| Abstract
| - Five X-ray crystallographic structures of bianthrones (1, 2b−5b) revealed the presence of longcentral C(sp3)−C(sp3) bonds (≥1.60 Å) and mostly gauche conformation. DFT calculations werecarried out to study the structural features of bianthrones and related model molecules. Our B3LYP/6-31G* gas-phase calculation results suggest that steric effect plays a role in causing the long C−Cσ bond length (>1.60 Å), nonplanarity of the anthracenone moiety (>20°), and preferred gaucheconformation. However, when the gauche preference is weak, environmental effects such as solvationand packing forces can reverse it. As the degree of nonplanarity of the anthracenone moiety isconsistently larger in the anti conformation than in the gauche and the ease of achievingnonplanarity is determined by flexibility of the central six-membered rings directly connected tothe C(sp3)−C(sp3) bond, one way to control the structure and anti-gauche energy difference of closelyrelated compounds is by changing the clamping group (Z) in the central six-membered ring.
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