Fundamentals of van der Waals Interactions in Aromatic Clusters

Aromatic-aromatic interactions play important roles in many chemical and biological systems. They control, among others, the base-base interactions leading to the double helical structure of DNA, the function of the special pair in photosynthetic reaction centers, the packing of aromatic crystals, the formation of aggregates, and the conformational preferences of polyaromatic macrocycles and chain molecules. The proper description of the interactions between the monomers forming clusters of aromatic molecules is critical for the fundamental understanding of these phenomena. The molecular systems ideally suited for a detailed study of the intermolecular potentials are van der Waals (vdW) dimers and higher clusters of aromatic hydrocarbons that are experimentally generated by free jet expansion techniques. Since these species are formed as a direct consequence of intermolecular interactions, the geometrical structures of the vdW molecules not only reveal the nature of the forces between the molecules but also provide an understanding of the cluster's other properties, including dynamics. Although significant efforts have been dedicated to the experimental characterization of these clusters, the results are usually inconclusive. Given the maturity reached by quantum chemistry and the improvement in the algorithms used in quantum chemical calculations, it is logical to expect that highly correlated ab initio electronic structure methodologies could be a valuable tool to complement the experimental efforts. However, since these systems are large and the number of possible structures dramatically increases with the size of the cluster, reliable theoretical calculations have been limited to small clusters of benzene (dimer and trimer). Therefore, it is necessary to assess the validity of these methodologies in the case of larger clusters of different aromatic molecules and possibly determine if alternative methodologies that incur in lower computational expenses can be devised in order to apply them to larger systems. So far, the results indicate that the combination of molecular dynamics simulations using the MM3 force field, followed by full geometry optimizations at the MP2/6-31G level of theory appear to provide a reliable tool for the study of van der Waals aromatic clusters. In general, our theoretical calculation on aromatic vdW clusters of various sizes containing benzene, naphthalene, and anthracene are consistent with the spectroscopy and excited-state dynamics data available in the literature. We are currently conducting studies to assess the basic set dependence of the theoretical treatment. Studying these methods with increasing computational efficiency, we have also implemented a newly developed Hartree-Fock Dispersion (HFD) methodology that adds empirical corrections to Hartree-Fock in a perturbative manner so dispersion forces can be properly described. So far, the results obtained using this efficient method agree with the MP2 results. We are using this methodology along with ab initio molecular dynamics simulations in order to understand the dynamics of these vdW systems.

(C6H6)2	(C10H8)2