All-trans conformation

Syndiotactic polystyrene, s-PS, is a highly stereoregular, semicrystalline vinyl polymer that normally melts at 270 °C [45,46], S-PS shows a large number of crystalline polymorphs [47-50] obtained by melt crystallization and solvent-exposure techniques. However, s-PS assumes only two distinct crystalline conformations [all trans, planar zigzag (... tttttttt...) and 2 -helical (... ttggttgg. ..) t= trans and g = gauche], which are characterized by fiber repeats of... 

Rotation about C-C bonds does not occur freely. In PE, the carbon atoms are at their lowest energy in the planar zig-zag conformation (all-trans structure). Rotation through 120° in either direction brings the carbon atom to another stable position, which was previously occupied by hydrogen (gauche position). 

The y-form which arises on stretching the polymer and which has an all-trans, i.e. a zig-zag, conformation. 

In summary, these results were interpreted to support rate-determining electrocyclization for the ring closure, starting from the all-trans iminium 11, via the cis conformation 13 of the neutral form, followed by fast proton transfer and elimination of aniline (Scheme 8.4.6). 

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

Table 17 contains the enthalpies of activation and reaction for the three propagation steps in the gas phase and in solution. They were calculated by using classical cations possessing all-trans conformation. 

Typically in solution, a polymer molecule adopts a conformation in which segments are located away from the centre of the molecule in an approximately Gaussian distribution. It is perfectly possible for any given polymer molecule to adopt a very non-Gaussian conformation, for example an all-trans extended zig-zag. It is, however, not very likely. The Gaussian set of arrangements are known as random coil conformations. 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

Figure 16.2a and b show schematic drawings of top and side views of the SAMs of alkanethiols on the Au(lll) surface, respectively [28]. The basic molecular arrangement is (v S X - /3)R30° with respect to the Au(lll) surface. Closer inspection of the structure revealed the existence of a c(4 x 2) superlattice of (v 3 x - /3)R30°. The alkyl chain is tilted from the surface normal by about 30° with all-trans conformation. This... 

Wilkes BC, Nguyen TM-D, Weltrowska G, Carpenter KA, Lemieux C, Chung NN, Schiller PW. The receptor-bound conformation of H-Tyr-Tic-(Phe-Phe)-OH related -opioid antagonists contains all trans peptide bonds. J Peptide Res 1998 51 386-394. 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

The square-planar complex (383) shows a monotropic smectic A-phase, and the X-ray diffraction study of the yellow form shows that the whole molecule adopts an extended rod-like shape with an all-trans conformation of the decycloxy chain.616... 

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

---