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排序方式: 共有411条查询结果,搜索用时 46 毫秒
401.
Klippenstein SJ Georgievskii Y Harding LB 《The journal of physical chemistry. A》2011,115(50):14370-14381
We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments. 相似文献
402.
403.
Oxa-Pictet–Spengler cyclization and microwave-assisted C–H arylation have been implemented as key steps in the synthesis of new isochroman heterocycles containing a 4,5,6a,7-tetrahydrodibenzo[de,g]chromene motif. These isochromans may be easily transformed to phenanthrene alkaloids via acidic cleavage of the isochroman ring and standard synthetic manipulations thereafter. The route described is attractive in that it provides access to two biologically interesting scaffolds in simple and high yielding synthetic steps. 相似文献
404.
Johnson CJ Harding ME Poad BL Stanton JF Continetti RE 《Journal of the American Chemical Society》2011,133(49):19606-19609
We report vibrationally resolved photoelectron spectra of internally cold HOCO(-) and DOCO(-) anions at wavelengths near and well above the detachment threshold. These spectra are dominated by a strong Franck-Condon progression of three low-energy modes of the cis isomer, the first gas-phase measurement of these vibrations. Using highly resolved, near-threshold spectra we are able to reassign the electron affinities (EAs) of cis- and trans-HOCO to 1.51 ± 0.01 and 1.37 ± 0.01 eV, respectively. Using these EAs, well depths with respect to OH + CO are determined to be 1.07 ± 0.02 eV for trans-HOCO and 0.99 ± 0.02 eV for cis-HOCO. High-level ab initio calculations show excellent agreement with all experimental results. These values will be of direct use in thermochemical calculations and will help to aid in the identification of the HOCO radical in complex reactions. 相似文献
405.
C. Kerpal D. J. Harding G. Meijer A. Fielicke 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,63(2):231-234
The adsorption of carbon monoxide on neutral iridium clusters in the size range of
n = 3 to 21 atoms is investigated with infrared multiple photon
dissociation spectroscopy. For each cluster size only a single ν(CO) band
is present with frequencies in the range between 1962 cm-1
(n = 8) and 1985 cm-1 (n = 18) which can be
attributed to an atop binding geometry. This behaviour is compared to the CO binding
geometries on clusters of other group 9 and 10 transition metals as well as to that on
extended surfaces. The preference of Ir for atop binding is rationalised by relativistic
effects on the electronic structure of the later 5d metals. 相似文献
406.
Hosey TP Harding SG Carpenter TA Ansorge RE Williams GB 《Magnetic resonance imaging》2008,26(2):236-245
A Markov chain Monte Carlo (MCMC) algorithm has been reported which is capable of determining the probabilistic orientation of two-fibre populations from high angular resolution diffusion-weighted data (HARDI). We present and critically discuss the application of this algorithm to in vivo human datasets acquired in clinically realistic times. We show that by appropriate model selection areas of multiple fibre populations can be identified that correspond with those predicted from known anatomy. Quantitative maps of fibre orientation probability are derived and shown for one- and two-fibre models of neural architecture. Fibre crossings in the pons, the internal capsule and the corona radiata are shown. In addition, we demonstrate that the relative proportion of anisotropic signal may be a more appropriate measure of anisotropy than summary measures derived from the tensor model such as fractional anisotropy in areas with multi-fibre populations. 相似文献
407.
Arpesella C Back HO Balata M Bellini G Benziger J Bonetti S Brigatti A Caccianiga B Cadonati L Calaprice F Carraro C Cecchet G Chavarria A Chen M Dalnoki-Veress F D'Angelo D de Bari A de Bellefon A de Kerret H Derbin A Deutsch M di Credico A di Pietro G Eisenstein R Elisei F Etenko A Fernholz R Fomenko K Ford R Franco D Freudiger B Galbiati C Gatti F Gazzana S Giammarchi M Giugni D Goeger-Neff M Goldbrunner T Goretti A Grieb C Hagner C Hampel W Harding E Hardy S Hartman FX Hertrich T Heusser G 《Physical review letters》2008,101(9):091302
We report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV 7Be neutrinos is 49+/-3stat+/-4syst counts/(day.100 ton). The hypothesis of no oscillation for 7Be solar neutrinos is inconsistent with our measurement at the 4sigma C.L. Our result is the first direct measurement of the survival probability for solar nu(e) in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of 7Be, pp, and CNO solar nu(e), and the limit on the effective neutrino magnetic moment using solar neutrinos. 相似文献
408.
Schimmel u. Co. H. G. A. Harding J. L. Turner R. C. Holmes F. D. Dodge H. G. Smith und J. C. Earle 《Fresenius' Journal of Analytical Chemistry》1923,62(10):408-410
Ohne Zusammenfassung 相似文献
409.
410.
August Harding 《Fresenius' Journal of Analytical Chemistry》1883,22(1):79-80
Ohne Zusammenfassung 相似文献