Conformational energy torsional potentials

The local conformational preferences of a PE chain are described by more complicated torsion potential energy functions than those in a random walk. The simulation must not only establish the coordinates on the 2nnd lattice of every second carbon atom in the initial configurations of the PE chains, but must also describe the intramolecular short range interactions of these carbon atoms, as well as the contributions to the short-range interactions from that... 

In the previous section, the adaptation of the RIS model was based on the distance between next-nearest neighbor beads. This approach is obviously inadequate for CH3-CHX-CH2-CHX-CH3, because it necessarily abandons the ability to attribute different conformational characteristics to the meso and racemo stereoisomers. Therefore a more robust adaption of the RIS model to the 2nnd lattice is necessary if one wants to investigate the influence of stereochemical composition and stereochemical sequence on vinyl polymers [156]. Here we describe a method that has this capability. Of course, this method retains the ability to treat chains such as PE in which the bonds are subject to symmetric torsion potential energy functions. 

What is the origin of the energy difference between the polyproline II and /J-strand backbone conformations Brant and Flory (1965b) emphasize the important roles of steric clash, dipole-dipole interactions (see also Avbelj and Moult, 1995), and the torsional potentials governing rotation about the backbone ,t/i angles (see also Flory, 1969). An ab initio quantum mechanics study (Han et al., 1998 see also references therein to earlier work) finds that solvation by water is important. The authors examine the predicted stabilities of eight conformers of... 

Fig. 14. Conformations of s,czs-cyclooctadiene-l,5 with calculated angles (inner values) and torsion angles (deg force field of ref. 19) (83). The additional calculated information given is (from top) symmetry, AV, AH (kcal mole-1 T = 298 K reference C2-symmetric distorted boat conformation). The three upper conformations correspond to potential energy minima, the two lower to onedimensional partial maxima (transition states).

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

Figure 4. Symmetry breaking of the ethanol torsion potential (top, two gauche and one trans conformation) by interaction with a chiral acceptor molecule (dimethyl oxirane, bottom), in this case RR trans 2,3 dimethyloxirane [128]. Note that trans ethanol is less stable in the complex and that the two gauche (g) forms differ in energy.

Lipkind et al. present their calculations of the conformational motion of maltose [84, 85] based on the assumption that only non bonded interactions and a torsion potential around the glycosidic bond are necessary to describe the energy of a glycoside. They conclude that for maltose there are four conformers with

coupling constants are better described by this four state approach than by a single conformation. 

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