Site-Selective Solvation in Supercritical CO2 Observed by Raman

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Site-Selective Solvation in Supercritical CO2 Observed by Raman Spectroscopy: Phenyl Group Leads to Greater Attractive Energy than Chloro Group Daisuke Kajiya† and Ken-ichi Saitow*,†,‡ Natural Science Center for Basic Research and DeVelopment (N-BARD) and Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan ReceiVed: August 18, 2010; ReVised Manuscript ReceiVed: NoVember 2, 2010

Vibrational Raman spectra of the CdC stretching modes of cis-stilbene and cis-1,2-dichloroethylene (C2H2Cl2) were measured in supercritical CO2 in a density range of 0.08 < Fr ) F/Fc < 1.5 at an isotherm of Tr ) T/Tc ) 1.02. As the fluid density increased, the peak frequencies of cis-stilbene and cis-C2H2Cl2 shifted toward the low-energy side. The shifted frequencies of cis-stilbene were consistently greater than those of cis-C2H2Cl2 in all density regions, by a factor of 4. By analyzing these density dependencies using the perturbed hardsphere theory, the shifted frequencies were decomposed into attractive and repulsive components. By quantifying these components as a function of fluid density, we investigated how each solute is solvated in supercritical CO2. The results indicate that the attractive energy between cis-stilbene and CO2 is twice that between cisC2H2Cl2 and CO2. A local density augmentation around the solute molecule was not observed in the cisC2H2Cl2/CO2 system, but it was observed in the cis-stilbene/CO2 system because of site-selective solvation around the phenyl group of cis-stilbene. To the best of our knowledge, this is the first time that the siteselective solvation of a solute molecule has been observed using Raman spectral measurements of a solution system. Based on theoretical calculations and Raman spectral measurements of cis-stilbene in the supercritical fluid of dipolar CHF3, it is concluded that a driving force for site-selective solvation is the dispersion force. Introduction The temperature of a supercritical fluid is above the critical point, inhibiting the vapor-liquid phase transition. In the absence of such a transition, it is possible to “fine tune” the density of the fluid across a continuum, from gaslike to liquidlike. Using this property, intermolecular interactions in supercritical fluids have been investigated through experiments and simulations over a wide range of densities,1-4 especially through spectroscopic studies.5-13 Raman scattering measurements enable the short-range structure around a vibrating molecule to be probed,14-16 and the vibrational Raman spectra of neat supercritical fluids have been measured to examine the local structures of supercritical fluids.17-29 Supercritical solutions have also been studied by Raman spectroscopy, and solutesolvent interactions have been investigated in various systems.30-42 In our previous studies, we investigated neat supercritical fluid structures using dynamic light scattering,43-46 terahertz absorption,47,48 vibrational Raman spectroscopy,49-53 and small-angle X-ray scattering.54 These studies were then extended to nanoparticle generation in supercritical fluids.55-58 Recently, vibrational Raman spectroscopy has been used to examine the density dependencies of the attractive and repulsive interactions between solute and solvent molecules in supercritical solutions.59-61 By measuring the Raman spectra of CdC stretching modes of the cis and trans isomers of 1,2-dichloroethylene (C2H2Cl2) in various supercritical * Corresponding author. Address: Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan. Phone and fax: +81-82424-7487. E-mail: [email protected]. † Natural Science Center for Basic Research and Development (N-BARD). ‡ Department of Chemistry, Graduate School of Science.

