Conformational analysis of ethylene oxide and ethylene imine oligomers by quantum chemical calculations: solvent effects

Abstract

Conformational analyses using quantum chemical calculations were carried out for 1- to 4-mers of ethylene oxide (EO) and ethylene imine (EI) oligomer models (EO-x and EI-x, x = 1 - 4) in the liquid phase using four solvents (permittivity: e = 2.0 ~80.1). The results were compared against those obtained in the gaseous phase. The calculations involved either RHF/6-31+G(d,p) or B3LYP/6-31G(d) // SCRF/IPCM, based on the observed and calculated results for the energy difference between trans- and cis-dichloroethane. The conformations repeated for a unit of X-C, C-C and C-X bonds (X: O or N) were examined. For both oligomers, the energies of every conformer decreased with increasing e values, and were linear against the Kirkwood function (Kf = (e-1)/(2e+1)). For the EO oligomers, the (ttt)x conformer was most stable in the gaseous phase. In liquid phases, however, the preference for the gauche-conformation (gauche preference) of the C-C bonds increased with higher values of e. In the case of EO-3, the (tg+t)x conformer was most stable above an e value of 8.9, which were in good agreement with those observed for triglyme solutions using NMR analysis. For the EI oligomers, the (tg+t)x conformer was most stable in either gaseous or liquid phase, and the gauche preference of the C-C bonds in both phases were comparable. These results were in good agreement with those observed for di-MEDA solutions using NMR analysis. It was estimated that such small solvent effects on gauche preferences of the EI oligomers result in weakening for hydrogen bonds (NH-N) of neighboring imino groups by solvents.