The effect of hydrogen bonding on the thermodynamic and spectroscopic properties of molecular clusters and liquids

Quantum cluster equilibrium (QCE) theory is presented for water, ammonia, hydrogen sulfide and phosphine. It is based on full quantum-mechanical treatment of structures, binding energies and vibrational frequencies of H-bonded clusters, which are considered the basic microscopic units of both gaseous and liquid phases. The molecular clusters for all systems were calculated at the hybrid density functional (B3LYP) and Moller-Plesset (MP2) level of theory using 6-31+G* and 6-311++G** basis sets. QCE theory provides a remarkably detailed picture of the equilibrium structures, thermodynamic and spectroscopic properties of these strongly different associated fluids. The varying H-bond strength of these systems leads to characteristic features of the supramolecular clusters in terms of binding energies, geometries and spectroscopic properties. The validity of the resulting QCE model is demonstrated by comparison with experimental thermodynamic and spectroscopic properties: the heat of vaporization, the heat capacity, NMR chemical shifts and quadrupole coupling constants.