Clusters containing open-shell molecules: Minimum-energy structures and low-lying isomers of ArnCH (X2Π), n = 1 to 15

Abstract

The size evolution of the equilibrium structures of open-shell Ar_nCH (X²Π) Van der Waals clusters is investigated for n = 1 to 15. We describe a method for combining pair potentials for Ar–CH and Ar–Ar interactions to obtain potential energy surfaces for Ar_nCH clusters. For each cluster size considered, the global and a few energetically close local minima are calculated using simulated annealing followed by a direct minimization scheme. Ar₂CH is found to have an unusually stable planar structure, which persists as a motif in larger Ar_nCH clusters and has a strong effect on their optimal geometries. The lowest-energy isomers of Ar_nCH with n = 3 to 11 have all Ar atoms in a shell around CH. The only exception is Ar₄CH, where the fully solvated isomer is 3 cm⁻¹ higher in energy than the optimal isomer with CH bound to the surface of the Ar₄ tetrahedron. For n = 7 to 11, the minimum-energy structure of Ar_nCH derives from the global minimum of the Ar_n+1 cluster, by replacing the Ar atom at the bottom of the pentagonal bipyramid with CH. The lowest-energy structure of Ar₁₂CH is that of the optimal icosahedral Ar₁₃ cluster, with CH replacing one of the Ar atoms on the cluster surface. This structure supports the proposition based on the spectroscopic data, that for Ar_nCH clusters with about 10 to 50 Ar atoms CH resides on the surface of Ar_n.