heat capacity of CO2 rotational states:

eps = 0.000049;  k = 1.38*10^-21;

z = 
      Sum[(2  j + 1) Exp[-j (j + 1)*b*eps]/2, {j, 0, 
          6}]

energy = -(1/z) D[z, b]  /.  
          b ->  1/(k*T);

heatcap = D[energy, T]  /.  T ->  t*eps/k

Plot [heatcap/k, {t, 0.0001, 3}]

Alternatively, calculate the individual terms as follows:

Table[(2  j + 1) Exp[-j (j + 1)/3]/2,  {j, 0, 6}]  // 
      N