Spin-Restricted Descriptions of Singlet Oxygen Reactions from XMS-CASPT2 Benchmarks

Reactions of singlet oxygen are numerous, some of which
are desired
but many are unwanted. Therefore, the ability to correctly predict
and interpret this reactivity for complex molecular systems is essential
to our understanding of singlet oxygen reactions. DFT is widely used
for predicting many reactions but is not suited to degenerate electronic
structures; application to isolated singlet oxygen often uses the
spin-unrestricted formalism, which results in severe spin contamination.
In this work, we demonstrate that spin-restricted DFT can correctly
describe the reaction pathway for four prototypical singlet oxygen
reactions. By careful benchmarking with XMS-CASPT2, we show that,
from the first transition state onward, the degeneracy of the 1Δg state is broken due to differing interactions
of the (degenerate) π* orbitals with the organic substrate;
this result is well replicated with DFT. These findings demonstrate
the utility of using spin-restricted DFT to explore reactions, opening
the way to confidently use this computationally efficient method for
molecular systems of medium to large organic molecules.