Fluoroform critical point

Critical points vary widely. Table 6.1 shows a representative sample of critical parameters and it is immediately obvious why carbon dioxide is widely used. With a critical temperature just above room temperature and a critical pressure that is relatively low, the amount of energy needed to render carbon dioxide supercritical is comparatively small. Fluoroform (CHF3) and difluoromethane also have easily attainable critical parameters, but they are much more expensive than carbon dioxide. Despite its high critical temperature and pressure, supercritical water (SCH2O) is used widely as a destructive medium since it is highly acidic. 

Kamat et al. found that the initial transesterification rate with Candida cylin-dracea lipase decreased markedly upon increasing the pressure from 80 to 120 bar [5]. They ran the reaction in fluoroform at 50 °C and calculated the apparent activation volume of the reaction from initial reaction rates at different pressures. The apparent activation volume showed a maximum near the critical point of fluoroform. As the pressure increased from 60 to 180 bar the apparent activation volume approached zero and the reaction rate decreased to one tenth. 

The majority of SCFs also show a sharp increase in the dielectric constant (e) with increasing pressure in the compressible region (around the critical point). This behavior reflects, to some extent, the change in density. The magnitude of the increase depends on the nature of the SCF whereas the dielectric constant varies little with pressure for non-polar substances such as SCCO2, dramatic increases are observed for more polar SCFs such as water or fluoroform (Figure 4.4). " ... 

The QTAIM approach mentioned here before is a very useful tool to describe La-Lb interactions since it is possible to have a deeper insight into changes in the electron charge distribution being the result of complexation especially the changes of hydrogen bonded systems. For example, the position of the C-H proton donating bond critical point (BCP) was analyzed recently [98] for the mentioned earlier here complexes of acetylene and fluoroform. It is possible to decompose the C-H bond into two radii dehned by the position of BCP (see Fig. 9.13). Hence there is the radius of the carbon atom which is the distance between BCP and the C-atom... 

The last point which can be evoked here is conceptually linked to the hot-spot theory. If the limit liquid layers around a bubble are in direct contact with the heated and pressurized bubble content, far above the critical point of the liquid, these layers should be in a supercritical state. 23 This attractive hypothesis (see p. 61) was used by Hoffmann et al to rationalize the sonolysis of nitrophenyl derivatives. Supercritical fluids are characterized by a very high flexibility of important parameters (density, dielectric constant, solubilizing power) as a function of pressure. Experts in the field distinguish gas-like and liquid-like media, in which the kinetics of a reaction can vary over a broad range. For instance, the conjugate addition of piperidine to methyl propiolate was studied in supercritical ethane or fluoroform (Fig. 5). ... 

Figure 3 Vibrational lifetimes for the asymmetric CO stretching mode of W(CO)6 vs. density along two isotherms of three polyatomic supercritical fluids ethane (34°C panel a and 50°C panel b), fluoroform (28°C panel c and 44°C panel d), and carbon dioxide (33°C panel e and 50°C panel f). The upper panel for each solvent is an isotherm at 2°C above the critical temperature. In all six data sets, error bars (representing one standard deviation) are approximately the size of the points.

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