Computational Investigation of Substituent Effects on the Alcohol + Carbonyl Channel of Peroxy Radical Self- and Cross-Reactions

Organic peroxy radicals (RO2) are key intermediates
in atmospheric chemistry and can undergo a large variety of both uni-
and bimolecular reactions. One of the least understood reaction classes
of RO2 are their self- and cross-reactions: RO2 + R′O2. In our previous work, we have investigated
how RO2 + R′O2 reactions can lead to
the formation of ROOR′ accretion products through intersystem
crossings and subsequent recombination of a triplet intermediate complex 3(RO···OR′). Accretion products can potentially
have very low saturation vapor pressures, and may therefore participate
in the formation of aerosol particles. In this work, we investigate
the competing H-shift channel, which leads to the formation of more
volatile carbonyl and alcohol products. This is one of the main, and
sometimes the dominant, RO2 + R′O2 reaction
channels for small RO2. We investigate how substituents
(R and R′ groups) affect the H-shift barriers and rates for
a set of 3(RO···OR′) complexes. The
variation in barrier heights and rates is found to be surprisingly
small, and most computed H-shift rates are fast: around 108–109 s–1. We find that the barrier
height is affected by three competing factors: (1) the weakening of
the breaking C–H bond due to interactions with adjacent functional
groups; (2) the overall binding energy of the 3(RO···OR′),
which tends to increase the barrier height; and (3) the thermodynamic
stability of the reaction products. We also calculated intersystem
crossing rate coefficients (ISC) for the same systems and found that
most of them were of the same order of magnitude as the H-shift rates.
This suggests that both studied channels are competitive for small
and medium-sized RO2. However, for complex enough R or
R′ groups, the binding energy effect may render the H-shift
channel uncompetitive with intersystem crossings (and thus ROOR′
formation), as the rate of the latter, while variable, seems to be
largely independent of system size. This may help explain the experimental
observation that accretion product formation becomes highly effective
for large and multifunctional RO2.