fluids, such as CO2, CHF3, SF6, and Xe, the attractive energy of each isomer was quantified. Note that supercritical Xe, as a “nonpolar fluid”, demonstrates the greatest attractive energy of these fluids.61 This phenomenon was ascribed to the charge-transfer state between the solute and Xe. It was also found that the attractive energy of the “nonpolar” trans isomer was greater than that of the “polar” cis isomer.59 In this case, it was determined that the configurations and orientations of solvent CO2 molecules around the solute generated a greater attractive energy with a nonpolar solute than with a polar solute in supercritical CO2.60 The detailed solvation structures of supercritical CO2 around solute molecules in numerous systems, including those of aromatic compounds, have been investigated and clarified by various researchers.30,37,62-72 A large negative value of partial molar volume was observed in naphthalene that had been dissolved in supercritical CO2,62 which has been interpreted from a macroscopic or thermodynamic approach viewpoint as CO2 solvation around aromatic compounds by attractive forces.30,37,63-66 From a microscopic viewpoint, it was also reported that CO2 lies perpendicular to the aromatic ring.30,63-66 Molecular dynamics simulation elucidated that there is an attractive energy between CO2 and the oxygen or nitrogen atoms in a solute molecule as a Lewis acid/base type of interaction.67,68 In the present study, we investigated site-selective solvation in supercritical CO2 from the microscopic viewpoint, in which the “attractive” and “repulsive” interactions between solute and solvent CO2 molecules were investigated by analyzing the vibrational Raman spectra of solute molecules with different functional groups. By changing the solvent density under isothermal conditions, solvation structures can be evaluated by considering electronic interactions. To explore these issues, two solutes were chosen as probe molecules, namely, cis-stilbene

10.1021/jp107820j  2010 American Chemical Society Published on Web 11/29/2010

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Figure 1. Molecular structures of cis-stilbene (left) and cis-C2H2Cl2 (right).

and cis-C2H2Cl2 (Figure 1). The Raman spectra of the CdC stretching modes of each solute molecule were measured in supercritical CO2 and analyzed using the perturbed hard-sphere theory. The results showed that the attractive energy of the cisstilbene/CO2 system was twice that of the cis-C2H2Cl2/CO2 system, and local density augmentation was observed only in the cis-stilbene/CO2 system. The greater attractive energy observed in the cis-stilbene/CO2 system was ascribed to siteselective solvation around the phenyl group. To the best of our knowledge, this is the first time that the site-selective solvation of a solute molecule has been observed by Raman spectral measurements. By comparing our results to both the theoretical calculations and the Raman spectral measurements of cisstilbene in the supercritical fluid of dipolar CHF3, we confirmed that a driving force for site-selective solvation is the dispersion force.

Figure 2. Typical examples of Raman spectra of CdC stretching modes of cis-stilbene in supercritical CO2 at Tr ) 1.02. Solid lines are curve fits using the Lorentzian functions. The term Fr is the reduced density represented by Fr ) F/Fc.

Experimental Section Raman spectra of the supercritical fluids were measured using a previously described instrument.52,59 In brief, the light source was a diode-pumped solid-state laser operated with an excitation wavelength of 532 nm, at a single frequency output of 200 mW, in front of an optical cell. The laser was incident on the cell, and a camera lens collected the scattered light at an angle of 90°. Using a monochromator and a photomultiplier tube, Raman spectra were recorded using the photon counting method. The frequency of each Raman spectrum was calibrated by that of the exciting laser. The frequency repeatability of the present spectrometer was confirmed to be within (0.02 cm-1 over a 4-h period. The high-frequency stability enabled very precise peak position measurements in the Raman spectra. The obtained spectra were accumulated in 15 min at high CO2 density or in 45 min at low density. Although laser irradiation over a 45min period seems extensive, it was confirmed that spectral shapes of Raman spectra and UV/vis absorption spectra did not change over a 9-h period. Supercritical solutions were prepared as follows: An accurate volume of cis-stilbene or cis-C2H2Cl2 was introduced into a highpressure container. The container was then filled with highpressure CO2 by measuring the weight of the container during CO2 injection. CO2 was injected until the solute mole fraction was 0.01. To increase the optical purity, the solution was filtered through a polytetrafluoroethylene (PTFE) membrane filter with a pore size of 0.1 µm. The high-pressure solution generated in the container was then transferred into a Raman optical cell, and the densities of the supercritical solutions were adjusted by releasing the high-pressure supercritical solution from the Raman cell. The temperature was maintained at an isotherm of reduced temperatures, Tr ) T/Tc ) 1.02, by using a setup comprising a proportional-integral-derivative (PID) controller, heaters, and a thermocouple. The pressure was monitored using a strain gauge backed up with a strain amplifier. Both temperature and pressure fluctuations were within the range of (0.1% during the measurements. Densities were calculated from the empirical state equation by using P and T values.73 The critical constants

