Bonding or Nonbonding? Description or Explanation? “Confinement Bonding” of He@adamantane

Insight into bonding phenomena depends on whether one analyzes the whole bond‐formation process or just the final product. Examination of the formation of the inclusion complex He@adamantane by energy partitioning reveals that the high energy cost of “He bonding” originates in strong He? C Pauli repulsions. The postulate of attractive He? C bonds requires a very specific view.

     

Reactions in clusters of acetone and fluorinated acetones triggered by low energy electrons

Electron attachment to clusters of acetone (A), trifluoroacetone (TFA) and hexafluoroacetone (HFA) is studied in a crossed beam experiment with mass spectrometric detection of the anionic products. We find that the electron attachment properties in A change dramatically on going from isolated molecules to clusters. While single acetone is a very weak electron scavenger (via a dissociative electron attachment (DEA) resonance near 8.5 eV), clusters of A capture electrons at very low energy (close to 0 eV). The final ionic products consist of an ensemble of molecules (M) subjected to the loss of two neutral H₂ molecules ((M_n−2H₂)⁻, n ≥ 2). Their formation at low energies can only be explained by invoking new cyclic structures and polymers. In clusters of TFA, anionic complexes containing non-decomposed molecules (M_n⁻) including the monomer (M⁻) and ionic products formed by the loss of one and two HF molecules are observed. Loss of HF units is also interpreted by the formation of new cyclic structures in the anionic system. HFA is a comparatively stronger electron scavenger forming a non-decomposed anion via a narrow resonant feature near 0 eV in the gas phase. In HFA clusters, the non-decomposed parent anion is additionally observed at higher electron energies in the range 3–9 eV. The M⁻ signal carries signatures of self-scavenging processes, i.e., inelastic scattering by one molecule and capture of the completely slowed down electron by a second molecule within the same cluster. The scavenging spectrum is hence an image of the electronically excited states of the neutral molecule.     

Sonochemical degradation of cyclic nitroxides in aqueous solution

The sonochemical degradation of eight five- and six-membered nitroxides has been studied by EPR spectroscopy after exposure to ultrasound at a frequency of 354 kHz in argon-saturated aqueous solution. Concentration vs. time profiles do not follow a simple rate law. Octanol/water partition functions have been determined for all eight nitroxides, and an excellent linear correlation has been found between initial decomposition rates and hydrophobicity (log K(octanol/water)). Variation of initial rate with concentration was investigated for one compound (TEMPONE) and is largely consistent with an equilibrium distribution of substrate between bulk solution and the gas/liquid interface.     

Analysis of the hydrogen bonding in (2-hydroxypropyl)cellulose-poly(acrylic acid) composites by Raman spectroscopy

Novel anisotropic polymer composites obtained by the photopolymerisation of acrylic acid (AA) in lyotropic solutions of (2-hydroxypropyl)cellulose (HPC) were investigated. Raman spectroscopy was used in the study of hydrogen bonding formed by carboxylic groups of poly(acrylic acid) (pAA) between themselves and with the hydroxyl groups of HPC macromolecules. These investigations show that the supramolecular structure of pAA obtained by photopolymerisation contains mainly inner oligomeric forms and cyclic dimer forms, as a consequence of the “matrix polymerisation” of AA. The presence of HPC affects the process of the photopolymerisation of AA, by a redistribution of hydrogen bonding within the system, leading to a different supramolecular structure of pAA. With increasing amounts of HPC the inner oligomeric forms characteristic of bulk pAA disappear. In the composites, the pAA chains probably form the terminal oligomeric structure, with some free COOH groups, as well as hydrogen bonds with HPC.     

Hierarchical Constraint-Based Singularity Avoidance for Multi-Axis Robots

A generalized reflexive approach for Hierarchical Constraint-Based Singularity Avoidance (HCB-SA) is proposed and demonstrated for the multi-axis robot control of a six Degrees Of Freedom (DOF) hybrid manipulator system. The concept utilizes a dynamic adaptation of virtual constraints by introducing virtual damped actuation on the velocity control level and an anisotropic reflexive trajectory deflection depending on the robot constraints on the position control level. Redundant or low priority DOFs can be used to minimize the pose error in the more important DOFs reflexively without calculating new trajectories. Accordingly, the end-effector can be safely controlled in the vicinity of singularities and all constraints in the task and joint space can be surely hold. Furthermore, the proposed cascaded feedback control with the generalized HCB-SA algorithm is able to react on the presence of external disturbances which is validated using software-in-the-loop simulation on the real-time control system. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)