Influence of Crystal Effects on Molecular Charge Densities in a Study of 9-Ethynyl-9-fluorenol

The experimental charge density distribution in crystalline 9-ethynyl-9-fluorenol has been determined in a refinement of a pseudo-atomic multipolar expansion (Hansen−Coppens formalism) against extensive low-temperature (T = 100 K) single-crystal X-ray diffraction data and compared with a selection of theoretical DFT calculations on the same complex. The molecule crystallizes in the centrosymmetric space group C2/c with two independent molecules in the asymmetric unit. A range of multipole models of varying complexity were tested and found to describe the observed intensities almost equally well in terms of the usual residual values presented; however, the resulting total density distributions and derived properties, such as atomic charges and molecular electrostatic potentials, showed significant differences and it is emphasized that care has to be taken when parameters used for the refinements are selected. In addition to the two medium-strength OH---O hydrogen bonds, which are found to differ between mo...