dipole moment

Examples of dipole moment in the following topics:

• Dipole Moment

  • The bond dipole moment uses the idea of the electric dipole moment to measure a chemical bond's polarity within a molecule.
  • Bond dipole moments are commonly measured in debyes, represented by the symbol D.
  • Molecules with only two atoms contain only one (single or multiple) bond, so the bond dipole moment is the molecular dipole moment.
  • At one extreme, a symmetrical molecule such as chlorine, Cl2, has 0 dipole moment.
  • Symmetry is another factor in determining if a molecule has a dipole moment.

• Bond Polarity

  • The Debye unit, D, is commonly used to express dipole moments.
  • In molecules containing more than one polar bond, the molecular dipole moment is just the vector addition of the individual bond dipole moments.
  • The linear shape of the CO2 molecule results in the canceling of the dipole moments of the two polar C=O bonds.
  • The net, molecular dipole moment of CO2 is therefore zero, and the molecule is nonpolar.
  • Apply knowledge of bond polarity and molecular geometry to identify the dipole moment of molecules

• Dipole-Dipole Force

  • Dipole-dipole interactions are intermolecular attractions that result from two permanent dipoles interacting.
  • Dipole-dipole forces: electrostatic interactions of permanent dipoles in molecules; includes hydrogen bonding.
  • For example, a water molecule (H2O) has a large permanent electric dipole moment.
  • Molecules often contain polar bonds because of electronegativity differences but have no overall dipole moment if they are symmetrical.
  • Dipoles may form associations with other dipoles, induced dipoles or ions.

• Properties of Aldehydes and Ketones

  • Because of the greater electronegativity of oxygen, the carbonyl group is polar, and aldehydes and ketones have larger molecular dipole moments (D) than do alkenes.
  • The resonance structures in the first diagram below illustrate this polarity, and the relative dipole moments of formaldehyde, other aldehydes and ketones confirm the stabilizing influence that alkyl substituents have on carbocations (the larger the dipole moment the greater the polar character of the carbonyl group).

• Heteronuclear Diatomic Molecules

  • Consequently, the molecule has a large dipole moment with a negative partial charge δ- at the chlorine atom and a positive partial charge δ+ at the hydrogen atom.
  • To determine exact polarity, dipole moment (in Debye) can be calculated as the product of the separated charges (Q) and distance between them (r) in Angstroms:
  • Sample problem: What is the dipole moment of the Cl-F molecule with a bond length of 163 picometers (163 x 10-12 m) and an 11 percent ionic character?

• Background and Properties

  • Indeed, if hydrogen bonding is not present, the boiling points of comparable sized compounds correlate reasonably well with their dipole moments.

• The Shape of Molecules

  • One way in which the shapes of molecules manifest themselves experimentally is through molecular dipole moments.
  • A molecule which has one or more polar covalent bonds may have a dipole moment as a result of the accumulated bond dipoles.
  • The bond dipoles are colored magenta and the resulting molecular dipole is colored blue.
  • In the linear configuration (bond angle 180º) the bond dipoles cancel, and the molecular dipole is zero.
  • In a similar manner the configurations of methane (CH4) and carbon dioxide (CO2) may be deduced from their zero molecular dipole moments.

• Bond Polarity

  • In chemistry, bond polarity is the separation of electric charge along a bond, leading to a molecule or its chemical groups having an electric dipole or dipole moment.
  • The unequal sharing of electrons within a bond leads to the formation of an electric dipole (a separation of positive and negative electric charge).

• The Structure and Properties of Water

  • The presence of such a charge on each of these atoms gives a water molecule a net dipole moment.
  • The electrical attraction between water molecules caused by this dipole pulls individual molecules closer together, making it more difficult to separate the molecules, and therefore raising the boiling point.
  • An object with such a charge difference is called a dipole (meaning "two poles").
  • The oxygen end is partially negative, and the hydrogen end is partially positive; because of this, the direction of the dipole moment points from the oxygen toward the center position between the two hydrogens.
  • Owing to the electronegativity difference between hydrogen (H) and oxygen (O) atoms, and the bent shape of the H2O molecule, a net dipole moment exists.

• Ion-Dipole Force

  • Ion-dipole and ion-induced dipole forces operate much like dipole-dipole and induced dipole-dipole interactions.
  • However, ion-dipole forces involve ions instead of solely polar molecules.
  • Ion-dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole; the strength of the ion-dipole force is proportionate to ion charge.
  • Ion-dipole bonding is also stronger than hydrogen bonding.
  • Like a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud in the non-polar molecule, causing a temporary partial charge.