Electron Density Redistribution as a Descriptor for Excited-State Reactivity

This study introduces the electronic redistribution function, Δρij(r), as a descriptor for analyzing excited-state reactivity in photochemical
processes. Building upon the Quantum Theory of Atoms in Molecules
(QTAIM) and state-specific density partitioning, Δρij(r) quantifies electron
density changes between two electronic states (i and j) and identifies reactive sites by integrating charge redistribution
over atomic basins. This approach includes state weights (ci2, cj2) to ensure
physically meaningful interpretations of density differences. Applied
to diverse systemsincluding the [2 + 2] cycloaddition of ethylene,
Paternò-Büchi reactions of carbonyl/thiocarbonyl compounds,
and photocyclizations of 1,3-butadienethe descriptor reveals
distinct reactivity patterns. For instance, Δρij(r) delineates charge
depletion (δ+) and accumulation (δ–) sites, predicting interactions that drive photoproduct formation.
The framework is further extended to analyze conical intersection
regions, offering insights into bond-forming mechanisms for processes
such as benzene isomerization and ethylene cycloaddition.