Conformational energies and the random-coil dimensions and dipole moments of the polyoxides CH3O[(CH2)yO]xCH3

Abstract

Conformational energies of the first four members 0, = 1-4) of the polyoxide series CH3O[(CH2),,-O-IxCH3 were calculated using semiempirical potential energy functions in the usual manner. Experimental values of the same energies were also obtained by means of critical analysis of the unperturbed dimensions and dipole moments of these chain molecules in terms of the rotational isomeric state theory of chain configurations. The semiempirical methods do not in general successfully predict conformational energies in this class of chain molecules, as was pointed out in several earlier but more limited comparisons of this type. The present study implicates the intramolecular interactions involving oxygen atoms as the origin of this disagreement between theory and experiment and gives a quantitative estimate of the magnitude of the discrepancy for each of the interactions thus involved. The energy differences thus established should be included, as carrectims, in conformational energy calculations on such molecules until more reliable potential energy functions can be derived. In any case, the energies obtained in the present investigation by direct correlation of theoretical and experimental values of the dimensions and dipole moments should be useful in the prediction of any configuration-dependent property of any member of the polyoxide series.In recent years a variety of configuration-dependent properties of polymeric chains have been succesqfully treated within the framework of the rotational isomeric state approximation, provided that the neighbor-dependent character of the conformational energies is taken into accounts2 Of fundamental importance in such treatments is information by means of which one may locate suitable rotational states and establish their relative energies. The most reliable information of this type is often obtained by direct measurements, either sgectroscopic or thermodynamic, on small molecules having structural features similar to those of the chain molecule under in~estigation.~ In addition, semiempirical conformational energy calculations frequently provide supplementary information useful in accessing the overall conformatiopal energy surface as a function of the skeletal rotational angles. Such calculations also provide very important information regarding the high energy conformations, which generally elude direct observation by conventional techniques. Since the potential energy functions used in such calculations ale semiempirical,2a however, their reliability must be carefully tested against appropriate experimental data. A typical example of this approach is the calculation4 of the conformational energies of the n-alkanes, including polymethylene (PM) [CH2-Ix. The results thus obtained were found to be in satisfactory agreement with those required to reproduce the observed values of the unperturbed dimension ( r2)o and its temperature coefficient d In (r2)o/dT for the PM chain.The PM chain may be converted schematically to any member of the polyoxide series [(CH2)y-O-]x by simply ...

Rotational Isomeric State Analysis of Poly(oxyethylene). Conformational Energies and the Random-Coil Configuration

“…4 (2) where the pre-exponential factors are the same as those adopted in paper I. 3 In the following calculations, the value of Ew was set equal to 0.53 kcal mol-1 , otherwise noted.…”

“…In aqueous salt solutions, POE exhibits a lower critical solution temperature. The unperturbed dimension of the POE chain has been determined in a variety of solvents: the results are usually expressed as the characteristic ratio C= <r 2 ) 0 /n/ 2 , where r is the end-to-end distance, these three solvents when the hydrodynamic constant ¢ is taken to be 2.6 x 10 23 mol-1 .1, 2 Recently, Kugler et a/. 5 quoted a larger value, C = 6.8-7.5, from Peuscher's unpublished work.…”

“…Thus, the NMR measurements have provided a support to the theoretical calculation presented in our previous paper. 2 Allen and Maconnachie 7 were the first to report the small angle neutron scattering (SANS) data on POE in the molten state. They obtained a value of C=9.7, which is exceedingly larger than those estimated in solution.…”

Simulation of the Phase Behavior of the (EO)13(PO)30(EO)13(Pluronic L64)/Water/p-Xylene System Using MesoDyn

“…23, 24 The second term in the freeenergy functional represents the Gibbs entropy of the distribution -k B TΨ ln Ψ. The third term F nid [F 0 ] is the mean-field nonideal contribution.…”

Novel stereochemical equilibria in crown ethers. Detection of additional conformations of cis-syn-cis- and cis-anti-cis-dicyclohexano-18-crown-6 by 100.6-MHz 13C nuclear magnetic resonance at 173 K

“…This leads to the expectation that the C-10,23 site will resonate ca. 1-2 ppm upfield from the resonance for C- 13 C-1,9 C-14,22 C-3,7 C-16,20 C-4,6 C-17,19 C-13,23 C-10,26 C-12,24 C-11 0.75 pprn for the major conformations. Interestingly, in the minor conformers this difference decreases, to only 0.16 ppm.…”