Thermally reversible precipitation

This thermally reversible precipitation suggested potential applications to the technology of reaction product isolation. It could be used as a tool to allow isolation of a specific product from a totally soluble reaction by raising the temperature. 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

The clear solutions of the polyhydrofuUerenes in various organic solvents become inhomogeneous upon the formation of a precipitate. This, together with the broad peaks in the NMR spectra, shows the instability of these CjqH s. Thermal treatment of CjqH in the solid state at 550 °C leads to a complete reversion to Cjq (Scheme 5.3). Sublimation ofC oHjg and CjqHjj at lower temperatures (273 12 °C) was accompanied by partial loss of hydrogen. Decomposition of CjqHjj was confirmed to be a stepwise process with formation of CggHjg as an intermediate product [64]. 

The reverse of this process, in which the same quantity of heat is added to the aqueous solution with spontaneous re-emergence (precipitation) of the salt crystal, is also fully compliant with the first law. However, such thermally induced reverse solvation has never been observed. 

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