Dipole Chemistry

6 Key excerpts on "Dipole Chemistry"

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  Prediction of Transport and Other Physical Properties of Fluids

  International Series of Monographs in Chemical Engineering

  • S. Bretsznajder, P. V. Danckwerts(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  This phenomenon is known as polarization and is associated with the following pro-cesses: 1. The formation of an induced dipole owing to the displacement of electrons (electron polarization). 2. The shift or rotation of the atoms and polar groups of atoms in the molecules (atom polarization). 3. If the molecules present in the field have dipole moments, the orientation of the molecules to the position corresponding to the minimum energy of the system (orient-ation polarization). A measure of the ease with which a substance may be polarized is provided by the so-called molar polarization P. This quantity may be defined by the equation _ 4πΝ(χ α is the polarizability of the molecules and is discussed further below. The three processes E S T I M A T I N G P H Y S I C O -C H E M I C A L P R O P E R T I E S 21 listed above gives rise to corresponding contributions P eU P at and P or to the quantity P, so that Ρ = P el +P at +P or (1.9) (All these quantities refer to 1 mole of the substance.) A relationship exists between the molar polarization Ρ and the dielectric constant ε. (ε is equal to the ratio of the capacity C of a condenser containing the given compound between the plates to the capacity C 0 of the same condenser with vacuum between the plates.) For gases and dilute so-lutions of polar molecules in non polar solvents the relationship is ρ = cm 3 /mole (1.10) e+2 ρ where M is the molecular weight, and ρ is the density. If the induced dipole is produced by a field of intensity F, then £ = a F (1.11) where a denotes the polarizability of the molecule and is equal to the magnitude of the dipole moment induced by a field of unit intensity. The polarizability a, like the polari-zation P, has the dimension of volume (cm 3 ).

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  Fundamentals Of Atomic Force Microscopy - Part I: Foundations

  Part I: Foundations

  • Ronald G Reifenberger(Author)
  • 2015(Publication Date)
  • World Scientific

    (Publisher)

  The near-field local electric fields generated by molecular dipoles are often sufficiently strong to remove atoms from compounds with high melting temperatures. As an example, common table salt (NaCl) is known to melt at 1074 K (800 ◦ C) yet it readily dissolves at room temperature in water, a polar solvent having a molecular dipole moment of 1.8 D. Also, the boiling point of common polar solvents is correlated with increasing dipole moment as shown in Fig. 2.11, indicating that dipole– The Force between Molecules 41 dipole inter-molecular interactions play an important role in determining the physical properties of liquids. 2.6 Dipole Moments in External Electric Fields Before ending this review, it is useful to mention a few of the electro-static consequences of a molecule with a permanent dipole moment. First, the electric field that develops around a dipole has interesting properties. Because the dipole has charges of equal but opposite polarity, the electric field far from the dipole will be small but in close proximity to the dipole, the electric fields can be large and highly non-uniform. This is schematically illustrated in Fig. 2.12 by plotting the dipolar electric field that develops when two charges + q , − q are separated by a distance d . The magnitude of the dipole moment is given by | p | ≡ p = | q | d. (2.20) The direction of p is defined from the negative to the positive charge. It is also useful to consider the work required to rotate a dipole, fixed at a point in space but free to rotate about its midpoint when placed in Fig. 2.12 An electric dipole is formed when two equal but opposite charges are displaced and rigidly held apart by a distance d . Such a charge configuration can be characterized by an electric dipole moment of magnitude p = qd . The direction of p points from negative to positive charge. The spatial dependence of the electric field that develops in close proximity to the dipole is schematically plotted.

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  Klein's Organic Chemistry

  • David R. Klein(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  All intermolecular forces are electrostatic—that is, these forces occur as a result of the attrac- tion between opposite charges. The electrostatic interactions for neutral molecules (with no formal charges) are often classified as (1) dipole-dipole interactions, (2) hydrogen bonding, and (3) fleeting dipole-dipole interactions. Dipole-Dipole Interactions Compounds with net dipole moments can either attract each other or repel each other, depending on how they approach each other in space. In the solid phase, the molecules align so as to attract each other (Figure 1.43). 34 CHAPTER 1 A Review of General Chemistry FIGURE 1.43 In solids, molecules align themselves so that their dipole moments experience attractive forces. H 3 C H 3 C O C δ+ δ+ δ+ δ− δ− δ− O C O C H 3 C H 3 C H 3 C H 3 C In the liquid phase, the molecules are free to tumble in space, but they do tend to move in such a way so as to attract each other more often than they repel each other. The resulting net attraction between the molecules results in an elevated melting point and boiling point. To illustrate this, com- pare the physical properties of isobutylene and acetone: Isobutylene Melting point = –140.3°C Boiling point = –6.9°C CH 2 Acetone Melting point = –94.9°C Boiling point = 56.3°C O C H 3 C CH 3 C H 3 C CH 3 Isobutylene lacks a significant dipole moment, but acetone does have a strong net dipole moment. Therefore, acetone molecules will experience greater attractive interactions than isobutylene mol- ecules. As a result, acetone has a higher melting point and higher boiling point than isobutylene. Hydrogen Bonding The term hydrogen bonding is misleading. A hydrogen bond is not actually a “bond” but is just a specific type of dipole-dipole interaction. When a hydrogen atom is connected to an electronega- tive atom (usually O or N), the hydrogen atom will bear a partial positive charge (δ+) as a result of induction.

