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

Abstract

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, V0=36.38(3) cm3 mol−1, bulk modulus, K0=9.59(9) GPa, and the first derivative of the bulk modulus with respect to pressure, K0′=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 (NH4OH) at 5.0(4) GPa. The equation of state of ammonium hydroxide is, V0=31.82(5) cm3 mol−1, K0=14.78(62) GPa, K0′=2.69(48). We calculate the reaction enthalpy, ΔH(NH4OH,s→NH3⋅H2O,s)=−14.8(5) kJ mol−1 at absolute zero, and thus estimate the enthalpy of formation, ΔfH⊖(NH4OH,s)=−356 kJ mol−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.