Conformational energy map

The different chain conformations observed in different polymorphic forms of a polymer are generally associated to nearly equivalent minima in the conformational energy maps, calculated for isolated chain models [2, 3],... 

As an example we report in this paper the conformational energy maps of two already cited stereoregular polymers, which have been obtained very recently, syndiotactic polystyrene s-PS and syndiotactic polybutene s-PB (Fig. 4 and 5, respectively). In fact, the energy map calculated for s-PS shows... 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.16 reports the conformational energy maps as a function of the torsion angles 0i and 02 of the two single bonds adjacent to the double bonds for 03 = T = 180° for cis-1,4-poly (1,3-butadicnc) (cisPBD),69 tranx-l,4-poly(l,3-butadiene) (transPBD),70 ds-l,4-poly (isoprene) (cisPI),68 trans-1,4-poly(isoprene) (transPI),71 ds-l,4-poly(2,3-dimethyl-l,3-butadiene) (cisPMBD),68 and lrans-, 4-poly(2,3-dimethy 1-1,3-butadicnc) (transPMBD).68 These polymers are representative examples of polydienes with A = A = H... 

Figure 7. A "snapshot" of a typical cellulosic chain trajectory taken from a Monte Carlo sample of cellulosic chains, all based on die conformational energy map of Fig. 6. Filled circles representing glycosidic oxygens, linked by virtud bonds spanning the sugar residues (not shown), allow one to trace the instantaneous chain trajectory in a coordinate system that is rigidly fixed to the residue at one end of the chain. Projections of the chain into three mutually orthogonal planes assist in visualization of the trajectory in three dimensions.

Figure 10. Relaxed (adiabatic) conformational energy map for p-maltose as computed by Brady and coworkers.i3 Contours are drawn at 2,4,6, 8, and 10 kcal/mol above the minimum near < ), y = -60°, -40°. The p-maltose structure may be derived from that of p-cellobiose in Fig. 1 by inversion of the stereochemical configuration at Cl.

The relaxed conformational energy map for Ag-Ig with the IdoA ring in the form is shown in Figure 1. The map for the form... 

Calculations of conformational energy made by means of molecular mechanics fully confirm these conclusions. Such calculations were first introduced into the examination of synthetic crystalline polymers by Liquori and co-workers (175, 176) and were extensively used by Natta, Corradini, Allegra, Ganis, and co-workers (168, 177-179). The conformational energy map of isotactic poly-... 

N 037 "Further Comparison with Experiment of the Calculated Results Obtained from Semiempirical and Quantum Mechanical Conformational Energy Maps... 

Steric information is most easily summarised in a conformational energy map. Such calculations have been made by a number of workers but they require semi-empirical potentials or the effective van der Waals radii which are subject to error and dispute. We have carried out calculations of this sort for polyethylene units using different potentials and find that although the relative energies of the minima vary considerably, their locations are not particularly sensitive to the different potentials. Thus the conformations for which the scattering will be calculated may be restricted to those of low energy without loss of generality. 

An important point to remember is that any calculated energy is useful only in comparison with other energies that are similarly computed. For this reason, computational chemists like to deal with relative energies. Thus, a fairly accurate conformational energy map can be obtained for many organic and pharmaceutical compounds because it involves relative energies obtained by one method on the same species. 

A careful analysis of conformational energy maps (Ramachandran plots) revealed that both enantiomers of P-tetralin amino acids were compatible with the right handed a-helical conformation.109 This was an important prerequisite for the development of N- and C-caps since the ( -configuration of the P-tetralin amino acid is needed for N-terminal helix induction, whereas the (R)-enantiomer was used in the C-cap series. The fact that both configurations were compatible with a-helical conformations made these amino acids our first choice as building blocks for... 

The values from these methods represent averages from conformational energy maps. 

---