Bonds, trans

This particular sequence of conformations-trans bonds that advance the helix along the axis, alternating with gauche bonds which provide the twist-takes the chain through a series of relatively low potential energy states and generates a structure with minimum steric hindrance between substituents. If the polymer series is extended to include bulkier substituents, for example. 

Variation of c/s-bond lengths has been noted in some cases and is believed to reflect steric interaction. Therefore, in the series PtX(PEt3)3 (X = H, Cl, F) the trans-bond to X shows a dependence upon the electronegativity of X, while the cz s-Pt—P bond shows no such dependence (Figure 3.91) [148], instead increasing as X becomes larger. 

In several cases the transitions under tension involve the transformation of gauche-bonds to trans-bonds for instance, for PBT the transition is from a form with dihedral angles, relative to the bonds of the tetramethylene group, of the kind GGTGG toward a form with the same dihedral angles of the kind TTTTT. Analogously for PVDF, under tension, there is a transition from the a form (TGTG chains) toward the 3 form (TTTT chains). 

Bismuth phosphine complexes represent a substantial component of the established phosphine complexes of heavier p-block elements, and an excellent overview has presented an important bonding model for these systems (7). The observed structures are considered as trigonal-pyramidal BiX3 units with three secondary trans bonds. If the acceptor orbitals are the Bi-X trans arrangement is expected, as the relationship between the trans X-Bi-P bond distances. The shortest Bi-P distance [2.7614(2) vs 2.866(3) A] is trans to the longer Bi-Br distance [3.403(1) vs 2.9916(1) A], as the only arrangement that will allow the phosphine ligands to occupy trans... 

It is of interest to note that the addition of nitrosyl chloride to a molecule such as cis,trans, trans-1,5,9-cyclododecatriene takes place only at one of the trans bonds [72], It would appear from this that, in a competitive reaction between cis and trans double bonds, the reaction at the trans bond is favored. However, further work is required to substantiate this generalization, particularly in view of the fact that in an experiment involving both cis- and trans-stilbene the nature of the nitrosyl chloride adduct was not fully determined [55]. 

The nitrosodecalin dimers were also prepared by this method. They were tra/js-a-nitrosodecalin dimer (m.p. 148°-149°C), Ira/zs-jS-nitrosodecalin dimer (m.p. 133.5°-134.5°C), and c/r-/3-nitrosodecalin dimer (m.p. 132°-133.5°C). In these dimers, the prefixes cis and trans refer to the conformation about the bridgehead carbons. The nitrogen-nitrogen bond of the dimer is presumed to be the usual trans bond [47]. 

Model structures using octahedral Co(Ni) indicate that all bonding between the four extending orbitals on each lOlO face must be through adjacent positions (cis) on the Co(Ni). Trans bonding is possible for some configurations with larger clusters of Mo or stacked crystals. As the concern is with the Type I Co(Ni)-Mo-S site, discussions here will concentrate on cis attachment. 

Smith et al. (1978) have described a procedure for the GLC determination of cis and trans isomers of unsaturated fatty acids in butter after fractionation of the saturated, monoenoic, dienoic, and polyenoic fatty acid methyl esters by argentation TLC. Total trans acids were much higher, as measured by infrared spectrophotometry than by GLC, probably because some of the acids could have two or more of the trans bonds designated as isolated by infrared spectrophotometry. Enzymatic evaluation of methylene-interrupted cis, cis double bonds by lipoxidase resulted in lower values than those obtained by GLC. The authors mention that the lipoxidase method is difficult, requiring considerable skill, and suggest that their method is suitable for the determination of the principal fatty acids in complex food lipids such as bovine milk fat. 

Deman and Deman (1983) have investigated the determination of trans unsaturation in milk fat by infrared analysis and found values of 7.4% (winter) to 9.9%(summer) when the TGs were analyzed. These are higher than the quantities found by infrared analysis of methyl esters of the fatty acids. These quantities are isolated total trans bonds and do not give an estimate of the positional and polyunsaturated isomers which are present. The trans contents obtained by Deman and Deman are higher than the 4% found by Smith et al. (1978). 

In complex (40) the dimethylmethyleneiminium cation, isoelectronic with H2C=CMe2, is i72-bound to the NiClPPh3 moiety.199 Short trans bonds (o-type) are generally found in the [NiL heteroalkene)] complexes, as for the alkene complexes. 

Calculation of overlap between the appropriate donor orbitals and the Pt 6po orbitals led to the sequence6 495 H" > PR3 > —SCN > 1 CHf CO CN" > Br > CC > NH3 > H20 Since this effect occurs in the substrate, it should be observable also as a non-kinetic effect and, indeed, studies of the trans influence (ground state trans bond weakening) considerably outnumber... 

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

CuL2C12(H20)2],3H20 ligand, 02 from neighbouring ligand distorted octahedral, trans-bonded ... 

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