First neutron diffraction analysis of an O-H center dot center dot center dot pi hydrogen bond: 2-Ethynyladamantan-2-ol

The neutron diffraction analysis of crystals of 2-ethynyladamantan-2-ol (I) provides confirmation of the presence of an unusual O-H ...pi hydrogen bond, together with more common O-H ... O and C-H ... O bonds, as proposed previously. This is first neutron analysis of an O-H ...pi hydrogen bond, which is shown to be short and linear, with the O-H vector clearly directed towards the mid-point (X) of the triple bond (O ... X = 3.221 Angstrom, H ... X = 2.258 Angstrom, O-H ... X = 179.0 degrees) rather than toward either of the two alkyne carbon atoms. An exceptionally short and straight C-H ... O bond is also present in the network which probably owes its characteristics to a cooperative effect with an O-H ... O hydrogen bond. The three types of hydrogen bond in this crystal structure link molecules to form supramolecular synthons which are defined as spatial arrangements of intermolecular interactions which incorporate the geometrical and chemical recognition features of molecules.