Abstract
Parallel interactions of aromatic and heteroaromatic molecules are very important in chemistry and biology. In this review, recent findings on preferred geometries and interaction energies of these molecules are presented. Benzene and pyridine were used as model systems for studying aromatic and heteroaromatic molecules, respectively. Searches of Cambridge Structural Database show that both aromatic and heteroaromatic molecules prefer interacting at large horizontal displacements, even though previous calculations showed that stacking interactions (with offsets of about 1.5 ?) are the strongest. Calculations of interaction energies at large horizontal displacements revealed that the large portion of interaction energy is preserved even when two molecules do not overlap. These substantial energies, as well as the possibility of forming larger supramolecular structures, make parallel interactions at large horizontal displacements more frequent in crystal structures than stacking interactions.
Highlights
Parallel interactions of aromatic and heteroaromatic molecules are very important in chemistry and biology
Interactions between two pyridine molecules were calculated at high levels of quantum chemistry, giving two energy minima, similar to ones found for benzenebenzene dimer [43]
The peak of this distribution is for offsets between 4.5 and 5.5 Å; since in benzene molecule hydrogen atoms are at 2.5 Å of benzene center, at these offset values only hydrogen atoms of benzene molecules overlap (Figure 3). These results show that the most stable geometry of stacking interactions, with offsets at 1.5 Å, is not typical for benzene–benzene
Summary
Parallel interactions of aromatic and heteroaromatic molecules are very important in chemistry and biology. Second minimum has the energy of -2.73 kcal/mol ( at CCSD(T)/CBS level) [43], with geometry of stacking interaction, where two benzenes are mutually parallel, with horizontal displacement of approximately 1.5 Å (Figure 1). The analysis of geometrical parameters revealed that benzene molecules prefer interactions at large horizontal displacements (Figure 4a), with 64% of all contacts (1173 of 1824) at offsets values larger than 4.5 Å.
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