The Multiplicity Forbidden Absorption Bands in the Spectrum of Pyridine

Abstract

The two singlet‐triplet transitions of pyridine have been studied using very long optical paths. The first of these, a π‐electron transition, which was previously observed in the liquid phase only as a region of continuous weak absorption at the long wave end of the main absorption system, has been observed in the gas phase as a well‐defined band system between 3000 and 3300A with a maximum molar absorption coefficient, ε=0.10. The sixty observed bands have been fitted into a vibrational scheme which suggests that the transition responsible for them is allowed by symmetry. The second transition which involves the non‐bonding pair of electrons on the nitrogen atom was observed previously as a low temperature phosphorescence emission. Attempts to observe this transition in absorption using very long paths in the gas phase were unsuccessful, but it has been observed weakly using an optical path of 45 cm of liquid pyridine.