 In that temperature range, the two additional degrees of freedom that correspond to vibrations of the atoms, stretching and compressing the bond, are still "frozen out".At about that temperature, those modes begin to "un-freeze", and as a result formula_2 starts to increase rapidly at first, then slower as it tends to another constant value.It is 35.5 J/K/mol at 1500 °C, 36.9 at 2500 °C, and 37.5 at 3500 °C.