Luminescence Properties of Tetrahedral Coordinated Mn2+; Genthelvite and Willemite Examples

The cause of the split of 4A4E(4G) Mn2+ excited level measured on minerals spectra is discussed. It is our view that ∆E = |4E(4G) − 4A(4G)| should be considered an important spectroscopic parameter. Among the possible reasons for the energy levels splitting taken under consideration, such as the covalent bond theory, the geometric deformation of the coordination polyhedron and the lattice site’s symmetry, the first one was found to be inappropriate. Two studied willemite samples showed that the impurities occur in one of the two available lattice sites differently in both crystals. Moreover, it was revealed that the calculated crystal field Dq parameter can indicate which of the two non-equivalent lattice sites positions in the willemite crystal structure was occupied by Mn2+. The above conclusions were confirmed by X-ray structure measurements. Significant differences were also noted in the Raman spectra of these willemites.

Chemical Bonding 2: Modern Theories of Chemical Bonding

Chemical Bonding II: Modern Theories of Chemical Bonding explains four bonding theories related to molecular geometry and bonding. Lewis structures and the Valence-Shell Electron-Pair Repulsion (VSEPR) model are used to describe and predict the electron group geometry, molecular geometry and shapes of molecules. The VSEPR model is then used to predict molecular polarity as a function of shape. This leads to Valence Bond Theory, which uses the principles of orbital overlap and hybridization of atomic orbitals to describe chemical bonding. Finally the Molecular Orbital Theory (MOT) based on electron delocalization is discussed in terms of bonding and anti-bonding molecular orbitals.

Real space electron delocalization, resonance, and aromaticity in chemistry

Chemists explaining a molecule’s stability and reactivity often refer to the concepts of delocalization, resonance, and aromaticity. Resonance is commonly discussed within valence bond theory as the stabilizing effect of mixing different Lewis structures. Yet, most computational chemists work with delocalized molecular orbitals, which are also usually employed to explain the concept of aromaticity, a ring delocalization in cyclic planar systems which abide certain number rules. However, all three concepts lack a real space definition, that is not reliant on orbitals or specific wave function expansions. Here, we outline a redefinition from first principles: delocalization means that likely electron arrangements are connected via paths of high probability density in the many-electron real space. In this picture, resonance is the consideration of additional electron arrangements, which offer alternative paths. Most notably, the famous 4n + 2 Hückel rule is generalized and derived from nothing but the antisymmetry of fermionic wave functions.

Gluten Adhesion and Shearing in a Contact-Based Coarse-Grained Model

We study adhesion of the gluten proteins during the shear and normal deformations as described by a coarse-grained molecular dynamics. We show that the two types of deformation have a different impact on the proteins. We also calculate the dynamic shear modulus and critical strain and find the results to be consistent with the slip-bond theory which assumes that the gluten proteins can be treated as an interconnected network of polymers. Graphical Abstract