Molecular Freedom of the Ammonium Ion. Heat Capacity and Thermodynamic Properties of Ammonium Perchlorate from 5°–350°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 ${\mathrm{(C}}_p)$ , entropy $\mathrm{(S°)}$ , enthalpy function ${\mathrm{(H°-H°}}_0\mathrm{)\hspace{0.17em}/\hspace{0.17em}T}$ , and Gibbs energy function ${\mathrm{(G°-G}}_0\mathrm{°)\hspace{0.17em}/\hspace{0.17em}T}$ 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.