The cloud points of various amorphous polyether, polyacrylate, and polysiloxane homopolymers, and a variety of commercially available block copolymers, were measured in CO2 at temperatures from 25 to 65 °C and pressures of ca. 1000−6000 psia. Almost without exception, the solubility of amorphous polymers increases with a decrease in the cohesive energy density, or likewise, the surface tension of the polymer. With this decrease in surface tension, the polymer cohesive energy density becomes closer to that of CO2. Consequently, solubility is governed primarily by polymer−polymer interactions, while polymer−CO2 interactions play a secondary role. The solubility is strongly dependent upon molecular weight for the less CO2-philic polymers. The solubilities of high-molecular-weight poly(fluoroalkoxyphosphazenes) in CO2 were comparable to those of poly(1,1-dihydroperfluorooctylacrylate), one of the most CO2-soluble polymers known.
Interactions between carbon dioxide and ethane or hexafluoroethane were examined using ab initio calculations which were performed at the restricted Hartree−Fock level of theory using the STO-3G and 6-31G* basis sets. Computations at the 6-31G* level have identified key differences between the interaction of hydrocarbons and fluorocarbons with carbon dioxide. The interaction of the fluorocarbon with carbon dioxide is predominantly electrostatic in nature, with the positively charged CO2 carbon atom having a strong attraction to the negatively charged fluorine atoms of the fluorocarbon, resulting in a favorable interaction energy of 0.75−0.8 kcal/mol for each CO2 molecule in the first solvent shell. The interaction of CO2 with hydrocarbons is minimal due to the neutral nature of the hydrocarbon molecule. Calculations on CO2/hydrocarbon systems show a clustering of CO2 molecules away from the hydrocarbon, whereas the calculations on the CO2/fluorocarbon systems indicate that the CO2 molecules orient around the C2F6 by sandwiching the positively charged CO2 carbon between two negatively charged fluorine atoms. These molecular modeling computations have brought to light differences between the interaction of hydrocarbons and fluorocarbons with carbon dioxide which may help to explain the different solubilities of these types of molecules in supercritical carbon dioxide.
A novel manganese(IV) monomer, [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)), has been synthesized in methanol by the reaction of MnCl(2) with the ligand, N,N',N"-trimethyl-1,4,7-triazacyclononane (Me(3)TACN), in the presence of Na(2)O(2). The resulting product was isolated as the red/brown crystalline hexafluorophosphate salt. The compound crystallizes in the space group P2/c with the cell dimensions a = 15.652(2) Å, b = 8.740(1) Å, c = 15.208(2) Å, beta = 108.81(1) degrees, V = 1969.4(4) Å(3), and Z = 4. The structure was solved by the heavy-atom method and was refined by full-matrix least-squares techniques to a final value of R = 0.067 (R(w) = 0.097) based upon 3087 observations. The manganese atom in the molecule is six-coordinate in an N(3)O(3) ligand environment with the triazacyclononane facially coordinated. Pertinent average bond distances and angles are as follows: Mn-O, 1.797(5) Å; Mn-N, 2.116(5) Å; O-Mn-O, 97.8(2) degrees; N-Mn-N, 81.4(2) degrees; O-Mn-N, 167.8 degrees (2); O-Mn-N, 86.8(2) degrees; O-Mn-N, 92.8(2) degrees. The complex was further characterized by UV-vis and EPR spectroscopies, solution magnetic susceptibility measurements, FAB-MS, and electrochemistry. [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)) was found to catalyze the oxidation of water-soluble olefins using hydrogen peroxide as the oxidant in an aqueous medium. The catalyzed rates of oxidation of these olefins indicate at least a 12-fold rate enhancement over oxidant alone. The unusual stability of the catalytic species was demonstrated by the repeated additions of substrate and oxidant while maintaining a constant catalytic rate of oxidation.
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