Fluoroform critical density

In this chapter, we describe the density- and temperature-dependent behavior of the vibrational lifetime (TO of the asymmetric CO stretching mode of W(CO)6( 2000 cm-1) in supercritical ethane, fluoroform, and carbon dioxide (C02). The studies are performed from low density (well below the critical density) to high density (well above the critical density) at two temperatures one close to the critical temperature and one significantly above the critical temperature (68-70). In addition, experimental results on the temperature dependence of Ti at fixed density are presented. Ti is measured using infrared (IR) pump-probe experiments. The vibrational absorption line positions as a function of density are also reported in the three solvents (68,70) at the two temperatures. 

C, for ethane, fluoroform, and C02, respectively. The near-critical isotherm temperatures for the three solvents were chosen so that the reduced temperatures (T/Tc) are essentially the same. For the near-critical isotherm, the lifetime decreases rapidly as the density is increased from low density. As the critical density, pc, is approached, the rate of change in the lifetime with density decreases (pc = 6.88, 7.56, and 10.6 mol/L, for ethane, fluoroform, and C02, respectively). The change in slope is more pronounced for C02. This is even more evident when the gas phase (zero density) contribution to the lifetime is removed from the data (see below). The difference between the C02 data and the other data will be discussed in Section V. The higher temperature isotherm data show a somewhat more uniform density dependence. Although the slope decreases with increasing density, there is a somewhat smaller change in slope than in the near-critical isotherm data. 

Figure 12 shows Ti(p, T) measured in fluoroform. Figure 12a shows data on the near critical isotherm of 28°C, i.e., 2 K above Tc. The calculated curve is scaled to match the data at the critical density, 7.5 M. (In... 

Figure 12 (a) Ti (p, T) data measured in fluoroform on the near critical isotherm (28°C), 2 K above Tc, and the calculated curve, which is scaled to match the data at the critical density, 7.56 mol/L. The fluoroform hard sphere diameter was adjusted since a good value at experimental temperatures is not available. A diameter of 3.28 A yielded the optimal fit. co is not adjustable. It is set equal to 150 cm-1, the value obtained in the fit of the ethane data. The theory does a very good job of reproducing the shape of the data with only the adjustment in the solvent size as a fitting parameter that affects the shape of the calculated curve, (b) Ti(p, T) data taken at 44°C, which is the equivalent increase in temperature above Tc as the higher temperature data taken in ethane (Fig. 10b). The theory curve is calculated using the same scaling factor, frequency, and solvent hard sphere diameter as at the lower temperature. Considering that there are no free parameters, the theory does an excellent job of reproducing the higher temperature data.

Equation 6 indicates that the solvent strength, 6, is pressure-dependent, providing a potential route to improved selectivity and rate by "pressure-tuning the solvent. A number of attempts to demonstrate reactivity control in su rcritical CO2 for Diels-Alder (75-77) and organic photoreactions (78,79) have exhibited very small effects. Andrew and coworkers have recently demonstrated dramatic solvent cage effects on selectivity of a photo-Fries reaction close to the critical density.(80) More polar SCF s have shown more promising results control of esterification rates and polyester molecular weight distribution via enzymatic catalysis in fluoroform has been demonstrated. (81,82)... 

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.

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 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). ... 

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