The bonding in magnesium oxide. A discussion based on X-ray emission and X-ray photoelectron spectra

Previously published X-ray emission spectra (Mg Kβ_1,3; MgL_2,3M and OKα) from magnesium oxide are aligned on a common energy scale using atomic orbital ionisation energies determined by X-ray photoelectron spectroscopy. All three spectra show two principal components which derive from the same two bands of molecular orbitals. The two components of both magnesium spectra have equal intensity; in the oxygen spectrum the high energy peak is the most intense: these observations are rationalised using a simple molecular orbital model based on the MgO₆^10?unit. It is necessary to consider the interactions of this unit with further magnesium and oxygen atoms and also π -type perturbations between Mg—O σ bonds in order to explain the structure of the observed emission spectra (but there is no recourse to forbidden 3d → 1s transitions).     

The complex cyanides of chromium(II) and chromium(0)

Summary Chromium(II) cyanide dihydrate has been isolated from the reaction between aqueous chromium(II) and KCN; it can be dehydrated at 100°in vacuo. From the red solutions containing an excess of cyanide, crystalline hexacyanochromates(II) M₄[Cr(CN)₆] (M=Na or K) have been isolated: these are dihydrates at room temperature, but become anhydrous when dried at 100°in vacuo. In liquid ammonia, the hexacyanochromates(II) are reduced to the chromium(O) compounds M₆Cr(CN)₆ by the respective alkali metals. All of these compounds are extremely air-sensitive and are low spin; i.r. and reflectance spectra are reported as well as x-ray powder data for Na₄[Cr(CN)₆], K₄[Cr(CN)₆] and K₆Cr(CN)₆.     

Structures of carbohydrate-boronic acid complexes determined by NMR and molecular modelling in aqueous alkaline media

The structures of thermodynamically stable aromatic boronic acid : cyclic carbohydrate chelates in aqueous alkaline media have been studied using 1H NMR spectroscopy and molecular modelling. It is found that interacting saccharides must necessarily possess a synperiplanar diol functionality for complexation to occur. While this is possible for furanose structures which tend to have a puckered planar geometry, for pyranose forms it is postulated that bis-complexation occurs with twist conformers of the pyranose ring, providing the ring has the requisite 1,2 : 3,4 polyol stereochemistry; specifically axial,equatorial : equatorial,axial or equatorial,axial : axial,equatorial orientations. In this respect it is possible to be predictive with regard to individual carbohydrate boronic acid interactions and to give reasonably comprehensive structural assignments to complexed components. In this paper twenty four polyhydroxy compounds have been screened using 1H NMR to monitor complexation along with computational techniques on a model system to substantiate proposed structures. It has been found that all of these materials interact with aromatic mono boronic acids as expected and structures for the resulting chelates are proposed.     

What do we know and when do we know it?

Two essential aspects of virtual screening are considered: experimental design and performance metrics. In the design of any retrospective virtual screen, choices have to be made as to the purpose of the exercise. Is the goal to compare methods? Is the interest in a particular type of target or all targets? Are we simulating a ‘real-world’ setting, or teasing out distinguishing features of a method? What are the confidence limits for the results? What should be reported in a publication? In particular, what criteria should be used to decide between different performance metrics? Comparing the field of molecular modeling to other endeavors, such as medical statistics, criminology, or computer hardware evaluation indicates some clear directions. Taken together these suggest the modeling field has a long way to go to provide effective assessment of its approaches, either to itself or to a broader audience, but that there are no technical reasons why progress cannot be made.