R, K, B and E Bands in Electronic Spectra
R, K, B and E Bands in Electronic Spectra
The bands due to n → π* transitions in compounds having a single chromophoric group and having at least one non-bonding pair of electrons on the hetero atom are called R-bands(German: Radikal). Such bands are forbidden bands due to less intense nature and also undergoes a red shift with little change in intensity with εmax value less than 100 e.g. acetone shows R band at λmax 270 nm with εmax 15.K, B and E bands arise due to π → π* transitions. K-bands (German: Konjugierte) are usually more intense having εmax value more than 104. The K-band in butadiene appears at 217 nm. The benzenoid i.e. B-bands are characteristic of aromatic and hetero aromatic compounds. The B-band in benzene appears at 256 nm. Acetophenone shows a K-band at 240 nm with εmax 13,000 and a B-band at 278 nm with εmax 1110. The ethylinic i.e. E-bands are characterisitic of benzenoid system with three close cyclic conjugated double bonds. These are further characterised as E1 & E2. E1 band appears at lower wavelength than E2 e.g. E1 & E2 bands of benzene appear at 184 & 204 nm respectively.
