Correlation between the Anisotropy of the Electronic Polarizability of Molecules and the Anisotropy of the van der Waals Atomic Radii

A procedure was developed for determining the anisotropy of the van der Waals atomic radii of halogens, hydrogen, oxygen, and sulfur in polyatomic molecules from the experimental characteristics of the anisotropic electronic polarizability of these molecules. The results are consistent with the published van der Waals radii determined by independent procedures.     

Van der Waals radii of metals from spectroscopic data

The lack of information about the van der Waals radii of metals can be compensated for by using the results of spectroscopic investigations of van der Waals molecules. It has been shown that the interatomic distances in these molecules obey an additive scheme if one allows for the polarization effects. The van der Waals radii of the alkali metals, Ag, Mg, Zn, Cd, Hg, B, Al, In, and Si, have been determined from the interatomic distances in their heteroatomic molecules with atoms of noble gases. Use of the obtained radii for crystal chemistry is discussed.Translated fromIzyestiya Akademil Nauk. Seriya Khimicheskaya, No. 8, pp. 1374–1378, August, 1994.     

Neutron scattering from a deformed “vander Waals chain”-quasi Gaussian approximation

We calculate the neutron scattering form of an affinely deformed van der Waals chain, i.e., a labeled chain embedded in a network, whose deformation behavior can be described by an effective van der Waals equation in an effective Gaussian approximation. This provides microscopic information about the deformation behavior of van der Waals networks and complements the macroscopic information given by stress-strain relations.dedicated to Prof H.-G. Kilian on occasion of his 66th birthday     

Anisotropy of the van der Waals Configuration of Atoms in Complex,Condensed, and Gas-Phase Molecules

Anisotropic van der Waals (vdW) radii of 5b–7bsubgroup elements were determined from structures of gas-phase van der Waals complexes and crystal molecular compounds. The anisotropy of the van der Waals configuration of atoms was shown to decrease when going from isolated molecules to the condensed state. Variations in intermolecular distances, which are usually explained in terms of the formation of hydrogen bonds, are substantially governed by the anisotropic effect.     

Adsorption of square and rectangular plate-like molecules on a planar surface

A mean-field statistical thermodynamic analysis of monolayer adsorption of rigid square and rectangular plate-like molecules on a homogeneous planar surface is developed. The analysis is simplified by only considering facewise and edgewise modes of adsorption in restricted orthogonal orientations parallel to the surface. The free energy density, adsorbate population distribution and surface spreading pressure are obtained as a function of adsorbate density and compared for square plate molecules using three different sequences of adsorbate molecule placement on the surface to evaluate the configurational degeneracy. It is found that edgewise adsorbed molecules can be anisotropically ordered if the edge length of square and rectangular plate-like molecules exceeds three length units in the absence of anisotropic dispersion interactions. If intermolecular dispersion interactions are present and of sufficient strength, the spreading pressure-density isotherms can exhibit one or two van der Waals loops for square plate molecules with three van der Waals loops possible for rectangular plate adsorbate molecules. The phase transitions for the adsorbed monolayer corresponding to the appearance of these van der Waals loops are discussed.     

On the role of van der Waals interaction in chemical reactions at low temperatures

It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.     

Dynamical and energetic properties of hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the dynamical and energetic properties in hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 30 and 200 K and 0.05 kbar, and also at intermediate temperatures, using SPC/E and TIP4P-2005 water models. The potential model is found to have a large impact on overall density, with the TIP4P-2005 systems being on average 1% more dense than their SPC/E counterparts, due to the greater guest-host interaction energy. For the lightly-filled mixed H(2)-THF system, in which there is single H(2) occupation of the small cage (1s1l), we find that the largest contribution to the interaction energy of both types of guest is the van der Waals component with the surrounding water molecules in the constituent cavities. For the more densely-filled mixed H(2)-THF system, in which there is double H(2) occupation in the small cage (2s1l), we find that there is no dominant component (i.e., van der Waals or Coulombic) in the H(2) interaction energy with the rest of the system, but for the THF molecules, the dominant contribution is again the van der Waals interaction with the surrounding cage-water molecules; again, the Coulombic component increases in importance with increasing temperature. The lightly-filled pure H(2) hydrate (1s4l) system exhibits a similar pattern vis-à-vis the H(2) interaction energy as for the lightly-filled mixed H(2)-THF system, and for the more densely-filled pure H(2) system (2s4l), there is no dominant component of interaction energy, due to the multiple occupancy of the cavities. By consideration of Kubic harmonics, there is some evidence of preferential alignment of the THF molecules, particularly at 200 K; this was found to arise at higher temperatures due to transient hydrogen bonding of the oxygen atom in THF molecules with the surrounding cage-water molecules.     

