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Supercritical ammonia: A molecular dynamicssimulation and vibrational spectroscopic investigation
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Image of FIG. 1.
FIG. 1.

Phase diagram of ammonia. The density–pressure coordinates of our infrared experimental spectra (red crosses) and MD simulations (blue squares) are displayed.

Image of FIG. 2.
FIG. 2.

Experimental infrared absorption spectra of supercritical ammonia.

Image of FIG. 3.
FIG. 3.

Normalized distributions of the intramolecular geometric properties of the ammonia molecule obtained from simulations: nitrogen–hydrogen distance (a), hydrogen–nitrogen–hydrogen angle (b), and perpendicular distance between nitrogen and the plane define by the hydrogen atoms (c). In this last figure, the distributions of the molecular dipole moment are also shown as inset. The labels refer to the liquid (liq) or supercritical (SC) phases at a given density (in g cm−3) for the molecular dynamics simulations performed as described in the Table I.

Image of FIG. 4.
FIG. 4.

Nitrogen–nitrogen radial distribution function and running coordination numbers (a), nitrogen–hydrogen (b), and hydrogen–hydrogen (c) radial distribution functions obtained from simulations and experimental results of Ricci et al. (Ref. 22) (T = 213 K, ρ = 0.71 g cm−3). The intramolecular contribution obtained from simulation is only shown for the simulation liq/0.71 to allow the comparison with experiment.

Image of FIG. 5.
FIG. 5.

Fourier transform of the hydrogen velocity autocorrelation functions obtained from simulations in the stretching region (a) and in the bending region (b). The inset shows a comparison between simulation (solid line) and experiment (dashed line) under the same thermodynamic conditions (T = 213 K, P = 140 bar, ρ = 0.71 g cm−3). The intensities of the spectra have been scaled to make easier the comparison.

Image of FIG. 6.
FIG. 6.

Fraction of molecules having a certain number of accepted (a) or donated hydrogen bonds (b).

Image of FIG. 7.
FIG. 7.

Evolution of the coordination number nNN(r) computed as the integral of the gNN(r) radial distribution function up to 5 Å (dashed line) and average number of hydrogen bond per molecule nH (solid line) in respect with the fluid density. The dotted line is just a guideline for the eyes (see text).


Generic image for table
Table I.

Characteristics of the molecular dynamics simulations performed. The densities correspond to the experimental values (Ref. 15) according to the imposed temperatures and desired pressures. The self-diffusion coefficients have been computed from the root mean square displacement of the molecular centers of mass. n H is the average number of hydrogen bond per molecule. 〈E inter〉 and 〈E intra〉 are the average inter- and intra-molecular potential energies, respectively. For the simulation MD3, the values between parentheses refer to a simulation using Ladd summation technique to take into account the long range coulombic interactions (see the text).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Supercritical ammonia: A molecular dynamicssimulation and vibrational spectroscopic investigation