Vibration spectra and rotational isomerism of chain molecules. IV. Diethyl sulfide, ethyl propyl sulfide, and butyl methyl sulfide.

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The Raman and infrared spectra of diethyl sulfide, ethyl propyl sulfide and butyl methyl sulfide were measured for the gaseous, liquid, glassy and crystalline states. The normal vibration frequencies were calculated, a consistent set of force constants explaining the frequencies of basic aliphatic sulfides being assumed. The rotational isomerism was studied and the following conclusions were obtained. (1) Only the all-trans form exists in the crystalline state. (2) Many forms coexist in the liquid state and most of them persist in the glassy state even at the liquid nitrogen temperature. (3) In the liquid state, the gauche conformation about the S–C axis is more stable than the trans conformation, and the gauche conformation about the C–C axis directly adjoining the S–C axis is as stable as the trans conformation. These results confirm that the repulsive force between nonbonded hydrogen atoms is one of the important factors influencing the stability of molecualr conformations. The stable conformations of the sulfide molecules were correlated with those of polythioether chains.
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