Triatomic linear CO2 molecule has three atoms. (n = 3). Therefore, the molecule has 3n degrees of freedom. For linear molecules there will be (3n - 5) fundamental vibrations. Therefore, $$ 3n - 5 = 3(3) - 5 = 9 - 5 = 4 $$ CO2 molecule will have 4 fundamental vibrations. These modes of vibrations are:
(i)symmetric stretching (1330 cm-1): In symmetrical stretching there is a partial charge separation due to two oxygen atoms. The magnetic moments get cancelled and the molecule is non-polar. Therefore polarisability does not changes. Thus Symmetric stretching is IR inactive but Raman active.
(ii)asymmetric stretching (2349 cm-1): In unsymmetrical stretching, one end is stretched and at the other end there is compression. This produces fluctuating dipole moment. This means polarisability changes and therefore this vibration is IR active but Raman inactive.
(iii) two types of bending vibrations (667 cm-1): There are two bending vibrations and they differ only in the direction and have the same frequency. In this case also polarisability does not changes. Hence, these vibrations will be IR active and Raman Inactive.
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