Can Accretion Products Be Formed at the Interface of Freshly Nucleated Particles?

Oligomerization reactions from RO₂ + R'O₂ radicals, occurring via a triplet (RO···³O₂···OR') cluster, are an important gas-phase reaction for the formation of low-volatile ROOR' accretion products. However, it remains unknown whether such reactions can occur at the interface of freshly nucleated particles (FNPs). For instance, FNPs coated with a shell of organic compounds could potentially form accretion products at the surface, further stabilizing the particle. Using quantum chemical methods, we here study how the RO₂ + R'O₂ reaction is influenced by interaction with FNP precursors such as sulfuric acid (SA), ammonia (AM), and dimethylamine (DMA). For the RO₂'s, we tested simple branched hydroxyl peroxy radicals (HO-RO₂) as the tether to the FNP components. Cluster structures were obtained using a systematic conformational sampling approach based on the ABCluster program and CREST. We calculated the final structure and vibrational frequencies at the ωB97X-D/6-31++G-(d,p) level of theory. Energy levels for intersystem crossing calculations were carried out at the XMC-QDPT2/6-311++G-(d,p) level of theory, and spin-orbit coupling matrix elements were calculated using CASSCF-(6,4)/6-311++G-(d,p). Our calculations show that the rate of the intersystem crossing needed to form ROOR' accretion products at the FNP model clusters lies in the range of 10⁶-10⁹ s^-1, similar to the rate previously computed in the gas phase. We also find that both the intermediate (RO···³O₂···OR') clusters and the resulting ROOR' accretion products interact strongly with the FNP components, leading to suppressed evaporation if formed at the surface. Unfortunately, the formation is limited by the requirement of two RO₂ radicals being involved. We hypothesize a pathway where the RO₂/R'O₂ are formed via oxidation reactions at the surface and recombine via a Langmuir-Hinshelwood mechanism. However, this must still be considered an extremely rare event.