Towards extracting the charge density from normal-resolution data

The limiting factor for charge-density studies is crystal quality. Although area detection and low temperatures enable redundant data collection, only compounds that form well diffracting single crystals without disorder are amenable to these studies. If thermal motion and electron density rho(r) were deconvoluted, multipole parameters could also be refined with lower-resolution data, such as those commonly collected for macromolecules. Using the invariom database for first refining conventional parameters (x, y, z and atomic displacement parameters), de-convolution can be achieved. In a subsequent least-squares refinement of multipole parameters only, information on the charge density becomes accessible also for data not fulfilling charge-density requirements. A critical aspect of this procedure is the missing information on the correlation between refined and non-refined parameters. This correlation is investigated in detail by comparing a full multipole refinement on high-resolution and a blocked refinement on 'normal-resolution' data sets of ciprofloxacin hexahydrate. Topological properties and dipole moments are shown to be in excellent agreement for the two refinements. A 'normal-resolution' data set of ciprofloxacin hydrochloride 1.4-hydrate is also evaluated in this manner.