Excess Heat Capacities Induced by Partial Deuteration of Methyl Groups in Methanol and Methyl Iodide

Heat capacity measurements were made between 0.35 K and 200 K by adiabatic calorimetry and relaxation calorimetry for methanol and methyl iodide and their methyl-deuterated analogs (-CH2D, -CHD2, and -CD3). A Schottky type heat capacity was found for the α-phase (the low temperature crystalline phase) of CH2DOH and CHD2OH, which is explained by an energy scheme of two levels. The upper one of the levels for CH2DOH and the lower one for CHD2OH are doubly degenerated. These excess contributions are due to orientational ordering of the methyl groups. CH2DI and CHD2I, on the other hand, only showed the high-temperature tail of the excess contribution below 5 K, which indicates that the spacing of the levels are much smaller than those of methanol.

(by H. Suzuki & A. Inaba)

	Fig. 1. Low temperature heat capacities of CH2DOH.
	Fig. 2. Excess heat capacities of CH2DOH and CHD2OH in the α-phase. The solid curves stands for the Schottky type heat capacities obtained by the energy schemes shown.
	Fig. 3. Molar heat capacities of methyl iodide and its deuterated analogs.