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Spectroscopic Identification of Water-Propane Interaction: Implications for Clathrate Nucleation Tuan Hoang Vu,† Sarah Dai Ka¨lin,‡ and Mary Jane Shultz*,† Pearson Research Laboratory, Department of Chemistry, Tufts UniVersity, Medford, Massachusetts 02155, and AmniSure International LLC, 30 JFK Street, Cambridge, Massachusetts 02138 ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: May 7, 2010
Propane is one of several hydrocarbons known to form a clathrate hydrate. To probe interactions leading to clathrate nucleation, the water-propane interaction is investigated in carbon tetrachloride with infrared spectroscopy. Isotopic substitution provides compelling evidence that the water-propane interaction occurs between the propane methylene hydrogen atoms and the water lone pair. In addition, interaction between propane and water results in clustering of water molecules, a clustering that is identified by the appearance of a peak between the symmetric and asymmetric stretches of water. Introduction Natural gas clathrate hydrates are inclusion compounds consisting of a hydrocarbon encased in a water-molecule cage. Clathrate hydrates, henceforth referred to simply as clathrates, are potential key players in numerous global environmental issues including ocean drilling, gas transportation, and greenhouse gas sequestration.1 Naturally occurring clathrates contain more carbon (∼10 Gtons) than all known fossil fuel sources found on earth.2-4 With this large store of carbon, clathrates could be either a huge energy source of a global-warming catastrophe depending on how they are managed. Understanding the nature of clathrate formation is likely to be important for the proper management of this great resource. Most previous investigations of clathrate formation fall into three broad categories: determining the structure or composition of the product,5-7 investigating formation conditions,8-12 or measuring growth kinetics.13-18 Visual inspection of clathrate growth indicates that initial growth likely occurs at the gas-water interface.1,12,13,16,19,20 X-ray and neutron diffraction studies5,6,14,17,21,22 provide valuable structural information. There is a high degree of cage occupancy resulting in a hydrocarbon concentration in the clathrate that is nearly three-orders of magnitude greater than the solubility in water. The high hydrocarbon concentration in the clathrate suggests the existence of an attractive interaction between water and the guest molecule. Theoretical calculations found such an attraction between water and methane.23 Methane acts as an H-bond acceptor, resulting in pentacoordinate carbon. Other calculations24,25 have shown that the water cage is not stable in the absence of a guest molecule. The aim of this work is to experimentally probe the attractive interaction between a hydrocarbon and a small number of water molecules. This paper focuses on propane, which forms a metastable hydrate under moderate pressure near the freezing point of water. Propane clathrate has a cubic structure II geometry. Structure II clathrate consists of large cages with 12 pentagonal and 4 hexagonal rings of water molecules, called a 51264 cage, mixed with smaller icosahedral, 512, cages. (See Sloan and Koh1 for a
general discussion of clathrate notation.) Propane occupies the larger cages leaving the smaller cages for helper gases. Laboratory samples are often synthesized via snow-gas mixtures.26 In contrast to macroscopic sample synthesis or product property studies, monitoring the molecular-level attraction leading to nucleation of clathrates requires a technique capable of probing molecular-level interactions. Infrared spectroscopy is particularly appropriate for this task since interactions between water and a guest molecule can be readily identified via changes in the O-H stretch spectral features and rovibrational frequency shifts. The huge oscillator strength of hydrogen-bonded water, however, renders bulk aqueous samples opaque. Thus using infrared spectroscopy to unravel fundamental aspects of the attraction between water and a hydrocarbon requires circumventing the opacity problem. This work employs a kind of matrix isolation technique in which water is dispersed in carbon tetrachloride. In contrast to classic matrix isolation techniques, carbon tetrachloride provides isolation with thermal energies closer to ambient;in this case, near the freezing point of water. Water is known to exist as monomers in CCl4 at room temperature (saturated concentration 7.5 mM27,28). The water monomer signature consists of the symmetric stretch, the asymmetric stretch, and rotational wings associated with the asymmetric stretch. The rotational wing structure28,29 is due to a nearly free symmetry-axis rotation and highly damped rotation perpendicular to the symmetry axis. Cluster formation is easily detected via the well-known O-H frequency shift and oscillator strength increase of H-bonded water. On the basis of detection limits, in CCl4 dimers constitute less than 2 parts in 10 000 of the water present.28 Relevant for this work, CCl4 also dissolves hydrocarbons offering an excellent environment for probing the water-propane interaction. The next section of this paper describes the high-pressure optical cell designed for this work as well as sample preparation. The results are set out in the third section. The results are interpreted and the interaction between propane and water is diagnosed in the Discussion section. The final section is a summary.
