MOLECULAR FREEDOM OF AMMONIUM ION . HEAT CAPACITY AND THERMODYNAMIC PROPERTIES OF AMMONIUM PERCHLORATE FROM 5 DEGREES -350 DEGREES K

The heat capacity of NH4ClO4 has been determined by adiabatic calorimetry from 5°-350°K and found to be of simple sigmate character without thermal anomalies. The heat capacity (Cp), entropy (S°), enthalpy function (H°-H°0)/T, and Gibbs energy function (G°-G0°)/T evaluated at 298.15°K from these data are 30.61, 44.02, 20.24, and -23.78 cal/(gfm °K). Combination of these values with aqueous NH4ClO4 thermochemical data suggests the absence of zero-point entropy. Comparison with the heat capacity of isostructural KClO4 permits resolution of the molecular dynamics of the ammonium ions and leads to the conclusion that these ions are restricted rotators, prevented from freely rotating by comparatively low-energy barriers.