Displacement of electrons

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

These various effects combine to result in different rate and equilibrium constants for homologous reactions. The substituents influence these parameters in part by displacements of electron density of the first two types and in part by geometric effects of the last type. When some of the possible complications are considered, it is not surprising that there is no generally useful method for correlating the Gyxn terms with satisfactory accuracy. However, there are a number of special cases in which useful correlations can be developed. This is particu-... 

Inductive or polar effects. These effects involve electron displacements that are transmitted along a chain of atoms without any reorganization of the formal chemical bonds in the molecule. For example, the introduction of a methyl group in a pyridine ring involves a displacement of electrons to the nitrogen atom from the methyl group. This effect falls off rapidly with separation distance. 

The materials and design of the various photoelectric detectors available are such that the absorption of radiation results in the displacement of electrons and hence in the development of a potential difference between two electrodes. The main types of photoelectric detectors may be classified as either photovoltaic or photoconductive (Figure 2.24). 

Thallium(III) substituents (see 36) exert even larger effects on a-caibon atoms than mercury does (141) (see 37 and 38) this has been ascribed to displacement of electrons in the carbon-metal a-bond toward the metal atom, due to its increased nuclear charge. 

The second shortcoming of a minimal (or split-valence) basis set... functions being centered only on atoms. . . may be addressed by providing d-type functions on main-group elements (where the valence orbitals are of s and p type), and (optionally) p-type functions on hydrogen (where the valence orbital is of s type). This allows displacement of electron distributions away from the nuclear positions. 

Si02, AI2O3, XH2O has beensuggested by Hansford (82) (Fig.21b), Tamele (80), in a development of the Thomas interpretation, pictures a displacement of electron along Al-O-Si bonds, the acidity arising because of the tendency of aluminium to accept an electron pair, thus completely filling the p-orbitals (Fig. 21c). All these concepts are based on the assumption that the acid centers important in these catalytic reactions exist independently on the catalyst. On the other hand, Milliken et al. (81) consider that the acid centers are only stable in the presence of the basic substrate. 

Protons that could logically be involved in a membrane Bohr effect are those present on imidazole rings coordinated to Fe or Cu in redox proteins. Removal of an electron from the metal ion could be accompanied by displacement of electrons within the imidazole, within a peptide group that is hydrogen-bonded to an imidazole, or within some other acidic group. A hypothetical example is illustrated in Eq. 18-12 in which a carboxyl group loses a proton when "handed" a second. If the transiently enolized peptide linkage formed in... 

The inductive effect is die ability of a substituent or group near die acidic proton to alter the electron distribution at die reaction center by through-bond displacement of electrons. The result is diat substituents which withdraw electrons from die reaction center by die inductive effect stabilize anions and thus increase the acidity of die conjugate acids of diose anions. Conversely, groups which donate electrons make die reaction center more electron rich and thus make die formation of die anion at diat center more difficult. The conjugate acid is dius a weaker acid. 

The displacement of electrons belonging to alkyl groups towards the aromatic ring, and the consequent increase in electron density of the ring reduce the rate of exchange as compared with benzene. This is shown by the following rate constants (sec-1) for C KNDj = 0-02n at 25° ... 

Ultraviolet, visible, and infrared spectroscopies refer to analysis of the absorption characteristics of a sample that are linked to various electronic and vibrational transitions within a molecule [17]. These involve relative displacements of electrons and nuclei that are able to couple to incident light if they induce a dipole in the material. The strength of this coupling is measured by the transition dipole moment [17],... 

Absorption in the ultraviolet (k 100 - 400 nm) and the visible (A. 400 - 800 nm) is primarily the result of transitions in the electronic state of the molecule. In such a process, the transition dipole moment would be proportional the overlap in the densities of the charge distributions between the two electron orbitals involved in a transition. The periodic displacement of electrons from one state to another will cause the charge distribution to be anisotropic, with net negative and positive contributions in certain locations within the molecule. The result is the formation of a dipole moment. Very often, dye molecules that absorb in the visible are dispersed within a sample or attached to the molecules of a sample and are used to monitor its degree of alignment. However, since the relative orientation of such a dye molecule to the molecular axes of the constituent sample molecules is often unknown, the interpretation of these measurements can be difficult. 

In this equation m and x are the mass and displacement of electron, respectively and Eext an intensity of an external electric field. The term... 

Permanent displacement of electrons along a certain chain when some atom or group of atoms with different electronegativity than carbon is attached to carbon chain is called inductive effect. For illustration consider a carbon chain which has a chlorine atom attached to one end. 

