Natural Bond Orbital-Resolved Antibonding Channels Govern Neutral Selective Bond Dissociation in Electron–Molecule Collisions

Neutral selective bond dissociation under low-energy electron irradiation is difficult to predict because the molecular states accessed by electron–molecule collisions do not, by themselves, reveal which chemical bond is locally activated. Here, we establish a natural bond orbital (NBO)-resolved antibonding-channel framework that connects electron-impact excitation to bond-specific neutral dissociation. Using 1-bromo-4-chlorobenzene as a model system containing competing C–Br and C–Cl cleavage pathways, we calculate state-resolved electron-impact excitation cross-sections with Quantemol-EC to quantify the accessibility of molecular excited states. Dissociative electron attachment analysis, incorporating bond dissociation energies, fragment electron affinities, and dissociative vibrational frequencies, predicts preferential C–Br cleavage in agreement with experimental trends. In contrast, potential energy curve-based neutral dissociation analysis along bond elongation coordinates shows configuration-dependent selectivity and does not directly identify the localized antibonding character responsible for bond weakening. We therefore project the same electron-impact-accessible excited states onto localized σ*(C–Br) and σ*(C–Cl) NBO channels. The resulting bond-resolved antibonding weights distinguish scattering accessibility from localized dissociation propensity, thereby revealing how an electronically accessed state is translated into selective neutral bond cleavage. Because the present postprocessing uses phase-insensitive excitation cross-sections rather than phase-resolved channel amplitudes, we adopt a diagonal-ensemble approximation in which no fixed relative-phase correlation is retained between distinct MO-pair channels; the channels are therefore combined incoherently at the probability level. This integrated DEA, PEC, and NBO analysis provides a chemically resolved framework for predicting electron-induced bond selectivity and identifies localized antibonding channels as the missing link between electron–molecule collision physics and neutral chemical reactivity.