Trans and gauche conformations

In addition to existing as helices in crystals, there is evidence that certain vinyl polymers also show some degree of regular alternation between trans and gauche conformations in solution. In solution, the chain is free from the sort of environmental constraints that operate in a crystal, so the length of the helical sequence in a dissolved isotactic vinyl polymer may be relatively short. 

In a class of reahstic lattice models, hydrocarbon chains are placed on a diamond lattice in order to imitate the zigzag structure of the carbon backbones and the trans and gauche bonds. Such models have been used early on to study micelle structures [104], monolayers [105], and bilayers [106]. Levine and coworkers have introduced an even more sophisticated model, which allows one to consider unsaturated C=C bonds and stiffer molecules such as cholesterol a monomer occupies several lattice sites on a cubic lattice, the saturated bonds between monomers are taken from a given set of allowed bonds with length /5, and torsional potentials are introduced to distinguish between trans and "gauche conformations [107,108]. 

Conformational Changes and Orientation of Trans and Gauche Conformers... 

This type of calculation was carried one stage further by Cunningham, Manuel and Ward n). On the assumption that the contributions to the overall second moment from structural units with trans and gauche conformations can be regarded as additive, we have... 

Table 6. Total Lattice Sums for Trans and Gauche Conformations in PET and Contributions from Intramolecular Interactions...

We can now attempt something more ambitious. Specifically, we shall try to predict the relative stability of the cis, trans and gauche conformers of HOOH by drawing Newman projections and specifying the key sigma conjugative interactions in each case. 

The Apparent Enthalpy Difference Between the trans and gauche Conformers for C mim[BE4] [22]... 

An example of the use of microwave spectra is provided by Figure 9-8, which shows separate rotational absorptions observed for trans and gauche conformations of propyl iodide (cf. Section 5-2). ... 

Histamine (1) is a flexible molecule that can adopt numerous conformations (Figure 2). These can be described in terms of three torsional angles Xj, X2 and x3. Quantum mechanical calculations on conformations for histamine [15,17-19] reveal that with respect to t2, the histamine monocation is found basically in the trans and gauche conformation. The values for X] and x3 are less restricted. 

Figure 2. The conformations of histamine (1) described by 3 torsion angles (tj, q and 3, respectively). Torsion angle t2 is found fundamentally in the trans and gauche conformations.

FIG. 5.—(a) Three-bond motion (top) and four-bond motion (bottom) of a hydrocarbon chain distributed on a tetrahedral (or diamond) lattice, (b) Crankshaft motion of five bonds around two collinear bonds. P and Q represent the tails of the polymer chain t and g denote the trans and gauche conformations, respectively. [Reproduced with permission from Figs. 2 and 3 of F. Heatley, Progr. 

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

---