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171.
Moss CL Chung TW Wyer JA Nielsen SB Hvelplund P Tureček F 《Journal of the American Society for Mass Spectrometry》2011,22(4):731-751
Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a
tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both
gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a
hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of low-energy electrons by doubly charged ions
in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation
channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the
primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H2PO4 radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways
and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of
the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces
a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations
and H- atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the
strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach
to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations. 相似文献
172.
Hansen M Jacobsen NW Nielsen FK Björklund E Styrishave B Halling-Sørensen B 《Analytical and bioanalytical chemistry》2011,400(10):3409-3417
This paper presents the development, optimization and validation of a methodology to determine nine key steroid hormones (viz.
pregnenolone, progesterone, dehydroepiandrosterone, androstenedione, testosterone, dihydrotestosterone, estrone, 17α-estradiol
and 17β-estradiol) expressed in the steroidogenesis in biological fluids. The analytical method allows for the determination
of steroid hormones in blood plasma and serum down to 0.08–0.16 ng/mL for estrogens, 0.20–0.36 ng/mL for androgens and 0.36–0.43 ng/mL
for progestagens. These limits of detection were obtainable using a two-step solid-phase clean-up for fractionation and elimination
of interfering lipids (fatty acids, phospholipids, glycerides and sterols) from the steroid hormones. The accuracy of the
method was 50–112% in the range 0.10 to 2.00 ng/mL. 相似文献
173.
Motivated by service levels in terms of the waiting-time distribution seen, for instance, in call centers, we consider two
models for systems with a service discipline that depends on the waiting time. The first model deals with a single server
that continuously adapts its service rate based on the waiting time of the first customer in line. In the second model, one
queue is served by a primary server which is supplemented by a secondary server when the waiting of the first customer in
line exceeds a threshold. Using level crossings for the waiting-time process of the first customer in line, we derive steady-state
waiting-time distributions for both models. The results are illustrated with numerical examples. 相似文献
174.
Weighted Overlap Dominance – A procedure for interactive selection on multidimensional interval data
We present an outranking procedure that supports selection of alternatives represented by multiple attributes with interval valued data. The procedure is interactive in the sense that the decision maker directs the search for preferred alternatives by providing weights of the different attributes as well as parameters related to risk attitude and weighted dominance. The outranking relation builds on pairwise comparisons between optimistic and pessimistic weighted values as well as weighted dominance relations supported by volume based measures. The suggested procedure is referred to as the Weighted Overlap Dominance procedure (WOD). 相似文献
175.
Southon PD Price DJ Nielsen PK McKenzie CJ Kepert CJ 《Journal of the American Chemical Society》2011,133(28):10885-10891
The metal-organic host material [{Co(III)(2)(bpbp)(O(2))}(2)bdc](PF(6))(4) (1·2O(2); bpbp(-) = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato; bdc(2-) = 1,4-benzenedicarboxylato) displays reversible chemisorptive desorption and resorption of dioxygen through conversion to the deoxygenated Co(II) form [{Co(II)(2)(bpbp)}(2)bdc](PF(6))(4) (1). Single crystal X-ray diffraction analysis indicates that the host lattice 1·2O(2), achieved through desorption of included water guests from the as-synthesized phase 1·2O(2)·3H(2)O, consists of an ionic lattice containing discrete tetranuclear complexes, between which lie void regions that allow the migration of dioxygen and other guests. Powder X-ray diffraction analyses indicate that the host material retains crystallinity through the dioxygen desorption/chemisorption processes. Dioxygen chemisorption measurements on 1 show near-stoichiometric uptake of dioxygen at 5 mbar and 25 °C, and this capacity is largely retained at temperatures above 100 °C. Gas adsorption isotherms of major atmospheric gases on both 1 and 1·2O(2) indicate the potential suitability of this material for air separation, with a O(2)/N(2) selectivity factor of 38 at 1 atm. Comparison of oxygen binding in solution and in the solid state indicates a dramatic increase in binding affinity to the complex when it is incorporated in a porous solid. 相似文献
176.
Johansson HA Zettergren H Holm AI Haag N Nielsen SB Wyer JA Kirketerp MB Støchkel K Hvelplund P Schmidt HT Cederquist H 《The Journal of chemical physics》2011,135(8):084304
The loss of C(2)H(2) is a low activation energy dissociation channel for anthracene (C(14)H(10)) and acridine (C(13)H(9)N) cations. For the latter ion another prominent fragmentation pathway is the loss of HCN. We have studied these two dissociation channels by collision induced dissociation experiments of 50 keV anthracene cations and protonated acridine, both produced by electrospray ionization, in collisions with a neutral xenon target. In addition, we have carried out density functional theory calculations on possible reaction pathways for the loss of C(2)H(2) and HCN. The mass spectra display features of multi-step processes, and for protonated acridine the dominant first step process is the loss of a hydrogen from the N site, which then leads to C(2)H(2)/HCN loss from the acridine cation. With our calculations we have identified three pathways for the loss of C(2)H(2) from the anthracene cation, with three different cationic products: 2-ethynylnaphthalene, biphenylene, and acenaphthylene. The third product is the one with the overall lowest dissociation energy barrier. For the acridine cation our calculated pathway for the loss of C(2)H(2) leads to the 3-ethynylquinoline cation, and the loss of HCN leads to the biphenylene cation. Isomerization plays an important role in the formation of the non-ethynyl containing products. All calculated fragmentation pathways should be accessible in the present experiment due to substantial energy deposition in the collisions. 相似文献
177.
