Microwave Spectroscopy - Obtaining info when isotope absorptions are known

[Horseb0x]

Heres the question
[PLAIN]http://img213.imageshack.us/img213/2669/16772870.jpg [Broken]
I know that I'm supposed to derive these values from the fact that I know the absorptions of 2 different isotopes of the diatomic molecule but I don't know how to go about doing it. I can obtain the masses of hydrogen, deuterium and iodine from the periodic table but I don't know how I can use this data to get the internuclear distances.

 

[ehild]

You need the formula for the rotational energy levels of diatomic molecules.

ehild