Dipole momentary

Figure 11.7 van der Waals bond caused by the creation of an instantaneous dipole. Momentary variations in the electron charge distribution of an atom causes a momentary dipole attraction between the asymmetric negative charge and the positive nuclear charge of another atom. 

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... 

This intermolecular attraction occurs in all substances, but is significant only when the other types of intermolecular forces are absent. It arises from a momentary distortion of the electron cloud, with the creation of a very weak dipole. The weak dipole induces a dipole in another nonpolar molecule. This is an extremely weak interaction, but it is strong enough to allow us to liquefy nonpolar gases such as hydrogen, H2, and nitrogen, N2. If there were no intermolecular forces attracting these molecules, it would be impossible to liquefy them. 

A van der Waals bond (B) is formed between apolar molecular groups that have come into close proximity. Spontaneous transient distortion of electron clouds (momentary faint dipole, 55) may induce an opposite dipole in the neighboring molecule. The van der Waals bond, therefore, is a form of electrostatic attraction, albeit of very low strength (inversely proportional to the seventh power of the distance). 

London Force intermolecular force that arises when momentary temporary dipoles form in nonpolar molecules Long-Range Order general term to describe regular repeating structure found in crystals... 

As noted above, London dispersive interactions occur even between molecules of apolar compounds like alkanes, that on average over time exhibit a rather smooth distribution of electrons throughout the whole molecular structure. This interaction occurs in all chemicals because there are momentary (order of femtosecond timescales) displacements of the electrons within the structure such that short-lived electron-rich and electron-poor regions temporarily develop. This continuous movement of electrons implies the continuous presence of short-lived dipoles in the structure. This fleeting dipole is felt by neighboring molecules whose electrons respond in a complementary fashion. Consequently, there is an intermolecular attraction between these molecular regions. In the next moment, these attractive interactions shift elsewhere in the molecule. 

Even in atoms in molecules which have no permanent dipole, instantaneous dipoles will arise as a result of momentary imbalances in electron distribution. Consider the helium atom, for example. It is extremely improbable that the two electrons in the Is orbital of helium will be diametrically opposite each other at all times. Hence there will be instantaneous dipoles capable of inducing dipoles in adjacent atoms or molecules. AnothCT way of looking at this phenomenon is to consider the electrons in two or more "nonpolar" molecules as synchronizing their movements (at least partially) to minimize electron-electron repulsion and maximize electron-nucleus attraction. Such attractions are extremely short ranged and weak, as are dipole-induced dipole forces. The energy of such interactions may be expressed as... 

VAN DER WAALS FORCES. Interatomic or intermolecular forces of attraction due to the interaction between fluctuating dipole moments associated with molecules not possessing permanent dipole moments. These dipoles result from momentary dissymmetry in the positive and negative charges of the atom or molecule, and on neighboring atoms or molecules. These dipoles tend to align in antiparallel direction and thus result in a net attractive force. This force varies inversely as the seventh power of the distance between ions. 

Movement of an electron from the ground electronic state of a molecule to an excited state creates a momentary dipole, called an electric transition dipole. Thus, associated with each electric transition is a polarization (electric transition dipole moment) that has both direction and intensity which vary according to the nature of the chromophore and the particular excitation. When two or more chromophores lie sufficiently close together, their electric transition dipoles may interact through dipole-dipole (or exciton) coupling. Exciton coupling arises from the interaction of two (or more) chromophores through... 

Electrophilic addition is an important reaction for alkenes. When you see an alkene on the MCAT, check for electrophilic addition. An electrophile j.s an eiec-LrOn-loving species, so il will have at leas I a partially positive charge, even if it is only from a momentary dipole. The double bond of an alkene is an electron-rich environment and will attract electrophiles,... 

Induced dipole-induced dipole interactions, also called London forces, result when a nonpolar molecule undergoes a distortion, perhaps as a result of a collision, which results in a momentary separation of the centers of positive and negative charge. The resulting dipole induces a dipole in a neighboring molecule, resulting in an attractive interaction. 

London dispersion forces are the weakest of the intermolecular forces and occur between all molecules. These are the only types of intermolecular forces that are possible between nonpolar molecules and are caused by momentary dipoles. Experimental evidence suggests that electrons are not symmetrically distributed about the nucleus at all times. On average, the electrons may be spread out evenly around the nucleus, but there are brief instants when the electron density may be greater on one side of the atom than another. During these periods of time, the atoms develop a temporary or instantaneous polarity. The temporary polarity (which is the cause of the momentary dipole) allows for attraction between particles that are normally nonpolar. London dispersion forces tend to increase as the size and mass of the molecule increase. 

The idea of correlating momentary multipoles stands behind the customary modeling of dispersion interaction in the form of a multipole expansion, including dipole-dipole (D-D), dipole-quadrupole (D-Q), quadrupole-quadrupole (Q-Q), and so on, terms. We owe the earliest variational treatments of this problem not only to Slater and Kirkwood [34], but also to Pauling and Beach [35], However, when the distance R decreases and reaches the Van der Waals minimum separation, the assumption that electrons of A and B never cross their trajectories in space becomes too crude. The calculation of the intermonomer electron... 

A qualitative interpretation of solvent shifts is possible by considering (a) the momentary transition dipole moment present during the optical absorption, (b) the difference in permanent dipole moment between the ground and excited state of the solute,... 

Note from Table 16.2 that the freezing point rises going down the group. The principal cause for this trend is that as the mass (and the atomic number) increases, the number of electrons increases, so there is an increased chance of the occurrence of momentary dipoles. We say that large atoms with many electrons exhibit a higher polarizability than small atoms. Thus the importance of London dispersion forces greatly increases as atomic size increases. 

London dispersion forces the forces, existing among noble gas atoms and nonpolar molecules, that involve an accidental dipole that induces a momentary dipole in a neighbor. (16.1) Lone pair an electron pair that is localized on a given atom an electron pair not involved in bonding. (13.9)... 

The momentary dipoles set up a field which polarizes the adjacent molecules or atoms and the induced moments tend to keep in phase with the oscillation of the original dipoles. Calculation for the ideal case of the interaction of two harmonic oscillators leads to the following expression for the energy of interaction (see Chapter i8). 

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. 

Figure 1.8.2. The sodium ion is inducing a temporary dipole In an atom with a normally spherical electron cloud. The induced dipole is causing another atom to have a momentary dipole. These temporary dipole attractions are sufficient to cause even noble gases to condense at low temperatures.

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