Although nitrobenzene is the smallest nitro-aromatic molecule, the nature of its electronic structure is still unclear. Most notably, the lowest-energy absorption band was assessed in numerous studies providing conflicting results regarding its charge-transfer character. In this study, we employ a combination of molecular dynamics and quantum chemical methods to disentangle the nature of the lowest-energy absorption band of nitrobenzene. Surprisingly, the charge-transfer transition from the benzene moiety to the nitro group is found to be quenched by a flow of charge into the opposite direction induced by vibrational motion. Beyond clarifying the charge-transfer character of nitrobenzene, we show that the widely used approach of analyzing the ground-state minimum-energy geometry provides completely wrong conclusions.
Keywords: QM/MM; charge transfer; excited states; photochemistry; vibrational motion.