Intermolecular potential of argon

Fig. 4. Intermolecular potential of argon. Symbols are those of Fig. 3. The open circles represent the Barker potential.

Maitland GC, Smith EB (1971) The intermolecular parr potential of argon. Mol Phys 22 861... 

The extensive experimental and theoretical work on the OH-Ar system in the past year has provided a wealth of new information on the intermolecular potential between argon and the hydroxyl radical in the ground X 113/2 and excited A electronic states. The intermolecular potential undergoes a dramatic change with electronic excitation of the OH moiety. The... 

Rgure9.15 The Lennard-Jones Representation of the Intermolecular Potential of a Pair of Argon Atoms. 

Straight self-consistent field calculations have been carried out on the interaction of water with neon and argon.30 Here it is possible to obtain some of the attractive contribution to the intermolecular force since there will be an inductive second-order interaction caused by the large dipole of the water molecule. Such interactions appear at the SCF level and the authors find a minimum in the potential at an O -Ne distance of 3.63 A, with a binding energy of 0.71 kJ mol-1. There is a shallower minimum in the case of argon. Calculations at a similar level of sophistication have been carried out on the H2----He system,31 primarily, however, from the... 

A key question about the use of any molecular theory or computer simulation is whether the intermolecular potential model is sufficiently accurate for the particular application of interest. For such simple fluids as argon or methane, we have accurate pair potentials with which we can calculate a wide variety of physical properties with good accuracy. For more complex polyatomic molecules, two approaches exist. The first is a full ab initio molecular orbital calculation based on a solution to the Schrddinger equation, and the second is the semiempirical method, in which a combination of approximate quantum mechanical results and experimental data (second virial coefficients, scattering, transport coefficients, solid properties, etc.) is used to arrive at an approximate and simple expression. 

Direct Determination of the intermolecular Potential Function for Argon... 

Figure 3. Experimental nonlinearities observed in H2/Ar mixtures as a function of global density p. —Hj infinitely diluted in argon o—relaxation of Hj by Hj in argon. The two curves are almost parallel as the nonlinearity at high density is primarily determined by the size of the argon atom and the displacement is due to different well depths of Hj -Hj and H -Ar intermolecular potentials.

In the dense phase the intermolecular potential consists mainly of a two-body term to which small three-body contributions should be added. This problem is poorly documented for molecular systems, and the classic example remains that of argon where an effective two-body Lennard-Jones potential accounts fairly well for the thermodynamic data simply as a result of cancellation of errors. For vibrational energy relaxation one is not directly concerned with the whole intermolecular potential, but rather by its vibrationally dependent part. As mentioned earlier, three-body effects are not usually observable and may be masked by inadequate knowledge of the true potential. Nevertheless one can expect some simply observable solvent effects describable by changes of either the intermolecular or the vibrational potentials. 

At 20°C and a pressure of 1 atm, 1 mol argon gas occupies a volume of 24.0 L. Estimate the van de Waals radius for argon from the onset of the repulsive part of the argon intermolecular potential curve in Figure 9.18, and calculate the fraction of the gas volume that consists of argon atoms. 

Recently, considerable effort has been devoted to obtaining quantum mechanical intermolecular potentials suitable for fluid simulations. For some examples see 75 for (HF)2 76 for (N2)2 77-80 for (CO)2 81 for the benzene dimer 82 for the SiH4 dimer 83 for water-argon and water-methane potentials 84 for the formamide dimer and N,N-dimethylformamide 85 for lithium iodide in dimethylsulfoxide and 86 for Ni2+ in aqueous solution. Rather than discuss each of these studies, here we will focus on a few important developments that we anticipate could alter the capacity or approach to development... 

When one adds the attractive van der Waals potential terms to the repulsive term, one obtains the Lennard-Jones expression for the intermolecular potential energy for a simple fluid such as an inert gas like argon. On the basis of the above, the Lennard-Jones potential function may be written... 

The behavior of the self-diffusion and viscosity coefficients of liquid argon during the increase of the external pressure in the interval (1,3-52) GPa was an object of the molecular-dynamic investigation performed in ref f]. The molecular motion was simulated as a motion of spheres characterized by the Buckingham intermolecular potential... 

Zwanzig, Kirkwood, Oppenheim, and Alder38 have numerically evaluated Eq. 47 for liquid argon at its normal boiling point, using the same Bom-Green radial distribution function g0<2) and the same numerical values for the frictional coefficient and for the constants in the intermolecular potential function shear viscosity coefficient for the same substance. The value obtained was x — 4.1 x 10-4 cal/g—sec—°K which is in reasonable agreement with the experimental value of 2.9 x 10-4. 

This formula can be used to calculate the surface energy of a liquid when both the intermolecular potential and the distribution function are known. Thus, the distribution function of liquid argon obtained by Eisenstein and Gingrich10 and the known intermolecular potential give Ucaic = 27 erg/cm2, while the observed value, that is, the value obtained from the observed values of surface tension by Eq. 11.12, is U0bs = 35 erg/cm2. For mercury the distribution functions obtained by Boyd and Wake-ham5 and the interatomic potential obtained by Hildebrand, Wakeham, and Boyd16 can be used. The calculated value is Ucaie = 490 erg/cm2, which can be compared with the observed value Uohs = 500 erg/cm2. 

Intermolecular Potentials and Macroscopic Properties of Argon and Neon from Differential Collision Cross Sections... 

The differential cross sections of argon and neon have been measured by using refinements of the modulated molecular-beam technique. From these measurements the intermolecular potentials were found. These potentials differ significantly from the Lennard-Jones potential. The neon and argon potentials have different shapes and are not related by any simple scaling factor. The macroscopic properties have been calculated and are in reasonable agreement with experiment. The face-centered cubic structure was found to be the most stable crystal lattice for neon. The effect of the argon potential on the critical properties and saturation pressures is also discussed. 

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