Mapping the Molecular Pathway during Photoaddition of Guest Acetophenone and p-Fluoroacetophenone to Host Deoxycholic Acid as Studied by X-ray Diffraction in Systems Undergoing Single-Crystal-to-Single-Crystal Transformation

Article abstract of DOI:10.1021/ja00247a011

The structures of the partially reacted crystals of the two complexes (5:2) deoxycholic acid (DCA)-acetophenone and (8:3) DCA-p-fluoroacetophenone were determined by low-temperature (-170 deg C) X-ray diffraction.The structure analyses indicated that both independent guest acetophenone molecules G' and G in DCA-acetophenone react and that in DCA-p-fluoroacetophenone the guest molecule G' reacts whereas the other guest molecule G remains unreacted because of packing of G and G' molecules along the channel.The observed guest photoconversion in DCA-acetophenone is 40percent; the maximum value according to packing considerations is 50percent.In DCA-p-fluoroacetophenone the observed guest photoconversion is 35percent, very close to the maximum theoretical yield of 33percent based on packing considerations.The analysis demonstrated that during the course of photoconversion there is minimal motion of the (guest) phenyl ring, pronounced rotation of the acetyl group about the exocyclic C-C bond, and displacement of unreacted and reacted molecules.Atom-atom potential energy calculations showed that in DCA-acetophenone a guest ketyl radical with a pyramidal geometry may rotate by a full 180 deg without inducing prohibitively short intermolecular contacts.