Hydrogen bond network fluctuations in small (H2O)(n) clusters (n=8-12-24)

Molecular dynamic studies of three (H2O)(n) clusters (n=8-12-24) were perfo rmed using our recently developed many-body model TCPE [J. Chem. Phys. 107, 9105 (1997)] in the microcanonical ensemble. The trajectories were analyze d using a new structural local index derived from one of the many-body ener getic term of TCPE. In the energy domain where the clusters are in a liquid like state, a dynamical equilibrium is theoretically predicted to exist amo ng molecules in the PHB1 and the PHB2 state (i.e., among molecules which on e of their protons is involved in one hydrogen bond, the PHB1 state, and mo lecules which both of their protons are involved in two hydrogen bonds, the PHB2 state). The enthalpy and entropy changes corresponding to that equili brium for the three clusters range from 0.75 to 1.10 kcal mol(-1) and from -7 to -3.8 cal mol(-1) K-1. Such an equilibrium between two species of hydr ogen bonded molecules could be related to that experimentally observed in t he case of liquid water at ambient conditions. In particular, the entropy c hanges corresponding to PHB2/PHB1 equilibrium in the case of the three clus ters are very close to those experimentally reported for liquid water (abou t 6.6 +/- 0.5 cal mol(-1) K-1), suggesting that the equilibrium observed in the case of liquid water could correspond to a PHB2/PHB1 equilibrium. The analysis of hydrogen bond networks in terms of PHBm states appears thus to be an encouraging way in characterising the dynamical properties of water s ystems. (C) 1999 American Institute of Physics. [S0021-9606(99)52235-6].