NMR Studies and Conformational Energy Calculations of 1,2-Dimethoxyethane and Poly(oxyethylene)

Abstract

ABSTRACT:Conformational studies on 1,2-dimethoxyethane (DME), a monomer model compound of poly(oxyethylene), have been carried out by using NMR technique. The observed temperature dependence of the vicinal coupling constants was analyzed on the basis of the rotational isomeric state scheme. For the C-C bond, the 13 CH satellite side bands provide the desired information. Conformational energy E., representing the energy of the gauche state expressed relative to the trans, was found to vary in a range -0.5 to -I .2 kcal mol-t, depending on the solvent system. Values of JT and fa for the vicinal 1 H-1 H coupling were determined concomitantly: JT =I 1.4 ± 0.3 and fa= 2.3 ± 0. I Hz. For the rotation about the C-0 bond, the vicinal 13 C-1 H coupling constant associated with the terminal methyl group was studied. Adoption of fa= 1.3 Hz leads to an estimate of E" = 0.8 to 1.1 kcal mol-1 for the energy difference between the gauche and trans states. In these treatments, the neighbor-dependent character of the bond rotation along the chain was rigorously taken into account. The values of E. and EP derived for the polymer are comparable with those of DME. The results are compared with those reported previously by Viti et a/. and Mastuzaki et a/.KEY WORDS 1,2-Dimethoxyethane I Poly(oxyethylene) I 13 CH Satellite NMR 1 13 C NMR I Vicinal Coupling Constant I Potential Energy Calculation I Conformational Energy I In a series of papers, 1 -3 we have investigated conformational characteristics of polyoxide chains having a general formula fCH 2 C-(R1)(R2)0tn• in which gauche 0 · · · 0 interactions take place around the skeletal C-C bond. The major purpose of these studies is to evaluate the gauche oxygen effect associated with these polymers in terms of the extra stabilization energies, which have been defined as the difference between the conformational energies calculated by using conventional semiempirical expressions 1 and those derived from the analysis of the experimental data on some relevant conformation-dependent properties such as the unperturbed dimension. dipole moment, and bond conformation. In the case of poly(oxyethylene) (POE), where R 1 = R 2 = H, the semiempirical energy calculation (0.6 kcal mol-1 ) failed to reproduce the observed preference (-0.4 kcal mol-1 ) for the gauche over the trans state. 4 · 5 The discrepancy amounts to ca. I kcal mol-1 • In the poly-(oxypropylene) (POP) chain, 2 two gauche states occurring around the skeletal C-C bond are sterically unequivalent to each other. The gauche oxygen effect estimated for the ga conformation, in which the articulated methyl group is situated trans to the preceding oxygen, is in the same order of magnitude as that of POE. Such a stabilization effect, however, was found to be largely suppressed in the sterically more hindered g fJ conformation, in which the preceding oxygen atom is syn to both the following oxygen and the methyl group. A value of Eg=0.5kcal mol-1 (expressed relative to trans) was derived from the 641

Rotational Isomeric State Analysis of Poly(oxyethylene). Conformational Energies and the Random-Coil Configuration

“…3 The leastsquares treatment of the experimental data was performed as set forth in paper I. 3 The value of Ea was thus estimated to be -0.6 kcal Polymer J., Vol. 17, No.7, 1985 Table I) was found to be well reproduced by calculation with the Ea value given above.…”

“…In these analyses, conformational energy Ew for the four-bond interaction C-0--tf,l-C-0 was set equal to the value (0.53 kcal mol-1 ) estimated by the calculation. 3 It should be important to note that values of Err and EP derived from the monomer model (DME) and the polymer are comparable with each other when compared for the same solvent system. The values of Err, EP, and Ew derived in this manner are generally in agreement with those adopted previously 2 in the analysis of the unperturbed dimension (C=5.2).…”

“…2 The rotational angles were chosen from the energy calculations presented in paper 1. 3 The conformational energies listed are those established for pure DME in the preceding section.…”

“…In our recent paper 3 (hereafter referenced as paper 1), conformational characteristics of 1,2-dimethoxyethane (DME) and POE were studied by the NMR method, supplemented with conformational energy calculations. The observed temperature dependence of the vicinal coupling constants was analyzed on the basis of the rotational isomeric state (RIS) scheme.…”

Phase Behavior and Ordering Characteristics of Some Chain Molecules Dissolved in a Nematic Liquid Crystal, 4′-Methoxybenzylidene-4-n-butylaniline

“…A detailed conformational analysis has been carried out for 1,2-dimethoxyethane (DME) and poly-(oxyethylene) (POE) in various conventional solvents on the basis of NMR vicinal coupling constant measurements. 7 The energy difference (LIE) between the gauche (g) and trans (t) state varies from -O.S to -1.1 kcalmol-1 with the polarity of the medium. The values were found to be nearly identical between DME and POE in a given solvent.…”