Clusters containing open-shell molecules: minimum-energy structures and low-lying isomers of ArnCH (X 2 pi), n = 1 to...

The size evolution of the equilibrium structures of open-shell ArnCH (X 2 pi) 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 ArnCH 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 ArnCH clusters and has a strong effect on their optimal geometries. The lowest-energy isomers of ArnCH 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 ArnCH derives from the global minimum of the Arn + 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 ArnCH clusters with about 10 to 50 Ar atoms CH resides on the surface of Arn.