DIPOLE MOMENT;

A dipole moment is the measurement of the separation of two opposite electrical charges.   

In most of the organic molecules there is a charge separation which may be either due to the difference in  electronegativities of the atoms. As in hydrogeh chloride or due to the presence of formal charges as in trim-ethyl amine oxide.

H+          Cl-

In each case the positive and negative charges are equal in magnitude and the molecule as a whole is neutral. The centers of gravity of the positive and negative charges may be consider as lying with in the atoms themselves. As in case of hydrogen chloride . At some point in space between the atoms as in the case of dichloromethane.

Definition of Dipole Moment:

Such molecules in which the concentration of the positive and the negative charges is at different points or polar in nature i-e;. They behave as dipoles and have dipole mo-ment which can be defined as The product of the charges q and the distance d between the centers of gravity of the positive and negative charges i-e
                                                            µ=q×d

dipole-moment-digram-or-representation-



Where q is of the order of 10-10esu and d 10-8  ,  therefore µ is of the order of 10-18esu centimeter. This unit is known as Deby (D) after the name of the discoverer of this property. Thus the dipole mo-ment of a molecule of which the positive charge os 0.8 × 10-10 esu is separated from the negative charge by a distance of 1.5×10-8 cm. will be 1.2×10-18 esu cm or 1.2D.

Direction of dipole moment:

In fact, the dipole mo-ment of a molecule is the vectorial sum of the individual bond moments which are  directional. The direction of the overall dipole moment of the molecule is indicated by an arrow. This arrow is parallel to the line joining the centers of the positive and the negative charges.with its head pointing toward the negative end.

For example,

The individual moments of the bond within molecule can not be 

measured:

We can only measure the overall dipole moment of the molecule. However, the individual bond moments in a molecule can be calculated if the bond moments of the other bonds and the dipole mo-ments of the molecule as a whole are known.

Values of bond moments in dipole moment:

The values of the bond moments of various bonds are roughly the same in all molecules, and therefore can be used to calculate the expected dipole moments of the molecules.

For example,

The toluene dipole moments are 0.43D and the nitro benzene moments are 3.39D. The dipole moment of p-nitrotoluene is therefore predicted to be 4.36D. The actual value is 4.39D which is reasonably close to the predicted value.

Direction of the dipole moment of

C- H bond:

The direction of the dipole moment of C H bond is toward hydrogen due to the directional hybrid orbital of carbon in which the centers of the electronic cloud;

i-e the negative charge  does not lie at the nucleus of carbon atom but at some distance from it.

This distance decreases  with increasing non directional s character of the hybrid orbital.Therefore the magnitude of C-H bond moment depends upon the nature of hybridization  and decreases in order sp3 sp2 sp.

individual bond moment:

Given the individual moment of bonding. Some molecules do not exhibit dipole mom-ent  because the individual bond moment are cancelled. This is due to the symmetry of the molecules. E.g trns-1,2-dichloroethylene does not exhibit any dipole mo-ment  although its cis isomer has a di-pole moment of 1.90D.

Shape:

Similarly the dipole mo-ment of methane ,ethylene , carbondioxide  etc. is zero . in these molecules the center of the positive and negative charges coinside, i.e d is zero such molecules are non polar. General ,dipole moment is the measure of polarity of moleculers . The greater the value of the dipole moment ,greater the polarity. The dipole mo-ment of some the common molecules are;

 
 

Read more;

What is spectroscopy,importance and types of spectroscopy