Alkyl Shifts in Absorption Spectra of Azulene and other Aromatic Molecules

An inquiry is made into the factors which cause shifts in the ultra-violet spectra of aromatic molecules on substitution. The method of molecular orbitals is used, and the discussion is confined to π-electrons. It is shown that there are two chief effects: the first is essentially electrostatic, or inductive; the second is essentially conjugative, or mesomeric. In alternant hydrocarbons the second predominates and causes a red shift. In other molecules (e.g. azulene, quinoline) the first effect is considerably larger and now predominates over the second. Different positions of substitution will lead to shifts, sometimes to the red and sometimes to the blue. Electron donors will cause shifts in opposite directions to electron acceptors. The importance of these variations in characterizing the electronic transitions is stressed. Detailed investigation of the various methyl azulenes confirms the assignment of the long wavelength band with almost complete certainty.