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What is 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 behaviors of the molecule observed. These behaviors and their effects we are going to study in this blog.

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.

Electromagnetic Phenomina

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 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 SO2has 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.

Net dipole moments of CO2 and SO2

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 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.

The net dipole moment of BF3 and NH3

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.

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 PE and that of nuclear charge is called atomic polarization PA. The sum of two values together is called distortion or induced polarization. PD and is given by the equation.

PD = PE + PA

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

µi = α E   

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



What is molecular spectroscopy used for?

Spectroscopy is used in chemistry because all atoms and molecules have their own unique spectra. As a result, these spectra can be used to detect, identify and quantify the required information about the atoms and molecules.

What is an example of molecular spectroscopy? What are the types of molecular spectroscopy?

Rotational Spectroscopy, Vibrational Spectroscopy, Raman Spectroscopy, etc.

What is the principle of molecular absorption spectroscopy?

Atoms and molecules are excited from the ground state and undergo either resonant rotations, vibrations or electronic transitions, or excitations.

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