The calculation of the Gibbs free energy, enthalpy, and entropy of hydration of ammonia, methylamine, dimethylamine, trimethylamine, water, methanol, dimethyl ether, hydrogen sulfide, methanthiol, and dimethylsulfide is presented to illustrate the usefulness of the enthalpy and entropy of solvation in studying microscopic phenomena affecting the thermodynamics of the hydration of simple organic molecules. The free energy perturbation (FEP) method is used in conjunction with constant temperature and constant pressure molecular dynamics (MD) configurational sampling. The hydration free energies are studied as a function of the temperature in order to evaluate the hydration entropy by finite differences (FD). The TIP3P water model is used for the solvent water and revised AMBER parameters for the solutes. Partial charges of the solutes are obtained from fitting the electrostatic potential obtained from electronic structure calculations. Discrepancies with the experiments, especially noticeable for the amines, are observed for the hydration enthalpies and entropies even in cases where the hydration free energies are in agreement with the experiments. We conclude that this molecular force field requires additional parametrization against experimental entropies and enthalpies of hydration. Other molecular force fields may also need reparametrization.
Chloride ion solvation in supercritical water is investigated by computer simulations, including water polarizability explicitly. Comparisons are made between the TIP4P fluctuating charge and the original TIP4P models. Particular attention is paid to the density dependence of the equilibrium structural and transport properties. The chloride ion hydration number slowly decreases with density reduction in a similar way for both the fluctuating charge and the fixed-charge water models. The diffusion coefficients for the two models also exhibit a similar density dependence, except at the lowest density examined (0.05 g/cm 3 ), where the chloride ion diffusion rate in polarizable water is significantly larger than that in nonpolarizable water because of a more rapid loss of the hydration shell in polarizable water at the lowest density. The remarkable similarity between the two models is related to the insensitivity of the local polarization in the first hydration shell to the bulk conditions. The results also suggest that the local viscosity rather than the long-range dielectric friction dominates the transport properties.
The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.
High-sensitivity Raman vibrational spectroscopic equipment was developed to study the hydrogen-bonding structure in supercritical fluids over a wide density range. Supercritical water was investigated to the very dilute region of 0.1 MPa (3×10−4 g cm−3) at 400 °C, and the spectroscopic profile at the isolated state was observed. For comparison, supercritical methanol was also investigated, and the effect of the alkyl group on the hydrogen bonding is elucidated. For both water and methanol, the OH stretching peak shifted toward a lower frequency with an increase in the density as a result of hydrogen-bond formation. The red shift relative to the isolated value was not always proportional to the density. In the case of water at 400 °C, the shift was nonlinear over a wide density range; it was almost linearly dependent on the density between 0.6 and 0.2 g cm−3, whereas at lower densities, the dependence became steeper. For methanol, a nonlinear density dependence was similarly observed at a corresponding reduced temperature. The density dependence then became more linear at higher temperatures. The density dependence of the spectroscopic profile is interpreted in terms of the matrix of force constants affected by the formation of hydrogen bonding.
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