INFRARED STUDY OF BETA-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 nu-C = O and a combination or overtone of a lower-lying fundamental (or fundamentals). However, for beta-propiolactone, three significant bands in the carbonyl stretching region of the spectrum are observed. These band result from nu-C = O in Fermi resonance between the combination tone nu-6 + nu-13,A' and 2-nu-10,A'. The extent of Fermi resonance interaction between nu-C = O and the combination and/or overtone is dependent upon the solvent system. Approximate unperturbed nu-C = O frequencies have been calculated, and these nu-C = O frequencies decrease in frequency in the solvent order n-hexane, carbon tetrachloride, and chloroform. The unperturbed nu-C = O frequencies were also calculated with the use of reported frequencies and band intensities for many other lactones, and the unperturbed nu-C = 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 nu-C = O frequencies for situations where three modes are in Fermi resonance, was developed and used to assign the unperturbed nu-C = O frequencies for beta-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 nu-C = O mode.