The Attractive Gauche Effect of Ethylene Oxides

Abstract

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

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

“…This behaviour of 10 mirrors the behaviour of low molecular weight dimethyl ethylene oxides [49]. Although the average bend angle of 10 is calculated to be comparable to that of 3 (3 = 103°, 10 = 105°), the values of T NÀN TB are vastly different.…”

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 17

“…In a gt-conformation, H pro-R ÀC(5'/II), usually resonating at higher field, is in close contact to O¼C(2/II) (see Fig. 5), and this may well lead to a downfield shift and to an inversion of the relative chemical shifts of H pro-R ÀC(5'/II) and H pro-S ÀC(5'/ II) 16 ). This revised assignment is in agreement with a 1.7 : 1 ratio of the volumes of the cross-peaks between HÀC(3'/II), and either H a ÀC(5'/II) or H b ÀC(5'/II) in the ROESY spectrum of 30.…”

Conformational Analysis of Ethylene Oxide and Ethylene Imine Oligomers by Quantum Chemical Calculation

“…Meanwhile, studies using quantum chemical calculations, which differ from RIS, MM and MD in that the experimentally obtained force field constants are not used, have been reported. [14][15][16][17][18][19][20] It is considered that this method is in a position to obtain accurate information of a single chain eliminating intermolecular interactions. Regarding PEO, Tsuzuki et al, 14 Smith et al 15,16 and Sasanuma et al 17 reported results using a monomer (dimethoxyethane) as a model of PEO.…”

“…Regarding PEO, Tsuzuki et al, 14 Smith et al 15,16 and Sasanuma et al 17 reported results using a monomer (dimethoxyethane) as a model of PEO. Sasanuma et al 17,18 reported those using a trimer and pentamer. For PEI, Boesch et al 19 and Sasanuma et al 20 reported results using the N,N 0 -dimethylethylenediamine monomer as a model of PEI.…”