Poly conformational energy calculations

Empirical conformational energy calculations are performed on helical poly(2,3-quinoxaline)s to predict stable conformations. Two energy minimum conformations are found by varying the dihedral angle, y, between two adjacent quinoxaline units from 5 to 180°. Circular dichroism spectra are calculated for the two stable conformations (v - 45 and 135°) on the basis of exciton theory. 

A polymer related to poly-L-proline, in the sense that the amide nitrogen is substituted and, therefore, cannot take part in hydrogen bonding, is poly-N-methyl-L-alanine. Conformational energy calculations for this... 

In addition to the order-disorder transition, observed for a helices, helical structures can also be induced to undergo transitions from one ordered form to another. For example, a crystalline form of poly[p-(p-chlorobenzyl)-L-aspartate] can be made to undergo a phase transition from an a-helical to an co-helical form by heating rotational entropy is computed to play a role in this process.68 Another order-order transition is the solvent-induced interconversion between polyproline 1 (with cis peptide bonds) and polyproline 11 (with trans peptide bonds), a process that has also been subjected to conformational energy calculations.85 The transition has been accounted for in terms of differences in the binding of solvent components to the peptide 0=0 groups. 

F. T. Hesselink, T. Ooi, and H. A. Scheraga, Macromolecules, 6, 541 (1973). Conformational Energy Calculations. Thermodynamic Parameters of the Helix—Coil Transition for Poly(L-lysine) in Aqueous Salt Solution. 

The semi-empirical conformational energy calculations of poly-cis-5-ethylproline (PC5EP) predict that the helical structure may exist in two conformational forms such as I and II. Experimental results confirmed that in solution two major conformations may be assumed by the poly-cis-5-ethyl-D-proline. However, the calculations for poly-trans-5-ethyl-D-proline indicated that only one form may be allowed.Spectroscopic data (Circular Dichroism, NMR) showed the polypeptide exists in a poly-L-proline form-I-type helix and changes slowly to some intermediate conformation. The slow muta-rotation is partially due to the steric interactions of the ethyl group with the carbonyl group of the amide during the mutarotation. 

Abe s semiempirical force field for alkyl sulphides is related to that used for polyethers, though gauche conformations of C—C bonds in poly(olefin sulphide)s seem to lack the extra stabilization of 0.7-1 kcal mor found in poly(olefin oxide)s. Conformational energy calculations have preceded many of the other RIS treatments noted in the next section. 

For syndiotactic polydienes, experimental data and energy calculations have been reported for ds-l,4-poly(l,3-pentadiene)83,99,100 and ds-l,4-poly(3-methyl-1,3-pentadiene).87 In both cases energy minima are obtained for the tc conformation (A cwA TA+cwA+T) 47,83,87 according to the observed chain axes of 8.5 and 8.6 A.87,99... 

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-... 

Isotactic poly(methyl methacrylate), also, is an intricate case, resolved only after a 20-year debate. The repetition period along the chain axis is 10.40 A corresponding to S monomer units the entire cell contains 20 monomer units (four chains). At first, the stmcture was resolved as a 5/1 helix (183) with = 180° and 62 — 108° but no reasonable packing was found using this assumption. Further conformational calculations showed that helices like 10/1 or 12/1 should be more stable than the 5/1 helix. The structure was solved by Tadokoro and co-workers (153b) who proposed the presence of a double helix. Two chains, with the same helical sense and the same direction but displaced by 10.40 A one from the other are wound on each other, each chain having 10 monomer units per turn [i(10/l)] and a 20.80-A repeat period. As a result, the double helix has a 10.40-A translational identity period, identical to that found in the fiber spectmm. The conformational parameters are Of = 179° and 2 = -148°. Energy calculations indicate that the double helix is more stable by 4.4 kcal per-mole of monomer units than two isolated 10/1 helices, a result that is in line with the well-known capacity of this polymer to form complexes in solution (184). 

Conformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Af-vinyl-2-pyrrolidone) are computed. The results of these calculations are used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coutombic interactions, which are governed by the dielectric constant of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before. 

Geometry-optimized CNDO/2 molecular orbital calculations are carried out on poly(5,5 -bibenzoxazole-2,2 -diyl-1,4-phenylene)- and poly(2,5-benzoxazole)-model compounds to determine conformational energies as a function of rotation about each type of rotationable bond within the repeat units. 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

Conformational calculations are carried out on poly(di-n-hexylsilanes). The most significant finding from the energy calculations is that the a -trans conformation is not the lowest energy structure for the symmetrically alkyl-substituted silane polymers. A helical structure is preferred for the isolated molecule. 

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