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Theoretical expressions for the overall values of the conventional discrepancy indicesR(F) andR(I) are derived for a non-centrosymmetric crystal with a centrosymmetric group by taking the centrosymmetric group and a part
of the other atoms in the unit cell as the trial structure. These results are used to obtain tables of values of these indices
in terms of the parameter σ
1c
2
and σ
1
2
which define the fractional contribution to the local mean intensity from the centrosymmetric group and all the known atoms
respectively.
Contribution No. 561 相似文献
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A. Nallini K. Saraboji M. N. Ponnuswamy S. B. Katti 《Crystal Research and Technology》2004,39(2):179-184
The crystal structure of N‐ (t‐butoxycarbonyl) ‐ L‐valine‐L‐phenylalanine‐methyl ester (Boc‐Val‐Phe‐OMe), C20H30N2O5 was determined by X‐ray diffraction methods. The dipeptide crystallizes in orthorhombic space group P212121, with cell parameters a = 5.0680(1) Å, b = 13.8650(1) Å and c = 28.2630(1) Å, V = 2143.8(5) Å3, F.W. = 378.46, Z = 4, Dcalc = 1.173 Mg/m3, μ = 0.687 mm‐1, F000 = 816, CuKα = 1.5418 Å. The structure was solved by direct methods and final R1 and wR2 are 0.0659 and 0.1654, respectively. The peptide unit is in trans conformation [ω = 177.4(9)°]. The conformation angles ϕ1, ψ1, ϕ2 and ψ2 for the peptide backbone are: ‐96.5(13)°, 101.2(13)°, ‐123.9(12)° and 34.0(15)°. The N‐H…O and C‐H…O hydrogen bondings influence the packing of the molecules in the dipeptide crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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N. Sampath S. M. Malathy Sony M. N. Ponnuswamy M. Nethaji 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(6):o346-o348
The piperidine ring in the title compound, C22H28N4S, exhibits a chair conformation. The thiosemicarbazone moiety adopts an extended conformation, and the planar phenyl rings are oriented equatorially with respect to the piperidine ring. Two intermolecular hydrogen bonds involving the S atom form molecular pairs, and the crystal structure is stabilized by weak C—H⃛π interactions in addition to van der Waals forces. 相似文献
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Ponnuswamy N Pantoş GD Smulders MM Sanders JK 《Journal of the American Chemical Society》2012,134(1):566-573
Amino-acid functionalized naphthalenediimides self-assemble into hydrogen-bonded supramolecular helical nanotubes via a noncooperative, isodesmic process; the self-assembly of ordered helical systems is usually realized through a cooperative process. This unexpected behavior was rationalized as a manifestation of entropy-enthalpy compensation. Fundamental insights into the thermodynamics governing this self-assembly were obtained through the fitting of the isodesmic model to (1)H NMR spectrometry and circular dichroism spectroscopy measurements. Furthermore, we have extended the application of this mathematical model, for the first time, to quantitatively estimate the effect of guests, solvents, and side chains on the stability of the supramolecular nanotube; most significantly, we demonstrate that C(60) acts as a template to stabilize the nanotube assembly and thereby substantially increase the degree of polymerization. 相似文献
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In this paper, we consider nonlinear Schrödinger (NLS) equations, both in the anomalous and normal dispersive regimes, which govern the propagation of a single field in a fiber medium with phase modulation and fibre gain (or loss). The integrability conditions are arrived from linear eigen value problem. The variable transformations which connect the integrable form of modified NLS equations are presented. We succeed in Hirota bilinearzing the equations and on solving, exact bright and dark soliton solutions are obtained. From the results, we show that the soliton is alive, i.e. pulse area can be conserved by the inclusion of gain (or loss) and phase modulation effects. 相似文献
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