Trans-planar conformation

Butadiene can exist in two planar conformations, s-trans and s-cis. They belong to the C2h and C2v symmetry point group, respectively. Obviously, both forms have symmetry planes. A skewed conformation, instead, has only a C2 axis. We can choose the reference frames depicted in Figure 12, characterized by z as the symmetry axis and x directed along the single bond. 

For 2-acetylpyrrole, 2-acetylthiophene, and 2-acetylselenophene, the presence in solution of the preferred X,0-cis conformation, planar or nearly planar, is widely accepted (81RCR336). Low-temperature NMR measurements C C) of 2-CO-alkyl derivatives of thiophene have provided evidence (85JCS(P2)1839) that the S,0-trans conformer, if present, is not detectable (<5%), at least in CHF2CI solution. The LIS method at room temperature gave (85JCS(P2)1839) 15% of this conformer in the case of the COMe derivative and 25% for CO-i-Pr. Better agreement between experimental and calculated LIS values was obtained (85JCS(P2)1839) when planar structures were assumed even for the conformers of the 2-pivaloyl derivative. 

Owing to the inequality of the bond angles, a decrease in the trans population increases 0lnl2. Hence, the positive temperature coefficient for this ratio denotes a lower energy for the trans state. This energy is attributed to favorable interaction between CH3 pairs separated by 380 pm in the planar (trans) conformation. The larger value observed for the ratio in a less polar medium is in the direction predicted for enhanced electrostatic interaction within a chain of partially ionic Si—0 bonds. 

All ester groups in the PET chain are assigned confidently to be planar trans. The restriction of bonds 1 and 3 to the trans states and bond 2 to a choice between cis and trans leads to the simple statistical weight matrices. The length of the span of the terephthaloyl residue in PET guarantees independence of the conformations of successive repeating units of the chain. 

In an extensive ab initio study, the preferred conformers for substituted pyrroles are predicted to be, inter alia NCCH trans and C(2)CCH cis for 2- and 3-methylpyrroles [i.e. a methyl C—H bond eclipsing the pyrrole C(2)—C(3) bond) NCCO cis and C(2)CCO cis for 2- and 3-hydroxymethylpyrroles pyramidal at nitrogen for aminopyrroles planar OH trans for hydroxypyrroles planar trans for vinylpyrroles, and planar cis (syn) for formylpyr-roles. In agreement with experiment the energy difference between the two rotamers for 3-formylpyrrole is much smaller than for the 2-isomer (79NJC473). 

Scheme 2.13 Rough sketch of the molecule 2,4-difluorobenzyl 2,2-dimethylpropionate (2,4-DFP) used as model compound for the repeating unit of the corresponding polymer. Rotations over CO— CH2Car (V/) ) and OCH2—Car Car (V/)2) bonds are drawn in their planar trans conformation which is represented by

It should be emphasized that these parameters should not be regarded as fixed, i.e. polypeptide structures are not necessarily rigid. However, departures from these values (and from the assumed planar trans conformation of the backbone amide group) can be taken care of by introducing appropriate energy terms to allow for such departures (see Section VE). 

In the case of poly-L-proline II (the amide group being in the planar trans conformation), the angle is fixed by the rigid geometry of the pyrrolidine ring hence, the conformational energy depends only on tfi. 

Conformational enantiomerism. trans-Cyclooctene is strained, unable to achieve a symmetric planar conformation. It is locked into one of these two enantiomeric conformations. Either pure enantiomer is optically active, with [a] = 430°. 

Even a simple strained molecule can show conformational enantiomerism. trans-Cyclooctene is the smallest stable trans-cycloalkene, and it is strained. If trans-cyclooctene existed as a planar ring, even for an instant, it could not be chiral. Make a molecular model of trans-cyclooctene, however, and you will see that it cannot exist as a planar ring. Its ring is folded into the three-dimensional structure pictured in Figure 5-18. The mirror image of this structure is different, and trans-cyclooctene is a chiral molecule. In fact, the enantiomers of iran.v-cyclooclcnc have been separated and characterized, and they are optically active. 

Table II. Values of the Band Gap E and Band Width BW for the Chains in the Planar trans Conformation...

Figure 1. TYie trans isomeric form of polyacetylene shovn in its planar trans and cis conformations, with pertinent si )stituents numbered.

In the ten-membered and higher lactams, the planar trans conformation can be strainless so that the contribution of conformational changes of the amide group to the AGp value will be minimum. Only the 10-membered lactam contains about 5% of cis form and a corresponding amount of energy will be released during polymerization. 

NPCl2)n show the lowest energy for a model in which the NP backbone has quasi-planar trans-cis (TC) conformation packed in a monoclinic cell. The difference between single and double NP bond was calculated to be 0.05 A. This agrees with results from density functional calculations and recently obtained X-ray data. Application of MD simulations and EDXD measurements to amorphous [NP(OC6H4Me-4)2]n leads to the conclusion that the backbone conformation can be described as [TC]n (trans-cis-trans-cis) rather than as [T3C]n trans-trans-trans-cis). ... 

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