# Predicting the generation of singlet oxygen

On Wikipedia, I saw the reaction $\ce{H2O2 + NaOCl -> O2 + NaCl + H2O}$, where the $\ce{O2}$ is in the $^1\Delta_g$ (singlet) state. Can anyone offer an explanation for why it isn't in the usual triplet state, maybe using the theory of molecular orbitals?

Molecular orbitals are actually, to the best of my knowledge, not really helpful in explaining this observation. Instead, a much more fundamental concept does a much better job: the conversion of (electronic) spin.

The reacion you mentioned can be analysed in terms of spin states. Nobody will disagree that hydrogen peroxide, sodium hypochlorite, sodium chloride and water are singlet compounds. Therefore, using generic Greek letters as placeholders for the different compound to illustrate the point that their true doesn’t matter much, we can write the equation as:

$$\ce{^1\unicode{x391}\ + ^1\unicode{x392}\ -> ^?\Gamma\ + ^1\Delta\ + ^1\unicode{x395}}\tag{1}$$

We see that we are reacting two singlet compounds two three compounds and two of the products are also singlets. This means, every spin on the left-hand side of the equation is paired and most of the spins on the right-hand side, too. To generate a triplet product, we would need to flip a few spins but that is a slow process on the timescale of a chemical reaction. (Note that the conversion from singlet oxygen back to triplet can be observed as a faint red glow demonstrating that it is not instant but delayed.) Thus, in the moment of the particle collision, generating all-singlet products from all-singlet reactants (if only a single product potentially can have an unpaired spin) is the only option. Thus:

$$\ce{^1\unicode{x391}\ + ^1\unicode{x392}\ -> ^1\Gamma\ + ^1\Delta\ + ^1\unicode{x395}}\tag{2}$$

• A small point, but if there was enough energy then S+S$\rightarrow$ T + T is a possible spin allowed process. Presumably to form water in a triplet excited state would need vast amounts more energy compared to what is available. – porphyrin Sep 25 '17 at 13:49
• @porphyrin But it still requires two triplets to be formed, doesn’t it? So in an oxygen case, you would need a double collision at least, no? – Jan Sep 26 '17 at 3:41
• not a double collision, just enough energy initially to form two triplets. Two triplets can be formed from two singlets in the collision just as can two singlets as you point out. The reverse process is more commonly observed; triplet-triplet annihilation. – porphyrin Sep 26 '17 at 8:29
• @porphyrin No, what I meant was like — We would need to have two species that are triplets at the end of the equation, so if it’s the generation of oxygen as discussed here, we would need an additional species capable of reaching a triplet state to take part here? Most likely two sets of reactants to give two triplet oxygens? – Jan Sep 26 '17 at 8:33
• yes you are correct :) – porphyrin Sep 26 '17 at 8:45