On the structure of cross-conjugated 2,3-diphenylbutadiene

The structure of the cross-conjugated compound 2,3-diphenylbutadiene was investigated by single-crystal X-ray diffraction and computational methods. In the crystal structure the central butadiene fragment adopts an s-gauche geometry [-55.6(2)° torsion angle φ around the essential single bond], whereas the styrene moieties are close to planarity. MP2/6-311G* calculations show that the s-gauche conformation represents the global minimum along the φ coordinate, but also revealed the existence of an s-trans local minimum. While the crystal structure seems to reflect dominance of styrene- like conjugation, the MP2/6-311G* calculations indicate that conjugation in both the styrene and butadiene π- systems is important. An NBO orbital deletion study shows that the structure is primarily determined by (hyper)conjugation and that steric effects play a minor role.