CCl4 and CS2 are having dipole moment zero or no dipole. But both are having characteristic vibrational bands in the mid IR range. What can be the reason/s for that?
The TRANSITION dipole moment must be different from zero. The are symmetric vibrations which are Raman active (the transition polarizability is not zero) and there are asymmetric vibrations (IR active) where the transition dipole moment is not zero. In the case of highly symmetric molecules it may happen that those modes which are Raman active are not Ir active or vice versa. If you look at the character table belonging to the symmetry of the given molecule those vibrations are IR active which transform as x, y or z and those are Raman active which transform as xy, yz or xz. See in more detail: https://www.chemie-biologie.uni-siegen.de/ac/be/lehre/ws1213/ir-ramanlong.pdf
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