A Monte Carlo study of isomers and structural evolution in benzene-cyclohexane clusters: (C6H6)(C6H12)n, n=3-7, 12.

Monte Carlo simulated annealing strategies, carried out on four different potential energy surfaces, are applied to benzene-cyclohexane clusters, BCn, n=3-7, 12, to identify low-energy isomers and to trace the evolution of structures as a function of cluster size. Initial structures are first heated to ensure randomization, and subsequent annealing yields optimized rigid, low-energy clusters. Five major structural isomers are identified for BC3: one assumes the form of a symmetric, modified sandwich; the remaining four lack general symmetry, assuming distorted tetrahedral arrangements. For BC4 and larger clusters, the number of low-temperature isomers is large. It is, nevertheless, feasible to classify isomers into groups based on structural similarities. The evolution of BCn structures as a function of cluster size is observed to follow one of two primary paths: The first maximizes benzene-cyclohexane interactions and places benzene in or near the BCn cluster center; the competing path maximizes cyclohexane-cyclohexane interactions and distances benzene from the cluster's center of mass. Results for BC3 and BC4 are discussed with reference to experimental results and models previously applied to interpret benzene-argon cluster spectra.

Monte Carlo studies of isomers, structures, and properties in benzene-cyclohexane clusters: computation strategy and application to the dimer and trimer, (C6H6)(C6H12)n, n = 1-2.

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.