Dipole temporary

Whichever name it is given, the origin of this attraction is the mushy electron cloud that surrounds the nitrogen molecule. Because the electrons can be considered mobile in the electron cloud, they can be pictured as congregating momentarily at one end of the molecule or the other. This momentary uneven distribution of electrons is termed a temporary dipole, but it acts in the same manner as a permanent dipole. It is attracted to other dipoles, temporary or otherwise. The redistribution of electrons may be spontaneous, or if there is an ion or a molecule with a permanent dipole in the vicinity, this species might induce a momentary dipole, too. This situation is shown in figure 1.8.2. 

The electric field of a molecule however is not static but fluctuates rapidly Although on average the centers of positive and negative charge of an alkane nearly coincide at any instant they may not and molecule A can be considered to have a temporary dipole moment... 

The neighboring molecule B feels the dipolar electric held of A and undergoes a spon taneous adjustment m its electron positions giving it a temporary dipole moment that is complementary to that of A... 

Figure 2.8 Attractive dispersion forces in nonpolar molecules are caused by temporary dipoles, as shown in these models of pentane, C5H12-...

Dispersion force. Temporary dipoles in adjacent molecules line up to create an electrical attraction force known as the dispersion force. Deeply shaded areas indicate regions where the electron cloud is momentarily concentrated and creates partial charges, indicated by (+) and (-). 

The most common type of intermolecular force, found in all molecular substances, is referred to as a dispersion force. It is basically electrical in nature, involving an attraction between temporary or induced dipoles in adjacent molecules. To understand the origin of dispersion forces, consider Figure 9.8. 

This temporary dipole induces a similar dipole (an induced dipole) in an adjacent molecule. When the electron cloud in the first molecule is at 1 A, the electrons in the second molecule are attracted to 2A. These temporary dipoles, both in the same direction, lead to an attractive force between the molecules. This is the dispersion force. 

This implies that Br2 is the electrophile. But how does Br2 function as an electrophile The bond between the two bromine atoms is a covalent bond, and we therefore expect the electron density to be equally distributed over both Br atoms. However, an interesting thing happens when a Br2 molecule approaches an alkene. The electron density of the pi bond repels the electron density in the Br2 molecule, creating a temporary dipole moment in Br2. 

At any given instant, because electrons move, the electrons and therefore the charge may not be uniformly distributed => a small temporary dipole will occur. 

This temporary dipole in one molecule can induce opposite (attractive) dipoles in surrounding molecules. 

Figure 2.8 Temporary dipoles and induced dipoles in nonpolar molecules resulting from a nonuniform distribution of electrons at a given instant.

These temporary dipoles change constantly, but the net result of their existence is to produce attractive forces between nonpolar molecules. 

Induced-dipole/induced-dipole forces, also called temporary dipoleAemporary dipole forces, are weak attractive forces that exist between all molecules. They arise when an instantaneous imbalance in the electron distribution in a molecule induces a corresponding imbalance in neighbouring molecules, leading to a weak electrostatic attraction. 

The existence of an attractive force between non-polar molecules was first recognized by van der Waals, who published his classic work in 1873. The origin of these forces was not understood until 1930 when Fritz London (1900-1954) published his quantum-mechanical discussion of the interaction between fluctuating dipoles. He showed how these temporary dipoles arose from the motions of the outer electrons on the two molecules. 

The great American scientist G. N. Lewis coined the word covalent, early in the 20th century. He wanted to express the way that a bond formed by means of electron sharing. Each covalent bond comprises a pair of electrons. This pairing is permanent, so we sometimes say a covalent bond is a formal bond, to distinguish it from weak and temporary interactions such as induced dipoles. 

Dispersion forces result from temporary dipoles caused by polarization of electron clouds... 

These types of attractions occur when the charge on an ion or a dipole distorts the electron cloud of a nonpolar molecule. This induces a temporary dipole in the nonpolar molecule. These are fairly weak interactions. Like an ion-dipole force, this type of force requires the presence of two different substances. 

This intermolecular attraction occurs in all substances. It is usually only significant for nonpolar substances. It arises from the momentary distortion of the electron cloud. This distortion causes a very weak temporary dipole, which... 

How dispersion forces develop between identical non-polar molecules. In A, neither molecule interacts with the other. In B, one molecule becomes, instantaneously, a dipole. At that moment, the dipolar molecule is able to induce a temporary charge separation in the other molecule, resulting in a force of attraction between the two. All the molecules within a sample undergo this same process, as shown in C. 

It is noteworthy that the neutron work in the merging region, which demonstrated the statistical independence of a- and j8-relaxations, also opened a new approach for a better understanding of results from dielectric spectroscopy on polymers. For the dielectric response such an approach was in fact proposed by G. Wilhams a long time ago [200] and only recently has been quantitatively tested [133,201-203]. As for the density fluctuations that are seen by the neutrons, it is assumed that the polarization is partially relaxed via local motions, which conform to the jS-relaxation. While the dipoles are participating in these motions, they are surrounded by temporary local environments. The decaying from these local environments is what we call the a-process. This causes the subsequent total relaxation of the polarization. Note that as the atoms in the density fluctuations, all dipoles participate at the same time in both relaxation processes. An important success of this attempt was its application to PB dielectric results [133] allowing the isolation of the a-relaxation contribution from that of the j0-processes in the dielectric response. Only in this way could the universality of the a-process be proven for dielectric results - the deduced temperature dependence of the timescale for the a-relaxation follows that observed for the structural relaxation (dynamic structure factor at Q ax) and also for the timescale associated with the viscosity (see Fig. 4.8). This feature remains masked if one identifies the main peak of the dielectric susceptibility with the a-relaxation. 

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. 

The major forms of van der Waals forces between molecules that are not bonded together are the permanent dipole-dipole interaction, the dispersion-induced temporary dipole interaction, and the hydrogen bond. They are short-range forces that operate only when two atoms or molecules are in close proximity. The Lennard-Jones potential of 6-12 is a model of this potential field ... 

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