Supercritical ammonia: a molecular dynamics simulation and vibrational spectroscopic investigation.

Combining infrared spectroscopy and molecular dynamics simulations, we have investigated the structural and dynamical properties of ammonia from liquid state (T = 220 and 303 K) up to the supercritical domain along the isotherm T = 423 K. Infrared spectra show that the N-H stretching and bending modes are significantly perturbed which is interpreted as a signature of the change of the local environment. In order to compare the experimental spectra with those obtained using molecular dynamics simulation, we have used a flexible four sites model which allows to take into account the anharmonicity in all the vibration modes particularly that of the inversion mode of the molecule. A good agreement between our experimental and calculated spectra has been obtained hence validating the intermolecular potential used in this study to simulate supercritical ammonia. The detailed analysis of the molecular dynamics simulation results provides a quantitative insight of the relative importance of hydrogen bonding versus nonhydrogen bonded interactions that governs the structure of fluid ammonia.

Investigation of the local structure in sub and supercritical ammonia using the nearest neighbor approach: a molecular dynamics analysis.

Molecular dynamics simulations of ammonia were performed in the (N,P,T) ensemble along the isobar 135 bar and in the temperature range between 250 and 500 K that encompasses the sub and supercritical states of ammonia. Six simple interaction potential models (Lennard-Jones pair potential between the atomic sites, plus a Coulomb interaction between atomic partial charges) of ammonia reported in the literature were analyzed. Liquid-gas coexistence curve, critical temperature, and structural data (radial distribution functions) have been calculated for all models and compared with the corresponding experimental data. After choosing the appropriate potential model, we have investigated the local structure by analyzing the nearest neighbor radial, mutual orientation, and interaction energy distributions. The change in the local structure was traced back to the change of the nonlinear behavior (which is more pronounced at low temperatures) of the average distance between a reference ammonia molecule and its subsequent nearest neighbor. Our results suggest to use the position of the maximum in the fluctuation of the average distance to define the border of the first solvation shell (particularly at high temperature when the minimum of the radial distribution is not well-defined). Indeed, the effect of the temperature on the position of this maximum shows clearly that the spatial extent of the solvation shell increases with a concomitant decrease of the involved number of ammonia molecules. Furthermore, our results show that the signature of the hydrogen bonding is mainly observed for temperature below 300 K. This signature is quantified by a short distance contribution to the closest radial nearest neighbor distribution, by a strong mutual orientation (defined by the angles between the axis joining the nitrogen atoms and the molecular axes) and by a strong attractive character of the total interaction energy.

Hydrogen bonding in supercritical tert-butanol assessed by vibrational spectroscopies and molecular-dynamics simulations.

We have investigated the state of aggregation in supercritical tert-butanol (T = 523 K,0.05 < rho < 0.4 g cm(-3)) by means of vibrational spectroscopies (infrared and Raman) and molecular-dynamics (MD) simulations. A quantitative band shape analysis of the spectra associated with the OH stretching mode of tert-butanol has been done using activities computed by ab initio calculations on small clusters. This allows us to determine the degree of hydrogen bonding and populations of oligomers. These latter quantities have been derived from MD simulations and very consistent results are found with experiments. These results show that hydrogen bond still exist in supercritical tert-butanol and that the fluid mainly consists of oligomers smaller than tetramers.