Conformational characteristics of poly(ethylene sulfide) (PES), polyethylene 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 H-1 and C-13 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 and E respectively, were evaluated as follows: in benzene, E-o = 0.41 kcal mol(-1) and E-o = -0.74 kcal mol(-1); in chloroform, E = 0.31 kcal mol(-1) and E-p = -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, E = 1.39 kcal mol(-1) and E-p = -0.24 kcal mol(-1); MP2, E-o = 0.89 kcal mol(-1) and E-o = -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(+/-)g(-/+) conformations for the C-C/C-O bond pairs. The MP2 calculations gave the first-order interaction energies (E-o and E-p) 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 E. 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: E-o = -0.25, E-p = 1.17, and E-w = -0.79 kcal mol(-1). Ours and Abe and Mark's data [E = -0.5, E-p = 0.9, and E-w = 0.4 kcal mol(-1), Abe, A.; Mark, J. E. J. Am. Chem. Soc. 1976, 98, 6468] show that the E and E-w, values of PEO so widely vary with solvent as to change the signs. Configurational entropies of 200 mers of PES and PEO were calculated to be 5.8-6.3 and 5.0-5.1 cal mol-1 K-1, respectively. Thus, the difference in melting point between PES (216 C) and PEO (68 C) was indicated to come from that in enthalpy (DeltaH(u)) of fusion: DeltaH(u) (PES) > DeltaH(u) (PEO). The natural bond orbital analysis for BMTE and DME revealed the following facts. For BMTE and DME, vicinal bond-antibond interactions around the C-C bond cause the gauche preference. For BMTE, however, a steric S...S repulsion considerably reduces the gauche stability, and hence the trans preference appears in the C-C bond. Bond-antibond and lone pair-antibond interactions around the C-X bond (X = S or O) stabilize the gauche conformation for BMTE but the trans state for DME. Both MO calculations and NMR experiments for BMTE showed that the most stable states are g(+/-)g(-/+), in which electron delocalization in the S-C-C-S antibonds is maximized and a large antiparallel dipole-dipole interaction is formed. Thus, the g(+/-)g(-/+) conformations have a smaller free energy than g(+/-)g(-/+) by ca. 0.2 kcal mol(-1), being found in BMTE and PES crystals.