Chain centrosymmetric

Sila-procyclidine, (cyclohexyl)phenyl[2-pyrrolidin-l-yl]silanol, may be prepared by hydrolysis of the corresponding methoxysilane (220) and is interesting in that it can form two types of hydrogen-bonded structure depending on whether it is enantiomerically pure or a racemate. In the racemate, the compound forms centrosymmetric dimers of (R)- and (S)-configuration molecules with an 0---N distance of 1.791 A. In the pure (R)-compound, however, the molecules are linked into infinite chains via intermolecular 0-H---N hydrogen bonds (0---N distance 2.792 A) (221), again similar to those in (2-morpholinoethyl)diphenylsilanol shown in Fig. 3. 

Sila-biperidin rac-(SiRS, C2 SR), 35, can be prepared by hydrolysis of the corresponding methoxysilane and behaves in a similar way to sila-procyclidine The exo form has centrosymmetric dimers formed via a pair of intermolecular OH---N hydrogen bonds (0---N distance 2.810 A), while the endo form has intermolecular OH---N hydrogen bonds linking the molecules into infinite chains (222). 

The structure of the centrosymmetric molecule Mo6010-(0-i-Pr)12 (3 1) has an S-chain of six molybdenum atoms and is... 

Quite intriguing is also the case of centrosymmetric homoato-mic bond alternated chains which get coupled into pairs in an asymmetric configuration (A). For such a system x can attain (22) values larger than those of the single heteroatomic bond alternated chain. 

Crystal structures of two hexitols, galactitol and D-mannitol, have been published. Galactitol is meso, but the permissible intramolecular center of symmetry is not utilized in the crystal.28 The molecules crystallize as enantiomorphic pairs that, in conformation, are almost centrosymmetric molecules the difference therefrom is of the same order of magnitude as the thermal motion of the atoms. The carbon atoms and terminal oxygen atoms form an approximately planar chain. All of the oxygen atoms are both donors and acceptors for an intricate network of hydrogen bonding. 

A 1 1 complex is possible because both ends of the N-methyl thiourea guest bind centrosymmetrically to separate host molecules. This results in long chains of alternating host/guest molecules. 

The crown ether adopts a D3d conformation and is coordinated on both faces by NH3 groups from centrosymmetrically related Cu++ complexes. The trans NH3 groups each bind to a crown ether forming long chain copolymers. In addition to the NH3... 

Here again the [18]crown-6 is situated on a center of symmetry with approximate D3d symmetry. Each of the centrosymmetrically related water molecules is linked to two oxygen atoms of the crown ether OH = 1.87, 1.92 A. Each end of the 4,4 -biphenyldiol units binds to a water molecule of a different crown ether. The bridging 4,4 -biphenyldiol leads to the formation of long chains. The spatial arrangement of the three constituents in the crystal are shown in Fig. 27. 

At present, it is common knowledge that not only the photoreactivity, but also the stereochemistry, of the photoproduct is predictable from crystallographic information of starting olefin substrates. This ability of olefinic crystals to dimerize has been widely applied to the topochemical photocycloaddition polymerization of conjugated diolefinic compounds, so called "four-center type photopolymerizations" (7,8). All the photopolymerizable diolefin crystals are related to the center of symmetry mode (centrosymmetric -type crystal) and thus give polymers having cyclobutanes with a 1,3-trans configuration in the main chain on irradiation. 

The synthetic work has been extended to di-jU-hydroxo-/ac-triammine derivatives (97) (Table 47 and Scheme 34). The crystals of (97) contain two different centrosymmetric cations, the trans diaqua and trans dihydroxo species, linked alternately in infinite chains by short (2.45 A) hydrogen bonds. The bond distances of the diaqua cation differ slightly from those of the dihydroxo cation.375... 

There are mercury compounds with mercury oxidation numbers lower than +1, e.g. +0.5,16,21 +0.6713,15,27 or +0.35.18,20 Yellow crystals of Hg3(AsF6)2 have been formed by the reaction of metallic mercury with AsFs in liquid S02.13 X-Ray structure determination showed a linear polycation Hg+—Hg—Hg+ with Hg—Hg distances of 255 pm.15 Metallic mercury and SbF5 react in liquid S02 to form Hg3(Sb2Fu)2.15,23 The Hg—Hg distances in the complex Hg3(AlCU)2 are 256 pm 14 the Hg—Cl distances are 251 and 256 pm the Hg—Hg—Hg angle is 174°. Dark red crystals of Hg4(AsF6)2 were obtained in liquid S02. This coordination compound contains centrosymmetric Hg4+ ions, which are connected to chains (see l).21... 

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