Bremsstrahlung isochromats from transition metal carbides: The cubic mono-carbides TiC,ZrC, HfC,VC, NbC,TaC

Bremsstrahlung isochromats of polycrystalline TiC, ZrC, HfC, VC, NbC, and TaC have been measured at a monochromator energy of 1,008.2 eV. The curves are similar to those of the pure metals, but show a secondary structure caused by the chemical binding of metal and carbon. The experimental results are compared with density-of-states calculations. Together with XPS-measurements, they allow to derive the widths of the total conduction bands of the carbides, which are compared with thed-band widths of the pure metals and with band structure calculations.Dedicated to Prof. K. Ulmer on the occasion of his 65^th birthday     

Role of solvation in the energy stabilization inside the hydrophobic core of the protein rubredoxin

There are many forces that contribute to the stability of a protein; among these are dispersion interactions, hydrogen bonding, and solvation effects. In a recent work, Vondrasek et al. estimated the in vacuo stabilization energy of the hydrophobic core of the protein rubredoxin using high level ab initio methods (Vondrasek, J.; et al. J. Am. Chem. Soc. 2005, 127, 2615). In this work, we evaluate the effects of solvation on the stability of the hydrophobic core of this protein. Solvation calculations are made using the polarizable continuum method at the MP2/aug-cc-pVDZ level of theory. It is found that, in a protein-like environment (mimicked by a continuum solvent with a dielectric constant of approximately 4), the stability of rubredoxin's hydrophobic core is decreased by 40-50%. We also observed that the stabilization energy of the hydrophobic core is only slightly lower in a protein-like medium than in an aqueous one (DeltaGether-DeltaGwater approximately 1.0-3.5 kcal/mol).     

Discovery of Elusive K4O6, a Compound Stabilized by Configurational Entropy of Polarons

Synthesis of elusive K₄O₆ has disclosed implications of crucial relevance for new solid materials discovery. K₄O₆ forms in equilibrium from K₂O₂ and KO₂, in an all‐solid state, endothermic reaction at elevated temperature, undergoing back reaction upon cooling to ambient conditions. This tells that the compound is stabilized by entropy alone. Analyzing possible entropic contributions reveals that the configurational entropy of “localized” electrons, i.e., of polaronic quasi‐particles, provides the essential contribution to the stabilization. We corroborate this assumption by measuring the relevant heats of transformation and tracking the origin of entropy of formation computationally. These findings challenge current experimental and computational approaches towards exploring chemical systems for new materials by searching the potential energy landscape: one would fail in detecting candidates that are crucially stabilized by the configurational entropy of localized polarons.     

Differential infrared thermography for boundary layer transition detection on pitching rotor blade models

Differential infrared thermography (DIT) was investigated and applied for the detection of unsteady boundary layer transition locations on a pitching airfoil and on a rotating blade under cyclic pitch. DIT is based on image intensity differences between two successively recorded infrared images. The images were recorded with a high framing rate infrared camera. A pitching NACA0012 airfoil served as the first test object. The recorded images were used in order to investigate and to further improve evaluation strategies for periodically moving boundary layer transition lines. The measurement results are compared with the results of unsteady CFD simulations based on the DLR-TAU code. DIT was then used for the first time for the optical measurement of unsteady transition locations on helicopter rotor blade models under cyclic pitch and rotation. Image de-rotation for tracking the blade was employed using a rotating mirror to increase exposure time without causing motion blur. The paper describes the challenges that occurred during the recording and evaluation of the data in detail. However, the results were found to be encouraging to further improve the method toward the measurement of unsteady boundary layer transition lines on helicopter rotor models in forward flight.     

Assessment of density functional theory methods for the computation of heats of formation and ionization potentials of systems containing third row transition metals

The performance of several different density functional theory (DFT) methods, including GGA, hybrid-GGA, meta-GGA, and hybrid-meta-GGA methods, have been assessed in terms of their ability to accurately compute both heats of formation and ionization potentials of systems containing third row transition metals. Two different basis sets were used in this study: 6-31G** and TZVP. It is found that the triple-zeta quality TZVP basis set generally produces the best results for both heats of formation and ionization potentials. One important observation made in this study is that the inclusion of exact exchange terms in DFT methods generally results in more consistently accurate results for both heats of formation and ionization potentials of transition metal systems. In general, DFT methods do not yield good ionization potential results for systems containing titanium or zinc. For heats of formation, it is found that the hybrid-meta-GGA functional, TPSS1KCIS, yields the best overall results when combined with the TZVP basis set, while PBE1PBE (hybrid-GGA) gives the best results for the 6-31G** basis. The hybrid-GGA functional, B3LYP, is found to produce the lowest overall errors for ionization potentials when combined with both 6-31G** and TZVP.     

Molecular dynamics study of ethanolamine as a pure liquid and in aqueous solution

Molecular dynamics simulations have been carried out for ethanolamine as a pure liquid and in aqueous solution at 298 and 333 K. The ethanolamine force field has been parametrized to reproduce intramolecular energies from quantum mechanical calculations and experimentally determined properties of the liquid. The results are presented for conformer distributions, density, enthalpy of vaporization, self-diffusion constant, dielectric constant, and radial distribution functions. The results strongly suggest that the main (O-C-C-N) dihedral tends to stay in its gauche conformers in solution and that the ethanolamine molecules populate conformers with a significant degree of intramolecular hydrogen bonding. This result is also supported by results from a continuum solvation model. Simulation of a 10 mol % aqueous ethanolamine system suggests that ethanolamine is preferentially solvated to by water molecules. The results suggest that ethanolamine dimer formation in aqueous solution is very limited. Simulations were also carried out for CO2 in an aqueous ethanolamine system. The results suggest that CO2 has a comparable level of attraction to ethanolamine and water. The degree of interaction between CO2 and the amine and alcohol functionalities in ethanolamine also appear to be of comparable strength.     

The hydrolysis of amides and the proficiency of amidohydrolases. The burden borne by kw

The hydrolysis of small amides has garnered major attention due to its relevance to peptide hydrolysis, one of the most fundamental reactions of biology. Both experimental and theoretical research efforts have studied the reaction in different media, and a consensus has been reached regarding the specific acid- and base-catalyzed reaction pathways. Nevertheless, for the water reaction, large discrepancies between theoretical and experimental results are found in the literature. Herein, we report the results of theoretical calculations of formamide and urea hydrolysis at different pH values. Model systems have been built clustering one and two water molecules with the reactive amide. A careful analysis of the reaction pathways at different temperatures has allowed us to accurately reproduce available experimental data and to separate the water reactions from their acid and base counterparts. The relevance of the results in providing an accurate definition of the proficiency of amidohydrolases is discussed.