Enthalpy data collection

Fig. 2.12. Enthalpy, entropy, and free energy differences for the ethane —> ethane zero-sum alchemical transformation in water. The molecular dynamics simulations are similar to those described in Fig. (2.7). 120 windows (thin lines) and 32 windows (thick lines) of uneven widths were utilized to switch between the alternate topologies, with, respectively, 20 and lOOps of equilibration and 100 and 500 ps of data collection, making a total of 14.4 and 19.2 ns. The enthalpy (dashed lines) and entropy (dotted lines) difference amount to, respectively, —0.1 and +1.1 kcalmol-1, and —0.5 and +4.1 calmol-1 K For comparison purposes, the free energy difference is equal to, respectively, +0.02 and —0.07kcalmol I, significantly closer to the target value. Inset Convergence of the different thermodynamic quantities...

The task remains to determine the mean annual enthalpy from plant physiognomy. An analysis is presented relating foliar physiognomic characters to mean annual values of enthalpy, temperature, specific humidity, and relative humidity that exploits the method and data in the Climate-Leaf Analysis Multivariate Program (Wolfe 1993). From present-day plant data collected from North America, Puerto Rico, and Japan, the leaf parameters are searched for linear combinations of the foliar characteristics that covary with the local climates. By doing so, the foliar characteristics can be determined that covary with one another and which best correlate with climate parameters. 

The DSC thermogram of spironolactone was obtained using a Du Pont TA-9900 thermal analyzer system, interfaced to the Du Pont data collection system. The thermogram shown in Figure 3 was collected over a range of 50 to 250°C, using a heating rate of 10°C/minute. It was found that the compound melted at 210.5°C, with an enthalpy of fusion equal to 43.40 J/g. 

The previous analysis shown that the initial values of most of the kinetic parameters obtained from DFT calculations provide a good description of the reaction kinetics data collected over a wide range of conditions. The principal difference between the values of the final kinetic parameters used in the model and the initial values obtained from DFT calculations is that the fitted enthalpy changes for the formation of C2Ha transition states involved in cleavage of the C-C bond are lower than the initial values predicted from DFT calculations. This difference may be explained by the structure sensitivity of the system and/or by the inherent error of the DFT calculations. 

It has been argued that EEC is a mere artifact (120). In an attempt to establish the existence of the EEC effect, a collection of thermodynamic data collected for the binding of 136 drugs to their receptors revealed strong correlation between enthalpy and entropy (116). Some have attributed these results to inaccuracies of the van t Hoff analysis that is used for the measurement of thermodynamic parameters (121). But isothermal titration calorimetry data (ITC) does not suffer from the same inaccuracies that plague the van t Hoff analysis. We have recently created a dataset that contains more than 400 entries containing structural and isothermal titration data (122). A plot of the enthalpy and entropy results in the following linear relationship ... 

Figure 6.2.1. Excess Gibbs free energy and excess enthalpy of the acetone and water binary mixture at 293 K. The excess Gibbs free energy was calculated from VLE data as described in Section 5.1, The excess enthalpy data are as reported in the DECHEMA Chemistry Data Series Heat of Mixing Collection, Christiansen et al. 19S4, Vol, l,Pt. lb, pp. 148-9.

The data, collected in Table 4, show the trend in Ag-olefin bond dissociation enthalpies in solution, the weakest bond being between Ag+ and cyclopentene and the strongest between Ag+ and cyclooctene. 

The ratio MiRTVA p H is called the ebulliometric constant. For the determination of solvent activities from ebulliometric data, tabulated ebulliometric constants should not be used, however. On the other side, it is sometimes recommended to use reference solutes to establish an experimental relationship for the equipment in use, i.e., unprecise data for the enthalpy of vaporization or perhaps some non-equilibrium effects cancel out of the calculation. Enthalpies of vaporization are provided by several data collections, e.g., by Maj er and Svoboda, or through the DIPPR database. 

In a DSC experiment lasting about 1 h, using as little as 2 pg of material, a protein can be thermally unfolded, allowing the enthalpy and entropy of the denaturation process to be measured, as seen in Figure 16.36. The thermodynamic data collected are presented in Table 16.5. As can be seen in this table, the thermodynamic data obtained were accurate even for the 2 pg sample. 

From the auxiliary data collected in step 1 and the reaction enthalpy from step 5, compute the enthalpy of formation of M at the selected temperature. 

Data from investigations applying to more than one chapter are divided and appear in the relevant chapters. Data are inclnded only if numerical values were published or authors provided their results by personal cormnnnication (and 1 wish to thank all those who did so). No digitized data have been included in this data collection. Finally, Chapter 6 covers a large mrmber of systems in a table in which additional information on enthalpy effects in polymer solntions can be foimd. 

In general, the accuracy of enthalpy of fusion from vapor pressttre measrrrements depends on the accuracy of the vapor presstrre measrrrement from which it is determined and on the acctrracy of heat capacities of the compoimd in the gas, liquid, and sohd phases. The accuracy of enthalpy of fusion from vapor pressure is at least 5 percent and upto 50 percent, according to our analysis of the data collected in this book. In addition, the vapor pressure method can not separate the enthalpies of a sohd transition from that of fusion if the temperatures of the transition and fusion are close. For example, the enthalpies of fusion for the long-chain alcohols determined from vapor pressure by Davies and Kybett [65-dav/kyb] are actually the total enthalpies of fusion and transition for these compounds. 

There are some determinant factors in the HPC ester derivatives such as, the length of the substituents, degree of esterification, degree of polymerization, that directly affects (i) the glass transition temperatures, Tg, of HPC and its derivatives, (ii) the clearing temperature, To, characterizing the transition from the cholesteric state to the isotropic fluid and (iii) transition enthalpy AHj, as observed from the data collected and summarized in Table 14.2. In this section the effect of the aforementioned factors in the thermal properties will be explored. 

Future absorbent solutions have to combine high carbon dioxide loading charges (moles of dissolved carbon dioxide per mole of amine) with low energies of regeneration. Characterization of new absorbent solution can be performed by calorimetric studies of gas dissolution. The experimental data collected are essential to develop thermodynamic models representative of the C02-absorbent solution systems that will be used to design the future capture units. The dissolution properties required are the mainly the gas solubility and the enthalpy of solution. However some other properties also have to be studied, such as heat capacity, vapor pressure, chemical and thermal degradations. Then specific calorimetric techniques were set up to provide the essential experimental data. 

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