benzene is non polar but give IR spectra? please explain to me why that?
Most non-polar molecules are in fact IR-active. For example CO
2 has D
∞h symmetry. As a consequence of its symmetry, it is non-polar. The number of degrees of vibrational freedom for CO
2 is 3n - 5 = 3*3 - 5 = 4 (where n is the number of atoms). Therefore, there are four modes of vibration. The symmetric stretch does not give rise to a change in dipole moment, however, both the asymmetric stretch and the bend do. Therefore, there are two fundamental absorptions which are observed in the IR spectrum of CO
2.
Just because it's nonpolar doesn't mean that it won't give an IR spectrum. Nonpolar substances like ethanol and acetone give IR spectra. As for a detailed explanation of why benzene gives an IR spectrum in the area of 1600 cm-1, here goes.
Ethanol & acetone = non-polar? Hmm...