Theoretical study on intermolecular interactions in the formation of aromatic hydrocarbon dimers

Abstract

The interaction between two closely lying and parallel aromatic hydrocarbon molecules, one of which is in its singlet excited state and the other in its ground state, leading to the formation of singlet excimers, is now well understood in terms of configuration interaction between the charge-transfer and the exciton states as well as the semiempirical molecular orbital theory of the ?-electrons in the super-molecule approximation. But the formation of stable dimers between two ground-state aromatic hydrocarbons is not yet clearly understood because the interaction of two closed-shell systems is generally believed to lead to repulsion owing to the operation of the Pauli exclusion principle. However, Chandross, Ferguson and McRae (1966) have observed that anthracene forms two types of weak dimers, at low temperatures, in which both the monomers are in their ground states. One form of dimer, prepared by photolytic dissociation of dianthracene, has a symmetrical sandwich conformation. The geometry of the other dimer, deduced from the vibronic analysis of the first absorption band, has a non-sandwich conformation, in which the long axes of the molecules are parallel, while their short in-plane axes make an angle of 55° (±5°). Chandross et al. concluded that the sandwich dimer was less stable than the non-sandwich one, although their binding energies were not determined. However, they observed that both the dimers were unstable above 190°K and dissociated into two monomers. The object of this work is to investigate, theoretically, the origin of the stability of these dimers. We have followed a perturbation approach to the study of intermolecular interactions. We have shown that the total interaction energy in a hydrocarbon dimer can be decomposed into the following important terms: (a) Electrostatic energy (b) ?–? overlap repulsion energy (c) Total non-bonded repulsion energy (d) ?-delocalisation energy (or the charge-transfer interaction energy) (e) ?–? dispersion energy (f) ?–? dispersion energy Results are given for both anthracene and naphthalene dimers. It is shown that interaction between two (identical) aromatic hydrocarbons can lead to formation of weak dimers of both sandwich and non-sandwich conformations. The stability of these 'weak' dimers is due to a combination of repulsive and attractive forces of the above kinds. The sandwich form of the dimer is more stable than the non-sandwich one, in contrast to the observations of Chandross et al. for anthracene dimer. The reason for this discrepancy is discussed. It is possible that the non-sandwich form can pass on to the sandwich form without an energy barrier.