Write down an 4. expression for the rotational energy of a molecule, defining terms used (b) from part (a), derive an expression for the wavelength of a pure rotational transition between two adjacent rotational levels. define any terms you introduce. (c) the longest wavelength in the pure rotational spectrum from co is 2.58 mm. use this information to deduce the separation between the c and o atoms in the molecule. the atomic masses of c and o are 12.0 and 16.0, respectively.