Figure 3. Density dependencies of shifts in peak frequencies for Raman bands of the CdC stretching modes of cis-stilbene (red solid circles) and cis-C2H2Cl2 (blue open circles) (from ref 60) in supercritical CO2. Dotted lines are fitting curves of a polynomial function as a visual guide. The quantity of Fr is expressed as Fr ) F/Fc.

of CO2 are reported to be Tc ) 304.1 K, Pc ) 7.4 MPa, and Fc ) 0.468 g cm-3.73 Results Figure 2 shows Raman spectra of the CdC stretching modes of cis-stilbene in supercritical CO2. The band at around 1635 cm-1 has been assigned to the CdC stretching mode.74 The spectral shapes did not change even when the fluid density changed. The spectra were analyzed using Lorentzian functions, and peak frequencies ν of CdC stretching modes were obtained. Figure 3 shows peak frequency shifts ∆ν as functions of fluid density, that is, ∆ν ) ν - ν0, where ν0 is the frequency obtained from the extrapolation of the peak frequencies to zero density. As the density of the supercritical CO2 increased, the peak frequencies of cis-stilbene shifted significantly toward the lower energy side, whereas the peak frequencies of the cis-C2H2Cl2 solute molecule shifted only slightly toward the lower-energy side, as reported previously.61 Note that a significant difference was observed in the magnitude of ∆ν of both solute molecules. That is, the ∆ν value of cis-stilbene was about 4 times greater

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than that of cis-C2H2Cl2. Also, cis-stilbene and cis-C2H2Cl2 have two phenyl groups and two chloro groups, respectively. Thus, the solute molecule having a phenyl group showed a frequency shift 4 times greater than the solute molecule having a chloro group. To quantify the above-mentioned functional-group effect, we analyzed the spectra using the perturbed hard-sphere theory,75,76 in which the solvent and solute molecules are regarded as a sphere and a pseudodiatomic molecule, respectively, consisting of two hard-sphere atoms. Using this theory, the shifted amounts of CdC stretching modes were decomposed into attractive and repulsive components. According to the perturbed hard-sphere theory, ∆ν consists of the sum of attractive and repulsive energies between solute and solvent molecules and is represented as75,76

∆ν ) ∆νR + ∆νA

(1)

TABLE 1: Molecular Parameters for Calculation of Repulsive Shifts parameter

cis-C2H2Cl2a

cis-stilbene

f (N/cm) g (N/cm2 × 109) ν0 (cm-1) GR/FR ) 1/4L (nm-1) L ) 0.0571σave (nm) σave ) (σ + σS)/2 (nm) σ (nm)g σS (nm) re (nm) m1 m2 m3 b1 b2 b3

7.65 -1.468 1592.7 10.9 0.023 0.40 0.40 0.40 0.133 2.66 4.41 4.37 -0.61 -1.62 -2.51

8.19b -1.392c 1639.5d 9.15e 0.027e 0.48f 0.55f 0.40f 0.133b 2.51e 4.40e 4.38e -0.03e -0.63e -1.53e

a

where ∆νR and ∆νA are the repulsive and attractive frequency shifts, respectively. The value of ∆νA was empirically derived by subtracting ∆νR from the experimentally obtained ∆ν value. The repulsive shift ∆νR is expressed as76

Reference 60. b Reference 74. c Reference 77. d Frequency obtained from extrapolation of the peak frequencies to density ) 0. e Reference 76. f Reference 78. g cis-Stilbene and cis-C2H2Cl2 are regarded as pseudodiatomic molecules consisting of two hard-sphere atoms of diameter σ of CH(C6H5) and CHCl, respectively.

