a What is the requirement for a molecule that it can be stud

a) What is the requirement for a molecule that it can be studied by purely rotational spectroscopy?

b) What is the requirement for a molecule that it can be studied by vibrational spectroscopy?

A pure rotation spectrum can only arise when the molecule possesses a permanent electric dipole moment. Whereas, in vibrational spectroscopy, the physical effect that couples to photons is a changing dipole moment