Dipole Moment and Molecular Strucutre
Dipole moment can determine the geometry of molecular structure. If there are two or more possible structures for a molecule, the correct one can be identified from their dipole moment.
Also read Applications or Significance of Dipole Moment
H2O has a Bent Structure
Water (H2O) molecule can have a linear or bent structure.
The dipole moments of the two O—H bonds in structure (A) being equal in magnitude and opposite in direction will cancel out. The net dipole moment (μ) would be zero.
In structure (B) the bond moment will add vectorially to give a definite net dipole moment. Since water actually has a dipole moment (1.85 D), so its linear structure is ruled out. Thus water has a bent structure as shown in (B).
CO2 has a Linear Structure and SO2 a Bent Structure
Carbon dioxide (CO2) has no dipole moment (μ = 0). This is possible only if the molecule has a linear structure and the bond moments of the two C = O units cancel each other.
On the other hand, SO2 has a dipole moment (μ = 1.63). Evidently, here the individual dipole moments of the two S = O bonds are not cancelled. Thus the molecule has a bent structure. The vector addition of the bond moments of the two S = O units gives the net dipole moment 1.63 D.
BF3 has a Planar and NH3 a Pyramid Structure
The dipole moment of boron trifluoride (BF3) molecule is zero. This is possible if the three B—F bonds are arranged symmetrically around the boron atom in the same plane. The bond moments of the three B—F bonds cancel each others effect and the net μ = 0.
Ammonia molecule (NH3) has a dipole moment (μ = 1.47 D). This is explained by its pyramidal structure. The three H atoms lie in one plane symmetrically with N atom at the apex of the regular pyramid. The dipole moments of the three N—H bonds on vector addition contribute to the net dipole moment. In addition, there is a lone pair of electrons on the N atom. Since it has no atom attached to it to neutralise its negative charge, the lone pair makes a large contribution to the net dipole moment. Thus the overall dipole moment of ammonia molecule is the resultant of the bond moments of three N—H bonds and that due to lone-pair.
CH4 has Tetrahedral Structure
Methane (CH4) has zero dipole moment, despite the fact that each C—H bond possesses a dipole moment of 0.4 D. This can be explained if the molecule has a symmetrical tetrahedral structure. Each C—H bond in the pyramidal CH3 group contributes (1/3)μ(μ cos 70.5) to the resultant dipole moment.
Thus the net dipole moment of CH3 group is equal to μ. This acts in a direction opposite to that of the fourth C—H bond moment, thereby cancelling each other.
Identification of cis and trans Isomers
The dipole moment can be used to distinguish between the cis and trans isomers. The cis isomer has a definite dipole moment, while the trans isomer has no dipole moment (μ = 0).
In the cis isomer, the bond moments add vectorially to give a net dipole moment. The trans isomer is symmetrical and the effects of opposite bond moments cancel so that μ = 0.
Identification of ortho, meta and para Isomers
Benzene has a dipole moment zero. Thus it is a planar regular hexagon. Let us examine the dipole moments of the three isomeric dichlorobenzenes (C6H4C12). Since the benzene ring is flat, the angle between the bond moments of the two C—Cl bonds is 60° for ortho, 120° for meta and 180° for para. On vector addition of the bond moments in each case, the calculated dipole moments are ortho 2.6 D, meta 1.5 D and para 0 D. These calculated values tally with the experimental values.
Thus the above structures of o-, m - and p-isomers stand confirmed. In general, a para disubstituted benzene has zero dipole moment, while that of the ortho isomer is higher than of meta isomer. This provides a method for distinguishing between the isomeric ortho, meta and para disubstituted benzene derivatives.
Also read Dipole Moment MCQs
Source: Essentials of Physical Chemistry by B.S. Bahl