The Origin of the Reactivity of the Criegee Intermediate: Implications for Atmospheric Particle Growth.

The electronic structure of the simplest Criegee intermediate, H2COO, is practically that of a closed shell. On the biradical scale (β), where 0 corresponds to the pure closed shell and 1 to a pure biradical, its β value is only 0.10, suggesting that its ground electronic state is best described as a H2C=O(δ+)-O(δ-) zwitterion. However, this picture of a nearly inert closed shell contradicts its rich reactivity in the atmosphere. It is shown that the mixing of its ground state with the first triplet excited state, which is a pure biradical state of the type H2C˙-O-O˙, is responsible for the formation of strongly bound products during reactions inducing atmospheric particle growth.