Polar Molecule

6 Key excerpts on "Polar Molecule"

• 

  eBook - ePub

  Intermolecular and Surface Forces

  • Jacob N. Israelachvili(Author)
  • 2010(Publication Date)
  • Academic Press

    (Publisher)

  4 Interactions Involving Polar Molecules

  4.1 What Are Polar Molecules?

  Most molecules carry no net charge, but many possess an electric dipole . For example, in the HCl molecule the chlorine atom tends to draw the hydrogen’s electron toward itself, and this molecule therefore has a permanent dipole. Such molecules are called dipolar or simply polar molecules. The dipoles of some molecules depend on their environment and can change substantially when they are transferred from one medium to another, especially when molecules become ionized in a solvent. For example, the amino acid molecule glycine contains an acidic group on one side and a basic group on the other. In water at neutral pH, the NH2 group acquires a proton and the OH group loses a proton to the solution to produce a diPolar Molecule:

  Quite often the magnitude of the positive and negative charges are not the same, and these molecules therefore possess a net charge in addition to a dipole. Such molecules are then referred to as dipolar ions . Polarity can also arise from internal charge displacements within a molecule, producing zwitterionic molecules or groups. In larger molecules, or “macromolecules,” such as proteins the net dipole moment is usually made up of a distribution of positive and negative charges at various locations of the molecules. It should already be apparent that the interactions and the solvent effects of Polar Molecules can be very complex.

  The dipole moment of a Polar Molecule is defined as

  (4.1) where l is the distance between the two charges +q and –q . The direction of the dipole moment is as shown in the above figure. For example, for two electronic charges q = ±e separated by l = 0.1 nm, the dipole moment is u = (1.602 × 10−19 ) (10−10 ) = 1.6 × 10−29 C m = 4.8 D. The unit of dipole moment is the Debye , where 1 Debye = 1 D = 3.336 × 10−30 C m, which corresponds to two unit charges separated by about 0.2 Å (~0.02 nm). Small Polar Molecules have moments of the order of 1 D, some of which are listed in Table 4.1 . Permanent dipole moments occur only in asymmetric molecules and thus not in single atoms. For isolated molecules, they arise from the asymmetric displacements of electrons along the covalent bonds, and it is therefore not surprising that a characteristic dipole moment can be assigned to each type of covalent bond. Table 4.1 also lists some of these bond moments , which lie parallel to the axis of each bond. These values are approximate but very useful for estimating the dipole moments of molecules and especially of parts of molecules by vectorial summation of their bond moments. For example, the dipole moment of gaseous H2 O, where the H—O—H angle is θ

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  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 ).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Molecular Physics

  • Dudley Williams(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  7. ELECTRIC PROPERTIES OF MOLECULES* A great many molecular phenomena have their origin in the electric properties of individual molecules. The most important electric proper-ties of a neutral molecule are its permanent dipole moment and polariza-bility. Through these the molecular motion is coupled to external electric fields giving rise to a host of effects including: absorption, emission, and scattering of radiation; refraction of light, polarization of dielectrics, and Stark effect. Absorption, emission and scattering of radiation were dis-cussed in Part 2. Refraction, polarization of dielectrics, and Stark effect are discussed in this part. Of the higher electric multipole moments only the molecular quadrupole can be singled out as producing observable effects. These are discussed in Chapter 7 .4. f An additional phenomenon which can be attributed to a characteristic of the molecular charge distribution which is not revealed by the lower multipole moments is optical activity. Optical activity is discussed in Chapter 7 .5. When atoms unite to form a molecule the atomic charge distributions are altered to such an extent that a permanent molecular dipole moment frequently results. This dipole moment is a vector quantity with com-ponents defined by equations of the form where qi is the charge on the ith particle, and Xi is its x coordinate. Mole-cules with permanent dipole moments are called Polar Molecules. If a Polar Molecule is placed in a static electric field it will be subject to a torque tending to align its dipole moment in the field direction. In addi-tion to exerting a torque on the molecule the electric field will slightly distort the molecular charge distribution and produce an induced dipole moment. The magnitude of this induced dipole moment depends on the structure of the molecule, the magnitude of the electric field, and the t See also Vol. 1, Chapter 8.5; Vol. 2, Section 10.6.3; and Vol. 6, B, Chapter 7.1.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Fundamentals Of Atomic Force Microscopy - Part I: Foundations

  Part I: Foundations

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

    (Publisher)

