Interactions between molecules electrostatic

Non-covalent interactions between molecules often occur at separations where the van der Waals radii of the atoms are just touching and so it is often most useful to examine the electrostatic potential in this region. For this reason, the electrostatic potential is often calculated at the molecular surface (defined in Section 1.5) or the equivalent isodensity surface as shown in Figure 2.18 (colour plate section). Such pictorial representations... 

For examining the interactions between molecules, use electrostatic charges. 

Approximating the intermoleculai interactions to only include two-body effects, e.g. electrostatic forces are only calculated between pairs of fixed atomic chai ges in force field techniques. Or the discrete interactions between molecules may be treated only in an average fashion, by using Langevin dynamics instead of molecular dynamics. 

Forces between macroscopic objects result from a complex interplay of the interaction between molecules in the two objects and the medium separating them. The basis for an understanding of intermolecular forces is the Coulomb1 force. The Coulomb force is the electrostatic force between two charges Qi and Q-2-... 

The third factor, ZR, in Eq. (5.1) is called the residual contribution in the chemical engineering notation and it arises from all kinds of non-steric interactions between molecules, i.e., usually from vdW, electrostatic, and hydrogen bond interactions. Despite its name, it is the most important contribution in most liquids. The basic assumption of surface-pair interaction models is that residual—i.e., non-steric—interactions can be described as local pairwise interactions of surface segments. The residual contribution is just the partition sum of an ensemble of pairwise interacting surface segments. 

The electrostatic part, Wg(ft), can be evaluated with the reaction field model. The short-range term, i/r(Tl), could in principle be derived from the pair interactions between molecules [21-23], This kind of approach, which can be very cumbersome, may be necessary in some cases, e.g. for a thorough analysis of the thermodynamic properties of liquid crystals. However, a lower level of detail can be sufficient to predict orientational order parameters. Very effective approaches have been developed, in the sense that they are capable of providing a good account of the anisotropy of short-range intermolecular interactions, at low computational cost [6,22], These are phenomenological models, essentially in the spirit of the popular Maier-Saupe theory [24], wherein the mean-field potential is parameterized in terms of the anisometry of the molecular surface. They rely on the physical insight that the anisotropy of steric and dispersion interactions reflects the molecular shape. 

In the development of the set of intermolecular potentials for the nitramine crystals Sorescu, Rice, and Thompson [112-115] have considered as the starting point the general principles of atom-atom potentials, proven to be successful in modeling a large number of organic crystals [120,123]. Particularly, it was assumed that intermolecular interactions can be separated into dispersive-repulsive interactions of van der Waals and electrostatic interactions. An additional simplification has been made by assuming that the intermolecular interactions depend only on the interatomic distances and that the same type of van der Waals potential parameters can be used for the same type of atoms, independent of their valence state. The non-electric interactions between molecules have been represented by Buckingham exp-6 functions,... 

The essential realization in this spontaneous ordering process is the importance of noncovalent bonding interaction between molecules, that is, supramolecular chemistry. These conformation-specific interactions are governed by weak forces including hydrogen bonding, metal coordination, van der Waals forces, pi-pi interactions, and electrostatic Coulombic effects. The cooperative action of multiple noncovalent interaction forces is precisely the path nature takes to produce shape and form. 

It is required to calculate the electrostatic interaction between molecules, winch possess a localized charge distribution, mathematically described by p(r") (where 7 = x,/ + x2j + x3 k in rectangular coordinates, and = x, x2 = y, and x3 = z), and are immersed in an external electric field with a potential, V( r ), given by a solid adsorbent. Subsequently, the electrostatic energy of the adsorbate-adsorbent system can be determined with the help of the following expression [46] ... 

Korona T, Moszynski R, Jeziorski B (2002) Electrostatic interactions between molecules from relaxed one-electron density matrices of die coupled cluster singles and doubles model. Mol Phys 100 1723-1734... 

Directional forces depend on the electrostatic interaction between molecules possessing a permanent dipole moment p due to their unsymmetrical charge distribution. When two dipolar molecules are optimally oriented with respect to one another at a distance r as shown in Fig. 2-3a, then the force of attraction is proportional to 1/r. An alternative arrangement is the anti-parallel arrangement of the two dipoles as shown in Fig. 2-3b. 

Dispersional Interaction between Molecules. We still wish to consider briefly energies due to interaction between fluctuating induced electric charge distributions of atoms and molecules. In constrast to electrostatic and induced interactions, these are present even when the molecules do not possess permanent electric moments. These dispersional interactions cannot be dealt with on a classical electrostatics level owing to their relation to London s quantum dispersion theory, they have been termed London dispersional interactions. 

For tliis class of mixtures, interactions between molecules of like species are different in kind for the two species. In particular, two molecules of the polar species experience a direct-electrostatic interaction and a (usually weak) induction interaction, in addition to the usual dispersion interaction here, the attractive forces are stronger than would be observed for a nonpolar species of similar size and geometry. Interactionbetween uiihke species, on the other liand, involves only the dispersion and (weak) induction forces. One therefore expects to be positive, only more so than for otlierwise similar NPNP mixtures. Experiment bears tliis out, on average (Fig. 16.5). 

The interaction between molecules that do not form chemical bonds arises from electrostatic, induction and dispersion effects. The energy of interaction... 

Although the structure of the PTCDA/Ag(l 11) interface is dominated by the molecule-substrate interaction (cf. the commensurate interface structure), the interaction between molecules does play an important role for the structural details and the energetics at the interface. On the one hand, there is the attractive electrostatic interaction between molecules. Because of this interaction PTCDA molecules always cluster in two-dimensional islands. However, at surface temperatures below 150 K, these islands do not yet exhibit the familiar herringbone structure [35] since the electrostatic interaction does not exhibit sufficient directional specificity, a considerable degree of structural disorder prevails, in spite of a clear propensity of the molecules to arrange in a T-like... 

As well as strong covalent bonds, there are weaker, electrostatic interactions between molecules that influence their properties, one of the most important of... 

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