Oxygen, addition effect triplet state

Since rapid reaction is expected and found between oxygen and molecules excited to their triplet state (i.e., 0.01 mm. oxygen has a marked effect) a less pronounced effect, such as is observed with 2-pentanone, may naturally be ascribed to a reaction with excited singlet states. The evidence is not conclusive, but the addition of 560 mm. oxygen to 2-hexanone at room temperature using 3130 A. radiation resulted in only a slight decrease in the acetone and propylene yields, so that Brunet and Noyes2 concluded that an excited triplet state was not involved in the type II process. Recent evidence from other sources seems to support this point of view. (Editor s Note Added in Proof However, a recent article by Ausloos and Rebbert [J. Am. Chem. Soc., 86, 4512(1964)] indicated that the type II process occurs via a triplet excited state.)... 

The addition of oxygen had no effect on the type II process at 105°C., but at room temperature oxygen affected the process adversely. This was ascribed to a wall reaction as the cell history affected the results. It is again concluded that an excited singlet state rather than a triplet state is involved in the type II process. 

In contrast to the absence of heavy-atom effects on both the formation and radiative decay of the triplet state, the ISC process can instead be assisted by molecular triplet oxygen [46,156,157]. The same mechanism is presumed to operate for other paramagnetic additives, e.g., the stable free radical TEMPO as quencher (see discussion of quenching rate constants below). 

Zahra and Noyes found that at 313 nm the addition of small amounts of oxygen (0.03-0.14 Torr) to 26 Torr of ketone (27°C) lowered the emission yield from 3-methyl-2-butanone by 21%, comparable to the effect seen in other small ketones. The emission yields observed by Zahra and Noyes in photolyses of pure ketone and in 2,3-butanedione-ketone mixtures, indicate clearly that the triplet state plays an important role in the photolysis of 3-methyl-2-butanone at 313 nm. There was no evidence of the triplet excited state observed in experiments at 253.7 nm. Lissi et al. (1973/1974) carried out experiments at 313 nm both with added 2,3-butanedione and with added cfi-l,3-pentadiene. These experiments confirm the importance of the triplet precursor to products for photolysis at 313 nm. 

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