction between the incorporated ethane molecules and the π-electron system of benzene. This combined evidence (emergence of the 2873 4089



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Figure 3. High-resolution Raman spectra of crystalline benzene before (black) and after (blue) incorporation of ethane in its lattice at 90 K. Note the emergence of a shoulder at 2873 cm−1 upon ethane incorporation. In comparison to pure solid benzene at 90 K (red), the υ7 mode at 3040−3050 cm−1 undergoes a red shift, while the frequency of υ1 mode at 3063 cm−1 remains unchanged. The spectrum of solid ethane at 80 K (yellow) is also included for reference. All spectra are vertically offset for clarity.

electron exchange−correlation effects play a significant role in the stabilization process.27 Density functional techniques, while very accurate for many systems, cannot reproduce dispersive interactions adequately.28,29 Hence, the second-order Møller− Plesset (MP2) perturbation theory, which performs relatively well for weak interactions,30,31 is used instead for binding energy calculations. Here the 6-31 G* basis set is chosen for geometry optimization in order to allow for direct comparison of our results with those of Boese et al.28 for benzene− acetylene co-crystals (high-level calculations of benzene− ethane dimers have also been performed by Fujii et al.32). For binary clusters between ethane and benzene, three different conformations can be considered: monodentate, bidentate, or tridentate structures in which one, two, or three C−H bond points to the center of the aromatic ring, respectively. The optimized geometries and electrostatic potential surfaces for all three isomers are displayed in Figure 4, showing the ethane moiety on top of the benzene ring in a staggered configuration. The monodentate isomer is found to be the most stable among the three structures, having the shortest contact distance and the strongest binding energy. Moreover, vibrational frequency calculations establish that only the monodentate isomer is at a true energy minimum, whereas the other two complexes are saddle structures with a few imaginary low-frequency modes after the simulations converge. It is somewhat surprising that the tridentate isomer, while having the highest number of binding sites, is the least favorable structure. However, these results are consistent with a recent study by Fujii et al.32 using a larger basis set, which affirms the preference of the monodentate contact in the C−H···π interaction between an alkane and a single phenyl ring. MP2 calculations also predict a red shift (indicated by the minus sign in Figure 4) for the υ1 (a1g) stretch of ethane when it is bound to a benzene molecule compared to that of a free

Table 1. List of Experimental Raman Shifts upon Co-Crystal Formationa Raman shift (cm−1) at 90 K molecule ethane

vibrational mode υ1 (a1g) CH3 sym. stretch υ11 (eg) CH3 deform. stretch

benzene

υ1 (a1g) CH sym. stretch υ7 (e2g) CH asym. stretch

co-crystal

Δυ̃

2884.8 (liquid)

2872.6

−12.2

2877.0 (solid −80 K) 1467.1 (liquid)

1454.8

−12.3

1461.5 (solid −80 K) 3063.3 (solid)

3063.6

0.3

3043.7 (solid)

3040.6

−3.1

3048.9 (solid)

3047.1

−1.8

pure component

a

For liquid ethane and solid benzene, temperature variation in the range 90−160 K does not change peak positions by more than 1.6 and 0.5 cm−1, respectively.

cm−1 peak, red shift of the υ7 mode, and the constancy of the υ1 stretch of benzene) strongly suggests that the attractive van der Waals interaction between benzene and ethane results in the formation of a co-crystalline structure. 2. Ab Initio Modeling of the Benzene−Ethane Interactions. To assess the validity of this model, ab initio calculations of isolated benzene−ethane heterodimers in the gas phase are carried out. Theoretical approaches have been widely used for determining structural, energetics, and vibrational properties of small clusters and have proved extremely useful in interpreting their bonding interactions.26 It is well-established that for van der Waals complexes, the 4090



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Figure 4. Electrostatic potential surfaces of various benzene−ethane dimers optimized at the MP2/6-31G* level. ΔE denotes the calculated binding energies (for comparison, values calculated by Fujii et al.32 for analogous complexes using the aug(d,p)-6-311G** basis are listed in the footnote). Δυ̃ is the frequency shift of the υ1 (a1g) mode of ethane upon complex formation relative to a free ethane monomer, where the minus sign represents a red shift. The observed Raman spectra is best accounted for by the presence of the monodentate isomer.