* To whom correspondence should be addressed. Phone: (617) 627-3477. Fax: (617) 627-3443. E-mail:
[email protected]. † Tufts University. ‡ AmniSure International LLC.
Experimental Methods A high-pressure optical cell was used both for sample preparation and for data acquisition. The 316-stainless steel cell
10.1021/jp101678z 2010 American Chemical Society Published on Web 05/17/2010
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Figure 1. Cross-section of the high-pressure optical cell with a 3/4 in. clear aperture.
(Figure 1) was equipped with calcium fluoride windows and had a 1-in. path length and a volume of 120 mL. A key design feature is that the internal pressure presses the windows against a flange to seal the optical ports. The cell-body seal consists of a gasket sandwiched between the stainless steel parts. The cell is easily disassembled enabling window replacement or substitution of alternate materials as needed. Carbon tetrachloride saturated with water is made by mixing 90 µL of H2O (18 MΩ · cm, UV radiated, Barnstead nanopure) with 90 mL of CCl4. The sample cell is sonicated and left to phase separate at room temperature for at least 24 h. The resulting mixture is stable for weeks giving a reproducible infrared absorbance. Carbon tetrachloride (Sigma-Aldrich, anhydrous, 99.5%) was used without further purification. The water-CCl4 system was frozen at -80 °C before propane injection. Twenty-five Torr of propane was introduced into the 30 mL head space and the cell warmed to -5 °C over a period of 24 h giving a final propane concentration of ∼0.5 mM. The system was pressurized with argon to the desired pressure. Pressure was measured with MKS Baratron model 722A (range: 0-1.3 atm) and model 890B (range: 0-207 atm) pressure transducers. All gases;ultrahigh purity argon, instrument-grade propane (Airgas East), and deuterated propane CD3CH2CD3 (C/ D/N Isotopes, 99.8% atom D);were passed through U-tubes containing Drierite and potassium hydroxide to remove water and CO2. All samples were incubated at -5 °C overnight. The FTIR (Nicolet Magna-IR 760; DTGS KBr detector; 64 scans; 1 cm-1 resolution) was purged with dry nitrogen for at least 24 h. The sample compartment was maintained at -5 °C with cold nitrogen gas. Ab initio density functional theory calculations were performed at the B3LYP 6-31G* level (Spartan ’08).30 Results The present work focuses on the interaction between water and propane. A picture of the interaction could guide efforts either to stabilize clathrate structures for energy and greenhouse gas sequestration purposes or to prevent clathrate formation in pipelines. This work is based on the existing picture of water in carbon tetrachloride at room temperature.27,28 Since propane clathrate formation requires subzero temperatures to reach the
Figure 2. Absorption spectrum of CCl4 saturated with water at -5 °C and 1 atm. The inset shows a magnified view of the hydrogenbonded region. Note the lack of discernible resonances in the hydrogenbonded region. The integrated intensity of the broad peak centered at 3400 cm-1 is approximately 2% of the remaining peaks. Based on a conservative estimated osciallator strength gain of one-order of magnitude, hydrogen-bonded water constitutes less than 0.2% of the total water.
quadruple point1 and carbon tetrachloride freezes at -23 °C, any temperature in between could be used: -5 °C was chosen as the operating temperature. The -5 °C spectrum of H2O in CCl4 (Figure 2) demonstrates that water exists primarily as monomers. The inset shows a magnified view of the hydrogenbonded region. Assuming a one-order of magnitude oscillator strength increase on hydrogen bonding (conservative), the dimer concentration is less than 0.2% the total water at -5 °C. On the basis of the known saturated concentration of water in CCl4 at room temperature (7.5 mM27,28), the concentration of water in CCl4 at -5 °C is 3 mM. The peaks at 3615 and 3705 cm-1 are assigned to the water O-H symmetric and asymmetric stretches, respectively. The broad feature underlying the stretches results from restricted rotational motion for water in carbon tetrachloride.28 The simplicity of the spectrum, relative to gasphase water, is a result of highly quenched rotational motions about the two axes perpendicular to the symmetry axis and a free rotation about the symmetry axis. The free symmetry-axis rotation is diagnostic of water monomers interacting via the lone pair, hence free of donor interactions.
