Quantitative EPR spectroscopy: Comparison between primary standards and application to MgO-MnO and α-Al2O3-Cr2O3 solid solutions

To study their reliability as primary standards in the quantitative EPR spectroscopy, a large series of pure paramagnetic compounds with known spin concentrations, whose spectra vary considerably in intensity, shape, structure and overall width are compared. The paramagnetic species examined as pure solid compounds and solutions, were free radicals (DPPH and TEMPO), vanadyl and Cu²⁺ ions (S = 1/2), Cr³⁺ (S = 3/2) and Mn²⁺ (5 = 5/2) ions. The quantitative EPR findings suggest that all theS = 1/2 paramagnetic compounds investigated and MnSO₄ · H₂O (S = 5/2) are reliable primary standards. By contrast, none of the pure Cr³⁺ compounds proved useful as primary standards because of their large fine-structure terms or high Néel temperature that invalidated the simple Curie law. Application of quantitative EPR in the study of dilute MgO-MnO and α-Al₂O₃-Cr₂O₃ solid solutions, focussing on the circumstances making paramagnetic species undetectable, is reported. In MgO-MnO solid solutions of high surface area, detection problems arising from the variation of local site symmetry can be circumvented and almost all Mn²⁺ are detected only by reducing the surface area. In concentrated α-Al₂O₃-Cr₂O₃ solid solutions, magnetic interactions lead to paramagnetic species being undetectable.     

Hierarchically Structured Allotropes of Phosphorus from Data-Driven Exploration

The discovery of materials is increasingly guided by quantum-mechanical crystal-structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data-driven approaches can vastly accelerate the search for complex structures, combining a machine-learning (ML) model for the potential-energy surface with efficient, fragment-based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic (“random”) structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one-dimensional (1D) single and double helix structures, nanowires, and two-dimensional (2D) phosphorene allotropes with square-lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures.     

Extreme value theory for singular measures

In this paper, we perform an analytical and numerical study of the extreme values of specific observables of dynamical systems possessing an invariant singular measure. Such observables are expressed as functions of the distance of the orbit of initial conditions with respect to a given point of the attractor. Using the block maxima approach, we show that the extremes are distributed according to the generalised extreme value distribution, where the parameters can be written as functions of the information dimension of the attractor. The numerical analysis is performed on a few low dimensional maps. For the Cantor ternary set and the Sierpinskij triangle, which can be constructed as iterated function systems, the inferred parameters show a very good agreement with the theoretical values. For strange attractors like those corresponding to the Lozi and He?non maps, a slower convergence to the generalised extreme value distribution is observed. Nevertheless, the results are in good statistical agreement with the theoretical estimates. It is apparent that the analysis of extremes allows for capturing fundamental information of the geometrical structure of the attractor of the underlying dynamical system, the basic reason being that the chosen observables act as magnifying glass in the neighborhood of the point from which the distance is computed.     

Wetting behavior of porous silicon surfaces functionalized with a fulleropyrrolidine

We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon (PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel's formula. This result has been explained by an extension of Wenzel's model in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid material, is substantially reduced below 90 degrees by surface roughness.     

Theoretical and spectroscopic study of the effect of ring substitution on the adsorption of anisole on platinum

The adsorption of anisole, 3,5-dimethylanisole, and 3,5-bis-(trifluoromethyl)-anisole on Pt(111) was studied theoretically and compared to the adsorption of benzene using relativistically corrected density functional theory. A cluster of 31 platinum atoms was used to simulate the surface. The three anisoles were found to be less strongly adsorbed than the parent molecule benzene, 3,5-bis-(trifluoromethyl)-anisole showing weakest adsorption, with an adsorption energy of only one-third that of benzene. The theoretical study was complemented by in situ ATR-IR spectroscopy of the adsorption of the anisole derivatives on a polycrystalline Pt film. The spectroscopic study indicated that the adsorption strength of the anisoles follows the same order as predicted by the calculations. In addition, catalytic hydrogenation tests showed that the propensity to aromatic ring hydrogenation can also be correlated to the mode and strength of adsorption of the anisoles. The degree of saturation followed the same order as the adsorption strength found by the calculations and indicated by spectroscopy. Although 3,5-dimethyl substitution on anisole resulted in only a partial loss of adsorption energy and reactivity toward ring hydrogenation as compared to anisole, the substitution by CF(3) groups led to a large loss of adsorption energy and complete loss of reactivity toward aromatic ring saturation. Along with the study of the substituent effect on the adsorption of aromatic molecules, the correlation between adsorption and propensity to saturation of aromatic substrates could be corroborated.     

Wettability of pristine and alkyl-functionalized graphane

Graphane is a hydrogenated form of graphene with high bandgap and planar structure insensitive to a broad range of chemical substitutions. We describe an atomistic simulation approach to predict wetting properties of this new material. We determine the contact angle to be 73°. The lower hydrophobicity compared to graphene is explained by the increased planar density of carbon atoms while we demonstrate that the presence of partial charges on carbon and hydrogen atoms plays only a minor role. We further examine the effects of graphane functionalization by alkyl groups of increasing chain lengths. The gradual increase in contact angle with chain length offers a precise control of surface wettability. A saturated contact angle of 114° is reached in butylated form. We find the saturation of contact angle with respect to the length of the functional groups to coincide with the loss of water's ability to penetrate the n-alkyl molecular brush and interact with carbon atoms of the underlying lattice. Since no experimental data have yet become available, our modeling results provide the first estimate of the wettability of graphane. The results also show how its alkyl functionalization provides the basis for a variety of chemical modifications to tune hydrophilicity while preserving the planar geometry of the substrate.