Solutions in supercritical fluids

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

Fluorescence Investigation of Cosolvent—Solute Interactions in Supercritical Fluid Solutions... 

Angus, S. Armstrong, B. de Reuck, K. M. eds., International Thermodynamic Tables of the Fluid State Carbon Dioxide. Pergamon Press, Oxford, 1976. Brennecke, J. F. Intermolecular Interactions in Supercritical Fluid Solutions from Fluorescence Spectroscopy. PhD Thesis, University of Illinois, Urbana, 1989. Brennecke, J. F. Eckert, C. A. AIChE J.. 1989, 35(9), 1409-1427. 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

The effect of cyclopentadienyl-ring substituents on the reactivity of the group VII half-sandwich complexes (t7 -C5R5)M(CO)2L[M = Mn and Re R = H, Me, and Et (Mn only) L = Kr and Xe] toward CO in supercritical fluid solution at room temperature has been investigated (70). The reactivity of the corresponding alkane complexes ( 7 -C5R5)Mn(CO)2(n-heptane) (R = H, Me, and Et) toward small molecules such as CO, N2, and H2 in n-heptane solution steadily increased in the order H < Me < Et (71). These results indicated that steric rather... 

There have been a number of modeling efforts that employ the concept of clustering in supercritical fluid solutions. Debenedetti (22) has used a fluctuation analysis to estimate what might be described as a cluster size or aggregation number from the solute infinite dilution partial molar volumes. These calculations indicate the possible formation of very large clusters in the region of highest solvent compressibility, which is near the critical point. Recently, Lee and coworkers have calculated pair correlation functions of solutes in supercritical fluid solutions ( ). Their results are also consistent with the cluster theory. 

Supercritical chemistry a review of chemical reactions in supercritical fluids solutions [63]... 

Poliakoff, M. Howdle, S.M. Supercritical chemistry a review of chemical reactions in supercritical fluids solutions. In Third International Symposium on Supercritical Fluids, International Society for the Advancement of Supercritical Fluids Nottingham, UK, 1999. 

Often, the unique and unusual molecular structure in SCF solutions profoundly affects the reactivity in these systems. Thus, the elucidation of this structure is one of the first requirements for developing a predictive capability for reaction rates and pathways. A powerful spectroscopic technique that has only recently been used for in situ characterization of the molecular structure in supercritical fluid solutions is X-ray absorption fine structure (XAFS). XAFS provides detailed structural information about the number of nearest-neighbor atoms, bond distances, and bond strengths (from the Debye-Waller factor). The application of XAFS to a wide range of SCF solutions provides another powerful tool to explore the detailed structure of SCFs. 

Van Alsten, J. G. and C. A. Eckert. 1993. Effect of entrainers and of solute size and polarity in supercritical-fluid solutions. Journal of Chemical and Engineering Data. 38, 605. 

Knutson, B. L., D. L. Tomasko, C. A. Eckert, P. G. Debenedetti, and A. A. Chialvo 1992, Local density augmentation in supercritical fluid solutions A comparison between fluoresence spectroscopy and molecular dynamics results . In F. V. Bright and M. E. McNally (eds.) Supercritical Fluid Technology, ACS Symposium Series 4 8. Washington ACS, p. 60. 

On the topics of solute-solvent and solute-solute clustering, there is a significant number of publications by research groups from around the world, demonstrating the tremendous interest of the international research community. This chapter is a review of representative literature results, especially those based on molecular spectroscopy and related experimental techniques. Discussion of the fundamental properties of supercritical fluids will be within the context of enhanced solute-solvent and solute-solute interactions in supercritical fluid solutions, and the current understanding of the reasonably well-established solute-solvent clustering model and the somewhat controversial solute-solute clustering concept will be presented. 

Numerous experimental studies have been conducted on solute-solvent interactions in supercritical fluid solutions. In particular, issues such as the role of characteristic supercritical solvent properties in solvation and the dependence of solute-solvent interactions on the bulk supercritical solvent density have been extensively investigated. Results from earlier experiments showed that the partial molar volumes 02 became very large and negative near the critical point of the solvent (4-12). The results were interpreted in terms of a collapse of the solvent about the solute under near-critical solvent conditions, which served as a precursor for the solute-solvent clustering concept. Molecular spectroscopic techniques, especially ultraviolet-visible (UV-vis) absorption and fluorescence emission, have since been applied to the investigation of solute-solvent interactions in supercritical fluid solutions. Widely used solvent environment-sensitive molecular probes include Kamlet-Taft jt scale probes for polarity/polarizability... 

