Here, we look at the probabilities of the rotational energy states of a diatomic molecule, such as n2 or o2. it is known that the rotational energy is given by (a.21), 1) erot j = 0,1, 2i where his planck's constant h divided by 27t, that is, h = h/(27t), and i is the moment of inertia about the center of mass, that is i = miri -m2r? + where mi is the mass and r is the distance from the rotation axis for the ith nucleus, explained m2 mass of an oxygen atom. just as there three components to the translational energies from the r-, y- and z-directions, there are three components of the rotational energy from the rotation about the three axes, with the corresponding inertia i, i and i. see fig. 1.5 and the text just above the figure. each degree of freedom, as you know, contributes kbt/2 of thermal energy, or equivalently, contributes r/2 see the contributions from the three translational degrees of freedom. the rotational degrees of freedom add 2 x r/2. that is, (for temperatures below 1000 k) there are only two rotational degrees of freedom o2 molecule, m1 = on page 376 of the textb0ok. for example, for an as moment of to the specific hcat, see fig. 1.13. from this figure, you use the information above to explain why, though there are three rotational even degrees of freedom, their contribution to the specific heat is only 2 x r/2