EXPERIMENTAL X-RAY DIFFRACTION EVIDENCE OF THE UNUSUAL BOND BENDING IN SMALL MOLECULES. M. Antipin*, and R. Boese,** Institute of Organoelement Compounds, Moscow, Russia.* Institute of Inorganic Chemistry, University of Essen, Germany**

Molecular structure and geometry are often the only kind of information that many chemists are interesting in as a final result of the X-ray analysis. Meanwhile, much more information may be extracted now from X-ray diffraction data using modern opportunities of this method. Thus, multipole representation of the electron density distributions and its topological analysis allow to describe the nature of some chemical bonds in new terms of the bond paths, critical point positions etc. This approach was found to be very fruitful for description bonding in small cyclic or strained cage molecules, where so called bent bonds are present.

In the deformation electron density (DED) maps the bending evidences as a shift of the DED bond maxima outward the internuclear lines connecting bonded atoms. Modern quantum chemical calculations show however, that bent bonds are much more abundant, some small acyclic and non-strained molecules having atoms with lone pairs (LP), may form bent bonds, but direct experimental evidence for this effect is absent.

In the present work a charge density study based on the high-resolution low temperature X-ray diffraction data was performed for series of crystals (s-tetrazines, ammonia, simplest carborane), where unusual bent bonds may be observed. It was found for s-tetrazines that DED maxima in N-N bonds are shifted to the centers of the 6-membered heterocycles because of repulsion between nitrogen LP's and electron density of the N-N bonds. The similar effect was found for ammonia molecule: DED maxima are shifted from the N-H lines inside the NH3 tetrahedron, and this result is in agreement with the bond critical point positions and quantum calculations. In the structure of the smallest carborane with the C2B3 central frame the bent C-B bonds were found together with the multicenter DED maxima in the middles of the 3-angles faces in this electron-precise molecule. The general conclusion about charge density analysis in molecules studied shows that straight bonds indicated"by lines" between atoms are more the exception than the rule.