Trivalent Atoms and Groups

Equally important isosteric replacements involve trivalent atoms and groups. These include -N= and -C=, especially in ring compounds, and they will be discussed in the appropriate chapters. 

Representative heats of formation predicted by the ECP/BAC-MP4 method are given in Table 1 (the complete published [88] thermodynamic data set used in the analyses below is available online [91]). Data are shown for a range of compoimds, including tetravalent, trivalent, and divalent coordination at tin. Values for the reference compounds SnCU, SnH4, and Sn(CH3)4 are also given. Finally, heats of formation for atoms and groups needed to calculate reaction enthalpies are given. These results are used in the analysis below to identify potential reaction pathways for MBTC and its decomposition products. 

Uric acid is odourless in spite of three carbonyl groups, four trivalent nitrogen atoms and a double bond, and that it is similarly colourless in spite of four chromophores. Measurements of its refractive and dispersive properties indicate that it is a saturated body which suggests that molecular attraction exists between the various groups. 

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the... 

The bridgeheads in a bimacrocycle must connect three bridges each. Therefore they must be trifunctional. This is given when trivalent atoms are used and also when trisubstituted groups are incorporated. 

The wide-rim CMPO calix[4]arenes are the most efficient compounds for the extraction of trivalent actinides and lanthanides. They are also the compounds that display the highest selectivity along the lanthanide series, provided that phosphorus atoms are linked to phenyl groups. They also display a higher selectivity than the calixarene-bearing diphosphine oxide, where the phosphorus atom is linked to phenyl units CPo21.176- ... 

Following Knight s discovery of P chemical shifts, ° attempts have been made to correlate changes in the observed shift parameters with the inductive effects of neighbouring atoms or groups, and the much lower field shifts observed for trivalent phosphorus compounds... 

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