Thermally Induced Hydrogen-Bond Rearrangements in Small Water Clusters and the Persistent Water Tetramer

Small water clusters absorb heat and catalyze pivotal atmospheric reactions. Yet, experiments produced conflicting results on water cluster distribution under atmospheric conditions. Additionally, it is unclear which "phase transitions" such clusters exhibit, at what temperatures, and what are their underlying molecular mechanisms. We find that logarithmically small tails in the radial probability densities of (H₂O)_n clusters (n = 2 - 6) provide direct testimony for such transitions. Using the best available water potential (MB-pol), an advanced thermostating algorithm (g-BAOAB), and sufficiently long trajectories, we map the "bifurcation", "melting", and (hitherto unexplored) "vaporization" transitions, finding that both melting and vaporization proceed via a "monomer on a ring" conformer, exhibiting huge distance fluctuations at the vaporization temperatures (T_v). T_v may play a role in determining the atmospheric cluster size distribution such that the dimer and tetramer, with their exceptionally low/high T_v values, are under/over-represented in these distributions, as indeed observed in nondestructive mass spectrometric measurements.