∆νR ) C1 exp(m1F*) + C2 exp(m2F*) - C3 exp(m3F*) ν0 (2) with

Ck ) kRRθz(1 - z)k-1 exp bk

(3)

and RR ) re[-(3g/2f) + (GR/FR)], where mk and bk refer to empirical parameters, which depend on the sizes of solute and solvent molecules. The quantity F* is represented by F* ) FSσS3, where FS is the number density and σS is the diameter of the solvent hard sphere. The quantity θ is given by θ ) kBT/fre2, where kB is the Boltzmann constant and re is the equilibrium bond length of the CdC bond. The value of z is expressed as z ) re/σ with two hard-sphere cavities of diameter σ. The values f and g are the intramolecular quadratic harmonic and cubic anharmonic force constants, respectively, of the CdC bond, and FR and GR are the linear and quadratic constants, respectively, indicating the forces that solvent molecules exert along the normal coordinate of the solute. All parameters required for the calculations are listed in Table 1. Figure 4a shows the repulsive shifts ∆νR of cis-stilbene and cis-C2H2Cl2, in which ∆νR increased with increased fluid density. This means that the increase in density causes an increase in repulsive energies for the CdC stretching modes exerted by solute-solvent interactions. Comparing the magnitudes of ∆νR for the two molecules, that of cis-stilbene was 1.2 times greater than that of cis-C2H2Cl2 at the same reduced density, due to the fact that the molecular size of cis-stilbene is larger than that of cis-C2H2Cl2. In other words, molecular size determined the repulsive-shifted amount, according to the relation between ∆νR and bk involved in eqs 2 and 3.76 Table 1 lists values of the molecular size and bk. Figure 4b shows the attractive shifts ∆νA of cis-stilbene and cis-C2H2Cl2. It is observed that the attractive shifts of both solute molecules increase with increasing solvent density. Comparing the attractive shifts of cis-stilbene with those of cis-C2H2Cl2 at the same density, the attractive shifts of cisstilbene were twice those of cis-C2H2Cl2. That is, the frequency

Figure 4. (a) Calculated repulsive shifts and (b) attractive shifts empirically obtained from the Raman spectral measurements of CdC stretching modes of cis-stilbene (red solid circles) and cis-C2H2Cl2 (blue open circles) in supercritical CO2. Dotted lines are linear functions. The quantity of Fr is expressed as Fr ) F/Fc.

shifts of CdC stretching modes due to attractive energies between the solute and solvent molecules were greater in the cis-stilbene/CO2 system than in the cis-C2H2Cl2/CO2, by a factor of 2. To evaluate the attractive and repulsive interactions between the solute and solvent molecules, we obtained the ratio of the attractive shift ∆νA to the repulsive shift ∆νR by using the values in Figure 4. Figure 5 shows the ratio, |∆νA/∆νR|, as a function of density, and the ratio is greater than 1 for both cis-stilbene and cis-C2H2Cl2. Differences in the values of both systems reached a maximum at around Fr ) 0.5, and the value for cisstilbene was 2.5 times greater than that for cis-C2H2Cl2. Accordingly, the solute molecule having a phenyl group showed a greater attractive energy than the solute molecule having a chloro group. The phenyl group was solvated preferably by CO2 molecules, and a site-selective solvation occurred.

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Figure 5. Ratios of attractive and repulsive shifts for cis-stilbene (red solid circles) and cis-C2H2Cl2 (blue open circles). The ratio was obtained by dividing the attractive shifts by the repulsive shifts. The quantity Fr is expressed as Fr ) F/Fc.

