Heat capacities of the six dimethylpyridines between the temperatures 10 K and 445 K and methyl-group rotational barriers in the solid state a,b

Heat capacities and enthalpy increments between the temperatures T = 10 K and 445 K were determined for the six dimethylpyridines by adiabatic calorimetry. (Chemical Abstract registry numbers: 2,3-dimethylpyridine [583-61-9]; 2,4-dimethylpyridine [108-47-4]; 2,5-dimethylpyridine [589-93-5]; 2,6-dimethylpyridine [108-48-5]; 3,4-dimethylpyridine [583-58-4]; and 3,5-dimethylpyridine [591-22-0].) Triple-point temperatures and enthalpies of fusion are reported for each material, and enthalpy increments and entropies relative to those of the crystals at T → 0 were derived. Lambda-type phase transitions in the crystals were observed for 2,6-dimethylpyridine and 3,4-dimethylpyridine. A small step or "bump" was observed in the heat-capacity-against-temperature curves for 2,6-dimethylpyridine and 2,3-dimethylpyridine. Barriers to methyl-group rotation in the solid state are estimated for each compound and compared with literature values.