If the electronegativity of the atom or group of atoms attached to the carbon chain is less than the electronegativity of the carbon then the displacement of electrons takes place away from group along the C-chain and this effect is called the Electron Releasing Inductive Effect or (+I)-Effect. This inductive effect can be represented as shown below ... 

It may be noted that in conjugated systems (having alternate single and double bonds) resonance causes displacement of electrons from one part of the system to another part creating centres of high and low electron density. This is called resonance effect. 

The polarizability of a polar molecule has two components an electronic polarizability, ye, due to the displacement of electrons, and an orientational polarizability, y, due to the rotation of the permanent dipole in an electric field. In the vapor phase, assuming that the molecule can rotate freely, the orientational polarizability is given by the Langevin function. Therefore, for low effective electric fields (pEe- kT)... 

Here aromatic bonds are designated with dotted curved lines. Plain curved lines are n- bonds, three dots signify weaker %- bonds (as in 2,3-butadiene), the dotted arrow (->) represents shifting of n- electrons into the cation field, and the curved arrow ( r )) the displacement of electrons. These denote the most important forms of the conjugations. 

A double bond in the aj8 position to the carboxyl group lowers the expansion temperature much less than in the middle of the chain (c). Hughes3 has shown that it approximately doubles the surface potential, however, presumably through the displacement of electrons near the carboxyl head rendering the a carbon more negative and the jS more positive than in the saturated chain. A double bond in the middle of the chain increases the surface potential of the molecule only slightly, but there is still a slight increase in dipole moment when a double bond is present in the middle of the chain. 

Fig. 8-3 Probable mechanism of bond cleavage by lysozyme. The thick solid line structure is the substrate, and the lightface groups are on the enzyme. The small arrows indicate displacement of electron pairs during the reaction.

When a substance is placed in an electric field, such as exists between the plates of a charged capacitor, it becomes to some extent electrically polarized. The polarization results at least in part from a displacement of electron clouds relative to atomic nuclei polarization resulting from this cause is termed electronic polarization. For molecular substances, atomic polarization may also be present, owing to a distortion of the molecular skeleton. Taken together, these two kinds of polarization are called distortion polarization. Finally, when molecules possessing permanent dipoles are present in a liquid or gas, application of an electric field produces a small preferential orientation of the dipoles in the field direction, leading to orientation polarization. 

If a material can be electrically polarized under an external electric field and the polarization inverts in response to the alternating electric field, the material is designated as a dielectric. The origins of the polarity inversion are classified as shown in Fig. 20 (a) electric polarization, which is caused by the displacement of electrons relative to the nucleus (b) ionic polarization caused... 

Upon surmounting this barrier, a complete displacement of electron pairs is achieved with the creation of a Mg-O bond and a C-C bond. The overall process is energetically favorable because (i) it ultimately leads to the formation of a very stable covalent C-C bond and an ionic Mg-O bond and (ii) its course is governed by the intrinsic tendency of magnesium to lose electrons and of oxygen to acquire electrons. 

Such a dethroning of carbocations and carbanions can be carried another step further. Certain purely covalent compounds possessing weakly polarized bonds can serve as ionic reagents if their electronic system is highly polarizable. In such molecules the approach of a charged particle, or even a dipole, induces a significant displacement of electrons. The electron displacement is to such an extent that the reaction intermediates may become almost fully ionized, as if an ionic reagent had been actually used. A typical example is represented by the previously mentioned reactions of electrophilic substitution in aromatic series, where a neutral molecule of an aromatic hydrocarbon, ArH, behaves as an efficient equivalent to a carbanion, Ar . 

An important achievement of quantum mechanics is the concept of band structure for the energies that electrons can assume, see fig. 3.68. Just as in atoms, where electrons are confined to certain orbitals, between which they can "Jump" across a forbidden zone, in crystals there are bands of a given width between which there are forbidden zones that under some conditions can be crossed. As is the case with atomic orbitals the number of electrons that can be accommodated in a band is limited by the Pauli principle. As displacement of electrons under the influence of an applied electric field is not possible for a completely filled band, such bands do not contribute to the conductivity. In the... 

These properties are ooUeoted in Table 31. The etructure of thiece 1.1-dioxide has been oonfirmed by nudear magiwtio resonance measure meints. o The eleolzon density on G-3 seems to be weaker than that on C-2. which suggests a displacement of electrons according to the following scheme ... 

---