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Vu et al. with the methyl groups of propane. The C-H region, on the other hand, exhibits a new peak at 2961 cm-1 upon addition of water. The new peak appears to blue shift with increasing pressure. The significance of this new feature is discussed in the next section. Discussion
Figure 3. Difference between the spectrum of water-propane and that of water in CCl4 at -5 °C and 1 atm. The inset is a magnified view of the 3100- 3400 cm-1 region showing emergence of hydrogenbonded resonances at 3162 and 3338 cm-1.
In contrast to neat water in carbon tetrachloride, the spectrum of the water-propane system at -5 °C (Figure 3) shows two important new features: (a) the difference spectrum shows negative peaks for water, hence an overall decrease in the freewater concentration and (b) emergence of hydrogen-bonded resonances at 3162 and 3338 cm-1 (shown in the inset). These features are examined in the Discussion section. Focusing on the water portion of the water-propane spectrum (Figure 4a) reveals an additional resonance between the symmetric and the asymmetric stretches of water. This feature is characteristic of a dangling O-H (d-OH) due to decoupling of the O-H oscillators when water acts as a hydrogen donor. The emergence of this 3643 cm-1 peak together with the decrease in the intensity of both the symmetric and asymmetric stretches indicate a transfer of free water to hydrogen-bonded water. On the basis of the relative concentrations of water and propane in solution, it is determined that, on average, each propane molecule is associated with two to three water molecules. To deduce interactions between water and propane, spectra of neat water in carbon tetrachloride (Figure 4b) were obtained under the same temperature and pressure conditions as those of water-propane (Figure 4a). Notice that while the waterpropane spectrum (Figure 4a) shows emergence of the dangling O-H peak, the neat water spectrum (Figure 4b) shows virtually no change in the spectral structure of water with pressure. The conclusion is that water does not self-associate or cluster due to pressure. Water does hydrogen bond in the presence of propane. This conclusion is described further in the Discussion section. To determine the site of interaction between water and propane, the spectrum of pure propane in CCl4 is compared to that of propane in the presence of water. These spectra are shown in Figure 5. Notice that addition of water causes splitting of the propane resonances at around 2930 and 2960 cm-1. The C-H stretches in propane are coupled modes involving all eight C-H bonds. Thus, to identify the site of the water-propane interaction, isotopic substitution was used. Since formation of clathrate cages is insensitive to the guest, partially deuterated propane and propane are expected to have similar characteristics. Hence, the spectra of methyl-deuterated propane, CD3CH2CD3, both with and without water (Figure 6) were examined. The vibrational modes of methyl-deuterated propane separate into two sets: the methylene C-H and the methyl C-D stretches. Examination of the latter shows virtually no structural change in the presence of water, suggesting that water does not interact
The goal of this work is to identify the interaction between water and propane that ultimately leads to clathrate nucleation and growth. At its heart, this interaction involves hydrogen bonds, and infrared spectroscopy is one of the most effective methods for probing them. However, the extremely high oscillator strength of the water hydrogen-bonded region makes it challenging to probe aqueous media with infrared spectroscopy. Opacity is avoided if water is dispersed in an infraredtransparent medium. For this application, it is desirable to choose a medium that is fluid under typical clathrate formation conditions: temperatures around 0 °C and pressure of tens of atmospheres. Carbon tetrachloride satisfies these criteria. In addition, the room-temperature dynamics of water in carbon tetrachloride are well characterized.27,28 