Algebraic topology is a young subject, and its foundations arenot yet firmly in place. I shall give some history, examplesand modern developments in that part of the subject called stablealgebraic topology, or stable homotopy theory. This is by farthe most calculationally accessible part of algebraic topology,although it is also the least intuitively grounded in visualizablegeometric objects. It has a great many applications to othersubjects such as algebraic geometry and geometric topology.Time will not allow me to say as much as I would like aboutthat. Rather, I shall emphasize some foundational issues thathave been central to this part of algebraic topology since theearly 1960s, but that have been satisfactorily resolved onlyin the last few years. 1991 Mathematics Subject Classification55P42, 55N20. 相似文献
The hydrolysis of T1(I) has been studied at 25°C using205T1-NMR spectroscopy and UV-Vis spectrophotometry in aqueous solutions with ionic strengths maintained by NaC104 at 2, 4, 6, and 8M. The formation constant and the spectral characteristics for the hydroxo complex, T10Hℴ have been determined.
At high hydroxide ion concentrations there is clear evidence from the UV-Vis data for the formation of a T1(OH)2-species. The spectrum and an estimated formation constant for this second hydroxo complex are also reported. 相似文献
We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on
the LAO (control) and LCMO side of the substrate. Critical current, Ic, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed
that while the control side showed a Jc of ∼ 2 × 106 A/cm2, the LCMO side showed about half the value for the same thickness (1800 ?). The difference in Jc indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of
Jc at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection
length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length
has been quantified. A control experiment carried out with LaNiO3 deposited by PLD on YBCO did not show any evidence of self-injection. 相似文献
Bis‐heteroaryl or bis‐aryl stereocenters were formed by an organocatalytic enantioselective conjugate addition using the respective trifluoroborate salts as nucleophiles. Control studies suggested that fluoride dissociation is necessary in the anhydrous conditions. This strategy is applicable to the synthesis of discoipyrrole D, an inhibitor of BR5 fibroblast migration. 相似文献
Condensed heterocycles such as quinazolines constitute the framework of many promising drugs. The great impact of the dramatic fluorine effect in pharmaceuticals prompted a great surge in the quest for fluorinated drug design resulting in over 20 % fluorine‐containing drugs in the market today. Therefore, finding an efficient and cost‐effective method for the direct synthesis of fluorine‐tagged quinazoline systems is of great significance in the pharmaceutical arena. For the first time, a one‐pot sequential condensation–cyclization reaction to form selectively the difluoro/trifluoromethylated tetrahydroquinazolines from simple components difluoro/trifluoroacetaldehyde hemiacetal and aromatic amines is reported. Our recent studies using difluoro/trifluoroacetaldehyde hemiacetal as simple and elegant difluoro/trifluoromethyl synthons and metal triflates such as gallium triflate as safe and stable Lewis acid catalysts led us to this direct synthesis protocol for the expedient and convenient synthesis of fluorinated quinazolines. DFT calculations at PCM/B3LYP/6‐31++G** were carried out for evaluating a possible reaction mechanism for this cyclization. According to the DFT calculations, product stereochemistry is thermodynamically driven, favoring the cis isomer as the major product, which is also confirmed experimentally. 相似文献
The polymerization of the photocleavable monomer, o‐nitrobenzyl methacrylate (NBMA), is investigated using photoinduced electron/energy transfer reversible addition‐fragmentation chain transfer polymerization. The polymerizations under visible red (λmax = 635 nm, 0.7 mW cm−2) and yellow (λmax = 560 nm, 9.7 mW cm−2) light are performed and demonstrate rational evidence of a controlled/living radical polymerization process. Well‐defined poly(o‐nitrobenzyl methacrylate) (PNBMA) homopolymers with good control over the molecular weight and polymer dispersity are successfully synthesized by varying the irradiation time and/or targeted degree of polymerization. Chain extension of a poly(oligo(ethylene glycol) methyl ether methacrylate) macro‐chain transfer agent with NBMA is carried out to fabricate photocleavable amphiphilic block copolymers (BCP). Finally, these self‐assembled BCP rapidly dissemble under UV light suggesting the photoresponsive character of NBMA is not altered during the polymerization under yellow or red light. Such photoresponsive polymers can be potentially used for the remote‐controlled delivery of therapeutic compounds.
A hybrid ion mobility-mass spectrometer (IM-MS) incorporating a variable-temperature (80–400 K) drift tube is presented. The
instrument utilizes an electron ionization (EI) source for fundamental small molecule studies. Ions are transferred to the
IM-MS analyzer stages through a quadrupole, which can operate in either broad transmission or mass-selective mode. Ion beam
modulation for the ion mobility experiment is accomplished by an electronic shutter gate. The variable-temperature ion mobility
spectrometer consists of a 30.2 cm uniform field drift tube enclosed within a thermal envelope. Subambient temperatures down
to 80 K are achievable through cryogenic cooling with liquid nitrogen, while elevated temperatures can be accessed through
resistive heating of the envelope. Mobility separated ions are mass analyzed by an orthogonal time-of-flight (TOF) mass spectrometer.
This report describes the technological considerations for operating the instrument at variable temperature, and preliminary
results are presented for IM-MS analysis of several small mass ions. Specifically, mobility separations of benzene fragment
ions generated by EI are used to illustrate significantly improved (greater than 50%) ion mobility resolution at low temperatures
resulting from decreased diffusional broadening. Preliminary results on the separation of long-lived electronic states of
Ti+ formed by EI of TiCl4 and hydration reactions of Ti+ with residual water are presented. 相似文献