1. The problem statement, all variables and given/known data This question is about microwave (rotational) spectroscopy. Which of the following molecules have a microwave spectrum? H2, N2, NO, N2O, CH4, CO2, OCS, H2O, Ethene, Benzene. 3. The attempt at a solution According to my textbook, it appears that all molecules have a rotational spectrum. However, Wikipedia, "Diatomic molecules such as dioxygen (O2), dihydrogen (H2), etc. do not have a dipole moment and hence no purely rotational spectrum." I'm not really sure what that's supposed to mean. Do all molecules have a rotational spectrum (I don't see why not, everything with a bond should be able to rotate)? Is the rotation considered relative to one of the atoms in the molecule, or to the centre of mass?