If I understand the equipartition theorem more or less well, if I heat up a gas then the energy I have spent to heat it up should be equally distributed to the rotational part and vibrational part of the molecule dynamics. Let's take the molecule of CO, I think it has 5 degrees of freedom. 3 vibrationals, 2 rotationals. I don't understand how does the equipartition theorem applies there: http://en.wikipedia.org/wiki/File:Vibrationrotationenergy.svg, taken from http://en.wikipedia.org/wiki/Rotational_spectroscopy#Structure_of_rotational_spectra. It seems that whenever I excitate the molecule, it will gain at least energy in the vibrational form. And if I'm lucky, in the rotational form too. But it seems forbidden to gain energy only in the rotational form. How does this agree with the equipartition theorem?!