Which of the following molecules will give a pure rotational
spectrum? Look for whether they have a permanent dipole moment or
not. N2 , NH3, BF3, o-difluorobenzene, p-difluorobenzene, H2O, O3,
CO2 , anti-butane, gauche-butane, cis-1,2-dibromoethene,
trans-1,2-dibromoethene.