Schottky Type Anomaly in Heat Capacity
Caused by Partial Deuteration
of Methyl Group in 2,6-Dichlorotoluene

Heat capacity measurements were made for 2,6-dichlorotoluene (C6H3Cl2-CH3) and its methyl-deuterated analogs, C6H3Cl2-CH2D, C6H3Cl2-CHD2, C6H3Cl2-CD3, between 0.35 K and 300 K by adiabatic calorimetry and relaxation calorimetry. A Schottky type anomaly was found for the two partially deuterated analogs, C6H3Cl2-CH2D and C6H3Cl2-CHD2, below 40 K. It is explained by an energy scheme involving three levels, g1/g0 = g2/g0 = 1. The spacing from the ground state is 267 μeV and 2950 μeV for C6H3Cl2-CH2D and 2400 μeV and 2700 μeV for C6H3Cl2-CHD2, respectively. The excess entropy amounts to Rln3 (= 9.13 J K−1 mol−1) which is due to the symmetry breaking of methyl group by partial deuteration.

(by H. Suzuki & A. Inaba)

	Fig. 1. Low temperature heat capacity of 2,6-dichrolotoluenes. The partially deuterated compounds (–CH2D and –CHD2) showed a broad anomaly in heat capacity, whereas the fully protonated and fully deuterated compounds showed no anomaly.
	Fig. 2. Excess heat capacity of partially deuterated 2,6-dichorolotoluenes (–CH2D and –CHD2). Solid curves indicate the Schottky type heat capacities; the energy schemes of the models are shown schematically in the figure.
	Fig. 3. Excess entropy of partially deuterated 2,6-dichorolotoluenes (–CH2D and –CHD2). Dashed line indicates the value of ΔS = Rln3 (= 9.13 J K−1 mol−1).