Clusters containing open-shell molecules. II. Equilibrium structures of Ar_nOH Van der Waals clusters (X~2II, n=1 to 15)

摘要

The equilibrium and low-lying isomeric structures of Ar_nOH (X~2II) clusters for n=1 to 15 are investigated by simulated annealing caluclations. Potential energy surfaces are obtained by a pairwise-additive approach, taking into account the open-shell nature of OH X~2II and including spin-orbit coupling. It is found that the spin-orbit coupling suppresses the Jahn-Teller effect, and many of the clusters have high-symmetry structures (C_nv with n>2) which would be forbidden in the absence of spin-orbit coupling. The structures are generally similar to those previously found for the closed-shell systems Ar_nCH. This is because Ar-OH (X~2II), like Ar-HF and Ar-HCl but unlike Ar-NO and Ar-CH, has a near-linear equilibrium structure. Ar_nOH clusters for n up to 6 have all Ar atoms in a single shell around OH. In the clusters with n=7 to 9, OH is under a pentagonal pyramid formed by six Ar atoms, while the others bind to its exterior, away from OH. For n=10 to 12, the minimum-energy structures have OH inside an Ar_n cage, which is essentially icosahedral for n=12 but has vacancies for n=10 and 11. For n>12, the extra Ar atoms begin to form a second solvation shell. The global minimum of Ar_nOH may be constructed from the minimum-energy structure of Ar_(n+1) by replacing one Ar atom with OH.