Discussion The nonlinear density dependence of ∆νA for cis-stilbene is shown in the medium-density region in Figure 4b, whereas this nonlinear feature is not observed for cis-C2H2Cl2. According to numerous studies on supercritical fluids, this nonlinear density dependence has been ascribed to the difference between bulk and local density. Local density augmentation (LDA) allows for a quantitative exploration of how much of the density is locally excess relative to the bulk density.79-82 Here, the LDA is defined as (Flocal - F)/Fc,83 where Flocal and F represent the local solvent density around a solute molecule and the average bulk density, respectively. The LDA value was obtained from Figure 6a by subtracting the density given by the measurement (F/Fc) from the density given by the linear fit (Flocal/Fc) at the same ∆νA/ν0 value corresponding to the LDA.84 The obtained LDA is shown in Figure 6b. We observe only the LDA for cis-stilbene, because only cis-stilbene exhibits a nonlinear feature. Figure 6b shows the LDA of cis-stilbene as a function of the density of supercritical CO2. The value reaches a maximum at around Fr ) 0.5, implying that the increase in the local density of CO2 around cis-stilbene becomes most significant at around Fr ) 0.5. In addition, the profile of the density dependence of LDA is in good agreement with that of the |∆νA/ ∆νR| in Figure 5. Thus, the strong attractive energy between the solute and CO2 causes the augmentation of the local density around the solute molecule having the phenyl functional group. According to an infrared spectroscopic study,70 the LDA was observed in the solute benzene dissolved in the supercritical CO2, and its value is reproduced in Figure 6c. Note that the profile of benzene is in good agreement with that of cis-stilbene in Figure 6b, in that LDA reaches the maximum at around Fr ) 0.5 in both systems. The LDA values of the solute cis-stilbene and benzene at Fr ) 0.5 are 0.3 in Figure 6b and 0.4 in Figure 6c, respectively.85 To further explore the strong interaction around the phenyl group, we carried out the Raman spectral measurements of cisstilbene in supercritical CHF3, and the obtained data were analyzed by the perturbed hard-sphere theory. According to our calculations in a previous study,61 the magnitude of the dispersion force of a CHF3 molecule is similar to that of a CO2 molecule. That is, the difference in dispersion forces in supercritical CHF3 and CO2 is less than 20% at the thermodynamic conditions.61 However, the magnitudes of the dipole moments of CHF3 and CO2 are significantly different: µ(CHF3) ) 1.65 D and µ(CO2) ) 0 D.86 Comparing the attractive shifts ∆νA of cis-stilbene in supercritical CHF3 and CO287 (Figure 7), we find that the values in supercritical CHF3 are the same as those in supercritical CO2, confirming that the attractive interaction of

Figure 6. (a) Relationship between the density (F/Fc) at measurement and density (Flocal/Fc) on the linear fit. The local density augmentation was obtained by subtracting F/Fc from Flocal/Fc at the same value of ∆νA/ν0. (b) Local density augmentation of cis-stilbene/CO2. (c) Local density augmentation of benzene/CO2 reproduced from an infrared spectroscopic study (from ref 70). The quantity Fr is expressed as Fr ) F/Fc.

Figure 7. Attractive shifts of cis-stilbene in supercritical CO2 (red solid circles) and supercritical CHF3 (blue solid circles).

cis-stilbene in CHF3 is due not to the dipole interaction but to the dispersion interaction. This means that the dispersion force is very important for the attractive interaction of cis-stilbene in supercritical CHF3 as well as in CO2. A similar phenomenon was recently reported in MD simulation of supercritical CO2. That is, the attractive interaction between the aromatic ring and the CO2 molecule is composed of 99% dispersion interaction (van der Waals energy) and 1% Coulomb interaction (electrostatic energy).69 To evaluate the dispersion force in the present systems, we calculated the dispersion term in the attractive interaction between solute and solvent molecules in the equation

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V(r) ) -C/r6 ) -3[R0RS(I0IS)/(I0 + IS)]/2r6

Kajiya and Saitow

(4)

