Ab initio simulation of ammonia monohydrate (NH3⋅H2O) and ammonium hydroxide (NH4OH)

We report the results of the first pseudopotential plane-wave simulations of the static properties of ammonia monohydrate phase I (AMH I) and ammonium hydroxide. Our calculated fourth-order logarithmic equation of state, at zero pressure and temperature, has molar volume, $V_0\text{=36.38(3)\hspace{0.05em}}{\mathrm{cm}}^3 {\mathrm{mol}}^{\mathrm{-1}},$ bulk modulus, $K_0\text{=9.59(9)\hspace{0.05em}}\mathrm{GPa},$ and the first derivative of the bulk modulus with respect to pressure, $K_0^{\prime }\text{=5.73(21).}$ Both this and the lattice parameters are in very good agreement with experimental values. The monohydrate transforms, via a solid-state proton transfer reaction, to ammonium hydroxide $({\mathrm{NH}}_{\mathrm{4}}\mathrm{OH})$ at 5.0(4) GPa. The equation of state of ammonium hydroxide is, $V_0\text{=31.82(5)\hspace{0.05em}}{\mathrm{cm}}^3 {\mathrm{mol}}^{\mathrm{-1}},$ $K_0\text{=14.78(62)\hspace{0.05em}}\mathrm{GPa},$ $K_0^{\prime }\text{=2.69(48).}$ We calculate the reaction enthalpy, $\mathrm{\Delta H(}{\mathrm{NH}}_{\mathrm{4}}\mathrm{OH}\mathrm{,s\to }{\mathrm{NH}}_3\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}\text{,s)=−14.8(5)\hspace{0.05em}}{\mathrm{kJ mol}}^{\mathrm{-1}}$ at absolute zero, and thus estimate the enthalpy of formation, ${\Delta }_f{H}^{\ominus }({\mathrm{NH}}_{\mathrm{4}}\mathrm{OH}\text{,s)=−356\hspace{0.05em}}{\mathrm{kJ mol}}^{\mathrm{-1}}$ at 298 K. This result places an upper limit of 84 kJ mol⁻¹ on the barrier to rotation of the ammonium cation, and yields an average hydrogen bond enthalpy of ∼23 kJ mol⁻¹.