2.5 Classification of Adsorption Bands
The absorption band[83] is the spectral band position of the absorption peak in the UV spectrum，it can be divided into four types：
R-band[84] （for radical-like）is formed by n→π* transition，the wavelength maximum（λmax）of R-band is more than 270 nm，the molar absorptivity of it is less than 100.R-band can be observed in unsaturated groups that contain atoms such as oxygen[85] or nitrogen[86].
K-band[87]（for conjugated，from German konjugierte）is formed by π→π* transition in conjugated system，and it is the most common band in UV spectrum.The wavelength maximum（λmax）of K-band ranges from 210 nm to 270nm，the molar absorptivity of it is more than 10000.
B-band[88]（for benzenoid band）is the characteristic band of aromatic compounds[89]，and caused by vibration[90] of benzene ring and π→π*overlap.Fine structure adsorption[91] can be found between 230 and 270 nm.
E-band[92]（For ethylenic[93] band）is caused by π→π* transition in ethylenic bond.There are E1-band and E2-band.E1-band is produced by π→π* transition in ethylenic bond of the benzene ring.The wavelength maximum（λmax）of E1-band is about 185nm，the molar absorptivity of E1-band is more than 10000.E2-band is produced by π→π* transition in conjugated diene[94] of the benzene ring.The wavelength maximum（λmax）of E2-band is about 204 nm，the molar absorptivity of E1-band is about 7400.When the chromophore substitutes the benzene ring and conjugates，E-band and B-band will shift to longer wavelength，E2-band and K-band will merge[95].
A molecule may give rise to more than one band in its UV spectrum，either because it contains more than one chromophore or because more than one transition of a single chromophore is observed.Thus the UV spectrum of benzene in cyclohexane[96] contains three observed bands（Fig.2-4）.
Fig.2-4 The UV spectrum of benzene in cyclohexane
图2-4 苯在环己烷中的紫外光谱