Deuterium N.M.R. of liquid crystal solutions with proton–proton dipolar decoupling

Abstract

Deuterium N.M.R. of solute molecules in liquid crystal solutions with the removal of proton–proton dipolar coupling has been explored. The deuterium N.M.R. spectrum of a partially deuterated compound dissolved in a liquid crystal solvent is usually complex or unresolved because of proton–deuterium coupling and extensive proton–proton dipolar coupling. When the latter is removed by a special dipolar decoupling sequence, the deuterium N.M.R. spectrum becomes first order, and deuterium–proton dipolar coupling constants can be readily obtained from the spectrum. Results of monodeuterated hexanes and heptanes dissolved in ZLI 2142 are reported.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

DEFORMATIONS OF HYPERCOMPLEX STRUCTURES ASSOCIATED TO HEISENBERG GROUPS

Let X be a compact quotient of the product of the real Heisenberggroup H_4m+1 of dimension 4m + 1 and the three-dimensional realEuclidean space R³. A left-invariant hypercomplex structureon H_4m+1 x R³ descends onto the compact quotient X. The spaceX is a hyperholomorphic fibration of 4-tori over a 4m-torus.We calculate the parameter space and obstructions to deformationsof this hypercomplex structure on X. Using our calculations,we show that all small deformations generate invariant hypercomplexstructures on X but not all of them arise from deformationsof the lattice. This is in contrast to the deformations on the4m-torus.     

Eliashberg function in amorphous superconductors

The conventional approach to tunnelling and critical temperature analysis for amorphous superconductors is discussed. Comparison is made on the effect of structural disorder on the electron-phonon coupling in nearly-free-electron and nearly-localized-electron metals. The results are in good agreement with tunnelling data.     

Generation of dangling bonds by high temperature annealing and hopping conduction in amorphous silicon films

By annealing evaporated a-Si films between 450°C and 620°C, it is found that new dangling bonds are generated. These dangling bonds can act as hopping centres. Quantitative analysis of experimental results on electrical resistivity and ESR measurements favours a conduction mechanism of hopping to nearest neighbours between room temperature and ?100°C. Films annealed just before crystallisation temperature have a density of states near Fermi level of about 3 × 10¹⁸ cm^?3 eV^?1.     

Review on theory and applications of wavefront recording plane framework in generation and processing of digital holograms

The wavefront recording plane (WRP), subsequently generalized to be known as the virtual diffraction plane (VDP), is a recent concept that has been successfully deployed in fast generation and processing of digital holograms. In brief, the WRP and its extension, the VDP, is a hypothetical plane that is located between the hologram and the object scene, and which is at close proximity to the latter. As such, the fringe patterns on the hypothetical plane are carrying the holistic information of the hologram, as well as the local optical properties of the object scene. This important property enables a hologram to be processed with classical image processing techniques that are normally unsuitable for handling holographic information. In this paper we shall review a number of works, that have been developed based on the framework of the WRP and the VDP.     

Synthesis, crystal structures, linear and nonlinear optical properties, and theoretical studies of (p-R-phenyl)-, (p-R-phenylethynyl)-, and (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes and related compounds

The (p-R-phenyl)dimesitylboranes (R=Me(2)N, MeO, MeS, Br, I), (p-R-phenylethynyl)dimesitylboranes (R=Me(2)N, MeO, MeS, H), (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes (R=Me(2)N, H(2)N, MeO, MeS, H, CN, NO(2)), (E)-[2-(2-thienyl)ethenyl]dimesitylborane, and (E)-[2-(o-carboranyl)ethenyl]dimesitylborane have been prepared through the reaction of the appropriate p-R-phenyl- and p-R-phenylethynyllithium reagents with dimesitylboron fluoride and by hydroboration of the appropriate p-R-phenylacetylene, 2-ethynylthiophene, and o-ethynylcarborane with dimesitylborane. Their UV/Vis absorption and emission spectra have been recorded in a range of solvents with the fluorescence maxima of the donor-substituted compounds in particular exhibiting large bathochromic shifts in highly polar solvents, indicative of charge transfer leading to large dipole moments in the excited state. The molecular structures of the (p-R-phenyl)dimesitylboranes (R=Me(2)N, MeO, MeS, Br, I), the (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes (R=Me(2)N, H(2)N MeO, MeS, H), (p-R-phenylethynyl)dimesitylborane (R=Me(2)N), and (E)-[2-(2-thienyl)ethenyl]dimesitylborane, which have been determined from single-crystal X-ray diffraction measurements, offer evidence of increased conjugation in the ground state with increased donor strength of the R substituent. Their first- and second-order molecular hyperpolarizabilities have been obtained from EFISH and THG measurements, the first-order hyperpolarizabilities being largest for the strongest R-substituent donors. AM1 calculations have been performed on these compounds, showing reasonable agreement with the experimentally obtained bond lengths and hyperpolarizabilities, as well as on several related hypothetical compounds containing multiple C==C bonds, most of which are proposed to have even larger hyperpolarizabilities.     

p-Siletanylbenzylidene acetal: oxidizable protecting group for diols

[reactions: see text] Hydrogen peroxide oxidation of benzylidene acetals (and derivative benzyl ethers) that incorporate a siletane ring at the para position creates a deprotection pathway without affecting other important chemical properties of the benzylidene acetal, such as regioselective reductive ring opening.     

Temperature-dependence of radical-trapping activity of phenoxazine,phenothiazine and their aza-analogues clarifies the way forward for new antioxidant design

The prediction and/or rationalization of diarylamine radical-trapping antioxidant (RTA) activity at the elevated temperatures where they are most useful presents a significant challenge, precluding the development of new technology. Whilst structure–activity relationships at ambient temperatures are well established, their predictive capacity at elevated temperatures is poor due to competing reactions. A striking example involves phenoxazine, which is a superior RTA relative to its sulfur analog phenothiazine at ambient temperature (e.g. k = 3.9 × 10⁷vs. 7.6 × 10⁶ M⁻¹ s⁻¹ at 37 °C, respectively), but is demonstrably inferior at elevated temperatures. Despite being inherently less oxidizable in electrochemical experiments and high-accuracy computations, phenoxazine is more rapidly consumed than phenothiazine in autoxidations at 160 °C – a result which we attribute to a lower reorganization energy barrier to oxidation. Given these observations, we surmised that incorporation of an electronegative N-atom into the phenoxazine ring system would decrease the driving force for oxidation and ‘rescue’ its activity. Indeed, this was found to be the case for nitrogen incorporation at any position, regardless of the impact on the inherent RTA activity. Analogous experiments were carried out on phenothiazines into which nitrogen atoms were incorporated, revealing little benefit at 160 °C. These results suggest that for highly reactive diarylamines (i.e. those with k > 10⁶ M⁻¹ s⁻¹), further enhancements in reactivity do not materially improve their ability to inhibit hydrocarbon autoxidation at elevated temperatures. Instead, their stability to one-electron oxidation determines their efficacy.

Studies of potent diarylamine antioxidants illustrate how their high-temperature activity can be limited by direct reaction with O₂, and how driving force and barrier to oxidation can be modulated for future antioxidant development.