S. H. Glarum

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

Raman Studies of Alkali-Metal Doped AxC60 Films (A = Na, K, Rb, and Cs; x = 0, 3, and 6)

The room temperature Raman spectra of the intramolecular modes between 100 cm–1 and 2000 cm–1 are reported for alkali-metal doped AxC60 films. For A = K, Rb, and Cs, phase separation is observed with the spectra of C60, K3C60, K6C60, Rb3C60, Rb6C60, and Cs6C60 phases reported. The x = 3 phases show only three Raman active modes: two of Ag symmetry and only the lowest frequency Hg mode. The other Hg modes regain intensity in the x = 6 films, with several mode splittings observed. For A = Na, such phase separation is not clearly observed, and reduced mode shifts are interpreted as due to incomplete charge transfer in these films.

Electron mobilities in organic semiconductors

Abstract The calculation of electron mobilities in organic molecular crystals is considered in terms of band and hopping models. It is concluded that, within the limits of the tight-binding approximation employed, maximum mobilities of 10–30 cm 2 V-sec exist for such systems.

Spin Exchange in Nitroxide Biradicals

The dependence of the electron spin‐exchange energy upon temperature and solvent has been studied for the carbonate, oxalate, and succinate diesters of tetramethyl‐piperidinol‐oxyl. An analysis is given of exchange effects on ESR line positions, intensities, and widths. For the carbonate diester the exchange energy is constant at low temperatures. As the temperature is raised this energy gradually decreases, passes through a minimum, and then increases exponentially. Interpretation suggests that at low temperatures exchange occurs through an indirect process involving the core of sigma electrons, whereas at high temperatures a direct exchange process is more important.

Paramagnetic Relaxation in Liquid‐Crystal Solvents

The influence of a liquid‐crystal solvent on paramagnetic relaxation processes involving Zeeman and hyperfine anisotropies is examined. Secular and pseudosecular relaxation mechanisms are reduced by molecular ordering. Secular contributions are most effectively quenched, and, for high degrees of order in viscous media, pseudosecular processes are most important.ESR studies were made of solutions of vanadyl complexes with six β‐diketones in p‐azoxyanisole. The linewidths in the spectra depend upon the 51V nuclear spin quantum number M, and vary as A+BM+CM2. As the degree of molecular ordering increases, one predicts and observes that first C and then B change in sign.It was also found that almost perfect ordering of a solute in an imperfectly ordered liquid crystal can be achieved, provided the solute molecule is sufficiently large and rigidly planar. Implications of ESR observations concerning the structure of liquid‐crystal phases are discussed.

The influence of oxygen on the Raman spectrum of C60 films

Abstract The influence of oxygen exposure on the Raman spectrum of C60 microscrystalline films under laser irradiation is described. In the frequency region of the symmetric Ag pentagonal pinch mode, exposure of vacuum grown C60 films to 16O2, 18O2, or air results in the appearance of a peak at 1467 cm-1, a mode previously attributed to pure C60. The time dependence of this mode in the laser beam, as well as the Raman spectrum of pristine C60 are discussed.

Resolution Enhancement for Spectra of Chemical and Physical Interest

When a spectrum is composed of a superposition of lines with known shape and uniform width, the signal output of a spectrometer may be transformed into a spectrum whose effective resolution is significantly improved. If the line shapes are not precisely known or the linewidths not exactly uniform, it is still possible to achieve a worthwhile resolution enhancement. Decomposition and analysis of complex spectra is often aided by this transformation. The theory of filters which produce the desired transformation is developed. Digital and analog implementation for these filters is given as well as several examples.

ESR of the Perinaphthenyl Radical in a Liquid Crystal

The electron spin resonance spectrum of the perinaphthenyl radical has been studied in the isotropic and nematic phases of p‐azoxyanisole. The g tensor and hyperfine splitting tensors for the two proton couplings and three of the four carbon−13 couplings have been determined, and were found to agree in most cases with theoretical estimates. Ordering of the radical was also deduced as a function of temperature in the liquid‐crystal range, and corresponded to the ordering of the solvent molecules.A general expression for the effective spin Hamiltonian of a radical in a liquid crystal is derived. Zeeman anisotropy and dipolar electron—nuclear couplings are shown to result in g shifts and changes in the hyperfine splitting coefficients.

Spin—Orbit Interactions in Molecular Radicals

Contributions to the g‐tensor and zero‐field parameters arising from spin—orbit interactions are derived for molecular radicals. Calculations are made for CH3, NH2, and CH2, and where experimental data are available, agreement is satisfactory. Polynuclear radicals are qualitatively discussed.

Electron Spin Resonance Study of the Sym‐Trinitrobenzene Anion and Related Radicals

The electron spin resonance spectra of anion radicals generated by the electrolytic reductions of sym‐trinitrobenzene, trinitromesitylene, trinitrobenzoic acid, trinitrotoluene, trinitroanisole, picramide, and the picrate ion are reported and discussed. Measurements were carried out in three solvents; dimethylformamide, dimethylsulfoxide, and acetonitrile. In addition, perturbations in the spectrum of the trinitrobenzene anion produced by the presence of various monovalent and divalent cations were studied. The trinitrobenzene and trinitromesitylene anions show an alternating linewidth effect which increases as the interaction between the anion and its environment increases. This effect implies correlated fluctuations in the spin densities of three equivalent nitrogroups.