Spectral Resemblances in Azulene and Naphthalene

Abstract

The spectrum of the odd‐ring compound azulene, C₁₀H₈, from 1730A to 7000A, is compared with that of its regular isomer naphthalene from 1600A to 3500A. The 5 band systems found in azulene seem to correspond, with respect to vibrational structure, intensity, and sequence to the 5 found in naphthalene, except that the former are all shifted to lower frequencies. The lowest band is shifted the most, about 17,000 cm⁻¹, and its absorption gives azulene its blue color. This band has quite different properties from those of the lowest band in the blue compound pentacene. The observed energy levels are compared with LCAO predictions of the lowest states in naphthalene (Coulson) and azulene. The qualitative agreement is good.