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 OH---O hydrogen bonds, which are found to differ between molecules, the crystal structure exhibits
a large array of weak intermolecular hydrogen bonds. These are assessed using a topological analysis of the
electron density distribution based on Bader's theory of atoms in molecules. Two discrete groups of hydrogen
bonds emerge that could only partly be differentiated using conventional structural crystallography.