Dong SS Nielsen RJ Palmer JH Gray HB Gross Z Dasgupta S Goddard WA 《Inorganic chemistry》2011,50(3):764-770
The electronic structures of metallocorroles (tpfc)M(NH(3))(2) and (tfc)M(NH(3))(2) (tpfc is the trianion of 5,10,15-(tris)pentafluorophenylcorrole, tfc is the trianion of 5,10,15-trifluorocorrole, and M = Co, Rh, Ir) have been computed using first principles quantum mechanics [B3LYP flavor of Density Functional Theory (DFT) with Poisson-Boltzmann continuum solvation]. The geometry was optimized for both the neutral systems (formal M(III) oxidation state) and the one-electron oxidized systems (formally M(IV)). As expected, the M(III) systems have a closed shell d(6) configuration; for all three metals, the one-electron oxidation was calculated to occur from a ligand-based orbital (highest occupied molecular orbital (HOMO) of B(1) symmetry). The ground state of the formal M(IV) system has M(III)-Cπ character, indicating that the metal remains d(6), with the hole in the corrole π system. As a result the calculated M(IV/III) reduction potentials are quite similar (0.64, 0.67, and 0.56 V vs SCE for M = Ir, Rh and Co, respectively), whereas the differences would have been large for purely metal-based oxidations. Vertically excited states with substantial metal character are well separated from the ground state in one-electron-oxidized cobalt (0.27 eV) and rhodium (0.24 eV) corroles, but become closer in energy in the iridium (0.15 eV) analogues. The exact splittings depend on the chosen functional and basis set combination and vary by ~0.1 eV. 相似文献
178.
Sørensen JK Fock J Pedersen AH Petersen AB Jennum K Bechgaard K Kilså K Geskin V Cornil J Bjørnholm T Nielsen MB 《The Journal of organic chemistry》2011,76(1):245-263
In continuation of previous studies showing promising metal-molecule contact properties a variety of C(60) end-capped "molecular wires" for molecular electronics were prepared by variants of the Prato 1,3-dipolar cycloaddition reaction. Either benzene or fluorene was chosen as the central wire, and synthetic protocols for derivatives terminated with one or two fullero[c]pyrrolidine "electrode anchoring" groups were developed. An aryl-substituted aziridine could in some cases be employed directly as the azomethine ylide precursor for the Prato reaction without the need of having an electron-withdrawing ester group present. The effect of extending the π-system of the central wire from 1,4-phenylenediamine to 2,7-fluorenediamine was investigated by absorption, fluorescence, and electrochemical methods. The central wire and the C(60) end-groups were found not to electronically communicate in the ground state. However, the fluorescence of C(60) was quenched by charge transfer from the wire to C(60). Quantum chemical calculations predict and explain the collapse of coherent electronic transmission through one of the fulleropyrrolidine-terminated molecular wires. 相似文献
179.
Kirketerp MB Ryhding T Støchkel K Brøndsted Nielsen M Brøndsted Nielsen S 《The journal of physical chemistry. A》2011,115(7):1222-1227
Oligo(p-phenyleneethynylene)s (OPEs) are conjugated oligomers of great interest within materials science and molecular electronics on account of their highly applicable electronic and optical properties. Here we use gas-phase action spectroscopy to elucidate how the intrinsic electronic properties of these chromophores are affected by nearby charges. An OPE3 chromophore with two nearby ammonium groups was synthesized. This molecule and a related OPE3 with only one amine protonation site were transferred to the gas phase by electrospray ionization and subjected to action spectroscopy. Ions were bunched in a 14-pole ion trap, accelerated to 50-keV kinetic energies, mass-to-charge selected by a magnet, and photoexcited in a crossed-beam configuration. Fragment ions were finally mass-analyzed by an electrostatic analyzer. The setup enables photodissociation mass spectrometry and action spectroscopy on the microsecond time scale. The gas-phase absorption of the mono- and dication was measured and compared to that of neutral chromophores in solution. Similar absorption was found for neutral chromophores (in solution) and the dication (in gas phase or solution), whereas the monocation absorbs at lower energies in the gas phase. Simple electrostatic considerations lead to an energy difference like the one found from the experiment. The work presented here addresses how the electronic properties of a π-conjugated system are affected by nearby charges, a question of fundamental interest in, for example, molecular electronics. 相似文献
180.
In SI Nielsen MG Vesborg PC Hou Y Abrams BL Henriksen TR Hansen O Chorkendorff I 《Chemical communications (Cambridge, England)》2011,47(9):2613-2615
We report here on a study of vertically aligned TiO(2) nanotube arrays grown by the one-step anodic oxidation technique and their photocatalytic performance for methane decomposition. Quantitative activity data as a function of film thickness is obtained. 相似文献