Chapter 2: Empirical Rules in CD Spectra and Absolute Configuration of Molecules

The most well known empirical rules for analysing CD spectra, and the information about the conformation or configuration of certain specific types of chiral system, are described. The octant rule for ketones, the best known empiral rule, is described in detail. Details of the analysis of the sign of the R band region of ketone spectra, which is associated with an electric dipole forbidden but magnetic dipole allowed n → π* transition, are explored. Cyclohexanone structures are used to illustrate the application of the octant rule. When viewed from the carbonyl oxygen atom along the z-axis, the saturated cyclohexanone ring and substituents can be divided into eight octants based on the xy-, xz- and yz-planes. Although cyclohexanone possesses a mirror plane and, therefore, lacks a chiral centre, significant CD intensity is often observed when substituents, referred to as perturbers, are introduced in an asymmetrical manner. The sign of the perturbation to the rotatory strength of the R band is usually determined by which octant the perturber lies in. Similar rules which have been developed to determine the sign of the CD signals of substituted benzene compounds based on the analysis of the electric and magnetic dipole forbidden ¹L_b band of the benzene ring are also described. Examples of compounds that deviate from the normal trends are used to emphasise that the known exceptions must be examined and firmly grasped before any detailed analysis is attempted.