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 ,

Conformational characteristics of poly(propylene sulfide) (PPS, [CH2C*H(CH3)S] x ) have been investigated. Proton and carbon-13 NMR vicinal coupling constants observed from its monomeric model compound, 1,2-bis(methylthio)propane (BMTP, CH3S−CH2−C*H(CH3)−SCH3), were analyzed to yield bond conformations of the S−C, C−C*, and C*−S bonds. Ab initio molecular orbital (MO) calculations at the MP2/6-311+G(3df,2p)//HF/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels were carried out for BMTP to evaluate free energies and dipole moments of all the possible conformers. Conformational energies and bond dipole moments of BMTP were estimated therefrom. Conformational energies of BMTP and PPS were also determined by simulations based on the rotational isomeric state scheme for experimental observations of bond conformations of BMTP, characteristic ratio of atactic PPS, and dipole moment ratios of isotactic and atactic PPS. The first-order interaction energies for the S−C (E σ) and the C−C* (E α and E β) bonds were obtained as follows:  E σ = −1.0 to −0.60 kcal mol-1, E α = 0.5−0.6 kcal mol-1, and E β = 1.1−1.2 kcal mol-1. The second-order ω1 and ω2 interactions, representing intramolecular C−H···S interactions, are repulsive:  E ω 1 = 0.6−0.9 kcal mol-1 and E ω 2 = 1.0−1.2 kcal mol-1. The S−C, C−C*, and C*−S bonds were found to prefer the gauche, trans, and trans states, respectively. The conformational characteristics of unperturbed PPS are similar to those of poly(ethylene sulfide) but significantly different from those of its corresponding polyether, poly(propylene oxide) (PPO), although isotactic PPS and PPO are isomorphous. The conformational characteristics of PPS are discussed in terms of solvent effect, crystal structure, and thermal properties.