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21.
The acid-catalyzed rearrangements of three model tricyclo[4.3.2] propellane derivatives have been explored. In each case the observed products are proposed to arise via a concerted 1,2-shift of the cyclobutane bond having an antiperiplanar alignment with the leaving group (i.e., stereoelectronic control).  相似文献   
22.
High chemoselectivity can be achieved in the addition of lithium dithiane anions to vinyl epoxides exploiting the steric nature of the dithiane substituent. Unencumbered dithiane anions afford SN2 adducts, whereas sterically encumbered anions lead primarily to SN2' adducts. Furthermore, the SN2' addition occurs syn to the vinyl epoxide.  相似文献   
23.
Summary The x-ray crystal structure of the title complex is described Crystals are monoclinic, space groupP21/n, with unit-cell dimensions:a=18.070(2),b=13.471(2),c=6.788(2) Å,=94.70(1),Z=4. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by least-squares techniques toR=5.0% for 2451 independent reflections. It consists of complex molecules, in which the copper atom square planar coordination comprises the chlorine atom, Cu-Cl=2.240(3) Å, and the organic ligand which acts as terdentate through the oxygen atom [Cu-O=1.948(3) Å] and a nitrogen atom, [Cu-N=1.933(5) Å] from the hydrazidic chain and the oxygen atom, [Cu-O = 1.894(4) Å] from the pyridoxal group.  相似文献   
24.
The convergence of the class of direct interpolatory iterationsI n for a simple zero of a non-linear operatorF in a Banach space of finite or infinite dimension is studied.A general convergence result is established and used to show that ifF is entire the radius of convergence goes to infinity withn while ifF is analytic in a ball of radiusR the radius of convergence increases to at leastR/2 withn.The research was supported in part by the National Science Foundation under Grant MCS 75-222-55 and the office of Naval Research under Contract N00014-76-C-0370, NR 044-422.  相似文献   
25.
[structure: see text] A second generation total synthesis of the potent antitumor agent (+)-phorboxazole A (1) has been achieved. The cornerstone of this approach comprises a more convergent strategy, involving late-stage Stille union of a fully elaborated C(1-28) macrocycle with a C(29-46) side chain. The second generation synthesis entails the longest linear sequence of 24 steps, with an overall yield of 4.2%.  相似文献   
26.
Vinyl fluoride was polymerized by photochemical initiation in a continuous-flow cylindrical reactor at room temperature and at pressures of up to 30 atm. Copolymers with vinyl acetate were prepared in order to improve the solubility and processability of poly(vinyl fluoride) (PVF). The copolymers were hydrolyzed to the corresponding vinyl alcohol copolymers and yielded hydrophilic films that are strong and flexible only when swollen by water. It was found that on hydrolysis the Tg, Tm, and heat of fusion as well as degree of crystallinity increased. It was suggested that PVF and the copolymers with vinyl alcohol are isomorphous.  相似文献   
27.
The approximation order provided by a directed set {S h } h>0 of spaces, each spanned by thehZ d -translates of one function, is analyzed. The nearoptimal approximants of [R2] from eachs h to the exponential functions are used to establish upper bounds on the approximation order. These approximants are also used on the Fourier transform domain to yield approximations for other smooth functions, and thereby provide lower bounds on the approximation order. As a special case, the classical Strang-Fix conditions are extended to bounded summable generating functions.The second part of the paper consists of a detailed account of various applications of these general results to spline and radial function theory. Emphasis is given to the case when the scale {s h } is obtained froms 1 by means other than dilation. This includes the derivation of spectral approximation orders associated with smooth positive definite generating functions.  相似文献   
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29.
A.T. Amos 《Molecular physics》2013,111(4):393-407
The theory of long waves originally developed by Born and Huang for ionic crystals and then adapted for molecular crystals with one molecule per unit cell by Hall is extended to include molecular crystals with two molecules per unit cell. A study is made of the non-analytic behaviour of the energy bands as the wave vector tends to zero and it is found that the energies of the optically excited levels will be different from the values usually assumed. The dispersion relation is obtained and used to discuss metallic reflectivity. The finite lifetime of the excited states can also be included in the theory and this has the effect of changing the position of maximum absorption as well as reducing the reflectivity. A comparison is made between the long wave theory, the simple form of Davydov theory and experiment.  相似文献   
30.
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