Photodynamics of the excited states of trimethylamine in a supersonic beam

The photophysics of trimethylamine (TMA) and rare gas-TMA van der Waals molecules has been studied under supersonic beam conditions. Dual exponential fluorescence decays observed for excitation of the second excited singlet state (S₂) are attributed to a novel S₂-S₁ relaxation induced by the vibrational predissociation of van der Waals molecules.     

A self-assembled organic capsule formed from the union of six hexagram-shaped amphiphile molecules

A discrete 2-nm-sized self-assembled organic capsule was formed from the union of six hexagram-shaped amphiphile molecules in aqueous methanol due to the hydrophobic effect, van der Waals forces, and CH-pi interactions between the chemical components. A couple of hexa-substituted benzene molecules were encapsulated inside the capsule in both solution and the solid state, as was evidenced by 1H NMR spectroscopy and single-crystal X-ray analysis.     

Van der Waals surface graphs and molecular shape

A van der Waals surface graph is the graph defined on a van der Waals surface by the intersections of the atomic van der Waals spheres. A van der Waals shape graph has a vertex for each atom with a visible face on the van der Waals surface, and edges between vertices representing atoms with adjacent faces on the van der Waals surface. These are discrete invariants of three‐dimensional molecular shape. Some basic properties of van der Waals surface graphs are studied, including their relationship with the Voronoi diagram of the atom centres, and a class of molecular embeddings is identified for which the dual of the van der Waals surface graph coincides with the van der Waals shape graph. This revised version was published online in July 2006 with corrections to the Cover Date.     

Second-order properties of bonds

By defining a localized hamiltonian for a saturated molecule its second order molecular properties may be regarded as a sum of bond contributions. Static and dynamic polarizabilities are calculated for the CH bond in methane and the latter one used to calculate the van der Waals interaction between two CH₄ molecules.     

1,1-Dichloroethane: a molecular crystal structure without van der Waals contacts?

Isochoric and isobaric freezing of 1,1-dichloroethane, CH3CHCl2, mp=176.19 K, yielded the orthorhombic structure, space group Pnma, with the fully ordered molecules, in the staggered conformation, located on mirror planes. The CH3CHCl2 ambient-pressure (0.1 MPa) structures were determined at 160 and 100 K, whereas the 295 K high-pressure structures were determined at 0.59 and 1.51 GPa. At 0.1 MPa, all intermolecular distances are considerably longer than the sums of the van der Waals radii, and only a pressure of about 1.5 GPa squeezed the Cl...Cl and Cl...H contacts to distances commensurate with these sums. The exceptionally large difference between the melting points of isomeric 1,1- and 1,2-dichloroethane can be rationalized in terms of their molecular-packing efficiency. It has been shown that the location of atoms in molecules affects their intermolecular interactions, and hence their van der Waals radii are the function of molecular structures.     

Van der Waals interactions: evaluations by use of a statistical mechanical method

In this work the induced van der Waals interaction between a pair of neutral atoms or molecules is considered by use of a statistical mechanical method. With use of the Schro?dinger equation this interaction can be obtained by standard quantum mechanical perturbation theory to second order. However, the latter is restricted to electrostatic interactions between dipole moments. So with radiating dipole-dipole interaction where retardation effects are important for large separations of the particles, other methods are needed, and the resulting induced interaction is the Casimir-Polder interaction usually obtained by field theory. It can also be evaluated, however, by a statistical mechanical method that utilizes the path integral representation. We here show explicitly by use of this method the equivalence of the Casimir-Polder interaction and the van der Waals interaction based upon the Schro?dinger equation. The equivalence is to leading order for short separations where retardation effects can be neglected. In recent works [J. S. H?ye, Physica A 389, 1380 (2010); Phys. Rev. E 81, 061114 (2010)], the Casimir-Polder or Casimir energy was added as a correction to calculations of systems like the electron clouds of molecules. The equivalence to van der Waals interactions indicates that the added Casimir energy will improve the accuracy of calculated molecular energies. Thus, we give numerical estimates of this energy including analysis and estimates for the uniform electron gas.     

