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61.
The Hückel-London-Pople-McWeeny (HLPM) (topological) approach for calculating π-electron ring-currents and bond-currents in conjugated systems is applied to coronene and the 17 nonalternant isomers of it for which Balaban, Bean, and Fowler recently published ab initio ipso-centric π-electron current-density maps. In a parallel investigation to that conducted by Balaban et al., the key findings of their quantum-mechanical study are confirmed by this topological one: (a) all seven rings in all of the 18 structures studied are found to bear substantial diamagnetic ring-currents, greater than the benzene value; (b) all 18 conjugated systems have a pronounced, overall London diamagnetic-susceptibility, perpendicular to their respective molecular-planes; (c) a substantial diamagnetic current runs around the periphery of each of these structures; and (d) there is general agreement between the two studies as to which structures, of those treated, respect the annulene-within-an-annulene model for conjugated systems of this sort. The same ideas are also tested against ring-current and bond-current intensities recently calculated by Randi? et al. using purely graph-theoretical methods that are based on circuits of conjugation. Considerable qualitative agreement is found, and when there is disagreement, arguments are offered as to why the HLPM topological ring-currents and bond-currents are to be preferred. 相似文献
62.
B. Dickens 《Thermochimica Acta》1979,29(1):41-56
A scheme of automation of a thermogravimetry apparatus is described which was developed with the factor-jump method in mind. Temperature, pressure and flow rates of two gases are controlled; all components except the furnace are commercially available. This paper describes the details of the automation scheme and provides data on the quality of its performance. The scheme includes a mini-computer; if no feedback is required, a recording computer terminal can be used instead. 相似文献
63.
P. Dickens G. Thanheiser B. F. Braun M. H. Clapp C. W. Foulk A. T. Barden und I. A. Atanasiu 《Fresenius' Journal of Analytical Chemistry》1931,86(5-6):258-261
Ohne Zusammenfassung 相似文献
64.
65.
66.
D. A. Claridge P. G. Dickens J. B. Goodenough 《Applied Physics A: Materials Science & Processing》1989,49(1):65-68
The relaxation timesT
1,T
1q,T
1D, andT
2 for1H in the tetragonal-A phase of HxWO3 have been measured over the temperature range 190 to 490 K. The1H relaxation behaviour appears to be governed by diffusion over inequivalent jump distances, approximating to a short range planar diffusion and a long range isotropic diffusion. Parameters for the latter motion are estimated asE
a = 68 kJ/mol and
0=2.5×10–13 s. The powder X-ray diffraction pattern for this phase of HxWO3 has been studied over the temperature range 300–470 K. The tetragonal distortion diminishes with temperature and H0.43 WO3 becomes cubic at about 435 K. Volumetric studies of hydrogen evolution show that decomposition accelerates at approximately this temperature. 相似文献
67.
68.
Neuber A. Dickens J. Hemmert D. Krompholz H. Hatfield L.L. Kristiansen M. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1998,26(3):296-303
Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3-MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10-6 Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and X-ray emission. All quantities are recorded with 0.21-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to DC flashover across insulators, and similar methods to increase the flashover field are expected to be applicable 相似文献
69.
Selke W. Dickens P. Pring M. E. Spencer J. F. Thanheiser G. Bobtelsky M. Kaplan D. Heczko Th. Spindeck F. Sanfourche A. Hahn Fr. L. Clarke B. L. Wooten L. A. Compton K. G. Czaporowski L. Wierciński J. Prytz M. Holst G. Kahler H. de Eds Floyd Vosburgh W. C. Elmore K. L. Gane R. Ingold C. K. 《Analytical and bioanalytical chemistry》1932,89(7-8):283-292
Analytical and Bioanalytical Chemistry - 相似文献
70.
Brian Dickens 《Thermochimica Acta》1979,29(1):87-113
We have recently provided details of an apparatus and computer programs to implement the factor-jump method of thermogravimetry. This paper describes the refinement of the technique to the present status of quasi-automatic routine operation. The procedure was worked out using samples of polymethyl-methacrylate, polystyrene and polyurethane polymers. The activation energies obtained are measured to 0.2 kcal mole?1 in favorable cases. This is adequate for diagnosis of changes in mechanism but may sometimes be inadequate for scaling temperature accelerated tests to room temperature. 相似文献