Ferromagnetic spin alignment in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and Oligo(1,4-phenylenevinylene)s bearing pendant p-phenylenediamine radical cations

The intramolecular and long-range ferromagnetic coupling between p-phenylenediamine radical cations in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and oligo(1,4-phenylenvinylene)s between neighbors and next-nearest neighbors is described. UV/vis/near-IR experiments show that the radical cations are localized in the pendant p-phenylenediamine units of the conjugated oligomers. The ESR spectra of these oligo(1,4-phenyleneethynylene) and oligo(1, 4-phenylenvinylene) di(radical cation)s are consistent with those of a triplet state. A linear behavior is observed for the doubly integrated ESR intensity of the DeltaM(s) = +/-1 and DeltaM(s) = +/-2 signals with the inverse temperature (I approximately 1/T), consistent with Curie's law. This behavior indicates a triplet ground-state diradical with a large triplet-singlet energy gap or possibly a degeneracy of singlet and triplet states.     

Supramolecular phthalocyanine dimers based on the secondary dialkylammonium cation/dibenzo-24-crown-8 recognition motif

[formula: see text] New unsymmetrically substituted DB24C8-phthalocyanines, which are able to form complexes with suitable dialkylammonium cations, have been prepared. These complexes most probably have a pseudorotaxane geometry.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Structure elucidation of everninomicin-6, a new oligosaccharide antibiotic, by chemical degradation and FAB-MS methods

The structural characterization of a new oligosaccharide antibiotic, Everninomicin-6 (EV-6), is described. Detailed fast-atom bombardment mass spectrometry (FAB-MS) studies along with NMR and chemical degradation methods were conducted to elucidate the structure of EV-6. The effects of the use of various matrices, including salt addition, on the quality of the FAB-MS were explored. The use of 3-nitro benzyl alcohol, dimethyl sulfoxide (DMSO), and NaCl produced the best results: an intense sodiated molecular ion plus structurely informative fragmentation. FAB-MS yields information providing the complete sugar sequence information for everninomicins, which is quite valuable to the elucidation of the structure of this complex oligosaccharide antibiotic. In addition, the results of accurate mass work with the molecular ion are consistent with the assigned structure. The use of electrospray ionization mass spectrometry (ESI-MS) and ESI-MS/MS for the study of EV-6 was investigated and was found to produce an abundant molecular ion with limited structural information. These results revealed that EV-6 resembled EV-D quite closely except for the absence of the nitrosugar and the replacement on ring g of the -CH₂OCH₃ group with a -CH₂OH group.