ethane monomer. Aside from a smaller magnitude for the red shift, the results agree qualitatively with experimental observations (Table 1). Among the three isomers, the monodentate structure exhibits the largest shift (7 cm−1) and is the closest to the observed value (12 cm−1). This red shift indicates weakening of the stretching force constant of the ethane molecule and is consistent with an increase in the reduced mass for ethane upon complex formation. It is likely that the monodentate contact is the preferred host−guest configuration during co-crystal formation. The molecular complex with benzene exemplifies the characteristic behavior of ethane as a weak hydrogen donor. Attractive interactions with benzene’s π electron cloud (a Lewis base center) should thus play a significant role in stabilization of the complex. C−H···π interactions between aromatics and hydrocarbons have found broad interest owing to their importance in supramolecular aggregates, crystal packing, molecular recognition, and folding of proteins.6 An example of a complex with a similar mode of interaction is one between benzene and acetylene, in which the acetylene molecule resides on the axis perpendicular to the center of the aromatic ring.13 A bond distance of 2.367 Å and an interaction energy of 11.3 kJ/ mol have been reported for the 1:1 complex at the MP2/631G* level (X-ray diffraction data at 123 K finds a value of 2.447 Å for the C−H···π bridge).28 These parameters are comparable to our calculated values for the monodentate benzene−ethane complex (2.66 Å and 9.33 kJ/mol, respectively). It could be argued that ab initio predictions, while limited to a pair of molecules, can still be considered valid in condensed phases because complex formation can be probed on the observation time and spatial range of Raman spectroscopy.27

In Figure 2, the optical images show an apparent change in the morphology of the benzene crystals that is associated with ethane incorporation into the lattice. A possible explanation for this effect could be the disruption of the strong stacking interaction in solid benzene by the incorporated ethane molecules. Here the overall flatness of benzene would permit perpendicular approaches to its π electron cloud, thereby increasing the number of benzene−ethane intermolecular contacts. This imposes a geometrically and energetically unfavorable strain on benzene’s orthorhombic lattice and forces a change in the crystal structure to accommodate the guest ethane molecules. Even though our method does not allow for quantitative measurement of the amounts of incorporated ethane, we observed that, in general, smaller crystals with higher amounts of liquid ethane lead to more efficient co-crystal formation compared to those with less ethane. The ethane incorporation process appears to reach saturation within a few minutes at 140 K and in 1.5 h at 110 K, implying that this could occur quite readily under Titan’s ambient conditions in evaporite basins containing benzene and similar aromatics. The kinetics of benzene−ethane co-crystal formation and the activation energy for the process (10.2 kJ/mol) have been examined in detail and are reported elsewhere.33 3. Stability Measurements. To gain qualitative insights into the stability of the benzene−ethane co-crystals, their Raman spectra have been obtained as a function of temperature. In Figure 5, the yellow curve shows the spectrum of a benzene−ethane sample prepared at 105 K. Compared to the blue curve in Figure 3, it is clear that mixing solid benzene and liquid ethane at a higher temperature leads to a larger amount of ethane being incorporated, evidenced by the considerably higher relative intensity of the 2873 cm−1 peak compared to the 4091



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2800−3100 cm−1, the most significant change is observed in the υ11 (eg) CH3 deformation stretch of ethane around 1500 cm−1. In liquid ethane, this mode constitutes a relatively broad peak at 1467 cm−1 (fwhm = 23.1 cm−1). Upon incorporation of ethane into solid benzene at 95 K, a sharp shoulder emerges at 1455 cm−1, which is on the lower frequency side of the fundamental (black, inset). As this feature is clearly distinguishable from those in solid ethane (red, inset) and the magnitude of the red shift from the liquid phase is virtually identical to that observed for the υ1 stretch upon co-crystal formation (Table 1), the peak at 1455 cm−1 serves as another signature of ethane molecules incorporated in solid benzene. The sharpness of this resonance (fwhm = 5.6 cm−1) relative to the liquid ethane points to a more structurally confined environment for ethane in the co-crystals. At 135 K, the band at 1467 cm−1 vanishes as liquid ethane evaporates from the sample, consistent with the temperature behavior observed in Figure 5. In addition, it is instructive to examine how the benzene lattice responds to changes in temperature as the co-crystal disintegrates. Figure 7 plots the band center frequency of the υ7 (e2g) mode of benzene containing ethane during heating from 90 to 165 K. The components of the υ7 doublet (red dots and squares) have been analyzed with Voigt profiles, and the results are compared to that of pure solid benzene (blue dots and squares). It is observable that, as the temperature increases, both peaks in the doublet shift progressively toward higher frequencies and approach their values in pure solid benzene. Together with the decrease in intensity and eventual disappearance of the co-crystal peak upon warming in Figure 5, the shift toward higher frequencies indicates weakening of benzene−ethane interactions and relaxation of the benzene lattice as the co-crystals dissociate. The fact that the υ7 frequencies for the pure benzene and the benzene−ethane co-crystals do not coincide until 160 K is consistent with the

Figure 5. High-resolution Raman spectra of solid benzene and liquid ethane with increasing temperatures. Note that free liquid ethane evaporates from the sample after 125 K (as evidenced by the disappearance of the 2885 cm−1 peak), but ethane within the cocrystals (the 2873 cm−1 feature) remains present until 160 K.

