In this paper we extend our study of the conformational properties of poly(alkyl ethers) by considering poly(oxytrimethylene) (POM(3)). The conformational energies and geometries of the important conformers of the model molecule 1,3-dimethoxypropane (DMP) were determined from ab initio electronic structure calculations. Predicted NMR vicinal coupling constants in DMP, using conformational populations determined from quantum chemical energies calculated at the MP2 level with a D95+(2df,p) basis set, are in good agreement with experimental gas phase values. The conformer energies and geometries of DMP from quantum chemistry calculations have been utilized in developing a second-order rotational isomeric state (RIS) model for poly(oxytrimethylene) (POM(3)). Values obtained for the first-order O-C-C-C gauche energy E sigma (-1.0 kcal/ mel) and the second-order O-C-C-C-O gi(+/-) g(-/+) energy E(omega") (4.0 kcal/mol) differ significantly from values in previous RIS models that were parametrized to reproduce experimental data on POM(3) and DMP in solutions. Our model yields values for the characteristic ratio in fair agreement with experiment, but its temperature coefficient and mean-square dipole moment values are in poorer agreement than those obtained from quantum chemistry based RIS models derived for other polyethers. This discrepancy is attributed mainly to significant contributions from polymer-solvent interactions which depend on local chain conformations and the temperature dependence of such condensed phase effects in POM(3) solutions.