Aromatic van der Waals clusters: Structure and nonrigidity

More than a dozen cases of nonrigid van der Waals clusters are presented and discussed to demonstrate that cluster nonrigidity is a general phenomenon in all weakly bound systems. The interplay of structure and nonrigidity complicates cluster research and mandates a dynamical approach to cluster properties in which multiple stable configurations coexist and interconvert, and large amplitude nuclear motions are the rule rather than the exception. Empirical potential energy surface calculations are employed to yield physical insight into the structure and dynamics of nonrigid clusters, and molecular symmetry group theory is applied to analyze spectroscopic manifestations of cluster nonrigidity. Empirical potentials of various forms are successful in predicting most cluster structures, as well as estimating the potential surface barrier heights hindering the interconversion between different local minimum-energy structures. Such calculational approaches also emphasize the importance of large amplitude motion for one or more of the cluster vibrational degrees of freedom. The limits of these empirical calculations are discussed, and recent attempts to derive cluster structure and properties by ab initio techniques are reviewed. The aromatic/small molecule clusters considered in this presentation display two types of nonrigidity: local nonrigidity in which large amplitude motion involves the rotation of one of the molecular constituents, and global nonrigidity in which large amplitude motion involves displacement of the centers of mass of the molecular constituents. The former motion interchanges equivalent atoms, and the latter motion interchanges cluster conformations. The potential surface barriers for these large amplitude motions lend to increase with constituent molecule complexity.