A note on excess Gibbs energy models,equations of state and the local composition concept

A density-dependent local composition expression for the residual energy is derived from a generalized NRTL expression for the excess energy and the van der Waals fluid theory. Integration of this expression yields a volume-dependent expression for the Helmholtz energy from which equations of state utilizing the local composition concept are derived and which in the high-density limit contain the well-known activity coefficient models.The local composition versions of the Carnahan—Starling—van der Waals, the Redlich—Kwong—Soave and the Peng—Robinson equations of state are derived. It is further shown that the group contribution versions of the NRTL, the Wilson and the UNIQUAC excess models may be derived from the generalized NRTL expression for the residual energy when applied to groups instead of molecules.It is thus demonstrated that all current local composition activity-coefficient models can be derived from a local composition version of the van der Waals equation of state using different sets of assumptions. In the same way the van Laar, the Scatchard—Hildebrand and the Flory—Huggins activity coefficient models are obtained from the van der Waals equation of state using the original mixing rules.     

The van der Waals interactions as a tool for the interpretation of aromatic substitutions

The isoenergetic lines of van der Waals interaction between aromatic compounds and electrophilic or nucleophilic agents in parallel planes are drawn in order to detect the best approach channels for reagents in aromatic substitutions. The van der Waals interaction is comprised of the electrostatic, polarization and dispersion terms. It has been found that the channels depend on the molecules and on the positions of the centres attacked. The polarization term is shown to play a decisive role in electrophilic substitutions of phenanthrene and in nucleophilic substitutions of the pyridinium ion.     

Determination of van der Waals forces in monolayer films of lipids & biopolymers. Equation of state for two-dimensional films

Amphiphile molecules are characterized by the dual property arising from the interactions between the apolar [alkyl] and the polar part and the surrounding solvent, i.e., water. In assemblies which amphiphiles form in diverse systems, e.g., micelles, soap bubbles, monolayers or bilayers at interfaces, the attractive forces are attributed to the van der Waals forces. It is not easy to estimate the magnitude of van der Waals forces in some of these systems by any direct method.The magnitude of van der Waals forces in spread monolayers of lipids and biopolymers has been reported to be estimated from experimental data. The magnitude of these forces has been estimated by using an equation of state of a very general form, as delineated herein. In the current literature no such attempt has been reported in the analyses of these monolayers spread on aqueous surfaces. These analyses suggest that the predominant surface forces arise from van der Waals interactions, if the magnitude of electrostatic charge repulsions is weak. The equation-of-state as derived indicates that it is useful in providing information about the molecular interaction in monolayers, for both lipids and biopolymers.     

Donor–Acceptor Mechanism of Complex Formation

The gas-phase van der Waals complexes formed in the course of donor–acceptor interaction are shown to become coordination compounds only in the crystalline state. With close lengths of the covalent-ionic and coordination bonds, their energies differ by 100 kJ. This energy difference is due to the work spent to overcome van der Waals forces during the formation of complex ions from interacting molecules.     

An ab initio investigation of the O(3P)-H2(1sigma(g)+) van der Waals well

We report an ab initio study of the van der Waals region of the O(3P)-H2 potential energy surface based on RCCSD(T) calculations with an aug-cc-pVQZ basis supplemented by bond functions. In addition, an open-shell implementation of symmetry-adapted perturbation theory (SAPT) is used to corroborate the RCCSD(T) calculations and to investigate the relative magnitudes of the various contributions to the van der Waals interaction. We also investigate the effect of the spin-orbit coupling on the position and depth of the van der Waals well. We predict the van der Waals minimum to occur in perpendicular geometry, and located at a closer distance than a secondary well in colinear geometry. The potentials obtained in the present study confirm the previous calculations of Alexander [M. H. Alexander, J. Chem. Phys., 1998, 108, 4467], but disagree with the earlier work of Harding and co-workers [Z. Li, V. A. Apkarian and L. B. Harding, J. Chem. Phys., 1997, 106, 942] as well as with recently refitted surfaces of Brand?o and coworkers [J. Brand?o, C. Mogo and B. C. Silva, J. Chem. Phys., 2004, 121, 8861]. Inclusion of spin-orbit coupling reduces the depth of the van der Waals minimum without causing a change in its position.