On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions

The electronic nature of low-barrier hydrogen bonds (LBHBs) in enzymatic reactions is discussed based on combined low temperature neutron and x-ray diffraction experiments and on high level ab initio calculations by using the model substrate benzoylacetone. This molecule has a LBHB, as the intramolecular hydrogen bond is described by a double-well potential with a small barrier for hydrogen transfer. From an "atoms in molecules" analysis of the electron density, it is found that the hydrogen atom is stabilized by covalent bonds to both oxygens. Large atomic partial charges on the hydrogen-bonded atoms are found experimentally and theoretically. Therefore, the hydrogen bond gains stabilization from both covalency and from the normal electrostatic interactions found for long, weak hydrogen bonds. Based on comparisons with other systems having short-strong hydrogen bonds or LBHBs, it is proposed that all short-strong and LBHB systems possess similar electronic features of the hydrogen-bonded region, namely polar covalent bonds between the hydrogen atom and both heteroatoms in question.

Characterization of Actinide Bonding in Th(S(2)PMe(2))(4) by Synchrotron X-ray Diffraction

Extensive synchrotron (28 K) and conventional sealed-tube (9 K) X-ray diffraction data have been collected on Th(S(2)PMe(2))(4). Modeling of the electron density of the complex shows the bonding is quite ionic with little diffuse f or d type bonding density. Furthermore a large polarization of the Th core is observed revealing some 5d-like involvement in the bonding. High-quality ab initio density functional calculations are not able to reproduce these features and instead predict rather covalent bonding with considerable 6d-5f mixing. The study suggests that this theoretical method exaggerates the covalent nature of actinide bonds. It is shown that the most direct measure of covalence-charge transfer and electron distributions-can be usefully estimated by X-ray diffraction even in this most unfavorable of cases, where many actinide core electrons are present. The use of very low temperature data is crucial in the study of heavy metal complexes in order to minimize systematic errors such as thermal diffuse scattering and anharmonicity. The fact that accurate synchrotron radiation diffraction data can be measured within days makes studies of compounds beyond the first transition series more frequently within reach.

Experimental evidence for the existence of non-nuclear maxima in the electron-density distribution of metallic beryllium. A comparative study of the maximum entropy method and the multipole refinement method

The electron-density distribution (EDD) of metallic beryllium has been derived from the structure factors of Larsen & Hansen [(1984). Acta Cryst. B40, 169-179] using the maximum entropy method (MEM). Subsequent topological analysis reveals non-nuclear maxima (NNM) in the EDD. Plots of the gradient field of the electron density illustrates this finding. A possible critical-point network for the hexagonal close-packed (h.c.p.) structure of beryllium is suggested. It is thus demonstrated that it is possible to obtain detailed topological information about the electron density in metallic beryllium without the use of a structural model. In order to test the findings of the MEM, the same set of structure factors were analysed using the multipole refinement method (MRM). Use of the MRM also reveals NNM. The results of the two different approaches to electron-density analysis are contrasted and discussed. Expressed within the framework of the theory of atoms in molecules, our results suggest that the h.c.p. structure of beryllium has no Be atoms directly bonded to other Be atoms. The structure is held together through a three-dimensional network of bonds between the NNM and Be atoms as well as between different NNM. The topological analysis thus reveals that the beryllium structure has important interactions connecting Be atoms of different basal plane layers. The breaking of these interactions when forming a surface may explain the abnormally large expansion of the inter-layer distance in the beryllium surface structure.