The infra‐red absorption spectra of acetaldehyde, paraldehyde, α‐ and β‐trithioacetaldehyde

Abstract

Abstract The infra‐red absorption spectra of gaseous and liquid acetaldehyde (region 7‐14 μ); liquid paraldehyde (7‐20 μ); solid α‐ and β‐trithioacetaldehyde (7‐14 μ) and solutions of these compounds in CS 2 (7‐20 μ) have been determined. A discussion of the vibrations belonging to the frequencies found for acetaldehyde is given. It follows by comparison of the Raman spectrum with the infra‐red absorption spectrum, that the molecule of paraldehyde in the liquid state is not plane. The number of vibrations active in infra‐red absorption and in the Raman effect has been derived for a cis ‐configuration (with the three methyl‐groups on the same side of the ring, probable symmetry C 3v ) and a trans ‐configuration (one of the methyl‐groups is on the opposite side of the ring, probable symmetry C a ), separating the modes of vibration of the molecule into those characteristic of the C 3 O 3 ‐ring, the methyl groups etc. The supposition, that in liquid paraldehyde an inner equilibrium between cis‐ and trans‐molecules exists, with the cis‐form preponderating, seems the more probable one. In absence of a complete set of polarization measurements for the Raman lines of paraldehyde, no definite choice can be made. The molecules, of α‐ and β‐ trithioacetaldehyde also possess “staggered” rings. The evidence from the Raman effect and from infra‐red absorption is in favour of ascribing to the β‐form the cis ‐configuration, in agreement with the conclusion drawn by Chattaway and Kellett from their experiments on the oxidation of these compounds. The correspondence of the spectra of paraldehyde and the two forms of trithioacetaldehyde in the region from 2600‐3100 cm −1 is good, proving the reliability of the separation ‐ to a first approximation ‐ of the vibrations of these molecules as a whole, into those of the ring, the methyl‐groups etc.