Carbon tetrachloride was thus chosen as the medium for water dispersal. Note that carbon tetrachloride is essentially a matrix isolation medium with ambient thermal energies. Since water in carbon tetrachloride has not yet been characterized at subzero temperatures, the absorbance spectrum of water in carbon tetrachloride at -5 °C, the target temperature, was obtained (Figure 2). The peaks at 3615 and 3705 cm-1 are the well-known symmetric and asymmetric stretches of water. Thus, even at -5 °C, water in carbon tetrachloride still primarily exists as monomers with a distinct rotational structure similar to that at room temperature. Upon addition of propane (Figure 3), hydrogen-bonded features at 3162 and 3338 cm-1 emerge. The difference spectrum (water-propane minus water) shows negative peaks for the symmetric and the asymmetric stretches of water indicating loss of water monomers. These two observations;loss of water monomers and growth of H-bonded water;indicate that water monomers are incorporated into H-bonded clusters in the presence of propane. Pressurizing a water-carbon tetrachloride system in the absence of propane (Figure 4b) shows neither a reduction of water monomers nor any evidence for hydrogen bonding. Thus, water does not self-associate or cluster under pressure. The distinct peaks at 3162 and 3338 cm-1 thus must be due to hydrogen-bonded water structures that only exist in the presence of propane. The magnitude of the O-H frequency shift due to hydrogen bonding has been found to be related to the electric field along the O-H bond.31,32 Since these two peaks are shifted 480 and 305 cm-1 respectively from a free O-H, these resonances must be due to strongly hydrogen-bonded water. The interaction between water and propane is expected to be relatively weak. Thus the emergent resonances are attributed to water-water hydrogen bonds. Water-water hydrogen bonds are typically very broad due to a high degree of flexibility. The relatively narrow resonances at 3162 and 3338 cm-1 indicate that the hydrogen-bonded water structure has relatively restricted motion. It is thus concluded that the water-propane interaction is responsible for incorporation of water into a relatively inflexible structure featuring hydrogen bonds. In addition to the hydrogen-bonded features, the water region in the water-propane system (Figure 4a) reveals an additional peak at 3643 cm-1. The additional peak is between the water
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Figure 4. The O-H region of water in carbon tetrachloride at -5 °C from 1 to 55 atm. (Pressure is adjusted by addition of Ar.) (a) Presence of propane results in emergence of a resonance between the symmetric and asymmetric stretches of water, magnified in the inset. This resonance is characteristic of a d-OH due to water acting as a hydrogen donor. (b) Neat water shows no d-OH resonance. The near constancy of the spectra indicates no self-association or cluster formation with pressure for neat water.
Figure 5. Absorption spectra of propane (blue) and propane-water (red) in CCl4 at -5 °C and 1 atm. Note the altered peaks at 2930 and 2960 cm-1.
Figure 6. Deuterated propane in carbon tetrachloride at -5 °C from 1 to 55 atm. (Pressure is adjusted by addition of Ar.) Main spectrum: The C-H absorption modes of propane deuterated at the methyl positions with and without water. Inset: The C-D region shows no structural change with water.
symmetric and the asymmetric stretches indicating that the two O-H stretches of water are decoupled. Since the resonance occurs right between the symmetric and asymmetric frequencies, it is concluded that one O-H bond is pinned due to H-bond donation and the other is free. The 3643 cm-1 peak is thus assigned to the dangling O-H, d-OH, stretch of H-donor water. To gain insight into the site of the water-propane interaction, a density functional computation at the B3LYP 6-31G* level
Figure 7. DFT-calculated infrared spectrum of propane in the presence of a water dimer. The calculated frequency was corrected by 0.956.
