Conformational analysis of poly(methylene sulfide) and its oligomeric model compounds: Anomeric effect and electron flexibility of polythioacetal

Conformational. characteristics of poly(methylene sulfide) (PMS) and its ol igomeric model compounds have been investigated. Carbon-13 NMR measurements for a dimeric model compound, bis(methylthio)methane, in the gas phase as well as in solutions were carried out, and the first-order interaction ener gy E-sigma representing the gauche stabilization of the C-S bond was determ ined from observed vicinal C-H coupling constants. For example, the E, valu e for the gaseous dimer was evaluated as -1.43 +/- 0.01 kcal mol(-1), being in good agreement with the ab initio molecular orbital calculation (-1.38 kcal mol(-1)) at the B3LYP/6-311+G(2d,p)//BSLYP/6-31G(d) level. The conform ational energy E, showed solvent dependence; polar solvents stabilize the t rans conformation, in which dipole moments are parallel to each other and h ence the molecule becomes polar. The characteristic ratio, dipole moment ra tio, and configurational entropy of unperturbed PMS were estimated by the r otational isomeric state scheme and compared with those of poly(methylene o xide) (PMO). The PMS chain was indicated to be more flexible than PMO. The difference in melting point between PMS (245 degreesC) and PMO (180 degrees C) was suggested to come mainly from that in enthalpy (DeltaH(u)) of fusion : DeltaH(u)(PMS) > DeltaH(u)(PMO). The geometrical parameters, electron den sities, and atomic charges of trimers of PMS and PMO, obtained from the MO calculations, showed that the gauche stability in the C-S bond of the PMS h omologues comes partly from antiparallel dipole-dipole interaction and n(s) --> sigma*(C-S) hyperconjugation formed in the gauche state, partly from s teric (SS)-S-... repulsion occurring in the trans form. It was also shown t hat sulfur electrons have such flexibility as to reduce the (SS)-S-... repu lsion and unfavorable (parallel) dipole-dipole interaction.