The crystal structure of a freibergite (Ag-tetrahedrite) from Keno Hill, Yukon, Canada, and a tetrahedrite from Turquetil Lake, NWT, Canada, have been refined. The freibergite, Cu5.84Ag4.22Fe1.59Zn0.43 Sb4.10AS0.13S13, a = 10.610(2) Å, and the tetrahedrite, Cu10.24Fe1.03Zn0.97Ag0.05Sb3.66As0.28S13, a = 10.364(2) Å, were both refined in space group I4̄3m. The refinements confirm that Ag does occupy the threefold coordinated metal site as first described by Kalbskopf (1972) and that Fe also occupies this site. The structural changes accompanying Ag substitution are described and a model to explain the upper limit of substitution at four atoms of silver for thirteen atoms of sulphur is proposed.

E.s.r, spectra have been obtained from the free radicals formed by photolysis of coniferyl alcohol and isoeugenol in low temperature ice matrices. Analysis of these spectra indicates that the unpaired electrons are delocalized over the entire radicals and the spin densities at the β carbon atoms are about a third of an electron.

In the ability of cycloalkyl groups to stabilize an adjacent carbocation centre, a cyclopropyl group is uniquely effective. Cyclobutyl, cyclopentyl, and cyclohexyl groups are about as effective as paraffinic substituents.

No abstract

The rearrangement of cis-1,2,3,4,5-pentamethylcyclopentenyl cation to the trans isomer in aqueous sulfuric acids has been found to follow Ho with unit slope (d log k,,,,,. = dHo). This rearrangement involves the intermediacy of a neutral compound, 1,2,3,4,5-pentamethylcyclopentadiene, which on reprotonation is only partly efficient in leading to the trans ion product. Under both steady-state intermediate and direct protonation conditions, this diene protonates to give about 55 % of the cis ion and only 45 % of the thermodynamically more stable trans isomer.