AB-INITIO STUDY OF THE GAS-PHASE EQUILIBRIUM BETWEEN (H2O)4 AND (H2O)8

Ab initio techniques inclusive of electron correlation (MP2) are used to study the gas-phase dimerization of water tetramers (T) into octame rs (O), which is relevant to a model for the specific heat of liquid w ater. The T species has one basic structure, whereas three different c onformations for the O species have been investigated, the most stable having D2d symmetry. The dimerization enthalpy at 298 K is almost-equ al-to -54 kJ mol-1, which is about half the value adopted in an applic ation of the model. Inspection of O structures shows that identity of the T species is most if favor of strongly interacting pairs.