Each X-ray diffraction experiment in principle probes the electron density of the sample. At standard resolution, local density maxima can be associated with atom sites; geometry data can be very precise, but no subtle conclusions with respect to electron density are drawn. All atoms of an element are treated as spherical scattering centers of the same type, regardless of their electronic situation and bonding environment. This limitation can be overcome: Given sufficient crystal quality and suitable equipment, diffraction experiments of high resolution can be performed. These experiments are conducted at low temperature in order to minimize thermal motion and allow to access reflections at high resolution with significant intensity. Such high order reflections owe their intensity to the inner-most electron shells and essentially define the position of the core, whereas the outer valence electrons may be polarized, engage in covalent bonding or act as lone pairs. Thus, the experimental electron density in each point of the unit cell is experimentally accessible and diffraction experiments can provide information concerning charge accumulation or depletion and the electron distribution along covalent bonds or in regions of short intermolecular contacts. Bader's Quantum Theory of Atoms In Molecules (QTAIM) was originally intended to analyze electron densities obtained from theory and today represents a particularly popular way to interpret such experimental charge densities.

	A focus area of our charge density studies are intermolecular interactions between Lewis bases and Cl, Br or I, usually addressed as halogen bonds. Our results indicate that such interactions come in very different flavours: they may resemble coordinative bonds with significant electron density along the short contact (example on the left) or they may correspond to largely electrostatic sigma hole interactions with charge depletions on the heavy halogen as in the example on the right.

The many flavours of halogen bonds - message from experimental electron density and Raman spectroscopy.
R. Wang, J. George, S. K. Potts, M. Kremer, R. Dronskowski, U. Englert; Acta Crystallogr. 2019, C75, 1190-1201. details and online article

Insight into trifluoromethylation - experimental electron density for Togni reagent I.
R. Wang, I. Kalf, U. Englert; RSC Adv. 2018, 8, 34287-34290. read online

Short is strong: experimental electron density in a very short N...I halogen bond.
R. Wang, D. Hartnick, U. Englert; Z. Kristallogr. 2018, 233, 733-744.

Charge-Assisted Halogen Bonds in Halogen-Substituted Pyridinium Salts: Experimental Electron Density.
A. Wang, R. Wang, I. Kalf, A. Dreier, C. W. Lehmann, U. Englert; Cryst. Growth Des. 2017, 17, 2357-2364.

Charge density of the biologically active molecule (2-oxo-1,3-benzoxazol-3(2H)-yl)acetic acid.
A. Wang, J. Ashurov, A. Ibragimov, R. Wang, H. Mouhib, N. Mukhamedov, U. Englert; Acta Crystallogr. 2016, B72, 142-150.

3-(4-Pyridyl)-2,4-pentanedione - a bridge between coordinative, halogen, and hydrogen bonds.
C. Merkens, F. Pan, U. Englert; CrystEngComm 2013, 15, 8153-8158.

Charge Density of Intra- and Intermolecular Halogen Contacts.
R. Wang, T. S. Dols, C. W. Lehmann, U. Englert; Z. Anorg. Allg. Chem. 2013, 639, 1933-1939.

Competing protonation sites in sulfadiazine: answers from chemistry and electron density.
F. Pan, R. Wang, U. Englert; CrystEngComm 2013, 15, 1164-1172.

The halogen bond made visible: Experimental charge density of a very short intermolecular Cl...Cl donor-acceptor contact.
R. Wang, T. Dols, C. W. Lehmann, U. Englert; Chem Commun. 2012, 48, 6830-6832.

Switching from Bonding to Non-Bonding - Temperature-dependent Metal Coordination in a Zn(II) Sulfadiazine Complex.
F. Pan, R. Wang, U. Englert; Inorg. Chem. 2012, 51, 769-771.

The Whole Range of Hydrogen Bonds in One Crystal Structure: Neutron Diffraction and Charge Density Studies of N,N-Dimethylbiguanidinium-Bis(hydrogensquarate).
M. Serb, R. Wang, M. Meven, U. Englert; Acta Crystallogr. 2011, B67, 552-559.

Weak interactions in chain polymers [M(µ-X)₂L₂]_∞ (M = Zn, Cd; X = Cl, Br; L = substituted pyridine) - an electron density study.