Stacked Clusters of Polycyclic Aromatic Hydrocarbon Molecules

Abstract

Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modeled using explicit all-atom potentials using a rigid-body approximation. The considered range of PAHs goes from pyrene (C10H8) to circumcoronene (C54H18) and clusters containing between 2 and 32 molecules are investigated. In addition to the usual repulsion-dispersion interactions, electrostatic point-charge interactions are incorporated, as obtained from density functional theory calculations. The general electrostatic distribution in neutral or singly charged PAHs is reproduced well using a fluctuating-charges analysis, which provides an adequate description of the multipolar distribution. Global optimization is performed using a variety of methods, including basin-hopping and parallel tempering Monte Carlo. We find evidence that stacking the PAH molecules generally yields the most stable motif. A structural transition between one-dimensional stacks and three-dimensional shapes built from multiple stacks is observed at larger sizes, and the threshold for this transition increases with the size of the monomer. Larger aggregates seem to evolve toward the packing observed for benzene in bulk. Difficulties met in optimizing these clusters are analyzed in terms of the strong anisotropy of the molecules. We also discuss segregation in heterogeneous clusters and vibrational properties in the context of astrophysical observations.

Effects of cluster formation on spectra of benzo[a]pyrene and benzo[e]pyrene

“…In BeP trimer the 180° rotation of the top two molecules is less stable than rotation through a smaller angle. This is the pattern seen in pyrene [20]. BeP also displays a slightly shorter intermolecular distance than BaP, indicating a more compact cluster structure.…”

“…The stacked, parallel structure is favored in most large PAHs [20]. Therefore the interplanar distances and rotation angles shown in Table 1 define the geometries relative to the global minimum energy structure.…”

“…In naphthalene dimer, and in the naphthalene-anthracene complex the T-shaped and stacked geometries are very close in energy [18]. In larger PAHs stacked, parallel displaced structures are expected to be the more stable form [19,20].…”

“…Force-field molecular mechanical methods have been notably successful in describing PAH stacking [20][21][22]. These latter can be parametrized in accurate calculations on smaller molecules [14,19].…”

“…The van der Waals interaction energy is represented as a Lennard-Jones potential. UFF and similar potentials have proven reliable in determining stacking geometries of polyaromatics [20,22].…”