An e.m.s. study of the cathodic H/D separation factor at gold electrodes in sulfuric acid

The cathodic H/D separation factor (S_D) at gold electrodes was studied with respect to its dependence on overpotential, time of electrolysis, and deuterium enrichment. All experiments were performed in sulfuric acid and under potentiostatic control. The evolved gas was on-line transferred through a porous hydrophobic electrode into a mass spectrometer there being analyzed directly. The results showed no influence of the applied overpotential between ?100 and ?600 mV RHE on the separation factor, which has a value of 5.3±0.2. A time dependence of S_D could not be noticed within 11 h of experiment. Between 0.5 and 4.7% deuterium enrichment the average separation factor remained constant with 5.2±0.4.     

Ordering and phase transitions in ising systems with competing short range and dipolar interactions

As a simple model of order-disorder ferroelectrics or dipolar magnets we consider a simple cubic Ising-system with nearest neighbor exchangeJ and dipolar interaction of strengthµ ²/a ³. ForJa ³/µ ²Ja ³/µ ²<0.1270 the ground state consists of ferromagnetic rows (in spin direction) arranged antiferromagnetically in the plane perpendicular to it. AtJa ³/µ ²=0.1279 the structure changes to a layered antiferromagnetic structure with a twocomponent order parameter, while forJa ³/µ ²>0.16429 the ferromagnetic phase becomes stable (with domain arrangements depending on the shape of the sample). For all critical values ofJa ³/µ ² where the bulk energies of two phases become equal also the interface energy between these phases is found to be zero. The ordering at nonzero temperature is studied by means of mean-field approximations (MFA) and Monte Carlo (MC) calculations. It turns out that forJa ³/µ ² of order unity the MFA overestimates ordering temperatures by about a factor of two, and predicts multicritical points (between the disordered and two ordered phases) at nonzero temperature, including two biaxial Lifshitz points which the MC work suggests to occur atT=0. In contrast to MFA the layered antiferromagnetic structure is found to be stable only at extremely lowT, because a metastable spin-glass phase (with random arrangement of ferromagnetic rows in the spin direction) has only slightly higher energy. The MFA also yields two regimes of helical phases which are “locked in” to the antiferromagnetic phases at uniaxial Lifshitz points occurring at the Brillouin zone boundary. In the MC-work various methods of treating the long-range interaction are investigated. While all kinds of truncations as well as compensating field methods are rather unsatisfactory in our case, Ewald summation techniques yield satisfactory results. Nevertheless strong fluctuations as well as strong finite size effects prevent us from making accurate exponent estimates, but arguments are given that there is no regime of broad visibility of Landaulike critical behavior. Finally the extension of our results to other lattices as well as experimental applications are briefly discussed.     

Thermal Activation of N?H Bonds by Transition‐metal Oxide Cations: Does a Hierarchy Exist in the First Row?

The thermal reactions of first-row transition-metal oxide cations [MO](+) (M=Sc-Ni, Zn) with ammonia have been studied by gas-phase experiments and computational methods. The activation of N-H bonds is brought about by the monoxides of the middle and late 3d metals Mn-Ni and Zn. The two primary reaction channels correspond to dehydration, which leads to [M(NH)](+), and hydrogen-atom abstraction to form [M(OH)](+). Oxygen-atom transfer from [MO](+) to NH(3) to produce neutral or ionized hydroxylamine was observed as a minor channel for some of the late transition-metal oxides. The computational analysis of these reactions, which was aimed at elucidating the reaction mechanisms and to uncover possible periodic trends across the first row, have been performed for the couples [MO](+) /NH(3) (M=Sc-Zn). Dehydration is found to be endothermic for the oxides of scandium to vanadium and exothermic for the other systems. Hydrogen-atom abstraction becomes exothermic starting with [MnO](+) and, finally, oxygen-atom transfer is feasible for the cationic oxides of nickel to zinc.     

One‐, Two‐, and Three‐Electron Reduction of a Cyclic Alkyl(amino)carbene–SbCl3 Adduct

A cyclic alkyl(amino)carbene readily reacts with SbCl₃ to form the corresponding Sb^III adduct. One‐electron reduction gives rise to the first example of a neutral antimony‐centered radical characterized in solution. Two‐electron reduction affords a Lewis base stabilized chloro‐stibinidene, whereas three‐electron reduction gives an antimony diatomic species capped by two carbenes. The radical has been characterized by EPR spectroscopy, while the structure of the other three species has been ascertained by single‐crystal X‐ray diffraction. In these four species, the formal oxidation state of the metalloid diminishes from III, to II, to I, and finally 0.     

CN and CC Bond Formations in the Thermal Reactions of "Bare" Ni(NH(2) )(+) with C(2) H(4) : Mechanistic Insight on the Metal-Mediated Hydroamination of an Unactivated Olefin

Strike! While pure ammonia is out of luck, Ni(NH(2) )(+) gets one strike after another at the "ethylene bowling championship". In fact, Ni(NH(2) )(+) is so effective that no pin, not even at the neighboring lane, is safe from it.     

Crystal and molecular structure of p-Toluquinone-monoguanylhydrazone,C8H10N4O

The crystal structure of p-toluquinone-monoguanylhydrazone is orthorhombic, space group Pna2₁ with 4 formula units C₈H₁₀N₄O in the unit cell. The lattice constants are a = 7.589, b = 20.117, and c = 5.699 Å. The structure was refined to the conventional R = 0.05. The configuration and conformation of the molecule as well as the unusual hydrogen bridge system are discussed.     

The crystal and molecular structure of 5α,8α-epidioxyergosta-6,22-dien-3β-acetate

From the mushroom species Tricholoma populinum LANGE which shows interesting pharmaceutical effects the steroid 5α,8α-epidioxyergosta-6,22-dien-3β-ol has been isolated. The acetate of this compound crystallizes in the monoclinic space group P 2₁ with two molecules in the unit cell of dimensions a = 19.788, b = 7.241, c = 9.914 Å, β = 103,97°. The final R-value is 0.079 for 1082 reflections.