Neutron Diffraction Study of Aromatic Hydrogen Bonds: 5-Ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol at 20K

The neutron diffraction crystal structure of the title compound, C₁₇H₁₂O, was determined at 20 K. One of the benzyl groups accepts intermolecular hydrogen bonds from a hydroxyl and an ethynyl group, one to each face of the ring. The bond donated by the hydroxyl group points almost linearly to an aromatic C atom with a H...C separation of 2.339 (6) Å. The bond donated by the ethynyl group points to the aromatic midpoint M with a H...M separation of 2.587 (5) Å. The average acetylenic C—H bond length determined by low- temperature neutron diffraction is only 1.062 (6) Å, appreciably shorter than observed for chemically different C—H bonds.

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