Studies on pyrazines. 15. A convenient synthesis of 2,5-dihydroxypyrazines

The reaction of 2,5-dimethoxypyrazines with iodotrimethylsilane followed by hydrolysis was investigated to prepare the title compounds.     

DC susceptibility of type-II superconductors in field-cooled processes

The DC susceptibility of oxide superconducting specimens in the field-cooled process has been experimentally found to depend not only on the applied DC magnetic field but also on the size of the specimens. The DC susceptibility is calculated using the critical state model in which the diamagnetism and the flux-pinning effect of superconductors are taken into account. It is shown that the saturated value of the DC susceptibility at sufficiently low temperatures, i.e., the so-called Meissner fraction, decreases with increasing DC field and/or increasing specimen size.     

INDO Studies on the structure of benzosemiquinone radicals

The spin densities of the radical anions from 4-methylcatechol, 4-carboxylic catechol, and 3-methylcatechol were calculated by means of the INDO method with the geometrical parameter adjusting method. The observed protion hyperfine coupling constants were exactly reproduced.     

Preparation and characterization of copper(II) and nickel(II) complexes with novel tetraaza-macrocyclic schiff bases

Novel tetraaza-macrocycles with N-substituted carbamoyl groups were prepared by the reaction of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,5,7,12-tetraene with isocyanates (RNCO, R = Ch₃ and C₆H₅). Their copper(II) and nickel(II) complexes were characterized by magnetic susceptibilities, electronic absorption spectra, and electrochemical properties. The complexing abilities and extractabilities of the ligands for the metal ions were investigated.     

A theoretical study on the photoisomerization of thiophene

The photoisomerization of thiophene (3–5 and 2–4 transpositions of carbon atoms) has been studied with ab initio SCF and CI calculations. A possible reaction mechanism from the lowest excited singlet state ¹B₂ of thiophene can be proposed from potential surface via Dewar thiophene. The ¹B₂ state of thiophene would easily convert to the biradical intermediate by almost one step. The internal conversion of this species to the S₀ state would cause to the transposition of carbon atoms. The effect of phenyl substituent is also discussed.