Modelling Gas-phase Accretion Product Formation Through Peroxy Radical Self- and Cross-Reactions

Field and laboratory studies indicate that the least volatile organic compounds participating in atmospheric new-particle formation are accretion products formed in the self- and cross-reactions of peroxy radicals (RO2). However, the actual mechanisms of the process has remained elusive.

Using multireference quantum chemical methods, we have demonstrated that RO2 + R'O2 reactions inevitably proceed via RO4R' tetroxides (as postulated already in the 1950s), which then rapidly decompose to yield O2 and a complex of two alkoxy radicals (RO...R'O). For the reaction to be thermodynamically feasible, the O2 must be formed in its triplet ground state. Spin conservation requires that the two alkoxy radicals are also coupled as a triplet, preventing direct recombination to ROOR' accretion products. ROOR' formation requires an intersystem crossing (ISC, a.k.a. spin-flip), to the singlet surface. Both the ISCs, as well as the competing channels leading to free alkoxy radicals or carbonyl and alcohol products, occur on sub-nanosecond timescales. In sufficiently complex systems, additional reaction channels, e.g. formation of ROR' - type accretion products through RO scission reactions, may also be competitive.

The results of our computational work can be condensed into four general key conclusions:

1)The overall rate of RO2 + R'O2 reactions is determined by the formation rate of the RO4R': subsequent reaction steps are very rapid.

2)For many primary and secondary RO2, RO4R' formation is associated with low or non-existent barriers. The overall rate is then controlled by the formation of a pre-reactive complex - which can be modelled by computationally cheap force-field based simulations.

3)For most tertiary RO2, RO4R' formation is associated with a substantial barrier. The overall rate can then be modelled using e.g. transition state theory, though high-level quantum chemical methods are needed to estimate the barrier height.

4)The branching ratio for ROOR' formation is determined primarily by the ISC rate of 3(RO...R'O) intermediates. This rate, which requires expensive multireference calculations to evaluate, depends both on the parent RO2s and on the 3(RO...R'O) conformer. The variation in ISC rates can be up to 10 orders of magnitude: much larger than the variation of the competing dissociation and H-shift rates.