ABSTRACT:Conformational energies of monomeric (1,2-dimethoxyethane, DME) and trimeric (triglyme) model compounds of poly(ethylene oxide) have been evaluated by accurate ab initio molecular orbital (MO) calculations at the MP2/6-311++G(3df, 3pd)//HF/6-31G(d) level. The first-order interaction energies (E 's) for gauche states around the C-C bonds of DME and the terminal repeating unit of triglyme are ca. þ0:1 kcal mol À1 , whereas the central unit of triglyme has a slightly negative E value of ca. À0:1 kcal mol À1 . For the C-C bond conformations of triglyme, The attractive gauche effect has been found in X-C-C-X bond sequences, where X stands for electronegative atoms such as F, Cl, and O; 1,2 the central C-C bond has been considered to have the inherent gauche preference. In a previous paper, 3 we have proposed a concept of the competitive balance between intramolecular and intermolecular attractions of ethylene oxides. The isolated (i.e., gaseous) ethylene-oxide chains form the intramolecular hydrogen bonds, which cause an apparent gauche stability of the C-C bond. In polar solvents, however, the O-C-C-O segment tends to prefer the tgt conformation because of attractive interactions with solvents (for the bond sequence, see Figure 1). These phenomena may be observed as variations in two conformational energies: E and E ! (for the interactions, see Figure 2). The former energy corresponds to the energy difference between trans and gauche states, and the latter represents the (C-H)Á Á ÁO interaction. The E and E ! values, depending on the polarity of environment, shift in the opposite directions: E ¼ þ0:32 and E ! ¼ À1:12 kcal mol À1 for 1,2-dimethoxyethane (DME) in the gas phase; 3 E ¼ À0:25 and E ! ¼ À0:79 kcal mol
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