Triplet State Radical Chemistry: Significance of the Reaction of 3 SO 2 with HCOOH and HNO 3 .

The triplet excited states of sulfur dioxide can be accessed in the UV region and have a lifetime large enough that they can react with atmospheric trace gases. In this work, we report high level ab initio calculations for the reaction of the a 3 B 1 and b 3 A 2 excited states of SO 2 with weak and strong acidic species such as HCOOH and HNO 3 , aimed to extend the chemistry reported in previous studies with nonacidic H atoms (water and alkanes). The reactions investigated in this work are very versatile and follow different kinds of mechanisms, namely, proton-coupled electron transfer ( pcet ) and conventional hydrogen atom transfer ( hat ) mechanisms. The study provides new insights into a general and very important class of excited-state-promoted reactions, opening up interesting chemical perspectives for technological applications of photoinduced H-transfer reactions. It also reveals that atmospheric triplet chemistry is more significant than previously thought.