ArnH2O (n=1–14) van der Waals clusters: Size evolution of equilibrium structures

Abstract

Size evolution of the equilibrium structures of ArnH2O van der Waals clusters with n=1–14 has been investigated. Pairwise additive intermolecular potential energy surfaces (IPESs) for ArnH2O clusters were constructed from the spectroscopically accurate Ar–Ar and anisotropic 3D Ar–H2O potentials. For each cluster size considered, we determined the global minimum of the respective IPES and several other lowest-lying ArnH2O isomeric structures. This was accomplished by using simulated annealing followed by a direct minimization scheme. The minimum-energy structures of all ArnH2O clusters considered in this work are fully solvated; up to n=12, the Ar atoms fill a monolayer around H2O. For n=12, the optimal Ar12H2O structure has the Ar atoms arranged in a highly symmetrical icosahedron, with H2O in its center. The icosahedral Ar12H2O structure is exceptionally stable; the energy gap separating it from the next higher n=12 isomer (289.55 cm−1) exceeds that for any other cluster in this size range. The observed preference for solvated ArnH2O structures was carefully analyzed in terms of the relative energetic contributions from Ar–Ar and Ar–H2O interactions. For n≤9, the monolayer, cagelike geometries are favored primarily by providing optimal Ar–H2O interactions, significantly larger than for alternative ArnH2O structures. For n≳9, the solvated ArnH2O isomers offer the best Ar–Ar packing, in addition to the strongest total Ar–H2O interactions. A detailed comparison was made with the minimum-energy structures of ArnHF clusters, determined by us recently [J. Chem. Phys. 100, 7166 (1994)], revealing interesting differences in the growth patterns of the optimal cluster structures.