Raman effect study of the two hexachloroketocyclopentenes C5Cl6O

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Abstract
The Raman frequency shifts of the two isomeric ketones C5Cl6O of melting points 28° and 92°, described for the first time by Zincke and Küster in 1888, were determined by a study of the Raman spectra of the undercooled liquid 28° isomer, the molten 92° isomer and of solutions of both of them in carbon tetrachloride as well as in carbon disulphide. Polarization measurements for liquid 28° and for solutions of the 28° and 92° isomers in CCl4 were also performed.Accelerated by the illumination, the molten 92° ketone shows an appreciable transformation in the direction of the equilibrium mixture corresponding to 28° in excess at that temperature. In undercooled liquid 28° the transformation is of subordinate degree at room temperature, even after long‐continued illumination.In the solutions no conversion took place to any measurable extent, as was proved by melting point determinations of the solids obtained by evaporation of the solvent after the Raman effect study.There are distinct differences between the Raman spectra of the two isomers. The Raman results do not agree either with possible open‐chain compounds of formula C5Cl6O, or with formulae containing heterocyclic six‐rings built up of five carbon atoms and one atom of oxygen. No simple decision can be reached between possible structures with four‐membered rings and the five‐ring formula accepted by Zincke and Küster on chemical grounds, when only the purely physical facts are considered.The Raman results (frequency heights, polarization results) agree best with cyclical structures with plane rings for both ketones, the 28° ketone corresponding to the less symmetrical structure II magnified image with a conjugated system of double bonds C = C with C = O, and the 92° ketone having the more symmetrical structure I. magnified image From the incomplete determinations of the composition of the equilibrium 28° ⇆ 92° in the liquid state at three temperatures by Küster, it follows that the 28° isomer possesses the lower energy content. The energy difference can be ascribed mainly to the extra resonance energy in the 28° ketone, caused by the conjugation of the double bonds C = C and C = O, which is also an argument for the structure II for the 28° form.

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