Chemical Processes Subsequent to n—π* and π—π* Transitions of Pyridine N-oxide and 2-Picoline N-oxide

Abstract

The gaseous photolysis of pyridine N‐oxide was carried out by irradiating with the 3261 A (Cd) or 2537 A (Hg) resonance line which corresponds to the absorption due to an n—π^* or a π—π^* transition of this substance, respectively. The main product was pyridine in both cases. However, there exists a remarkable difference in the relation between the quantum yield of pyridine produced and the temperature. In the case of 3261 A irradiation the quantum yield of pyridine increases steeply at temperatures greater than 80°C, which means that an activation energy would be required for the production of pyridine. On the other hand, the quantum yield of pyridine produced at 2537 A irradiation was nearly independent of the temperature. The activation energy necessary for the production of pyridine in the case of 3261 A irradiation is about 11.9 kcal mol^—1, which was estimated from the temperature‐dependence of the quantum yield of pyridine. The chemical processes succeeding to an n—π^* or a π—π^* transition of pyridine N‐oxide were interpreted under the assumption that the excited molecule dissociates to pyridine and atomic oxygen in a predissociative manner, in which a triplet state crossing with the first excited state exists as a short‐lived intermediate. The gaseous photolysis of 2‐picoline N‐oxide was also carried out by irradiating it with the 3261 or 2537 A resonance line. It was found that 2‐picoline N‐oxide undergoes a photochemical rearrangement to isomerize to 2‐pyridinemethanol at 3261 A (n—π^* transition) and a fission of N→O bond to produce 2‐picoline at 2537 A (π—π^* transition).