Oxygen atom bridge

Differing from atropine (Fig. 58) by only a single oxygen atom (bridging two earbon atoms in the distinetive seven-earbon ring), seopolamine has mueh greater ability to enter the eentral nervous system. 

The second structural motif, B (Figure 72), involves also a dimeric structure in which two alkoxy oxygen atoms bridge between two zinc atoms, but now an additional intramolecular coordinating substituent is present, resulting in the formation of four-coordinate zinc atoms. Most of these strucmral investigations are aimed at the elucidation of the mechanism of the enantioselective addition of organozinc compounds to aldehydes, a topic that will be discussed in Chapter 13. 

The ethylzinc alcoholate (149) prepared from racemic a-terr-butyl-6-phenyl-2-pyridyl-methanol and Et2Zn exists in the solid state as a hetero chiral dimer (Figure 78). Also in this case the alcoholate oxygen atoms bridge between two zinc atoms, affording a central Zn—O—Zn—O plane. The pyridyl nitrogen atoms are coordinated in such a way to the zinc atoms [Zn—N 2.185(2) A] that they are in awfi-position with respect to this plane. 

The white tetranuclear osmium compound 0s4(C0)1204 has been isolated on reaction of 0s04 in xylene with CO at 175 °C and 175 atm for 24 h27). X-ray analysis shows a cubane structure with oxygen atoms bridging a face by donating 4 electrons, resulting in there being no osmium-osmium bonds27), (dos.ru =... 

Radical A, shown here, is related to the triphenylmethyl radical by having oxygen atoms bridging the ortho positions of the phenyl groups. This radical dimerizes much faster than the triphenylmethyl radical. Explain this observation. 

The rhodium complex [Rh(acac) (C2Hi )2] reacts with the unsaturated triosmium compound to give the 60-electron cluster species [Rh0s3(y-H)2(y-acac)(CO)10J (A7) in which the acac ligand acts as a five-electron donor, one oxygen atom bridging an Os — Rh separation of 3.292(2) so that the four metal atoms adopt a butterfly configuration. 

Molecular mechanics (MM), molecular dynamics (MD), and Monte-Carlo (MC) methods were employed to simulate the adsorption of methane, ethane, propane and isobutane on silicalite and HZSM-5. The silicalite was simulated using the same cluster-model adopted in the diffusion calculations. The H-ZMS-5 structure was constructed according to the procedure suggested by Vetrivel et al. [32], which consists in replacing one atom at the channel intersection by and protonating the oxygen atom bridging the Ta and Tg sites in order to preserve the lattice neutrality. 

All of the atoms pictured in each case, plus millions more, make up one giant molecule, (a) In diamond, carbon atoms are connected in a three-dimensional structure, (b) In graphite, carbon atoms are connected in sheets or layers, (c) Silica (silicon dioxide) has a structure somewhat like that of diamond, except that it contains silicon and oxygen atoms instead of carbon atoms, and an oxygen atom bridges each pair of silicon atoms. 

Q denotes a silicon atom bonded to four oxygen atoms. The superscript n denotes the number of Q units bonded to that Q. For example is 03Si-0-Si03, where all the other oxygen atoms bridge from silicon fo another atom (e.g., Al). is 03Si-0-Si(0)2-0-Si03 (note the two terminal Si are units, etc.). 

There are now numerous, metal-linked oligomeric (and polymeric) systems that fall into this category. For example, the acetylacetonates of manganese(II), nickel(II) and zinc(II) have long been known to be trimeric while the cobalt(II) complex is tetrameric, with three (3-diketonate oxygen atoms bridging adjacent metal centres in a linear array in each case. Other more recent examples include systems built... 

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