There are many compounds with 1, 7-interaction in polycyclic aromatic hydrocarbons. The molecular structures of some of them have been determined by X-ray diffraction. They are largely deformed from the planar structure by the strain due to 1, 7-interaction. Such 1, 7-interactions are relaxed by two modes, which are defined as angle expansion mode and twisted mode. In these compounds, unusual physical and chemical properties have been also found, and are interpreted in terms of nonplanarity. Other interesting strained molecules are also briefly described.
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