Sun et al. carried out a more systematic investigation of the TICT molecules DMABN and ethyl p-(A,A-dimethylamino)benzoate (DMAEB) in supercritical fiuoroform, CO2, and ethane as a function of fluid density (1). They found that the absorption and TICT emission spectral maxima shifted to the red with increasing fluid density. The results were comparable to those reported by Kajimoto et al. (51-55). More importantly, the spectral shifts and the fractional contribution of the TICT state emission changed with fluid density following the characteristic three-density-region pattern (Figures 11 and 12) (1). In fact, these results furnished the impetus for the development of the three-density-region solvation model for solute-solvent interactions in supercritical fluid solutions (2,3). 

The K, Py, and TICT solvation scales discussed above have been the basic techniques used in the investigation of solute-solvent interactions in supercritical fluid solutions. In addition, other methods have been applied for the same purpose, including the use of unimolecular reactions and vibrational spectroscopy and the probing of rotational diffusion the results obtained have been important to the understanding of the fundamental properties of supercritical fluids. 

To account for the characteristic density dependence of the spectroscopic (and other) responses in supercritical fluids, a three-density-region solvation model was proposed, reflecting the different solute-solvent interactions in three distinct density regions (Figure 17) (1-3). According to the model, the three density-region solvation behavior in supercritical fluid solutions is determined... 

The three-density-region solvation model provides a generalized view of the solvation behavior in supercritical fluid solutions, providing a qualitative but global explanation of the available experimental results however, a theoretical basis for the model remains to be explored and established. 

An important topic in supercritical fluid research is the effect of solvent local density augmentation on solute-solute interactions in a supercritical fluid solution. The most important question seems to be whether the supercritical solvent environment facilitates solute-solute clustering, which may be loosely defined as local solute concenttations that are greater than the bulk solute concentration. Unlike solute-solvent clustering discussed in the previous section, solute-solute clustering in supercritical fluid solutions is a more complex and somewhat controversial issue. Following is a summary of the available experimental results and a review of the various explanations and mechanistic proposals on the clustering of solute molecules in supercritical fluid solutions. 

Formation of pyrene excimer (a complex between a photoexcited and a ground-state pyrene molecule Scheme 4) is an extensively characterized and well-understood bimolecular process (35). Because the process is known to be diffusion controlled in normal liquid solutions, it serves as a relatively simple model system for studying solvent effects on bimolecular reactions. In fact, it has been widely employed in the probing of the solute-solute clustering in supercritical fluid solutions (40-42,46,47,160,166-168). (See Scheme 4.)... 

An excimer is a special case of exciplex—a complex between an excited-state molecule and a ground-state molecule, where the two molecules have different identities. Exciplex formation has been used as a model bimolecular process in the study of solute-solute clustering in supercritical fluid solutions. Brennecke et al. reported the investigation of naphthalene-triethylamine exciplex formation in supercritical CO2 at 35 C and 50°C (166). Their results show that the exciplex emission can be observed, even at low triethylamine concentrations (5 X 10 -5 X 10 M). Similarly, Inomata et al. investigated the formation of pyrene-dimethylaniline excimer in supercritical CO2 at 45°C (169). They... 

Photodimerization reactions in supercritical fluid solutions have been used to probe the effects of possible solute-solute clustering. BCimura et al. investigated the dimerization of 2-methyl-2-nitrosopropane in CO2, chlorotrifluoromethane, fluoroform, argon, and xenon (173-176). Their results show that the density dependence of the dimerization equilibrium constant is rather complex, probably due to the existence of various dimerization mechanisms in different density regions. 

Electron transfer reactions have also been used in the probing of solute-solute interactions in supercritical fluid solutions. For example, Takahashi and Jonah examined the electron transfer between biphenyl anion and pyrene in supercritical ethane (192). Worrall and Wilkinson studied triplet-triplet energy transfer reactions for a series of donor-acceptor pairs, including anthracene-azulene in supercritical C02-acetonitrile and supercritical C02-hexane and ben-zophenone-naphthalene in supercritical C02-acetonitrile (193). The high efficiency of the energy transfer reactions at low cosolvent concentrations was attributed to the effect of solute-solute clustering. 

SG Kazarian, M Poliakoff. Can conformational equilibria be tuned in supercritical fluid solution An IR spectroscopic study of trans/gauche isomerism of hexafluoropropan-2-ol in supercritical SFa and CHF3 solutions. J Phys Chem 99 8624, 1995. 

RM Anderton, JF Kauffman. Rotational relaxation in the compressible region of CO2 Evidence for solute-induced clustering in supercritical fluid solutions. J Phys Chem 99 13759, 1995. 

MP Ekart, KL Bennett, SM Ekart, GS Gurdial, CL Liotta, CA Eckert. Cosolvent interactions in supercritical fluid solutions. AIChE J 39 235, 1993. 

CR Yonker, SL Wallen, JC Linehan. Reaction chemistries in supercritical fluid solutions. J Microcolumn Sep 10 153-160, 1998. 

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