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91.
As a first stage in the development of a suitable molecular orbital method for treating inorganic systems, we consider the possible integral approximations that may be made to reduce the complexity of the computation. The significance of invariance of the approximations to different transformations is discussed and the effect of various levels of neglect of differential overlap is analysed by the S-expansion technique. A method — the many-centre ZDO method — that is computationally feasible but contains more information than the widely used CNDO approximation is given particular consideration.
Zusammenfassung Als erste Stufe in der Entwicklung einer geeigneten Molekular-orbital-Methode für die Behandlung anorganischer Systeme untersuchen wir mögliche IntegralnÄherungen, die geeignet sind, die Berechnungen zu vereinfachen. Die Bedeutung der Invarianz der NÄherungen unter verschiedenen Transformationen wird diskutiert und der Einflu\ verschiedener Stufen der VernachlÄssigung der differentiellen überlappung wird mit der S-Entwicklungstechnik analysiert. Einer Methode, der Vielzentren-ZDO-Methode, die rechnerisch gut durchführbar ist, aber mehr an Information enthÄlt als die meistens benutzte CNDO-NÄherung, wird besondere Beachtung geschenkt.
Résumé Dans une première étape lors du développement d'une méthode d'orbitales moléculaires convenant aux systèmes inorganiques, nous envisageons les approximations possible pour les intégrales. On discute la signification de l'invariance des ces approximations par rapport à différentes transformations et l'on analyse par la technique du développement en S l'effet des différents niveaux d'approximation du recouvrement différentiel nul. On considère en particular une méthode — la méthode ZDO polycentrique-quis'avère réalisable numériquementtout en contenant plus d'information que l'approximation CNDO couramment utilisée.相似文献
92.
DESTRUCTION OF PHOTOREACTIVATING ENZYME BY 365 nm RADIATION* 总被引:3,自引:0,他引:3
Abstract— Following the observation that in vivo photoreactivation of 365-nm-induced pyrimidine dimers could not be observed chemically, a study was made of the inactivation of photoreactivating enzyme activity by this near-ultraviolet wavelength. It was observed that: (1) Dimers induced in extracted bacterial DNA by 365 nm radiation are completely photoreactivable and are monomerized as an exponential function of the photoreactivation time. (2) Photoreactivability of 254-nm-induced damage in Escherichia coli B/r Hcr is progressively destroyed in vivo as a function of the dose of 365 nm radiation. (3) The ability of the yeast photoreactivating enzyme to monomerize dimers induced at 365 nm in bacterial DNA is destroyed in vitro as a function of the dose of 365 nm radiation, and at a rate comparable to killing of E. coli. These results are consistent with biological measurements which indicate that photoreactivability of ultraviolet (near and far) lethal damage is reduced by exposure of the bacteria to 365 nm radiation. 相似文献
93.
Peter Brown Cecil L. Hornbeck John R. Cronin 《Journal of mass spectrometry : JMS》1972,6(12):1383-1399
The positive ion electron-impact mass spectra of a series of alloxazines, iso-alloxazines and some derivatives have been examined. The compounds employed were lumichrome (7,8-dimethylalloxazine), 1,3-dimethyllumichrome, lumiflavin (7,8,10-trimethyl-iso-alloxazine), 3-methyllumiflavin, riboflavin [7,8-dimethyl-10-(D-1′-ribityl)-iso-alloxazine], riboflavin tetraacetate, 3-methylriboflavin tetraacetate and riboflavin tetrapropionate. By using exact mass measurements, metastable ion defocusing and the mass/composition shifts occurring with derivatives, it has been possible to arrive at detailed interpretations of the mass spectra of all compounds. With lumichrome and lumiflavin, fragmentation commences by elimination of HNCO from the pyrimidine ring. With riboflavin and its derivatives the ribityl chain cleaves off first, followed by decomposition of the iso-alloxazine ring. Application of these methods and findings to the structural analysis of chemically interesting modified flavins is predicted to be rewarding. 相似文献
94.
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97.
We examine a new algorithm for finding jets ine
+
e
– annihilation, using a jet measure based on relative transverse momentum. We perform an analytic calculation of the three-jet fraction at lowest order, and compare our result with the standard jet-finding algorithm. For soft gluons in an abelian theory it is shown that the leading double logarithms exponentiate, unlike the situation for the commonly used algorithm based on invariant mass. In QCD we find that there are leading non-abelian logarithms, and we calculate these explicitly atO(
s
2
). We discuss the modifications to the algorithm which are needed when the mass of a parton cluster is taken into account. The hope is that the new algorithm will allow an improved theoretical analysis at smaller values of the resolution parametery
T
, and hence an improved fit to the experimental data.Nick Brown was tragically killed on 13th June, 1991. This paper is dedicated to his memory 相似文献
98.
Brown LS 《Physical review D: Particles and fields》1992,46(10):R4125-R4127
99.
Alam MS Kim IJ Nemati B O'Neill JJ Romero V Severini H Sun CR Wang P Zoeller MM Crawford G Fulton R Gan KK Kagan H Kass R Lee J Malchow R Morrow F Sung M White C Whitmore J Wilson P Butler F Fu X Kalbfleisch G Lambrecht M Skubic P Snow J Bortoletto D Brown DN Dominick J McIlwain RL Miao T Miller DH Modesitt M Schaffner SF Shibata EI Shipsey IP Battle M Ernst J Kroha H Roberts S Sparks K Thorndike EH Wang C Artuso M Goldberg M Haupt T Horwitz N Kennett R Moneti GC Playfer S Rozen Y Rubin P 《Physical review D: Particles and fields》1992,46(11):4822-4827
100.
Hard form factors for meson-baryon strong couplings as derived from deep inelastic lepton scattering
W. -Y. P. Hwang J. Speth G. E. Brown 《Zeitschrift für Physik A Hadrons and Nuclei》1991,339(3):383-389
As stimulated by earlier attempts for obtaining theNN andN form factors from the deep inelastic lepton scattering data, we extend the analysis by taking into account effects of additional mesons including, , ,K, andK
*, with the coupling constants fixed by the lowenergy nucleon-nucleon and hyperon-nucleon scattering data. Contrary to an earlier claim that the NN andN form factor must be very soft (e.g., with the cutoff mass less than 500 MeV in the monopole form), we find, for example, that with all form factors parametrized in the dipole form, a universal cutoff mass of 1150 MeV in the/N sector and 1400 MeV in the/ sector yields predictions in excellent agreement with recently published neutrino data on the momentum fractions carried by thes, , and¯d quarks, as well as consistent with the sea-to-valence ratio extracted from the CDHS data and the Femilab E615 experiment. Similar results can also be obtained by using exponential cutoffs for all couplings, or by using monopole forms for some vertices while retaining dipole forms for the rest. The success of the mesonexchange picture in generating the strangeness content in a proton suggests an alternative understanding of the origin of sea quarks in the proton.We wish to thank G. Garvey, K. Holinde, L.-C. Liu, M. B. Johnson, M. Strikman, A. W. Thomas and Jochen Wambach for helpful conversations. W-Y. P. Hwang wishes to acknowledge the Alexander von Humboldt Foundation for a fellowship to visit Jülich for conducting research. His research works was also supported in part by the National Science Council of the Republic of China. The work of G. E. Brown is supported in part by a Humboldt award, in part by NATO Grant RG85/0093, and in part by the U.S. Department of Energy. 相似文献