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1.
Semigroup Forum - John Meakin has had a distinguished career of over half a century in the theory of semigroups. This article gives a synopsis of his most important contributions. The author, a...  相似文献   
2.
In this paper, we introduce a combinatorial optimization problem that models the investment decision a political candidate faces when treating his or her opponents’ campaign plans as given. Our formulation accounts for both the time cost of traveling between districts and the time expended while campaigning within districts. We describe a polynomial-time algorithm that computes a (2+?)-approximation to the optimal solution of a discrete version of our problem by reducing the problem to another combinatorial optimization problem known as Orienteering.  相似文献   
3.
Let G be a finite, connected graph. An arithmetical structure on G is a pair of positive integer vectors d,r such that (diag(d)?A)r=0, where A is the adjacency matrix of G. We investigate the combinatorics of arithmetical structures on path and cycle graphs, as well as the associated critical groups (the torsion part of the cokernels of the matrices (diag(d)?A)). For paths, we prove that arithmetical structures are enumerated by the Catalan numbers, and we obtain refined enumeration results related to ballot sequences. For cycles, we prove that arithmetical structures are enumerated by the binomial coefficients 2n?1n?1, and we obtain refined enumeration results related to multisets. In addition, we determine the critical groups for all arithmetical structures on paths and cycles.  相似文献   
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Ketones 2 (MSDC-0160) and 3 (MSDC-0602) had been selected for clinical development, however their initial syntheses were considered suboptimal for application deep into clinical trials. Difficulties ranging from the nature of the starting material, alcohol oxidation problems, epoxide opening regioisomeric issues, and endgame ketone redox problems had been encountered. Direct ketone introduction/maintenance was desired for maximum efficiency and convergence was found to be critically dependent upon the acidity of the nucleophilic species (13, 18) and the use of pre- or post-alkylative oximino-ether/oxime protection (vide infra). Improvements in overall yield for the syntheses of 2 (MSDC-0160) and 3 (MSDC-0602) from 20% (2) and 31% (3) respectively, to 44% (2) and 59% (3) were realized.  相似文献   
6.
Journal of Chemical Crystallography - 2-Chloro-N-(2,4-dinitrophenyl) acetamide, 1, was synthesized and characterized by 1H and 13C NMR spectroscopy, ESI–MS, X-ray crystallography, and...  相似文献   
7.
The application of electron transfer and dipolar direct current induced collisional activation (ET‐DDC) for enhanced sequence coverage of peptide/protein cations is described. A DDC potential is applied across one pair of opposing rods in the high‐pressure collision cell of a hybrid quadrupole/time‐of‐flight tandem mass spectrometer (QqTOF) to induce collisional activation, in conjunction with electron transfer reactions. As a broadband technique, DDC can be employed for the simultaneous collisional activation of all the first‐generation charge‐reduced precursor ions (eg, electron transfer no‐dissociation or ETnoD products) from electron transfer reactions over a relatively broad mass‐to‐charge range. A systematic study of ET‐DDC induced collision activation on peptide/protein cations revealed an increase in the variety (and abundances) of sequence informative fragment ions, mainly c‐ and z‐type fragment ions, relative to products derived directly via electron transfer dissociation (ETD). Compared with ETD, which has low dissociation efficiency for low‐charge‐state precursor ions, ET‐DDC also showed marked improvement, providing a sequence coverage of 80% to 85% for all the charge states of ubiquitin. Overall, this method provides a simple means for the broadband collisional activation of ETnoD ions in the same collision cell in which they are generated for improved structural characterization of polypeptide and protein cations subjected to ETD.  相似文献   
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Electrospray ionization mass spectrometry (ESI‐MS) is a useful technique for solving organometallic and coordination chemistry characterization problems that are difficult to address using traditional methods. However, assigning the ESI mass spectra of such compounds can be challenging, and the considerations involved in doing so are quite different from assigning the mass spectra of purely organic samples. This is a tutorial article for organometallic/coordination chemists using ESI‐MS to analyze pure compounds or reaction mixtures. The fundamentals of assigning ESI mass spectra are discussed within the context of organometallic and coordination systems. The types of ions commonly observed by ESI‐MS are categorized and described. Finally, a step‐by‐step guide for the assignment of organometallic and coordination chemistry ESI mass spectra is provided along with two case studies.  相似文献   
10.
The three-fold HOMO-LUMO gap oscillation, typical of finite length armchair carbon nanotubes (CNT), has a major effect on the magnetic response of ultrashort, single-end-capped [5,5] carbon nanotubes to a perturbing magnetic field parallel to the main symmetry axis. For the CNT's containing 40, 70, and 100 carbon atoms, for which 100 % of the C=C double bonds can be grouped into aromatic-sextets, i. e., fully or complete Clar networks, large paratropic (antiaromatic) global circulations around the cylindrical axis are predicted at the DFT level of calculation. Local and semi-global diatropic (aromatic) currents of strengths not larger than that of the benzene molecule are determined for a perpendicular perturbing magnetic field. CNTs of intermediate lengths do not display this enhanced antiaromatic response. The paratropic current flow clearly shows that these complete Clar networks can be viewed as stacked cycloparaphenylene belts, each providing a double annulene circuit as a consequence of the quinoidal resonance structure that results from their closure. Paradoxically, the fully aromatic Clar structure itself is responsible for the enhanced global antiaromaticity.  相似文献   
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