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791.
Thallapally PK Katz AK Carrell HL Desiraju GR 《Chemical communications (Cambridge, England)》2002,(4):344-345
Conformational flexibility in a symmetrical tris-phenol leads to close packed structures that are also characterised by an extended though finite cooperative chain of hydrogen bonds. 相似文献
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We report a measurement of the excitation spectrum omega(k) and the static structure factor S(k) of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum in the local density approximation, even close to the long-wavelength limit of the region of applicability. Feynman's relation between omega(k) and S(k) is verified, within an overall constant. 相似文献
794.
The kinetics of chlorine atom abstraction from trichloromethyl groups of the haloethanes (XCCl3), CF3CCl3, CH3CCl3, C2Cl6, C2Cl5H, and CH2ClCCl3, by radiolytically generated cyclohexyl radicals was studied in the liquid phase by a competitive method. The chlorine atom abstraction data were put on an absolute basis by comparing the rates of the metathetical reactions with the known rate of addition of cyclohexyl radicals to C2Cl4. The following Arrhenius parameters were obtained: The error limits are the standard deviations from least mean square Arrhenius plots. It is shown that the neighboring group effect on the rate of chlorine atom abstraction from the trichloromethyl groups can be correlated with Taft polar substituent constants. 相似文献
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We discuss the effects of motion on the computation of the myocardial transverse relaxation time by use of magnetic resonance imaging. Equations describing its behavior are derived and illustrated graphically under different conditions. It is shown that the myocardial transverse relaxation time calculated from magnetic resonance images depends on the actual myocardial transverse relaxation time ex vivo (T2) as well as the phase of the cardiac cycle in which it is computed, heart rate, cardiac wall velocity, choice of spin-echoes used in the calculation, and the spin-echo times employed. In particular, the error in T2 decreases when both the first and third echoes are employed in the calculation, rather than only the first two echoes. However, the myocardial transverse relaxation time is more strongly dependent on heart rate in the former case rather than in the latter. Furthermore, the error in T2, when both the first and second spin echoes are used in the calculation, is seen to increase as the spin-echo time shortens. On the other hand, the error in T2 decreases for shorter spin-echo times when both the first and third spin echoes are used instead. The results are relevant to the noninvasive assessment of ischemia, cardiac transplantation rejection, and other myocardial disorders. 相似文献