Photochemistry of Cyclopentanone. I. Details of the Primary Process

Abstract

In the photochemistry of cyclopentanone, three processes are known to be important. These lead to CO + 2C2H4 (1), [formula omitted] (2) and 4-pentenal (3), respectively. Evidence is now presented to show that since there is no detectable light emission from cyclopentanone when excited at 3130 Å., and since oxygen does not suppress any of the processes, these processes probably do not arise from a triplet state. Photolysis of cyclopentanone in the presence of O218 does not lead to detectable amounts of cyclopentanone-O18. The effect of pressure or of the addition of a foreign gas on the distribution of the products has been interpreted to mean that the relative importance of the three photochemical processes (which probably occur from the same upper singlet state) is dependent on the vibrational energy possessed by the molecule at the instant of decomposition. It is estimated that the lowest vibrational level at which the processes leading to carbon monoxide, i.e., (1) and (2) can still occur differ by 2.5 kcal. from the lowest vibrational level from which process (3) can occur. The lack of a scavenging effect on the products through the addition of even 35.5 mm. of oxygen suggests that the previously postulated diradical hypothesis for (1) and (2) should be reconsidered. © 1961, American Chemical Society. All rights reserved.