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

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).