Taking the aromaticity out of aromatic interactions.

The phrase “aromatic interactions” is widely used to describe p stacking, cation–p, and anion–p interactions, among others. These interactions are central to many areas of modern chemistry and molecular biology, and are vital tools in the supramolecular armamentarium. The concept of aromaticity appears almost universally in definitions of these interactions, implying that they are somehow dependent on aromatic p delocalization in the interacting monomers. But does aromaticity actually enhance these interactions, or can stronger noncovalent interactions be achieved, for example, by using planar, nonaromatic polyenes? We show below, through robust ab initio studies of model systems, that the cyclic p-electron delocalization associated with aromaticity often hinders p stacking and anion–p interactions, although it strengthens cation–p interactions. The implication is that more favorable stacking interactions can be achieved in supramolecular complexes by exploiting interactions with nonaromatic polyenes rather than aromatic systems. In 2008, Grimme showed that stacking interactions in the parallel-displaced benzene and naphthalene dimers are comparable in magnitude to the corresponding saturated cyclic systems. However, for larger acenes (i.e., anthracene and tetracene), Grimme reported enhanced stacking interactions in the aromatic dimers that are not mirrored in the saturated systems. This was attributed to long-range correlation effects. The impact of aromaticity on these stacking interactions, however, was not directly addressed. Here, we quantify the effect of aromatic p-electron delocalization on the strength of p stacking, cation–p, and anion–p interactions. We first consider the sandwich dimers of benzene with the unsubstituted rings of 2-methylnaphthalene (1) and 2-methylene-2,3-dihydronaphthalene (2 ; Scheme 1 and Figure 1a). Isomer 2 provides a means of quenching the aromaticity present in 1 while conserving the number of p electrons. Isomers 1 and 2 can thus be used to quantify the electronic effects of aromatic p delocalization on stacking interactions. SCS-MP2/TZVPP interaction energies at equilibrium separations are provided in Figure 1, and are plotted as a function of inter-monomer distance (R) in Figure S1 of the Supporting Information. Across the full range of distances, the stacking interaction of benzene with the nonaromatic isomer 2 is more favorable than with 1. In other words, the nonaromatic isomer engages in stronger stacking interactions with benzene than does the aromatic isomer. This difference is not attributable to differential direct interactions between benzene and the methyl/CH or methylene/ CH2 groups in 1 and 2 (see the Supporting Information), but instead results from the localization of the p system in isomer 2. The homodesmotic dissection of benzene depicted in Scheme 1 provides an alternative means of quantifying the effect of aromatic p delocalization on stacking interactions (Figure 1b and Table 1). Comparing CCSD(T) interaction energies in the benzene sandwich dimer with the interaction between benzene and dissected benzene yields the same conclusion as above: p localization stabilizes sandwich stacking interactions by 0.31 kcal mol 1 at the corresponding Scheme 1. Molecular systems used to quantify the effect of aromatic p delocalization on stacking and other aromatic interactions: 2-methylnaphthalene (1), 2-methylene-2,3-dihydronaphthalene (2), and dissected benzene. In the dissected benzene, the nuclear positions are such that all interatomic distances exactly match those in benzene.