Oxygen atoms, spatial arrangement

B (hydrogen-bond acceptor) unit, as it is also present in many of the derivatives that are not sweet. Its presence in these derivatives promotes the cis orientation of the amide (or thioamide) bond formed with the aniline amino group, as it is known, from infrared and n.m.r. studies, that a bulky substituent on an amide (or thioamide) favors the cis orientation, owing to the steric interaction between the bulky group and the carbonyl oxygen atom. Such an arrangement will bring the AH,B system into the correct spatial separation from the D unit. 

In biological systems, H-bond donors and acceptors are predominantly nitrogen and oxygen atoms. However, the n electrons of aromatic systems can also act as acceptors, and H-bonds involving sulfur groups or metallic cofactors are also known. The presence of individual H-bonds in biomacromolecular structures is usually derived from the spatial arrangement of the donor and acceptor groups once the structure of a molecule has been solved by diffractive or NMR techniques. More detailed information about H-bonds... 

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. 

Figure 2. Spatial arrangement of oxygen atoms, cation, and adsorbed gas atom at site SII. The Li atom lies on the 6-membered ring.

One of the more prominent features of polar molecules and ionic species is their ability to dissolve in polar solvents. Water, in turn, is one of the more polar solvents, because the distribution of electrons is quite different from the spatial arrangement of protons the oxygen atom of H20 carries more negative charge, while the hydrogen atoms carry more positive charge (Figure 2.3). 

The carbonyl oxygen is cts with respect to the hydrogen of the Ca-atom. In this spatial arrangement (which has been termed cts ), rotation of the pyrrolidine ring about bond (it) is limited to about 15°. 

Reaction C in Fig. 4 is an aldol condensation between an achiral aldehyde and an ester enol borinate featuring a bidentate chiral substituent at the boron atom [24]. Upon enolate-boron/aldehyde-oxygen co-ordination, two chair-like TS can be formed, both featuring the aldehyde phenyl group in a pseudo-equatorial position. Preferential attack on the aldehyde Si face is determined by the spatial arrangement of the metal ligand. The almost exclusive formation of the anti diastereoisomers arises from control of the enolate geometry. 

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