Conformational Characteristics of Poly(ethylene sulfide) and Poly(ethylene oxide):  Solvent Dependence of Attractive and Repulsive Gauche Effects

Abstract

Conformational characteristics of poly(ethylene sulfide) (PES), poly(ethylene oxide) (PEO), and their oligomeric model compounds have been investigated by the rotational isomeric state (RIS) analysis of ab initio molecular orbital (MO) calculations, NMR vicinal coupling constants, characteristic ratios, and dipole moment ratios. Conformational energies of PES were determined from 1 H and 13 C NMR vicinal constants of its monomeric model compound, 1,2-bis(methylthio)ethane (BMTE), and ab initio MO calculations for BMTE at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) and MP2/6-311+G-(3df,2p)//HF/6-31G(d) levels. By the NMR analysis, the firs-order interaction energies for the gauche states around the C-C and C-S bonds, designated as E σ and EF respectively, were evaluated as follows: in benzene, Eσ ) 0.41 kcal mol -1 and EF ) -0.74 kcal mol -1 ; in chloroform, Eσ ) 0.31 kcal mol -1 and EF ) -0.41 kcal mol -1 . The C-C and C-S bonds were shown to prefer the trans and gauche conformations, respectively. These tendencies are consistent with the MO calculations: B3LYP, Eσ ) 1.39 kcal mol -1 and EF ) -0.24 kcal mol -1 ; MP2, Eσ ) 0.89 kcal mol -1 and EF ) -0.41 kcal mol -1 . Inasmuch as the MO calculations represent gaseous BMTE, the conformational energies were indicated to have large solvent dependence. Ab initio MO calculations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) and MP2/6-311+G-(3df,2p)//HF/6-31G(d) levels and by the complete basis set (CBS-Q) method were carried out for 1,2dimethoxyethane (DME), a model compound of PEO. All of the MO calculations showed the presence of the (C-H)‚‚‚O attraction in the g ( gconformations for the C-C/C-O bond pairs. The MP2 calculations gave the first-order interaction energies (Eσ and EF) for the gauche states around the C-C and C-O bonds as 0.32 and 1.22 kcal mol -1 , respectively. The conformational energy Eω representing the (C-H)‚‚‚O interaction was evaluated as -1.12 kcal mol -1 . In the RIS scheme, bond conformations of PEO in 1,4-dioxane and dipole moment ratios of PEO in benzene were simultaneously simulated, and the conformational energies of PEO in nonpolar organic solvents were determined: Eσ ) -0.25, EF ) 1.17, and Eω ) -0.79 kcal mol -1 . Ours and Abe and Mark's data [Eσ ) -0.5, EF ) 0.9, and Eω ) 0.4 kcal mol -1 ,

The Attractive Gauche Effect of Ethylene Oxides

“…¼ À0:79 kcal mol À1 . 3 These energy parameters are considered to represent the ethylene oxides in weakly polar solvents. From E i 's = 1.17, E !…”

“…For the C-C bond conformations of triglyme, The attractive gauche effect has been found in X-C-C-X bond sequences, where X stands for electronegative atoms such as F, Cl, and O; 1,2 the central C-C bond has been considered to have the inherent gauche preference. In a previous paper, 3 we have proposed a concept of the competitive balance between intramolecular and intermolecular attractions of ethylene oxides. The isolated (i.e., gaseous) ethylene-oxide chains form the intramolecular hydrogen bonds, which cause an apparent gauche stability of the C-C bond.…”

“…C-O antibond ( C{H ! Ã C{O ) delocalizations, 3 suggesting that the tgt state is more stable than ttt. Here, the designation tgt indicates that bonds 2, 3, and 4 take t, g, and t states, respectively (see Figure 1a).…”

“…In the previous study, 3 we calculated conformer free energies of monomer (DME) and trimer (triglyme) of PEO at the B3LYP/6-311+G(3df, 2p)// B3LYP/6-31G(d) level. The gauche energy of the central C-C bond seems to depend on the chain length: DME, 0.76 kcal mol À1 ; triglyme, 0.15 kcal mol À1 .…”

Conformational Analysis for Hydrated Ethylene Imine Oligomer Models by Quantum Chemical Calculations

“…[10][11][12] Furthermore, in contrast to the RIS, MM, and MD methods, recent studies involve quantum chemical calculations method (QCC). Although conformational analysis in the gaseous phase using QCC has been reported for dimethoxyethane (DME) [13][14][15][16] and its oligomers 13,17 (as models of PEO) and for di-MEDA 7,18 (as a model of PEI), comparable studies in the liquid phase, however, are little. Smith et al…”