Group frequencies interaction effects

This correlation does not apply to lactones with two carbonyl groups in the same six-membered ring. Wasserman and Zimmerman [23] have examined mesolactide and benzilide in which the carbonyl frequencies occur between 1767 cm" and 1757 cm" and Randall et al [17] quote a value of 1721 cm" for dehydracetic acid. There are compounds in which interaction effects might well be expected and are too complex for any interpretation of these shifts to be attempted. 

The purpose of this chapter is to discuss the vibrational origin of group frequencies with an emphasis on mechanical effects. A characteristic of a group frequency vibration is that mechanical interaction effects which control the form of the vibration are relatively constant from molecule to molecule, making the frequency readily predictable. Mechanical interaction effects can only occur between vibrations belonging to the same symmetry species. For example, in a molecule with a plane of symmetry such as vinyl chloride, in-plane vibrations will not interact with out-of-plane vibrations. All group frequencies mentioned in this chapter will be discussed and referenced in later chapters. 

See also Hydrogen Bonding and Other Physicochemical Interactions Studied By IR and Raman Spectroscopy IR Spectral Group Frequencies of Organic Compounds IR Spectroscopy, Theory Photoelectron Spectrometers Polymer Applications of IR and Raman Spectroscopy Surface Studies By IR Spectroscopy Symmetry in Spectroscopy, Effects of. 

Such characteristic and reliable frequencies provide a powerful means for identification of molecular subunits. By definition, they are largely insensitive to mechanical (vibrational) interactions and electronic effects from different adjacent atoms or groups. However, these perturbations can produce consistent shifts that are not so great as to destroy the characteristic nature of the group frequency, so that they provide information on the nature of the immediate chemical environment. 

Field ects will also perturb the carbonyl frequency. The classic case of a field effect is that of the chloroacetones. In those rotamers in which the chlorine atom is in the ecUpsed position with respect to the oxygen, repulsive lone-pair interactions occur. Chloro groups can generate effects that result in suppression of the contribution of dipolar carbonyl resonance forms, and therefore the system undergoes a rise ( 30 cm ) in the stretching frequency. 

---