Interaction between stacked aryl groups in 1,8-diarylnaphthalenes: Dominance of polar/π over charge-transfer effects
F. Cozzi, J. S. Siegel
1995
Pure and Applied Chemistry
Several 1,8-diarylnaphthalenes have been prepared, and the barrier to rotation around the arylhaphthalene bond has been measured. In these molecules steric congestion forces the aryls in a parallel stacked geometry. The barriers to rotation were used to evaluate the strength and to investigate the nature of the interaction between the arenes. The variance of the AG+ for the rotation upon arene substitution with electron donating or electron withdrawing groups indicates that polarh electrostatic
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... effects dominate over charge-transfer effects in determining the arene/arene interaction. Non covalent interactions between aromatic units (1) play a major role in determining the properties and the behavior of molecules in a variety of processes, such as stereocontrol of organic reactions (2) and molecular recognition (3), and in affecting the structure of many biologically relevant molecules (4). In order to establish the influence exerted by arene/arene interactions on these phenomena and to learn how to use them in molecular design, we must understand the relative importance of the factors contributing to the interaction. The energy of the interaction between two arene units divides roughly into a polar (electrostatic and induction) and a van der Waals term (dispersion) (1). The polar term depends on the relative charge distribution and on the interaction between the charge of one arene with the induced change in the charge of the other. The van der Waals term depends on the contact surface area. Since in the case of two interacting benzenes the surface area is relatively small, the polar term should dominate the interaction (1). The charge distribution in benzene can be described as an electron rich central core surrounded by an electron poor perifery (5). This description is in agreement with the preference shown by crystalline (6), liquid (7), and gaseous benzene (8) for the edge-to-face, T-shaped geometry, and predicts an unfavorable repulsive interaction for the face-to-face stacked disposition, and a favorable attractive one for the offset stacked arrangement. On this basis, the interaction between two parallel stacked benzenes can be studied only in those systems in which this geometry is imposed by steric factors. We reasoned that the steric congestion at the peri positions of naphthalene should indeed force two phenyl rings in the required orientation (9). This orientation is lost when a phenyl group rotates around the phenylhaphthalene bond (10,ll). Thus, the barrier to rotation should depend on the strength of the parallel stacked interaction. and. in a homologous series of comnounds. the variance of the hamer unon substitution of the aryls with electon donating (EDG) and electron withdrawing (EWG) groups should provide a new insight into the nature of the R/R interaction.
doi:10.1351/pac199567050683
fatcat:xfm7wxo6nzh4tgpiop566r5f5a