Vibrational spectra and a valence force field for conformers of diethyl ether and deuterated analogues

Abstract From a comparison of the high-resolution infrared and Raman spectra of diethyl ether (I) and the deuterated analogues CD3CH2OCH2CD3 (II), CH3CD2OCD2CH3 (III), and CD3CD2OCD2CD3 (IV), fairly complete vibrational assignments for the trans-trans (TT) and trans-gauche (TG) conformers of all four species have been deduced, together with a valence force field adjusted to give a weighted least-squares fit to the observed frequencies of the TT conformer. From the temperature dependence of the infrared spectra bands were identified as belonging to either the TT or TG conformer (or to both). Information from Raman spectra and infrared vapour phase band contours was used to guide the computer refinement of the force field. An acceptable set of frequencies for the TG conformation of all four species has been calculated. In the vapour phase, the pure liquid, and in CCl4 and CS2 solutions the TT conformer predominates and is more stable than the TG conformer by about 1.1 kcal/mole. The greater stability of the TT conformer is interpreted in terms of a stabilizing interaction between the oxygen lone pair orbitals and the nearest hydrogen atoms of the methyl group, making the methyl group unsymmetrical and removing the degeneracy of the asymmetric CH3 stretching and CH3 bending vibrations.

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