Vibration spectra of hydrocarbon molecules I. Frequencies due to deformation vibrations of hydrogen atoms attached to a double bond

An analysis has been made of the infra-red and Raman spectra of alkyl substituted ethylenes for ‘characteristic’ frequencies in the region between 1500 and 700 cm.-1 . The mean values of the frequencies characterizing the various types of substitution are as follows: asymmetrically disubstituted, 1415 and 890 cm.-1 ;trans -disubstituted, 1303 and 973 cm.-1 ;cis -disubstituted, 1260 and 973 cm.-1 ; mono-substituted, 1415, 1295, 990 and 910 cm.-1 ; trisubstituted, 1383 and 820 cm.-1 . These frequencies have all been assigned to specific modes of vibration, essentially localized in deformation motions of the hydrogen atoms directly attached to the double bond; the higher frequencies (>1000 cm.-1 ) are concerned with motions in the plane of the ethylenic double bond and the lower frequencies arise from motions out of that plane. Using a valency force field, a theoretical explanation has been given of the persistence of these characteristic frequencies in alkyl substituted ethylenes and detailed calculations have been made of the relevant force constants involved. The variation in certain of these characteristic frequencies when halogen atoms replace the alkyl groups has been considered and shown to be due to changes in the force constants and not to the changes in the masses of the substituents. For a given type of substitution, the force constant decreases progressively along the series alkyl group, iodine, bromine, chlorine and fluorine, i.e. with increasing electronegativity. The use of these methods to provide a means of following changes in the electronic structure of the double bond with substitution is discussed.