Hydration enthalpies simulated

Previous studies have shown that there is a correlation between the enthalpy of hydration of alkanes and their accessible surface area [30,31] or related magnitudes. Moreover, relationships between the hydration numbers calculated from discrete simulations for hydrocarbons and both the free energy and enthalpy of hydration of these molecules have also been reported [32] and have been often used to evaluate solvation enthalpies. Analysis of our results, illustrates the existence of a linear relationship between A//n eie and the surface of the van der Waals cavity,. SVw, defined in MST computations for the calculation of the non-electrostatic contributions (Figure 4-1). In contrast, no relationship was found for the electrostatic component of the hydration enthalpy (A//eie data not shown). Clearly, in a first approximation, one can assume that the electrostatic interactions between solute and solvent can be decoupled from the interactions formed between uncharged solutes and solvent molecules. 

FIGURE 10-28 Simulated Hydration Enthalpies of M Transition Metal Ions. 

When the computed do agree with experimental data, as in the case of Zn [211] and [212-214] it appears that an error compensation might have occurred, due to the use of a small basis set that underestimates two-body binding energies. Hydration enthalpies and free energies also are overestimated for mono- and divalent cations, if calculated in simulations with uncorrected two-body potentials. 

The effect of freezing the first hydration shell in the simulation on the hydration enthalpy of Zn " " has been debated in the literature [225-227]. The conclusion seems that hydration enthalpy is underestimated by 10 % and the increase of second shell structure is also minor. 

Choking, or expansion of gas from a high pressure to a lower pressure, is generally required for control of gas flow rates. Choking is achieved by the use of a choke or a control valve. The pressure drop causes a decrease in the gas temperature, thus hydrates can form at the choke or control valve. The best way to calculate the temperature drop is to use a simulation computer program. The program will perform a flash calculation, internally balancing enthalpy. It will calculate the temperature downstream of the choke, which assures that the enthalpy of the mixture of gas and liquid upstream of the choke equals the enthalpy of the new mixture of more gas and less liquid downstream of the choke. 

After this computer experiment, a great number of papers followed. Some of them attempted to simulate with the ab-initio data the properties of the ion in solution at room temperature [76,77], others [78] attempted to determine, via Monte Carlo simulations, the free energy, enthalpy and entropy for the reaction (24). The discrepancy between experimental and simulated data was rationalized in terms of the inadequacy of a two-body potential to represent correctly the n-body system. In addition, the radial distribution function for the Li+(H20)6 cluster showed [78] only one maximum, pointing out that the six water molecules are in the first hydration shell of the ion. The Monte Carlo simulation [77] for the system Li+(H20)2oo predicted five water molecules in the first hydration shell. A subsequent MD simulation [79] of a system composed of one Li+ ion and 343 water molecules at T=298 K, with periodic boundary conditions, yielded... 

Calculated (By Simulations) And Experimental Ionic Enthalpies And Free Energies of Hydration, in kcal/mole... 

While still widely accepted in some form, this perspective has been criticized (often in favor of a view that water s small size determines its unique beha-vior)/ More recently, simulations have been used to probe issues related to the heat capacity of solvation. A large heat capacity leads to enthalpy-dominated hydration thermodynamics at high temperatures and also to a high-temperature convergence of solvation entropies for various-sized solutes. 

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