MOLECULAR SPECTROSCOPY AND SOME BASIC CONCEPTS

What is the Principle of Spectroscopy?

Spectroscopy is the study of the interaction between matter and radiation. When the light has been absorbed by the molecule there is some change in the behaviors of the molecule observed. These behaviors and their effects are going to study in this blog. We will also learn some basic concepts relevant to this.

The word spectroscopy is derived from two different words: spectrum is a Latin word it means image and skopia is a Greek word that means observation.

Some Basic concepts–

Dipole Moment

Dipole Moment:-Every molecule is composed of positively charged nuclei and negatively charged electrons in such a way that the molecule as a whole is electrically neutral.

 It is  “the product of the magnitude of charge (positive or negative) and the distance between them (bond length). If positive charge + q is separated from a negative charge –q by a distance ‘r’  then the dipole moment it is given by

µ  =  q  x  r         Where  µ – Dipole moment, q- Electronic charge, r-Bond length

Dipole moment arises due to the formation of a bond between two atoms of different electronegativity where the charge distribution is not uniform e.g. HC1. The CGS unit for dipole moment is the Debye, symbolized by  D.

Bond Moment:- Any bond which has a degree of polarity has a dipole moment. This is called a bond moment.

H₂O has an angular structure | Why water has an angular structure?

A water molecule (H₂O) can have a linear or angular structure,  The dipole moments of the two O-H bonds in the structure being equal in magnitude and opposite in direction will cancel out. The net dipole moment (µ) would be zero.

 In structure (b) the bond moments will add vectorially to give a definite net dipole moment. Since water actually has a dipole moment (1.85 D); its linear structure is ruled out. Thus water has an angular structure as shown in above Fig. .       The dipole moments of the two O-H bonds in the structure being equal in magnitude and opposite in direction will cancel out. The net dipole moment (µ) would be zero.

CO₂ has a linear structure and SO₂ has an angular structure.

Carbon dioxide 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 canceled. Thus the molecule has an angular structure. The vector addition of the bond molecule of the two S=O units gives the net dipole moment 1.63 D.

BF₃ has a planar and NH₃ has a pyramidal structure.

The dipole moment of the boron trifluoride molecule is zero. The three B-F bonds are arranged symmetrically around the boron atom in the same plane.

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) For example,

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 in Benzene.

Benzene has dipole moment zero. Thus it is a planar regular hexagon. Let us examine the dipole moments of the three isomeric dichlorobenzenes (C6H4CI2). Since the benzene ring is flat, the angle between the bond moments of the two C- CI bonds is 60 for ortho, 120 for meta, and 180 for para. Fig.(a) (b), (c) respectively, 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.

CCl₄ has zero dipole moment.

It can be explained on the basis of the symmetric tetrahedral structure of the molecule in spite of the C-Cl bond being highly polarized.

The percentage of ionic character can be calculated using dipole moment it is given by,

 Ionic Character = 

Polarization of Molecules :

 When a molecule is placed in an electric field, the field distorts the electronic structure and changes the equilibrium positions of the nucleus giving rise to the separation of centers of positive and negative charges. The displacement of electronic charge is called electron polarization P_E and that of nuclear charge is called atomic polarization P_A. The sum of two values together is called distortion or induced polarization. P_D is given by the equation.

P_D = P_E + P_A

The electric field applied is responsible for induced polarization and the resulting dipole is called induced dipole moment u1, which is given by the equation.

µ_i = α E   

Where alpha is a constant called the polarizability of the molecule and E is the electric field applied.

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