Vibration Spectra and Rotational Isomerism of Chain Molecules. V. 2,5-Dioxahexane, 2,5-Dithiahexane, and 2-Oxa-5-thiahexane

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Abstract
The Raman and infrared spectra of 2,5-dioxahexane CH3OCH2CH2OCH3, 2,5-dithiahexane CH3SCH2CH2SCH3 and 2-oxa-5-thiahexane CH3OCH2CH2SCH3 were measured for the gaseous, liquid, glassy and crystalline states and were correlated with the normal-vibration calculations. The rotational isomerism was studied and the following conclusions were obtained. (1) The molecular form existing in the crystalline state is a non-extended form, in contrast with the cases of the unbranched ethers or sulfides containing one oxygen or sulfur atom which take the extended all-trans form. (2) Many forms coexist in the gaseous, liquid and glassy states. (3) The form existing in the crystalline state is the most stable in the liquid state. It was found that the repulsive force between non-bonded hydrogen atoms is one of the important factors influencing the conformational stabilities and for 2,5-dioxahexane the dipole-dipole interaction is another important factor. The stable conformations of these molecules were correlated with those of the polyether and polythioether chains. The stable isomers of 2-oxa-5-thiahexane could be explained on the basis of the knowledge of the conformational stabilities of the unbranched ethers and sulfides. The force constants of the ethers and sulfides were satisfactorily transferred to 2-oxa-5-thiahexane.