of theory was performed to calculate the propane-water dimer infrared spectrum for various configurations. Interaction at the C(2) position gives good agreement between the simulated spectrum (Figure 7) and the experimental spectrum (Figure 5). The energy of the system with interaction at the methylene group is calculated to be 460 J/mol more stable than that of water interacting at the methyl groups. Note that the normal modes of propane are collective modes involving motion of all C-H bonds. Thus to experimentally test the DFT picture, propane-water samples were prepared with deuterated methyl groups: CD3CH2CD3. Due to the large mass difference between deuterium and hydrogen, the methyl modes are decoupled from those of the methylene. Accordingly, methyl-deuterated propane in carbon tetrachloride (Figure 6) exhibits two sets of bands: the methylene modes around 2900 cm-1 and deuterated methyl modes from 2000 to 2250 cm-1. The methyl-d3 modes are unaffected by addition of water. In contrast, the two methylene modes, the CH2 symmetric stretch at 2870 cm-1, and the CH2 asymmetric stretch centered around 2930 cm-1 are joined by a third feature at around 2960 cm-1. The unaffected CD3 modes along with the new C-H mode suggest an interaction between water and the methylene C-H bonds. It could be argued that substitution of deuterium at the C(1) and C(3) methyl groups biases the water interaction due to the zero-point energy difference between a C-H and a C-D stretch. However, zero-point energy favors deuterium bonding over hydrogen bonding. For example, at the ice surface, a dangling OH is favored over a dangling OD.33 For water dimers
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in Kr matrices, D donation is favored over H donation by 60 cm-1.34 Thus, deuterating the methyl groups biases interaction toward those sites. Observation that water overcomes the bias in favor of deuterium to interact with the methylene hydrogen ensures it to be the site of interaction in C3H8 propane. Interaction at the methylene hydrogen is supported by a recent computer modeling35 of solvation free energies for nonpolar solutes in water. Modeling results found the center regions of alkanes form interaction “hot spots” for first-shell water molecules. In contrast, interactions at the methyl end-groups were less favorable. Note that the new methylene resonance that emerges upon interacting with water appears to the blue of the asymmetric stretch;a blue shift on interaction. There is precedence for such a blue shift due to a C-H · · · water interaction.36-38 Increased understanding of weak interactions such as that in C-H · · · O has resulted in an expansion of the classical definition of a hydrogen bond that normally results in a red shift. Specifically, interactions between the oxygen of water and an sp3 carbon transfer electron density from the oxygen lone pairs into the LUMO.36-38 If the LUMO is antibonding, the sp3 hybridization is modified toward sp2 hybridization. The resulting increased s content strengthens the C-H bond, hence produces a blue shift. Since a methyl group generally lacks the requisite antibonding LUMO to accept electron density, the blue shift of the C-H resonance by water in deuterated propane strongly supports the methylene C(2)-H as the site of interaction. Increasing pressure results in further blue shifting of the donor C-H without any structural changes in the C-D region (Figure 6). This suggests that additional water molecules either interact with the remaining methylene hydrogen or H-bond with the water already present. If additional water attached to the remaining methylene hydrogen atom, new resonances would appear and replace the lower frequency C-H stretches in contradiction to observation. It is thus concluded that additional water molecules hydrogen bond to water interacting with the methylene hydrogen atom. The configuration of the added water molecules is likely highly flexible, hence the oscillator strength is spread over tens or hundreds of cm-1 rather than producing distinct resonances like those shown in Figure 3. Conclusions As part of an ongoing effort to unravel the interactions leading to clathrate hydrate formation, the water-propane interaction is studied with infrared spectroscopy. Carbon tetrachloride is chosen as the isolation medium as it enables control and monitoring of both water and propane. The results show that, in the presence of propane, water molecules form an initial cluster: the first step to nucleate the crystal. Examination of the propane vibrational stretches by isotopic substitution and DFT calculations support diagnosis of a van der Waals attraction between the oxygen of water and a methylene hydrogen of propane. A picture consistent with the data is that at a pressure of 1 atm, some propane molecules associate with water while some are free. The reduction of free water indicates that, on average, there is one water molecule associated with each propane molecule. The spectrum of hydrogen-bonded water suggests that some propane molecules are associated with multiple water molecules while some still remain unassociated. As pressure increases, more and more propane molecules pick up water molecules, resulting in an average distribution of two or three water molecules per propane molecule at 55 atm.
