Theory of Color of Organic Compounds

Several structural formulae can be written for a colored compound, since the molecule usually contains an alternation of single and double bonds. The color is attributed to absorption bands corresponding to transitions between the levels which arise from a resonance among these different possible structures. A calculation is made for the levels of benzene, cyclohexadiene, fulvene, and azulene; fulvene being a yellow compound, azulene a blue one; the others, although colorless, have absorption bands in the near ultraviolet. One parameter only enters the calculation and is evaluated by data on heats of hydrogenation. The absorption bands of the molecules are then calculated without using any optical data, the results agreeing well with experiment. The calculation is amplified in the case of benzene so as to include polar structures and also triplet states in the linear combination representing the molecule. The symmetry of the states is discussed in connection with selection rules, which are found to be in agreement with the relative intensity of the band groups in benzene. An interpretation of the structure of the 2600A band is proposed which is shown to account for many properties of this band system.