Structure and Thermodynamics of H+(H2O)n (n = 9, 21, 40) Clusters between 0 and 300 K. A Monte Carlo Study

We present a Monte Carlo study of protonated water clusters, H + (H 2 O) n, with size n = 9, 21, and 40, and neutral clusters, (H 2 O) 20, in the temperature range 0 to 300 K. We study the structural differences between the solidlike and liquidlike phases, using an empirical polarizable water model. The transition between these two phases is particularly distinct in H + (H 2 O) 21, which attains the dodecahedral cage configuration at temperatures up to 150 K, a structure that does not survive above 170 K. The results support the idea that the “magic number” behavior of H + (H 2 O) 21 is restricted to temperatures below the melting point. We estimate the melting temperatures for H + (H 2 O) 9, H + (H 2 O) 21, and (H 2 O) 20 as predicted by the model to be 130, 160, and 160 K, respectively. The melting process in the protonated clusters thus appears to be governed mainly by water−water interactions.