Introduction:
The question asks why N2 does not show pure vibrational spectra. In order to answer this question, we need to understand the concept of vibrational spectra and the properties of N2.

Vibrational Spectra:
Vibrational spectra refers to the study of molecular vibrations, which involve the periodic motion of atoms within a molecule. When a molecule vibrates, it undergoes changes in bond lengths and bond angles, resulting in absorption or emission of infrared radiation. The vibrational spectra of a molecule can provide information about its structure and bonding.

Properties of N2:
N2 is a diatomic molecule consisting of two nitrogen atoms. It is held together by a triple bond, which is very strong due to the overlap of atomic orbitals. N2 is a symmetric molecule, meaning that the nitrogen atoms are identical and the molecule has a center of symmetry. As a result, N2 has a zero dipole moment.

Explanation:
The correct answer is option 'B' - the dipole moment of N2 is zero. This is because pure vibrational spectra are observed when a molecule has a dipole moment. A dipole moment occurs when there is an asymmetric distribution of charge in a molecule, resulting in a separation of positive and negative charges.

In the case of N2, the molecule is symmetric, and the nitrogen atoms are identical. As a result, the distribution of charge is symmetrical, and there is no separation of positive and negative charges. Therefore, N2 does not have a dipole moment.

Why is dipole moment important for vibrational spectra?
In vibrational spectroscopy, the electric field of the incident radiation interacts with the dipole moment of the molecule. This interaction leads to changes in the vibrational energy levels of the molecule, which can be observed as absorption or emission of infrared radiation.

If a molecule has a dipole moment, it can undergo a change in dipole moment during vibrational motion, resulting in a change in its electric field interaction with the incident radiation. This change in dipole moment leads to the absorption or emission of infrared radiation, which gives rise to vibrational spectra.

Conclusion:
In conclusion, N2 does not show pure vibrational spectra because it does not have a dipole moment. The symmetric nature of N2, along with the identical nitrogen atoms, leads to a symmetrical distribution of charge and a zero dipole moment. Therefore, N2 does not interact strongly with the electric field of incident radiation, resulting in the absence of pure vibrational spectra.