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

被引:46
作者
Fortes, AD
Brodholt, JP
Wood, IG
Vocadlo, L
Jenkins, HDB
机构
[1] Birkbeck Coll, Res Sch Geol & Geophys Sci, London WC1E 6BT, England
[2] UCL, London WC1E 6BT, England
[3] Univ Warwick, Dept Chem, Coventry CV4 7AL, W Midlands, England
关键词
D O I
10.1063/1.1398104
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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=36.38(3) cm(3) mol(-1), bulk modulus, K-0=9.59(9) GPa, and the first derivative of the bulk modulus with respect to pressure, K-0(')=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, V-0=31.82(5) cm(3) mol(-1), K-0=14.78(62) GPa, K-0(')=2.69(48). We calculate the reaction enthalpy, DeltaH(NH4OH,s --> NH3.H2O,s)=-14.8(5) kJ mol(-1) at absolute zero, and thus estimate the enthalpy of formation, Delta H-f(circle minus)(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 similar to 23 kJ mol(-1). (C) 2001 American Institute of Physics.
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页码:7006 / 7014
页数:9
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