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101.
To identify the reasons for the very low barrier that has been measured for ring inversion of 1,4,5,5-tetrafluorobicyclo[2.1.0]pentane (deltaG(double dagger) = 6.8 +/- 0.2 kcal/mol), CASSCF and CASPT2 calculations have been performed on ring inversion in this and other bicyclo[2.1.0]pentanes. The results of the calculations show that a cooperative interaction between the geminal fluorines at C2 and the fluorines at C1 and C3 in the singlet cyclopentane-1,3-diyl transition structure (TS) contributes 3.7 kcal/mol to lowering the barrier to ring inversion in the tetrafluoro compound. In contrast, a competitive substituent effect in the TS for ring inversion of 1,4-dicyano-5,5-difluorobicyclo[2.1.0]pentane is predicted to raise the barrier height by 6.1 kcal/mol. The origin of these cooperative and competitive substituent effects is discussed. 相似文献
102.
103.
A simple and efficient method for the preparation of 5′-O(N)-carbamyl and 5′-O(N)-polycarbamyl nucleoside derivatives is reported. The method consisted of treatment of 2′,3′-O-protected purine (Ado, Ino) or pyrimidine nucleosides (Thd, Urd) with trichloroacetylisocyanate, followed by cleavage of the trichloroacetyl moiety by silica-gel promoted methanolysis during column chromatography. Iterative application of this method gave mono, di, and tricarbamyl derivatives in good to excellent yields (ave = 80%). 相似文献
104.
105.
Jessica D. Posey-Dowty Thelma L. Watterson A. Kent Wilson Kevin J. Edgar Michael C. Shelton Larry R. LingerfeltJr. 《Cellulose (London, England)》2007,14(1):73-83
New carboxymethylcellulose esters were developed with useful properties for oral dosage forms in drug delivery. Normally,
commercial cellulose esters are used as the major excipients in oral dosage forms as a coating or a membrane. In applications
involving compression tablets, cellulose esters are usually mixed with other more hydrophilic matrix components to facilitate
dissolution of the active. In the present study, novel cellulose esters were single component matrix resins. Pharmaceutical
actives were cryogenically ground as a physical blend or an amorphous blend with the polymer. Subsequently, tablets were made
by direct compression using a single tablet press, or capsules were made by filling them with the ground material. Dissolution
tests were completed on the solid dosage forms at pH 1.2, 4.5, 6.8 or 7.4 in a United States Pharmacopeia (USP) II device
to determine the release profiles for up to 24 h. Carboxymethylcellulose esters provide an excellent matrix for controlling
both the rate of release and the pH at which pharmaceutical actives release into the aqueous environment. When used in suitable
quantities, dictated by the active of interest, carboxymethylcellulose acetate butyrate provided zero-order release over sustained
time up to 24 h. 相似文献
106.
MPW1K/6-31+G(d,p) calculations which include the effects of small curvature tunneling find that, around room temperature, thermally activated tunneling dominates the 1,5-hydrogen shift reactions of 1,3-cyclopentadiene (2a) and 5-methyl-1,3-cyclopentadiene (2c). The calculated temperature dependence of the H/D kinetic isotope effect (KIE) for the latter rearrangement agrees well with experimental measurements that were published nearly 40 years ago. It is argued that the experimental KIEs provide prima facie evidence for tunneling in this reaction. The calculations also predict that it should be possible, at least in principle, to confirm this conclusion by observing curvature in the Arrhenius plot for the rearrangement of 2c. 相似文献
107.
Valliant-Saunders K Gunn E Shelton GR Hrovat DA Borden WT Mayer JM 《Inorganic chemistry》2007,46(13):5212-5219
In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et(3)SiH, (i)Pr(3)SiH, Ph(3)SiH, or PhMe(2)SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO(4)((t)Bupy) oxidizes Et(3)SiH and PhMe(2)SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO(2)((t)Bupy)(2)](2)(mu-O)(2) (1a). With L = pyridine (py), only 40-60% yields of R(3)SiOH are obtained, apparently because of coprecipitation of osmium(VIII) with [Os(O)(2)py(2)](2)(mu-O)(2) (1b). Excess silane in these reactions causes further reduction of the OsVI products, and similar osmium "over-reduction" is observed with PhSiH(3), Bu(3)SnH, and boranes. The pathway for OsO(4)(L) + R(3)SiH involves an intermediate, which forms rapidly at 200 K and decays more slowly to products. NMR and IR spectra indicate that the intermediate is a monomeric Os(VI)-hydroxo-siloxo complex, trans-cis-cis-Os(O)(2)L(2)(OH)(OSiR(3)). Mechanistic studies and density functional theory calculations indicate that the intermediate is formed by the [3 + 2] addition of an Si-H bond across an O=Os=O fragment. This is the first direct observation of a [3 + 2] intermediate in a sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO(4)(L) and RuO(4). 相似文献
108.
DR. E. W. J. Mitchell 《Contemporary Physics》2013,54(3):217-222
The accurate measurement of the thermal conductivity of metals still poses considerable experimental difficulty. The reasons for this, and the main ways in which the problem has been approached, are summarized. A detailed examination of the development in technique and methods of measurement shows that while most of the steady-state methods introduced over 50 years ago are still being improved, the emphasis in the last few years has been on the development of methods which give more rapid results, particularly at high temperatures. 相似文献
109.
Dipanjan Pan Ewald Roessl Jens‐Peter Schlomka Shelton D. Caruthers Angana Senpan Mike J. Scott John S. Allen Huiying Zhang Grace Hu Patrick J. Gaffney Eric T. Choi Volker Rasche Samuel A. Wickline Roland Proksa Gregory M. Lanza 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2010,122(50):9829-9833
110.