where C is a coefficient depending on the solute and solvent molecules; r is the distance between the molecules; and R and I are the molecular polarizability and ionization energy, respectively. Their values are listed in Table 2. Subscripts 0 and S represent solute and solvent molecules, respectively. The calculated values of C for the cis-stilbene/CO2 system and the cis-C2H2Cl2/CO2 system were 5.9 × 10-77 and 2.9 × 10-77 J m-6, respectively, confirming that the dispersion energy for cis-stilbene is greater than that of cis-C2H2Cl2 by a factor of 2. This calculated result is consistent with the experimental observation of the greater attractive energy of cis-stilbene, as shown in Figure 4b. In conclusion, we found that the great dispersion energy of cis-stilbene becomes a driving force of CO2 solvation around the phenyl group and causes site-selective solvation in the supercritical state. Finally, we present a brief introduction of recent studies on solvation in supercritical CO2. In studies on molecular dynamics simulations,67-69 abinitiocalculations,71 andinfraredspectroscopy,72,90 the solvation of CO2 at the oxygen and nitrogen atoms of solute molecules is reported. To describe these site-selective solvations, it was considered that a solvent-solute Lewis acid/base complex is formed by attractive interaction between δ+ charge on carbon atom of solvent CO2 and δ- charge on oxygen atom of solute molecule.71 If this scenario were applied to the present system, CO2 solvation would occur significantly at the chlorine atom of cis-C2H2Cl2. However, the experimental data show a greater attractive interaction at the phenyl group rather than the chloro group, as shown in Figure 5. This result revealed that the solvation of the phenyl group rather than the chloro group is preferable for the CO2 molecule. A similar result was recently reported from the ab initio calculation for chlorobenzene in supercritical CO2.42 The result indicates that the site-selective solvation of CO2 molecule is not observed at the chloro group but at the phenyl group of chlorobenzene. This theoretical result is consistent with our experimental observation, that preferable solvation occurs at the phenyl group rather than at the chloro group. Conclusions Vibrational Raman spectroscopy of the CdC stretching modes of both cis-stilbene and cis-C2H2Cl2 was performed in supercritical CO2 at an isotherm of Tr ) 1.02 over the range of densities 0.08 < Fr < 1.5. The density dependencies of peak frequencies were analyzed by the perturbed hard-sphere theory, and the attractive and repulsive shifts were obtained. As a result, we conclude the following: (i) The peak frequencies of the CdC stretching modes of both cis-stilbene and cis-C2H2Cl2 shift toward the low-energy side as the density increases. (ii) The frequency shifts of cis-stilbene are 4 times those of cis-C2H2Cl2. (iii) The repulsive shifts of cis-stilbene are 1.2 times those of cis-C2H2Cl2. (iv) The attractive shifts of cis-stilbene are twice those of cis-C2H2Cl2. (v) The greater attractive shifts observed for cis-stilbene are ascribed to site-selective solvation around the phenyl group, and the site-selective solvation arises from TABLE 2: Molecular Polarizability and Ionization Energy

a

molecule

R (nm3 × 10-3)

I (J × 10-18)

cis-stilbene cis-C2H2Cl2 CO2

18.7a 8.03c 2.65b

1.25b 1.55b 2.21b

Reference 88. b Reference 86. c Reference 89.