Vu et al. Acknowledgment. The authors acknowledge support from the donors of the Petroleum Research Fund, administered by the American Chemical Society, grant no. 46671-AC6. T.H.V. also acknowledges the U.S. Department of Education for the GAANN fellowship. References and Notes (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Dartois, E.; Deboffle, D. Astron. Astrophys. 2008, 490, L19–L22. (3) Haq, B. U. Science 1999, 285, 543–544. (4) Max, M. D. Natural Gas Hydrate: In Oceanic and Permafrost EnVironments; Kluwer Academic Publishers: Boston, MA, 2000. (5) Yasuda, K.; Takeya, S.; Sakashita, M.; Yamawaki, H.; Ohmura, R. J. Phys. Chem. C 2009, 113, 12598–12601. (6) Buchanan, P.; Aldiwan, N.; Soper, A. K.; Creek, J. L.; Koh, C. A. Chem. Phys. Lett. 2005, 415, 89–93. (7) Cady, G. H. J. Phys. Chem. 1983, 87, 4437–4441. (8) Allison, S. A.; Barrer, R. M. Trans. Faraday Soc. 1968, 64, 549– 556. (9) Allison, S. A.; Barrer, R. M. Trans. Faraday Soc. 1968, 64, 557– 563. (10) Barrer, R. M.; Edge, A. V. J. Proc. R. Soc. London, Ser. A 1967, 300, 1–9. (11) Whitman, C. A.; Mysyk, R.; White, M. A. J. Chem. Phys. 2008, 129, 174502. (12) Lee, J. D.; Song, M.; Susilo, R.; Englezos, P. Cryst. Growth Des. 2006, 6, 1428–1439. (13) Peng, B. Z.; Dandekar, A.; Sun, C. Y.; Luo, H.; Ma, Q. L.; Pang, W. X.; Chen, G. J. J. Phys. Chem. B 2007, 111, 12485–12493. (14) Murshed, M. M.; Kuhs, W. F. J. Phys. Chem. B 2009, 113, 5172– 5180. (15) Wang, W.; Bray, C. L.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 11608–11609. (16) Smelik, E. A.; King, H. E., Jr. Am. Mineral. 1997, 82, 88–98. (17) Thompson, H.; Soper, A. K.; Buchanan, P.; Aldiwan, N.; Creek, J. L.; Koh, C. A. J. Chem. Phys. 2006, 124, 164508. (18) Kini, R. A.; Dec, S. F.; Sloan, E. D. J. Phys. Chem. A 2004, 108, 9550–9556. (19) Moudrakovski, I. L.; Sanchez, A. A.; Ratcliffe, C. I.; Ripmeester, J. A. J. Phys. Chem. B 2001, 105, 12338–12347. (20) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T. J. Phys. Chem. B 2003, 107, 10299–10311. (21) Wang, X.; Schultz, A. J.; Halpern, Y. J. Phys. Chem. A 2002, 106, 7304–7309. (22) Mayer, E.; Hallbrucker, A. J. Chem. Soc., Chem. Commun. 1989, 12, 749–751. (23) Raghavendra, B.; Arunan, E. Chem. Phys. Lett. 2008, 467, 37–40. (24) Jiang, H.; Jordan, K. D.; Taylor, C. E. J. Phys. Chem. B 2007, 111, 6486–6492. (25) McCarthy, V. N.; Jordan, K. D. Chem. Phys. Lett. 2006, 429, 166– 168. (26) Prado, M. P.; Pham, A.; Ferazzi, R. E.; Edwards, K.; Janda, K. C. J. Chem. Educ. 2007, 84, 1790–1791. (27) Kuo, M. H.; David, A.; Kamelamela, N.; White, N.; Shultz, M. J. J. Phys. Chem. C 2007, 111, 8827–8831. (28) Kuo, M. H.; Kamelamela, N.; Shultz, M. J. J. Phys. Chem. A 2008, 112, 1214–1218. (29) Danten, Y.; Tassaing, T.; Besard, M. J. Phys. Chem. A 2000, 104, 9415–9427. (30) Spartan ′08, v. 1.1.1; Wavefunction Inc.: Irvine, CA, 2008. (31) Corcelli, S. A.; Skinner, J. L. J. Phys. Chem. A 2005, 109, 6154– 6165. (32) Eaves, J.; Tokmakoff, A.; Geissler, P. J. Phys. Chem. A 2005, 109, 9424–9436. (33) Devlin, J. P. J. Chem. Phys. 2000, 112, 5527–5529. (34) Engdahl, A.; Nelander, B. J. Chem. Phys. 1987, 86, 1819–1823. (35) Fennell, C. J.; Kehoe, C.; Dill, K. A. J. Am. Chem. Soc. 2010, 132, 234–240. (36) Gu, Y.; Kar, T.; Scheiner, S. J. Am. Chem. Soc. 1999, 121, 9411– 9422. (37) Hobza, P.; Havlas, Z. Chem. ReV. 2000, 121, 4253–4264. (38) Masunov, A.; Dannenberg, J. J.; Contreras, R. H. J. Phys. Chem. A 2001, 105, 4737–4740.
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