Samala Nagaprasad Reddy, Agmon Noam
The Fritz Haber Research Center, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel.
ACS Omega. 2019 Dec 17;4(27):22581-22590. doi: 10.1021/acsomega.9b03326. eCollection 2019 Dec 31.
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 (HO) clusters ( = 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 ( ). may play a role in determining the atmospheric cluster size distribution such that the dimer and tetramer, with their exceptionally low/high values, are under/over-represented in these distributions, as indeed observed in nondestructive mass spectrometric measurements.
小水团簇吸收热量并催化关键的大气反应。然而，实验在大气条件下的水团簇分布上产生了相互矛盾的结果。此外，尚不清楚此类团簇会在何种温度下表现出哪些“相变”，以及其潜在的分子机制是什么。我们发现，(HO) 团簇（ = 2 - 6）的径向概率密度中对数形式的小尾巴为此类转变提供了直接证据。使用现有的最佳水势（MB - pol）、一种先进的恒温算法（g - BAOAB）以及足够长的轨迹，我们绘制出了“分叉”、“熔化”以及（此前未探索的）“汽化”转变，发现熔化和汽化均通过“环上单体”构象体进行，在汽化温度（ ）下表现出巨大的距离波动。 可能在决定大气团簇尺寸分布方面发挥作用，使得二聚体和四聚体因其极低/极高的 值，在这些分布中占比过低/过高，这正如在无损质谱测量中实际观察到的那样。