the dispersion force. (vi) The greater dispersion force generates the local density augmentation of solvent CO2 around the phenyl group of cis-stilbene. The profile of density dependence is in good agreement with that of benzene in supercritical CO2. Acknowledgment. K.S. acknowledges “Structure Control and Function” of PRESTO of the Japan Science and Technology agency (JST). PRESTO substantially supported this research. This study was also supported by a Grant-in-Aid for Young Scientists (B) (13740321) and Young Scientists (A) (16685001) from the Ministry of Education, Science and Culture of Japan and by the Sumitomo Foundation Award for Young Researchers. References and Notes (1) Supercritical Fluids: Fundamentals and Applications; Kiran, E., Debenedetti, P. G., Peters, C. J., Eds.; NATO ASI Science Series E, Applied Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; Vol. 366. (2) Supercritical Fluids: Molecular Interactions, Physical Properties, and New Applications; Arai, Y., Sako, T., Takebayashi, Y., Eds.; Springer: Berlin, 2002. (3) Kajimoto, O. Chem. ReV. 1999, 99, 355. (4) Tucker, S. C. Chem. ReV. 1999, 99, 391. (5) Kim, S.; Johnston, K. P. Ind. Eng. Chem. Res. 1987, 26, 1206. (6) Kajimoto, O.; Futakami, M.; Kobayashi, T.; Yamasaki, K. J. Phys. Chem. 1988, 92, 1347. (7) Sun, Y.-P.; Fox, M. A.; Johnston, K. P. J. Am. Chem. Soc. 1992, 114, 1187. (8) Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M. J. Chem. Phys. 1996, 105, 3121. (9) Zhang, J.; Roek, D. P.; Chateauneuf, J. E.; Brennecke, J. F. J. Am. Chem. Soc. 1997, 119, 9980. (10) Myers, D. J.; Shigeiwa, M.; Fayer, M. D.; Cherayil, B. J. J. Phys. Chem. B 2000, 104, 2402. (11) Bourne, R. A.; Han, X.; Chapman, A. O.; Arrowsmith, N. J.; Kawanami, H.; Poliakoff, M.; George, M. W. Chem. Commun. 2008, 37, 4457. (12) Kimura, Y.; Yamamoto, Y.; Fujiwara, H.; Terazima, M. J. Chem. Phys. 2005, 123, 054512. (13) Andanson, J.-M.; Jutz, F.; Baiker, A. J. Phys. Chem. B 2009, 113, 10249. (14) Buckingham, A. D. Proc. R. Soc. A 1958, 248, 169. 1960, 255, 32. (15) Oxtoby, D. W. AdV. Chem. Phys. 1979, 40, 1. Oxtoby, D. W. Annu. ReV. Phys. Chem. 1981, 32, 77. (16) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry; Wiley-VCH: Weinheim, Germany, 2003. (17) Garrabos, Y.; Tufeu, R.; Neindre, B. L.; Zalczer, G.; Beysens, D. J. Chem. Phys. 1980, 72, 4637. (18) Wood, K. A.; Strauss, H. L. J. Chem. Phys. 1983, 78, 3455. (19) Clouter, M. J.; Kiefte, H.; Deacon, C. G. Phys. ReV. A 1986, 33, 2749. (20) Ben-Amotz, D.; LaPlant, F.; Shea, D.; Gardecki, J.; List, D. In Supercritical Fluids Technology; ACS Symposium Series 488; Bright, F. V., McNanlly, M. E., Eds.; American Chemical Society: Washington, DC, 1992; p 18. (21) Okazaki, S.; Matsumoto, M.; Okada, I.; Maeda, K.; Kataoka, Y. J. Chem. Phys. 1995, 103, 8594. (22) Musso, M.; Asenbaum, A.; Keutel, D.; Seifert, F.; Oehme, K.-L. Phys. ReV. Lett. 1996, 77, 2746. (23) Ikushima, Y.; Hatakeda, K.; Saito, N.; Arai, M. J. Chem. Phys. 1998, 108, 5855. (24) Musso, M.; Matthai, F.; Keutel, D.; Oehme, K.-L. J. Chem. Phys. 2002, 116, 8015. (25) Lalanne, P.; Andanson, J. M.; Soetens, J.-C.; Tassaing, T.; Danten, Y.; Bersard, M. J. Phys. Chem. A 2004, 108, 3902. (26) Andanson, J.-M.; Soetens, J.-C.; Tassaing, T.; Besnard, M. J. Chem. Phys. 2005, 122, 174512. (27) Cabaco, M. I.; Besnard, M.; Tassaing, T.; Danten, Y. J. Mol. Liq. 2006, 125, 100. (28) Cabaco, M. I.; Longelin, S.; Danten, Y.; Besnard, M. J. Phys. Chem. A 2007, 111, 12966. (29) Yasaka, Y.; Kubo, M.; Matubayasi, N.; Nakahara, M. Bull. Chem. Soc. Jpn. 2007, 80, 1764. (30) Zerda, T. W.; Song, X.; Jonas, J. Appl. Spectrosc. 1986, 40, 1194. (31) Akimoto, S.; Kajimoto, O. Chem. Phys. Lett. 1993, 209, 263. (32) Devendorf, G. S.; Ben-Amotz, D.; de Souza, L. E. S. J. Chem. Phys. 1996, 104, 3479.

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