Dipoles transient

Van der Waals forces arise from attractions between transient dipoles generated by the rapid movement of electrons on all neutral atoms. Significantly weaker than hydrogen bonds but potentially extremely numerous, van der Waals forces decrease as the sixth power of the distance separating atoms. Thus, they act over very short distances, typically 2-4 A. 

Attractive forces arise from dipole interaction, a result of the fluctuations in the cloud of counterions. Although the mean distribution of counterions is uniform along the length of the polyion, there are fluctuations in the cloud of counterions which induce transient dipoles. When two polyions approach each other counterion fluctuations become coupled and enhance the attractive force. Since polyions have a high polarizability these attractive forces can be considerable. 

A further limitation exists because Equation 8.3 is correct only in a vacuum. For molecules in a polarizable medium characterized by the dielectric constant, er, the effective transient dipole moment is... 

In the quantum mechanical approach, a transition moment is introduced for characterizing the transition between an initial state and a final state (see Box 2.2). The transition moment represents the transient dipole resulting from the displacement of charges during the transition therefore, it is not strictly a dipole moment. 

To describe solvatochromic shifts, an additional energy term relative to the solute should be considered. This term is related to the transition dipole moment that results from the migration of electric charges during an electronic transition. Note that this transient dipole has nothing to do with the difference pe — pg between the permanent dipole moment in the excited state and that in the ground state. 

Nonpolar molecules such as heptane and PE are attracted to each other by weak London or dispersion forces that result from induced dipole-dipole interactions. The temporary or transient dipoles are due to instantaneous fluctuations in the electron cloud density. The energy range of these forces is fairly constant and about 8 kJ/mol. This force is independent of temperature and is the major force between chains in largely nonpolar polymers, for example, those in classical elastomers and soft plastics such as PE. 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Even if molecules do not possess a permanent dipole moment weak forces can exist between them. The movement of the valence electrons creates transient dipoles , and these in turn induce dipole moments in adjacent molecules. The transient dipole in one molecule can be attracted to the transient dipole in a neighbouring molecule, and the result is a weak, short-range attractive force known as the London dispersion force. 

London (dispersion) forces Attractive forces between transient dipoles caused by random changes in the electron distribution of a molecule (one part of a molecule temporarily becomes slightly positively charged while another part becomes slightly negatively charged). 

The dispersion or London forces [43] between adsorbed nonpolar molecules and any adsorbent emerges when the transient dipoles, become correlated. As a result, the instantaneous dipole of the adsorbed nonpolar molecule induces a dipole in the adsorbent atoms, and, subsequently, both interact to lower the energy of the adsorbate-adsorbent system. Due to the correlation, the attraction between the instantaneous dipoles, developed in the entire system, does not vanish, and produces an induced-dipole-induced-dipole interaction energy, described by the following equation ... 

London dispersion interactions between transient dipoles of nonpolar but polarizable bodies ... 

Here, the electrons on each molecule create transient dipoles. They couple the directions of their dipoles to lower mutual energy. "Dispersion" recognizes that natural frequencies of resonance, necessary for the dipoles to dance in step, have the same physical cause as that of the absorption spectrum—the wavelength-dependent drag on light that underlies the dispersion of white light into the spectrum of a rainbow. 

Coefficients for the polarization created on a single small particle a by an electric field of magnitude E. (The coefficient a is sometimes broken into a contribution that is due to the field orientation of permanent dipoles /xaipoie and a contribution that is due to the field s induction of a transient dipole on a polarizable particle.) Polarization = amks-Emks or cgs cgs in either unit system. Similarly fi or /3mps or Pegs for single small particle b. 

Permanent dipole -Permanent dipole Permanent dipole -Induced dipole Transient dipole -Induced dipole... 

All atoms and molecules attract one another as a result of transient dipole-dipole interactions. A molecule need not have a net charge to participate in a dipolar interaction electron density can be highly asymmetric if interacting atoms have different electronegativities. 

Energy calculations for ionic lattices show 6a = 1l6. Molten salts can form transient dipoles and multipoles as ion pairs and clusters, but it is unlikely that these contribute to a dielectric constant. For charge transfer in molten salts, the equivalent of an FC process is the change in electroneutrality length as valence changes. 

Van der Waals-London binding is caused by the attraction between atoms when they are brought together in close proximity. These mteractions are basically electrostatic in nature and are appHcable to polarizable, noncharged molecules, whose structure allows the electron cloud around the molecule to be distorted by outside forces in such a way that a transient dipole is produced. Such polai ization results in... 

Dipole-induced dipole interactions. A permanent dipole induces a transient dipole in a nearby molecule by distorting its electron distribution (Figure 3.7b). For example, a carbonyl-containing molecule is weakly attracted to a hydrocarbon. Dipole-induced dipole interactions are weaker than dipole-dipole interactions. 

Dispersion force The dispersion or London force arises from the instantaneous transient dipoles that all molecules possess as a result of the changes in the instantaneous positions of electrons. The dispersion force which in fact is an induced dipole-induced dipole interaction depends on the polarizability of the interacting molecules and is inversely proportional to the sixth power of separation. In the case of, e.g., two CH4 molecules at a separation of 3 A, the dispersion interaction energy is of the order of — 1.1 kcal/mol. 

In reality there are several molecular vibrations that can couple to an electronic transition although the basic phenomenology is retained, namely the resonant character of the S St) 0-0 transition and the mirror symmetry of the vibronic satellites. In case of a jr—Mt transition the dominant vibrational modes are those of the polymer backbone, notably of the phenyl ring. An example is the absorption and fluorescence of it-conjugated molecules, such as tetracene, in the gas phase [22], In fluid solution there is interaction between the transient dipole of the molecule with the permanent and induced dipoles of the solvent. It gives rise to (i) a bathochromic shift of the spectra, (ii) a Stokes shift between the... 

When any two atoms approach each other closely, they create a weak, nonspecific attractive force called a van der Waals interaction. These nonspecific interactions result from the momentary random fluctuations in the distribution of the electrons of any atom, which give rise to a transient unequal distribution of electrons. If two noncovalendy bonded atoms are close enough together, electrons of one atom will perturb the electrons of the other. This perturbation generates a transient dipole in the second atom, and the two dipoles will attract each other weakly (Figure 2-8). Similarly, a polar covalent bond in one molecule will attract an oppositely oriented dipole in another. 

Weak and relatively nonspecific van der Waals Interactions are created whenever any two atoms approach each other closely. They result from the attraction between transient dipoles associated with all molecules (see Figure 2-8). 

In practical systems, the second-order molecular polarizability can be approximated by the fourth term of Eq. (5.27). Assuming here a two-level system where the permanent and transient dipole moments have the same direction (the x-axis), then the second-order molecular polarizability is expressed by Eq. (5.28),... 

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