free ethane resonance at 2885 cm−1. As the co-crystals are exposed to higher temperatures, the intensity of both peaks decreases gradually due to loss of liquid ethane. At 135 K in particular, all excess liquid ethane that has not reacted with benzene evaporates completely, while the ethane fraction that has been incorporated in the co-crystal remains. The 2873 cm−1 feature continues to persist up to 150 K and eventually the cocrystal disintegrates at 160 K, resulting in a spectrum similar to that of pure solid benzene in Figure 3. The dependence of the benzene−ethane co-crystals on temperature is also apparent in other regions of the spectrum. Figure 6 displays the full-range Raman spectra of the co-crystal obtained at 95 K (black) and 135 K (blue) collated with control spectra of pure solid ethane at 80 K (red) and liquid ethane at 94 K (green curve). Aside from the aforementioned features at

Figure 6. Full-range Raman spectra of solid ethane at 80 K (red), liquid ethane at 94 K (green), and benzene−ethane co-crystals at 95 K (black) and 135 K (blue). Note the presence of a sharp resonance at 1455 cm−1 in the co-crystals (inset). Spectra are vertically offset for clarity. 4092



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Figure 7. Comparison of the temperature dependence of peak positions for the υ7 stretching mode in pure solid benzene and in the co-crystalline phase. In the co-crystal (red), this mode exhibits a blue shift as temperature increases, while it remains fairly constant with temperature in pure solid benzene (blue).

edges and evaporite basins on Titan may hold important quantities of ethane and could point toward a new hydrocarbon reservoir on this Saturnian satellite.

observation that the incorporated ethane molecules do not completely escape the benzene lattice until the sample achieves this temperature (Figure 5, orange). In addition, the υ1 (a1g) mode of complexed benzene at 3063 cm−1 remains flat throughout the entire temperature range (not shown) and is the same as that of pure solid benzene, confirming that the interaction in the co-crystal is between the hydrogen atoms of ethane and the π electron cloud of benzene. The behavior of both ethane and benzene features further reinforces the importance of specific guest−host interactions in the initial stabilization of the complex and provides compelling evidence for the stability of the co-crystal until a relatively warm temperature of 160 K. At this point, the co-crystals become destabilized as ethane is liberated and benzene crystals revert to their original structure. A movie showing the disintegration process at 165 K is available in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Video showing real-time disintegration of benzene−ethane cocrystals at 165 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(R.H.) E-mail: [email protected]. Phone: (818) 354-4321. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS We have presented a series of micro-Raman experiments to demonstrate that the mixing of liquid ethane (a major constituent of Titan’s lakes) with solid benzene (a putative Titan surface material) at ambient pressure and cryogenic temperatures can result in the formation of a stable cocrystalline structure containing both species. The main spectral signatures for incorporation of ethane molecules into the benzene lattice are associated with a 12 cm−1 red shift for both the υ1 (a1g) and υ11 (eg) stretching modes of ethane. Marked band shifts are also observed for the υ7 (e2g) mode of benzene upon complex formation, which is consistent with the obtained optical evidence showing complete recrystallization of the sample. The υ1 (a1g) in-plane vibration of benzene, however, remains unaltered in the presence of ethane, indicating that the stabilizing interaction in the co-crystal is between benzene as the electron donor and ethane as the electron acceptor. Ab initio calculations confirm this model and show that the most stable configuration corresponds to a geometry in which the ethane molecule rests along the axis perpendicular to the benzene molecular plane with one C−H bond facing the aromatic ring. A direct implication of these findings is that lake

ACKNOWLEDGMENTS This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Support from the NASA Astrobiology Institute (Titan node), the NASA Astrobiology Science and Technology Instrument Development Program (ASTID), the NASA Outer Planets Research Program (OPR), the NASA Postdoctoral Program (administered by Oak Ridge Associated Universities), and government sponsorship are gratefully acknowledged.

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(1) Stofan, E. R.; Elachi, C.; Lunine, J. I.; Lorenz, R. D.; Stiles, B.; Mitchell, K. L.; Ostro, S.; Soderblom, L.; Wood, C.; Zebker, H.; et al. The Lakes of Titan. Nature 2007, 445, 61−64. (2) Barnes, J. W.; Bowa, J.; Schwartz, J.; Brown, R. H.; Soderblom, J. M.; Hayes, A. G.; Vixie, G.; Mouélic, S. L.; Rodriguez, S.; Sotin, C.; et al. Organic Sedimentary Deposits in Titan’s Dry Lakebeds: Probable Evaporite. Icarus 2011, 216, 136−140. (3) Brown, R. H.; Soderblom, L. A.; Soderblom, J. M.; Clark, R. N.; Jaumann, R.; Barnes, J. W.; Sotin, C.; Buratti, B.; Baines, K. H.; Nicholson, P. D. The Identification of Liquid Ethane in Titan’s Ontario Lacus. Nature 2008, 454, 607−610.

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