Thermal dimerization of the conjugated 10‐trans, 12‐trans linoleate (250C, 5 hr) produced a dimer whose structure is shown to be that of the Diels‐Alder reaction between two molecules of monomer, with one molecule acting as diene, and either one of the two double bonds of the second molecule acting as dieneophile. This produces four skeletal isomers of a tetrasubstituted (1,2,3,4) cyclohexene structure with ॅ‐ॆ unsaturation on one chain. The isomers formed depend on whether the 10 or the 12 double bond acts as dieneophile, and whether the monomers add 舠head to head舡 or 舠head to tail.舡 Evidences for the structures include chemical analyses, ozonolysis, nuclear magnetic resonance, IR and UV spectrometry and particularly mass spectrometry of the distilled dimer, of the completely hydrogenated dimer, and of the aromatized dimer formed by catalytic dehydrogenation. The hydrogenated dimer can be separated into two components by TLC. These are probably related to 舠head to head舡 vs. 舠head to tail舡 addition.

b Mass spectrometric characterization of steroid intermediates has been reduced to a quick routine operation with an all glass inlet system. Unambiguous determination of molecular weight is usually easy, particularly if sample history is known. Additional information about the nature of substituents is contained in the spectrum. Thirty steroid spectra illustrate how fragmentation patterns change as a variety of substituent groups are attached to the steroid nucleus.

No abstract

As an acceptor dopant with a solid:liquid distribution coefficient ks<1, iron is an example of an impurity which can be used in modest amounts to ensure that an adequate fraction of EL2 midgap defects are ionized along the length of a melt-grown GaAs crystal, as desired for semi-insulating behavior. The results of such deliberate doping with iron (when NFe is in the mid-1015 cm−3 range) are reported for crystals grown by both the liquid encapsulated Czochralski and the vertical gradient freeze methods. Except in the very tail region of such crystals (when NFe≳NEL2 and high resistivity p-type behavior results), GaAs with this modest iron modification to the compensation balance behaves with quite ordinary semi-insulating properties. The iron acceptors are then all ionized, and are optically ‘‘invisible.’’