H2 (hydrogen) and N2 (nitrogen) molecules do exhibit rotational spectra, but these spectra are not typically observed under normal conditions due to certain factors:
Lack of a Permanent Dipole Moment: Rotational spectroscopy relies on the interaction between electromagnetic radiation and the dipole moment of a molecule. A molecule must possess a permanent dipole moment for rotational transitions to be observed. H2 and N2 molecules consist of identical atoms bound by a symmetric bond, resulting in a linear geometry with a zero permanent dipole moment. Therefore, their rotational spectra are typically weak or undetectable under normal conditions.
Pure Rotational Transitions: Rotational spectra are more commonly observed in molecules with more complex structures or in molecules that interact with an external electric field. In the case of H2 and N2, their rotational spectra mainly consist of pure rotational transitions, which involve changes in the rotational energy of the molecule without simultaneous changes in other energy levels (such as vibrational or electronic). These transitions typically occur at microwave frequencies, making them difficult to observe using conventional optical spectroscopy techniques.
Low Rotational Energy Levels: H2 and N2 molecules have relatively low rotational energy levels due to their light masses. As a result, the energy differences between adjacent rotational levels are small, leading to spectral lines that are closely spaced. This makes it challenging to resolve individual rotational transitions and observe distinct rotational spectra.
It's worth noting that although the pure rotational spectra of H2 and N2 are not commonly observed, they can be studied under specialized conditions or using advanced spectroscopic techniques. For example, rotational spectroscopy experiments at low temperatures or in the gas phase can provide better resolution and reveal the rotational structure of these molecules. Additionally, isotopic variants of H2 and N2, such as HD (deuterium) or N2O (nitrous oxide), can exhibit more observable rotational spectra due to differences in their molecular properties.