Charge transfer supercritical fluids

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

Kajimoto, O., Futakami, M., Kobayashi, T., and Yamasaki, K., "Charge-Transfer-State Formation in Supercritical Fluid (N,N-Dimethylamino)benzonitrile in CF3H," J. Phys. Chem., 1988, 92, 1347. 

In another investigation, picosecond absorption studies were conducted on TPE singlet excited state in supercritical fluids. Studies were also carried out on ethyl-(Af,Af-dimethylamino)benzoate TICT (twisted intramolecular charge transfer) state for comparison with the TPE studies in supercritical fluids [62]. The decay rates of excited singlet of TPE were found to be correlatable with the solvent-induced changes. The results of this study indicated that the singlet excited state of TPE is twisted and is associated with a polar or zwitterionic character. 

Solubility of some substances in the supercritical fluids, among different parameters, mostly depends on the vapor pressure, the substance polarity, and substance molar mass. Compounds of smaller molar mass and a higher vapor pressure at supercritical condition are more soluble in SFs, compared to other with lower vapor pressure and higher molar mass. An enhancement factor (dimensionless parameter) is defined as the ratio of solubility of substances in the SF (solvent) compared to its solubility in the ideal gas. Usually, this parameter has the common value between 10" and 10 [3]. Different mechanisms reported in the literature have been used for explaining the enhancement of solute solubility in supercritical fluids. They included the hydrogen bonding, the charge transfer complex formation, dipole-dipole noninduced and induced interactions, and solute-solvent with and without cosolvent interactions. 

Many other probes have been used to study supercritical fluid-cosolvent mixtures, including the charge transfer complexes Fe°(l,10-phenanthroline)3 " and Fe (2,4-pentadionate)3 (for C02-methanol mixtures) (154), Nile red dye (for Freon-13, Freon-23, and CO2 with the cosolvents methanol, THF, acetonitrile, and dichloromethane) (155), benzophenone (for ethane with the cosolvents... 

Y-P Sun, TL Bowen, CE Bunker. Formation and decay of the ethyl p- N,N-diethylamino)benzoate twisted intramolecular charge-transfer state in the vapor phase, supercritical fluids, and room-temperature solutions. J Phys Chem 98 12486, 1994. 

O Kajimoto, K Yamasaki, K Houma. Intramolecular charge-transfer reactions studied in a supercritical fluid of varying densities and in a molecular beam. Faraday Disc Chem Soc 85 65, 1988. 

O Kajimoto, M Futakami, T Kobayashi, K Yamasaki. Charge-transfer-state formation in supercritical fluid (V,V-dimethylamino)benzonitrile in CF3H. J Phys Chem 92 1347, 1988. 

O Kajimoto, T Nayuki, T Kobayashi. Picosecond dynamics of the twisted intramolecular charge-transfer state formation of 4-(V,A-dimethylarnino)benxonitrile (DMABN) in supercritical fluid solvent. Chem Phys Lett 209 357, 1993. 

Y-P Sun, CE Bunker. Twisted intramolecular charge transfer of ethyl p-(N,N-diethylaminojbenzoate in the gas phase and in the low-density non-polar supercritical fluids, a quantitative spectral resolution using principal component analysis and self modeling. J Chem Soc, Chem Commun 5, 1994. 

Y-P Sun, G Beimett, KP Johnston, MA Fox. Quantitative resolution of dual fluorescence spectra in molecules forming twisted intramolecular charge-transfer states. Toward establishment of molecular probes for medium effects in supercritical fluids and mixtures. Anal Chem 64 1763, 1992. 

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