Bridging oxygens

The ultrasonic relaxation loss may involve a thermally activated stmctural relaxation associated with a shifting of bridging oxygen atoms between two equihbrium positions (169). The velocity, O, of ultrasonic waves in an infinite medium is given by the following equation, where M is the appropriate elastic modulus, and density, d, is 2.20 g/cm. ... 

Fig. 16.4. Silicate structures, (a) The Si04 monomer, (b) The Si207 dimer with a bridging oxygen. ( ) A chain silicate. (d) A sheet silicate. Each triangle is the projection of on Si04 monomer.

When the ratio MO/SiOj is a little less than 2/1, silica dimers form (Fig. 16.4b). One oxygen is shared between two tetrahedra it is called a bridging oxygen. This is the first step in the polymerisation of the monomer to give chains, sheets and networks. 

With decreasing amounts of metal oxide, the degree of polymerisation increases. Chains of linked tetrahedra form, like the long chain polymers with a -C-C- backbone, except that here the backbone is an -Si-O-Si-O-Si- chain (Fig. 16.4c). Two oxygens of each tetrahedron are shared (there are two bridging oxygens). The others form ionic bonds between chains, joined by the MO. These are weaker than the -Si-O-Si- bonds which form the backbone, so these silicates are fibrous asbestos, for instance, has this structure. 

Resonance stabilization in the products is best illustrated by the reactant anhydrides (Figure 3.10b). The unpaired electrons of the bridging oxygen atoms in acetic anhydride (and phosphoric anhydride) cannot participate in resonance structures with both electrophilic centers at once. This competing resonance situation is relieved in the product acetate or phosphate molecules. 

Figure 13.11 Structure of the binuclear anion [Sbj OCle] " showing the bridging oxygen and chlorine atoms and the pseudooctahedral coordination about Sb the O atom is at the common apex of the face-shared square pyramids and the lone-pairs are trans- to this below the SbCU bases. The bridging distances Sb-Cl are substantially longer than the terminal distances Sb-Cl,.

This could account for the paramagnetism, but esr evidence shows that the 2 cobalt atoms are actually equivalent, and X-ray evidence shows the central Co-O-O-Co group to be planar with an 0-0 distance of l3l pm, which is very close to the 128 pm of the superoxide, 02, ion. A more satisfactory formulation therefore is that of 2 Co atoms joined by a superoxide bridge. Molecular orbital theory predicts that the unpaired electron is situated in a rr orbital extending over all 4 atoms. If this is the case, then the jr orbital is evidently concentrated very largely on the bridging oxygen atoms. 

For the most common series of corrosive agents, water, steam, acids, alkalis and salts, the hydrolytic processes peculiar to each determine the mechanism of attack. Thus, under the right circumstances, hydrolytic attack on the bridging oxygens can occur in the following way ... 

Molecules of the oxides of phosphorus P4Oe and P4O10 also contain bridging oxygen atoms. The predicted amounts of ir-bond character, 0.73 X 0.32 = 0.23 and 0.77 X 0.32 = 0.25, respectively, lead with use of the single-bond distance... 

Fig. 22.—Antiparallel packing arrangement of the 2-fold helices of (1— 3)-a-D-glucan (21). (a) Stereo view of two unit cells approximately normal to the aoplane. The two chains in the back (open bonds) are antiparallel and so are the chains in the front (filled bonds). Each helix is stabilized by 2-OH 0-4 hydrogen bonds across the bridge oxygen atoms. Interchain hydrogen bonds are formed in sheets along the a direction, (b) An axial projection of the unit cell shows that the sheets in the front and back are also joined by hydrogen bonds.

Glucose molecules can link together into chains, with each ring tethered to the next by a bridging oxygen atom. In one form, this is cellulose, the stiff material that gives the stalks of plants and the trunks of trees their structural strength. Chitin, a variation on cellulose, is an even stiffen material that forms the exoskeletons of crustaceans such as crabs and lobsters. 

C20-0110. Determine the Lewis structure and draw a ball-and-stick model showing the geometry of the dichromate anion, which contains one bridging oxygen atom. 

These spectral differences are related to different Ti-O-Si bond angle of the Ti sites. Indeed, an angle opening will shift the bridging oxygen hybridization from sp3 to sp2 and... 

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