  The electron wavefunctions are schematically indicated as uniform spherical charge clouds in this figure. When the atoms involved are identical, the final electron charge distri-bution must reflect this symmetry and hence the charge distribution must lie along the bisector between the two nuclei and be symmetrically dis-tributed about it. When the two atoms involved are dissimilar, one atom will inevitably attract electrons more than the other, resulting in a final electron charge distribution that is asymmetric along the bisector between the two nuclei. 34 Fundamentals of Atomic Force Microscopy, Part I Foundations Fig. 2.6 A schematic illustration of distortions in the electron cloud during the forma-tion of non-polar and dipolar covalent molecules. Any dipole moment p that develops provides a useful way to characterize the interaction between molecules. It is useful to identify atoms that strongly attract electrons because molecules comprised of those atoms are likely to be highly dipolar. The well-known electronegativity series for the elements provides this ranking as shown in Fig. 2.7. The chart is useful for qualitatively assessing the relative electronegativity of various elements one with respect to another. Example 2.3: Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. Would you expect hydrocarbon molecules to have a large dipole moment? By inspection of Fig. 2.7, the positions of H and C in the electronega-tivity chart lie roughly at the same vertical location. Therefore, neither atom is more electronegative than the other. As a result, you might expect hydrocarbon molecules to have relatively small dipole moments. Molecules with small dipole moments will not strongly interact with one another. As a consequence, when hydrocarbon molecules condense into the liquid state, you should expect the liquid to have a relatively high vapor pressure which leads to rapid evaporation.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Collected Works Of Lars Onsager, The (With Commentary)

  (With Commentary)

  • Per Chr Hemmer, Helge Holden, Signe Kjelstrup(Authors)
  • 1996(Publication Date)
  • World Scientific

    (Publisher)

  Further pertinent characteristics of a molecule are its polarizability. a, related to an internal refractive index n as follows »' - 1 »« + 2' (3) and a permanent electric moment IM> {in vacua). In an electric field, F, the total electric moment, (») Wymu. THIS JOOBMAL. M, 1482 (1938). 678 1488 LARS ONSAGER Vol.58 is the vector sum of the permanent and the induced dipole moments m - M*i + oF (4) where u denotes a unit vector in the direction of the dipole axis. The statistical a priori expecta-tion of u is isotropic. First, let us consider an unpolarized medium of dielectric constant, t, and introduce a rigid dipole of moment m into a cavity of radius a. For simplicity, let the dipole be a point singularity of the electric field, situated in the center of the spherical cavity. The potential The solution of this problem is . iu cos 9 _ . , , - j Rr cos 9, (r < a) , m* cos 9 . . . (6) whereby the coefficients m* and R must equal * 3. * m zT+~i m 2(« - 1) m 2« + 1 a« (7) The former may be called the external moment of the immersed dipole; it determines the force (modified by the intervening medium), which the dipole will exert upon a distant charge in the di-electric. The coefficient R measures the electric field which acts upon the dipole as a result of electric displacements induced by its own pres-ence, we shall refer to it as the reaction field. For a neutral, spherical molecule with an arbi-trary distribution of charges the above relations between m, m* and 2? still maintain. In this more general case m is the actual dipole moment of the molecule, while m* measures the dipole part of its external field, and R the homogeneous part of the reaction field.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemical Bonds and Bonds Energy

  • R Sanderson(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  FIVE Polar Covalence I: Electronegativity Equalization, Partial Charge, and Bond Length THE PRINCIPLE OF ELECTRONEGATIVITY EQUALIZATION The usual definition of electronegativity as the power of an atom in a molecule to attract electrons to itself leaves much to be desired in both clarity and significance. Electronegativity as listed is the property of an isolated atom. It changes when the atom is placed in a variety of conditions. A very simple model of the act of formation of a heteronuclear covalent bond can be very helpful in visualizing some of the fundamental consequences of chemical combination. Let us begin with atoms A and B y Β initially more electronegative than .4. Let us assume that each atom possesses at least one half-filled outermost orbital, giving it the capacity to form a covalent bond. When the two atoms come in contact, the single electron of A finds the vacancy of Β available and the single electron of Β finds the vacancy of A available. Thus both electrons come under the considerable influence of both nuclei and are shared between the two atoms. The attraction between the two atoms that results from this mutual sharing of the two bonding electrons is called a covalent bond. Since here the atoms are not alike, the bond is called heteronuclear. In the fact of sharing the same two electrons between two nuclei, the heteronuclear bond is just like a homonuclear bond. However, a difference arises from the fact that Β is initially more electronegative than A. This implies that the bonding electrons will be more strongly attracted to the nucleus of Β than to the nucleus of A. 75 76 5. Polar Covalence I A stable system cannot result unless in effect the bonding electrons are able to adjust to a condition of essentially equal attraction to both nuclei. Electrons are free to move according to the forces acting upon them, and they are certainly not to be expected to remain evenly distributed if they are not evenly attracted.

  Sign up to read

  Learn more about book