Conformational analysis for hydrated ethylene oxide oligomer models by quantum chemical calculations

Hydrate effects on the conformations of ethylene oxide oligomers (EO-x, x = 1–8 mers) were examined using quantum chemical calculations (QCC). Conformational analyses were carried out by RHF/6-31G. The models were constructed by locating a water molecule to each ether–oxygen in the structures optimized for non-hydrate oligomers. Hydrate ratio, h (h = H2Omol/Omol in oligomer), was set from 0 to 1.0. The six type conformations with repeated units of O–C, C–C and C–O bonds were examined. Conformational energy, Ec (HF), was calculated as difference between the energy of oligomer with water molecules and that of non-hydrogen and/or hydrogen bonding water molecules. Hydrate energies for each conformer, ∆μh (kcal/m.u., based on Ec in non-hydrate state), were negative and linearly decreased with the increase of h values, and such effects with the increase of h values were weaken with increasing x values. These results were consistent with our previous results calculated using the permittivity, ε (ε = 0–80.1), by QCC. In non-hydrate (h = 0), the (ttt)x conformers were the most stable independent of x. However, in hydrate states (h = 0.44–0.67), the (tg+t)x conformers were the most stable independent of x values, and in h = 1, the (tg+t)8 conformer (8-mer) was most stable [∆Ec(g) = −1.3 kcal/m.u., ∆Ec(g): energy difference between a given oligomer and the (ttt)x oligomer]. These results supported the experimental those based on NMR analyses using dimethoxyethane and triglyme solutions. Molecular lengths (l) of (tg+t)x, (tg+g−)x and (g+g+g+)x conformers having higher x values significantly decreased with increasing h values. Such contraction with hydration, however, was independent of ΔEc(g) values of each conformer.