The change in
dipole moment is first and foremost criteria to get IR spectrum of any
compound. The compounds which give rise to change in the dipole moment upon
absorption of IR radiation, will give IR spectra. The compounds which do not
give rise to change in the dipole moment upon absorption of IR radiation, will
not give IR spectra. For example, the symmetrical stretching of C=C bond in
ethylene will not produce any change in dipole moment of the molecule. Hence,
this mode of vibration is IR inactive. This also tell why trans-dichloroethane
does not show C=C stretching whereas cis-dichloroethane shows C=C stretching.
However, both cis and trans-dichloroethane show C-H and C-Cl stretching upon IR
absorption. Carbon monoxide and Iodine chloride (I-Cl) show IR absorption but
hydrogen (H2), Nitrogen (N2), Oxygen (O2), Chlorine (Cl2) and other symmetrical
diatomic molecules do not show IR absorption. A large change in dipole moment
gives rise to strong absorption.
In summary, only
those compounds give rise to IR absorption or IR spectra which have-
1.
Permanent dipole moment
2.
Which show change in dipole moment upon absorption of IR radiation
3.
Which do not possess center of symmetry
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