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  Theory of Electric Polarization

  Dielectrics in Static Fields

  • Bozzano G Luisa(Author)
  • 2012(Publication Date)
  • Elsevier Science

    (Publisher)

  We then have: m = le. (1.2) Therefore, the electric moment of a system of charges with zero net charge is generally called the electric dipole moment of the system. A simple case is a system consisting of only two point charges +e and — e at a distance I. Such a system is called a (physical) electric dipole, its moment is equal to el, the vector / pointing from the negative to the positive charge. In theoretical organic chemistry the dipole vector is generally taken as pointing from the positive to the negative charge. We prefer to use the physical definition given above. A mathematical abstraction derived from the above defined physical dipole is the ideal or point dipole. Its definition is as follows: the distance / between two point charges + e and — e is replaced by l/n and the charge e by en. The limit approached as the number η tends to infinity is the ideal dipole. The equations derived for ideal dipoles are much simpler than those obtained for non-ideal dipoles. Many neutral molecules are examples of charge systems with a non-ideal electric dipole moment, since in most types of molecule the centres of gravity of the positive and negative charge distributions do not coincide. Apart from these permanent or intrinsic dipole moments, a temporary or induced dipole moment arises when a particle is brought into an external electric field. Under the influence of this field the positive and negative charges in the particle are moved apart: the particle is polarized. In general, these induced dipoles can be treated as ideal; permanent dipoles, however, may generally not be treated as ideal when the field at molecular distances is to be calculated (see section 2). ELECTRIC DIPOLES AND MULTIPOLES 11 The values of molecular dipole moments are usually expressed in Debye units. The Debye unit, abbreviated as D, equals 1 0 1 8 electrostatic units (e.s.u.). The permanent dipole moments of non-symmetrical molecules generally lie between 0.5 and 5D.

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  Atomic Clusters

  Introduction to the Nano World

  • Michel Broyer, Patrice Mélinon(Authors)
  • 2024(Publication Date)
  • EDP Sciences

    (Publisher)

  Chapter 6 Dipole and electrical polarizability 6.1 Introduction The permanent electric dipole moment is a property of the ground state. It is non-zero in a cluster when the barycenters of the positive and negative charges do not coincide. This permanent dipole moment noted µ dip,e is the product of a charge (in Coulomb) by a length (in meter) (C.m). Indeed the simplest dipole is a couple of two charges q of opposite signs of distance d (µ dip,e = qd). It is most often expressed in Debye (1 D = 3.335 641 010 −30 C.m). This dipole always exists in heterogeneous systems like ionic compounds, iono-covalent sys- tems like SiC, bimetallic clusters, etc. except for particular symmetries (figure 6.1). In homogeneous clusters like A n , it is zero for a spherical symmetry struc- ture and depends on the geometry. For M n (M metal), it will depend on the geometry and on the screening of cations by delocalized electrons. If the metal is “perfect”, the screening is total and the electric dipole will be zero. The mea- surement of this (residual) dipole in a metallic system will be a fairly direct measure of the more or less metallic character of the bond, a very general ques- tion already addressed in the previous chapters. As we shall see, this dipole remains weak in the case of metals and is therefore difficult to measure. When an external electric field F is applied to a molecule or a cluster, it induces a dipole µ ind which is of first order proportional to the applied field: μ ind = α.F (6.1) α is the polarizability tensor. In general, this tensor is not a scalar, but in most of the systems studied (especially in this chapter) the anisotropic part of this tensor is small and one can settle for the average value during molecular rotation α avg and write: μ ind = α avg .F (6.2) with α avg = (1/3)T r(α) (trace of the matrix) The measurement of α avg is important because it will give information on the electronic and structural properties of the clusters.

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  Introduction to Electromagnetism

  • M. Sibley(Author)
  • 1995(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The dipole momentis a vector quantity with direction towardsthe positive charge. The reasonfor this choice ofdirection will becomeclearin the next section. In most materials,the separationbetweenthechargesis directly dependent on the magnitudeof the external E field. However,if we continuallyincrease the E field, therecomes apointwherewe cannotpolarizethe atomany more. 8 Dielectrics In. this chapte r we will re-examine capacitor s and, in particular , the effect of an electric field on dielectrics , As we saw in Chapte r 2» dielectrics have a permittivity greate r than air. Thus, when we use them as the insulating materia l in a capacitor , they increase the capacitance « If the potentia l across the plates is constant , the stored charge will increase and so it is generall y desirabl e to design a capacito r with a dielectric, The next section introduces the dipole momen t that occurs when an electric field distorts an atom, or molecule . 6,1 ELECTRIC DSPOLES AND DIPOLE MOMENTS If we have an. atom thai is well away from any externa l electric field, the orbit of the electrons will describe a spher e with the nucleus at the centre (see Fig. 6.1(a)). If we place this atom in an electric field, the field will distort the atom as Fig, 6.1(b) shows , In effect, the electron cloud moves away from the field» whereas the positiYely charged nucleus moYes in the direction of the field. The atom is said to be polarized by the electric field. Although the polarized atom is still electricall y neutral , on the microscopi c scal e the atom has an electric field, This becomes cleare r if we replace the electron cloud by a point source at a distance d from the positive nucleus .

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