Infrared Study of β -Propiolactone in Various Solvent Systems and other Lactones

Many lactones exhibit two significant bands in the carbonyl stretching region which result from Fermi resonance interaction between vC = O and a combination or overtone of a lower-lying fundamental (or fundamentals). However, for β-propiolactone, three significant bands in the carbonyl stretching region of the spectrum are observed. These bands result from vC = O in Fermi resonance between the combination tone v₆ + v₁₃,A′ and 2v₁₀,A'. The extent of Fermi resonance interaction between vC = O and the combination and/or overtone is dependent upon the solvent system. Approximate unperturbed vC = O frequencies have been calculated, and these vC = O frequencies decrease in frequency in the solvent order n-hexane, carbon tetrachloride, and chloroform. The unperturbed vC = O frequencies were also calculated with the use of reported frequencies and band intensities for many other lactones, and the unperturbed vC = O frequencies decrease significantly as the amount of ring strain is reduced in the lactone structures. An equation, based on perturbation theory for obtaining approximate unperturbed vC = O frequencies for situations where three modes are in Fermi resonance, was developed and used to assign the unperturbed vC = O frequencies for β-propiolactone in various solvent systems. This method assumes that all or nearly all of the intensity in the three bands in Fermi resonance arises from the vC = O mode.