6. (10 points) Using statistical mechanics, the absolute entropy of mono-deuterated methane, CH$_3$D, can be calculated at 90.41 K and 84.50 mm Hg to be 179.2 J/(mol K). However, experiments using calorimetry give a value, at the same temperature and pressure, of 167.6 J/(mol K), which is about 11.6 J/(mol K) smaller than the statistical mechanical value, which should be quite accurate. Assuming that the experiments were carefully done, what is the most likely origin of this discrepancy? Why is it 11.6 J/(mol K)?